CA1314964C - Navigation and tracking system - Google Patents
Navigation and tracking systemInfo
- Publication number
- CA1314964C CA1314964C CA000574251A CA574251A CA1314964C CA 1314964 C CA1314964 C CA 1314964C CA 000574251 A CA000574251 A CA 000574251A CA 574251 A CA574251 A CA 574251A CA 1314964 C CA1314964 C CA 1314964C
- Authority
- CA
- Canada
- Prior art keywords
- signals
- signal
- receiving
- phase
- phase difference
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
- G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves
- G01S5/14—Determining absolute distances from a plurality of spaced points of known location
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
- G01S5/0009—Transmission of position information to remote stations
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
- G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
- G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves
- G01S5/10—Position of receiver fixed by co-ordinating a plurality of position lines defined by path-difference measurements, e.g. omega or decca systems
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
- G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves
- G01S5/14—Determining absolute distances from a plurality of spaced points of known location
- G01S5/145—Using a supplementary range measurement, e.g. based on pseudo-range measurements
Abstract
ABSTRACT
The invention concerns a navigation and tracking system, for monitoring the movement of a roving object which carries one of a pair of receiving stations (D,E) the other receiving station being at a known position. Signals are transmitted by a number of independent transmission sources (A,B,C) equal at least to the number of dimensions in which movement is to be monitored. A link (F) passes a representation of the signal or signals received at one of the receiving stations (E) to the other receiving station (D) and means (R,S,T,U) at the other receiving station compares the signal representations and determines the phase difference or time delay between the received signals and hence the change in phase difference or time delay of the signals as the object moves. The movement of the object relative to the transmission source or sources and hence its position can thus be determined.
The invention concerns a navigation and tracking system, for monitoring the movement of a roving object which carries one of a pair of receiving stations (D,E) the other receiving station being at a known position. Signals are transmitted by a number of independent transmission sources (A,B,C) equal at least to the number of dimensions in which movement is to be monitored. A link (F) passes a representation of the signal or signals received at one of the receiving stations (E) to the other receiving station (D) and means (R,S,T,U) at the other receiving station compares the signal representations and determines the phase difference or time delay between the received signals and hence the change in phase difference or time delay of the signals as the object moves. The movement of the object relative to the transmission source or sources and hence its position can thus be determined.
Description
~31~6~
DESCRI PTION
NAVIGATION AND T~CKING SYSTBM ~
This lnvention relates to a navigation and tra~king system using radio or other broadcast transmissions.
A navigation system is a means by which the position of a navigator can be determined, either by the navigator himself, or at another location from which the information is relayed to the navigator or is used ~o direct his progress. A trackîng system is a means by which the position of a remote roving point may be tracked at one or more base stations, the coordinates deining the position of the rover being continuously available at the or each base station. There are many different systems.
For example, conventional radio navigation systems are known which use the signals from two or more pairs of special radLo transmitters, and fall into two distinct cateyories: (1) time referencing, and (2) phase referencing.
Systems in the first category include the Long Range Navigation System (LORA~ - C1 set up by the US
Coastguard. The radio transmissions take ~he form of radio pulses whose times of arrival a~ the roving point can be accurateiy measured. The navigator uses a special receiver to determine the time difference between the reception of pulses from each of a pair of such transmitters. This in~ormation defines a curve on which he must lie, along which all points have the same difference in distance from the two transmitters. The navigator then measureæ the time difference between the reception of pulses from a second similar pair of transmitters. The information defines a second curve and the point of intersection o~ the two curves is the position of the navigator.
1 3 ~
In general, there is more than one point of intersection, but the navigatvr usually knows roughly where he i9 and can select the correct one, or he can use signals from a third pair of transmitters to determine his position uniquely.
Systems in the second category include the Decca Navigation System. In such systems the radio transmissions from a pair of transmitters are coherent with each other, that is, they maintain a predictable phase relationship at all times. In the simplest case, they may be continuous sinusoidal transmissions of exactly the same frequency. The transmissions then form an interference, or 'fringe', pattern in which curves of minimum and maximum intensity may be identified~ The navigator measures the fluctuations in intensity as he moves around. He also measures the fluctuations in intensity of signals from another pair of coherent signal transmitters. If he knows his position at the start, he can determine his position at any subsequent tlme by counting the numbers of fringes of each pair that he has crossed, thereby establish1ng a point of intersection of ~he fringes and hence his location within the intersecting fringe patterns. A similar system is described in US-A-3889264 which shows the use of pairs o~ transmitters to create plural grids of isophase hyperbolic lines, two or more pairs being required for tracking purposes.
Other systems rely on signals transmitte~ by a roving object whose position is to be measured.
US-A-4651156 discloæes a system in which the position of a roving object is measured by comparing the phase of a range tone transmitted by the rover with that transmitted by a fixed transmitter. US-A-4169245 discloses a system for tracking a roving transmitter by estimating the differences in times of arrival o~
signals at two pairs of spaced receiver~ r using the .
1 3 ~
spread of frequences associated with modulation of the carrier transmission wave.
In US-A-3774215 the system ~hown uses OMEGA
~very low frequency) transmitters transmitting to a roving object r the roving object receiver detecting differences in phase between a reference signal and the received transmissions. Phase difference data is transmitted to a communication ~tation where it is analysed in a computer and positional data determined therein is transmitted back to the roving receiver.
US-A-4054880 discloses the use of three transmitters r the phases of which are compared with a local oscillator reference signal, measured phase difference data being passed to a central station for correction of phase drifts in the transmitters and reference oscillators.
The present invention is intended to overcome problems of the prior systems, particularly with regard to the necessity for dedicated transmitters and the need to provide these in pairsL
According to the present invention a navigation and tracking system, for monitoring the movement of a rovinq object, comprises a pair of receiving stations for receiving the signal~ transmitted by a number of independent transmis~ion sources equal at least to the number of dimensions in which movement is to be monitored r in use, one o the receiving stations being at a known position and the other being located on the roving objact; means for passing a representation of the signal or ~ignals received at one of the receiving stations to the other receiving station; means at the other receiving station for comparing the signal representation~ and for determining the pbase differellce or time delay between the received signals and hence the change in phase difference or time delay of the signals as the objec~ moves, in order to determine the movement o~
``` ~3~L~9~
the object relative to the transmission source or sources and hence its position.
When phase differences are measured the position of the roving object is not determined ~absolutely", but only relative to a known starting position. Furthermore, the phases must be monitored continuously so tha~ they may be tracked through ambiguous cycles of 360~ Each error of one cycle introduces an error in position at leas~ as large as one wavelength. A particular advantage of the system described is that the transmitters used may have any spectral characteristics provided that the signals are continuous (in the sense that there are no long gaps - pulsed transmissions are allowed if the repetition rate is high enough). Continuous wave unmodulated transmissions will therefore suffice. In principle, two transmitters are needed for navigation and tracking over a plane, but in practice three are required since the unknown and variable offset between the reference oscil]ators at the receiving stations must also be measurecl.
When the signals from the transmitters are modulated, the spread of frequencies within the bandwidth of the modulation enables timing delays ~etween the transmitted signals received at the two receiving stations to be determined and used instead of or in addition to phase difference measurements.
If the system is primarily a navigation system then the comparison and position determining means will preferably be located in the receiving station on the roving object; but if it is located at the base station then the position o~ the roving object can be signalled back to the roving object. If the system is primarily a tracking system then signalling back the position of the roving object to the roving object receiving station may not be required.
Although the transmission sources will ~c~9~
preferably be radio sources, particularly where the system is to operate say for tracking vehicles, transmission ak any frequency is envisaged, using waves of any type, such as sound signals, which might be used~ for example in laboratory systems.
ln a land based system for determining location over a localised area of the Earth's surface, two transmission sources at least will be required, but the system may be used to determine the distance moved by an object that is constrained to move along a fixed path, in which case a single transmission source only may be required.
Preferably, when usinq measurements of phase differences, the two receiving stations convert the signals received to baseband signals by the use of local oscillators which are phase-locked to local frequency standards or reference oscillators and the baseband signal from one of the receiving stations is transmi~ted to the other. ~n this case it i5 an advantaye if the system includes an additional transmis~ion source for measuring the drift of the reference oscillators or frequency standards, but an alternative is to lock the reference oscillators to one another using the transmission link between the two receiving stations.
The system may also include the transmission sources themselves, but since there does not need o be any specific phase relationship between them, the sources used by the system may be any exi6ting radio transmitters.
The system o the invention differs from conventional sy~tems such as those described above in ~he opening paragrapbs in that ~i) a single transmitter or transmitters are used rather than pairs o~ transmitters, (ii) the sîgnals are received at two points rather than one, (iii) the signals received at one point are relayed to the other and 6 ~
compared there to determine their phase or time delay difference, (iY) almost any radio transmitters may be used for this purpose provided that their positions are known.
Time delay measurements, for example, may use amplitude modulated signals in the long, medium or short wavebands which usually o~cupy bands of about lOk~z width each, frequency modulated VHF broadcasts which have widths of about 150k~z, or TV signals in the UHF band which occupy several M~z each~ The time delays between the arrivals of the signals at the two receiving stations from a modulated transmitter may be measured with an accuracy which i5 inversely related to the bandwidth (all other things being equal). The time delays are unambiguous quantities ln the sense that there are no recurrences of 360 as there are when determining phase differences and thus the time delays measured may b~ used to determine the pos~tion of the roving object without reference to a starting positionO
In either system there may be advantages ln placing the non-roving r~eceiving station (base station) at the same location as one of the transmitters. This is especially so when the system is used for nav~gation rather than tracking and correlation and processing wo~ld preferably take place on the roving object, the link being provided in the direction from the base station to the rovin~
object. If the base station which transmits the link signals to the roving object has an omni-directional radiation pattern then any number of separate roving objects can use the same link signals. This is equivalent to setting, say ~ = 0 in equation lC below~
Additionally, whether or not the base sta~ion is at the same location as one of the transmitters, the transmissions from the transmitter may be used to provide the link, by suitable modification, for ~3~9~
example, in a system in which the ba~e station receives the signals from all three transmitters and mixes tham to baseband and inserts them into the modulation o~ the transmissions from the transmitter at which it is located. It may be possible to do this without interference with the normal use o~ the transmissions; for examplej when the transmitter is a domestic broadcast FM repeater, the link signals may be inserted between the side band signals so that normal reception of the FM signals will not be affected, but the receiver on the or each roving object is able to extract the link signals from the modulation and use them to measure the phases or time delays as previously described.
There may also be advantages, in some applications, in using the same reference oscillator for the transmitter and co-located base station.
~here may also be advantages in modiying every transmitter in the way described in the immediately preceding paragraph. In that case, every roving receiving station then has a link built ln to every transmission it receives, thus giving considerable redundancy in the system and hence increased freedom from errors. When the system is being used to track the position of a roving object~ there may also be advantages in using the link transmitter itself on the roving object as one of the three triangulation transmitters. Thus, the roving object receives transmissions from two independent fixed transmitters, and transmits baseband representations of their signals over the link. The base station also receives signals from the two fixed transmi~ters; but additionally measures the link signals themselves as if they had come ~rom the third fixed transmitter~ This is equivalent to setting, say, ~ = L in equation lC below, and is also equivalent to using the link to lock ~he local oscillators.
In the examples given below, conventional broadcasting VHF transmitters are used, but naturally occurring transmission sources, such as radio stars, might also be used especially for space navigation.
Two examples of systems illustrating the principles of the invention and one example of a prototype system constructed in accordance with the invention will now be described with reference to the accompanying drawings in which:-Figure 1 shows a typical arrangement of~ransmitters and receiving pointsS
Figure 2 illustrates how the signals received from the transmitters are processed;
Figure 3 shows an arrangement for measuring the phase difference between the two signalE~
Figure 4 is a vector d:iagram of the system;
Figure 5 illustrates an arrangement for determining time delays;
Figure 6 illustrate~ the use of an interpolation function for determining time offsets.
Figures 7 and 8 i:llustrate, respectively a mobile receiver unit and a base station receiver unit of a prototype;
: 25 Figure 9 illustrates a correlation/computer system for analysis of the signals received by the roving and base station receiver units Figures lOA and lOB illustrate~ by way of flow charts, the signal processing which takes place in the base station computer to determine the path of the roving recelver; and Figure 11 shows a plot of a track of a mobile unit~
The particular systems shown are land based systems and~ rely on using transmitters A and B and, optionally a third transmitter C.
The first examplP relates to a system in which ~3~4~
phase differences are used to track the posi~ion of a receiver, The signals from the ~ransmitter A are received at a base statio~ D (which i6 a known length lAD away S from the transmitter A) and at roving point ~, whose position iæ to be tracked or determined (and which is an unknown distance lAE rom A). The receivers convert the incoming signals, within a reception band of finite bandwidth, to baseband Isee below) and those from the roving point E are then relayed to the base station D by means of a link F. This link F
might, for example, take the form of a dedicated telecommunications link, or non-dedicated one such aæ
a public cellular radio telephone nPtwork. At the base station D, the signals received directly from A
are compared with those received at E to determine their phase difference PA~ The phase difference PA, measured in degrees, i8 equal to the value:
(1AD--lAE).360/n-A~
where lAD is as beore, lAE is the distance fro~ A to E, ~A is the centre wavelength of the reception band, and n is an :integer. This process of determining PA is one of multiplication, averaging r and mathematical analysis (see below). ~he measured phase difference may be converted directly into a path length difference (modulo one wavelength~ as the centre wavelength ~A f the emissions received from A
can be determined by any conventional mean~.
Now, because the path length difference (lAD-lAE) can be determined, and as lAD i8 known, the possible values of the distanc* lAE can be calculated. Thus E lies on one of a number of spheres ~on a ground based system these can be considered to be circles) of radius lAD - PA.n.~/36~.
~ he signals received at D and E from the ~31~
transmitter B (in a band whose centre wavelength is different from that of A) are treated in the same fashion to determine their phase difference P~, and this defines a second set of circles of radiuR
lBD - PB.m.AB/360 centred on B on one o which E must also lie. m i an integer and AB is the centre wavelength of the transmission received from B. The location of E is then one of the points of intersection of two circles, one centred on ~ and one on B.
In order to determine the position of uniquely, the tracking or navi~ation process begins by calibration~ The rover E must know his initial position with respect to the base station D since the procedure outlined above produces an a~biguous result both because (i) there are two points of intersection of each pair of circles, and (ii) the phase differences PA and PB det~rmine the radii of the circles modulo one wavelength, and not uniquely. PA
and Pg may initially be any values and preferably are set to zero by starting E at D for example.
Thereaf~er, as the rover E moves away from the starting position at D, the values of PA and Pg are continuously monitored, and their values at any time define the current position of E uniquely~
For example, suppose that the transmissions from A and B are both centred on wavelengths close to 3 metres. Initially, PA and PB are both zero (the rover is at the base station) and after some movement of E they may take tbe values 1620 and -3240~
respectively, corresponding to differences of distance of 13.5 and -~7 metres. Thus we know tbat E
has moved from its (known) starting position in such a way as to increase its distance from A by 13 5 ; 35 metres, and to decrease its distance from B by 27 metres. Its new position is thereore determined.
During movement of E the phases must be monitored sufficiently often to ensure that no changes of more than 180 occur between one signal sample and the ne~t. If interruptions occur, either in the transmissions from A and B or in the link F~
the immediate histories of the phases might be used to interpolate over the interruption or an error~correcting transmission protocol may be used to overcome the problem if the error is in the transmission link. Otherwise, each 3609 of phase introduced, and not accounted for, results in an error in position of at least one wavelength (3 metres in this example~, The process of signal conversion to baseband at D and ~ is illustrated in Figure 2. The signals received from A by the antennas G and H are multiplied in mixers I and J with sinusoid~ generated by local oscillators R ancl L. Baseband products are output at P and M. Those from the roving station are then transferred to the base station via the link F
using transmitter ~ and receiver O, and appear at Q.
: The signals at P and Q can be compared (see below) to determine their phase difference. This will reflect the movement of E described above.
The signals received from B are treated in the same manner using a different set of mixers and local os~illators (not shown).
There is one combination of mixer and local oscillator for each transmitter to be monitored. All the local oscillators at each station are phase-locked to common reference oscillators W and X. However, as technology has not yet advanced sufficiently far for the construction of rèference oscillators W and X of sufficiently high stabili~y, any drift in either or ~oth of their frequencies results in a corresponding change in the calculated phase difference between P and Q. Thus it may appear that E is moving~ when in realit~ it is stationary, ~ 3 ~
This problem may be overcome using a third transmitter C (Fiyure 1). Its signals are received at D and E and are used to correct drifts ~n the reference oscillators W and X. In effect, the reference oscillators are both locked to C. If the frequency of C varies, it does not matter since both reference oscillator~ W and X are corrected by the same amount, their frequency difference remaining zero.
Note that the (changing) path of the link F
does not have a significant effect on the phase difference as the signals are converted to baseband (near zero frequency) before conveyance over the link. Furthermore, use of a double sideband transmission system offers immunity to errors of this sort.
An alternative is to lock one reference oscillator to the o~her using the transmiss~on link F.
In practice, the signals from A, Br and C are all treated in exactly the same manner to produce phases P~, PB, and Pc. Three non-linear equations involving the two unknown coordinates of E, and the unknown phase drift of W relative to X, can then be solved to determinQ the unknown qu ntitieæ. This can appreciated from tbe following, which ~hould be read in conjunction with Figure 4~
If A, B, and C are at vector positions ~, ~r and ~ relative to the base station D and the rover is :at a vector position ~ and if the three transmitting stations operate at centre wavelengths of AA~ AB~ and Ac~ then as PA, PB, and Pc are measured, r and P0 (the varying phase difference between the reference oscillators W and X) can be determined from the following equations:-, .
~ 3 ~
IL ~l l~l ~(qa ~qo) ~ ..... ~,............. lA
IL ~ (qb ~qo) = ~.. O.. ~............ lB
~ Icl ~(qc ~qo) ~ .......................... lC
where:
qa ~ ~APA~360l qb ~ ABPg/360~ qc ~ ACPC/360~ and qO = ~APo/360.
The phase diference between the signalsreceived by D and E can be measured using a process similar to that illustrated in Figure 3. The signals P and Q, which may have significant bandwidthsi are multiplied in a multiplier R. This could, for example, be a digltal correlator or a linear multiplier. Two products in phase ~uadrature are output and averaged in low-pass fil~ers ~ and T. It should be noted that, in practice, it may be simpler to produce two signal~, P and P', by mixing with separate o~cillators in phase-quadrature before correlating with Q in two separate multipliers R and R' to produce phase-quadrature product~ S and T. We can conveniently think o~these a~eraged products as 'sine' and Icosine'. A computer ~ performs the mathematical process of taking the inverse tangent of the fraction sine/cosina, giving the phase difference output at V. Thiæ will always be returned in the range -18Q to +180', and hence it i8 necessary to sample sufficiently often :to ensure that the change between ~uccessiYe values i8 less than 1805.
The example described above relates to ~he two-dimensional case where the transmitters~ base ; 35 station, and rover all lie roughly in one plane (eg~
the Earth's surface over sufficien~ly small ranges)O
With per~ect reference oscillators, two transmitters ' ' ~ `
1 3149~
would suffice to determine the position of the rover with respect to its starting point; in practice, three are needed as explained above. Greater accuracy and freedom from interruptions may be achieved by using more than this number. ~he same technique may be applied to the three dimensional case (eg. navigation in space) when a theoretical minimum of three transmitters, but four in practice, must be used.
The second example relates to a system that uses time delay measurements to track position of a receiver.
If the timing delay between the signal~
received at. the base station D directly from transmitter A and via ~he roving station E is determined as t~ and if the delay between signals received at base station D directly from transmitter B and via the roving st2ation E is tB then two non-linear equations can be solved once these ; 20 quantities have been measured, to determine the unknown coordinates of the roving station E~ This can be appreciated from the following which should be ; read in conjunction with Figure 4.
; If A and B are at a vector positions a and ` 25 relativ~ to the base station D and the rover is at vector position Lf then if t~ and tB are measured, can be found from the equations:
ct~ = 0 ..... ~........... 2A
Ll - ILI - ctg = 0 ................. 2 30 where c represents the speed of light.
In particular it should be note~ that the position of the rover can now be determined uniquely without reference to a known starting position so that, in principle at least, the method can be uæed : 35 to find the starting position for subsequent phase tracking D
Timing delays may be measured at any point in ~ 3 ~
time without any reference to any previous measurement and tracking is not required7 Furthermore, the transmitters are likely to have a broad range of spectral characteristics, but they must be modulated, as continuous wave unmodulated transmissions do not provide time delay ; information. The signals do not have to be continuous, and the wider the bandwidth o~ the transmissions the more accurately the timing delay may be measured. If there is more than one independent transmitter at the same transmission site, ~he total bandwidth occupied by the transmissions may be used as if the signals had all come rom one source.
In th~ method of measuring time delays described further below, the maximum bandwidth is used to obtain the highest accuracyO
In practice~ two transmitters are needed for navigation and tracking over a plane as previously described, the offset between the oscillators at the two receiYing stations being of almost no consequence. However, if three stations are being monitored for phase tracking purposes for greater accuracyJ then~ the accuracy of the position determined from timing delays alone can be improved by averaging. The accuracy with which a position may be determined in practice using time delays will be inferior to that determined by phase tracking by at least one order of magnitude.
; 30 Referring now to Figure 5, the baseband signals P, P' and Q are extracted as described in the first example (Figure ~ - it qhould be noted that in practice P as previously referred to ~omprises two signals Cnow P and P'] whlch have been mixed by separate oscillators in phase quadrature) and are digitized in one bit digitizers a, b and b' and the outputs of the digitizers are digital representations .
~L31~9~
1~
of the analog input signals. The sampling interval for digitization should be less than half the inverse of the bandwidth BW of the radio signals 7 i.e.
~ < (2BW)-l (the Ny~uist criterion) in order that minimal information is lost in the sampling process.
The bit streams from a, b and b' are delayed in shift registers c, d and d' respectively. The register c has parallel outputs e each corresponding to a delay of one unit of ts. The shift registers d and d' each have one output corresponding to half the maximum delay from c so that the outputs from d and d' correspond in time with the output from e which is half way along the register c, assumlng ~here is no further delay between the signals. Each of the outputs e is correlated (exclusive-OR operations) wlth the streams from d and d', two such correlations being shown by ~ and f'. The correlator X has two outputs corresponding to correlations in phase-quadrature. These are integrated in the low-pass digital filters g and h and the results passed to a computer U.
The computer ~ receives para]lel correlations, as above, corresponding to steps in the delay between the two signals in units of ts. A plot of the amplitude of the cross-correlation function verses time delay might be as shown for example in Figure 6. The large dots represent the numbers supplied to the computer and it can be seen clearly that the correlation is largest where the total time offset between the data streams is zero. In Figure 6 this corresponds to a time delay of ts + T~
It is unlikely that any of the discrete steps in delay which are actually measured will lie exactly on top of the required delay. Instead, the computer must fit an interpolation function (curve shown in Figure 6) to the measured points to determine the additional offset T. The accuracy with which the peak of the interpolation function can be found limits the accuracy of the method. The interpolation function may be calculated, or measured from the received signals.
Pigures 7 to 10 illustrates various components and features of a prototype system comprising a mobile unit 1 and a base station 2.
Figure 7 shows a rovlng or mobile unit rece~ver 100, which has a receiving antenna 101 which receives signals from three FM transmitters (A,B,C say - see Figure l)o A three-way splitter 102 supplies the signals from the an~enna to three substantially identical receivers 103A, 103B, 103C, each of which is tuned to receive the signals of one of the transmitters~ In this example the receivers 103A, 1O3B~ 103C are shown tuned to receive signals FA~ FB-FC of 96 M~z, 92.3MHz and 89.7 M~z respectively, corresponding to the nominal centre frequencies of three FM transmitters in the East Anglian region of England. Suitable receivers are Sony ICF 2001D~
Each of the receivers i8 phase locked to a master reference oscillator 104 via a local o~cillator 105 for frequency synthesis, the master oscilla~or being, for example a high-stability crystal oscillator or rubidium frequency standard, In ~he example the master oscillator outputs a 10 MHz signal and the phase-locked local oscillator has an output ~requency of 6.275 MHz. ~he receivers output signals at an intermediate frequency, in this case 10.7 MHz, to a 3-way combiner 106, and the combined signal, is mixed to baseband in a mixer 107 using a second local oscillator 105' at 10.7015 MHz which is also phase locked to the master reference oscillator 104. The baseband signal, which has an approximate band width of 10 K~z,i~ passed to a link transmitter 108 which thus transmitæ a signal containing representations of the three received signalæ, via a lB
link transmitter antenna lG9, to the base station 2 (see Figure 8), where analysis takes place.
The base station 2 has a receiver unit 200 (see Figure 8) which in most respectfi is identical with the mobile unit, reference numerals increased by 100 be~ng used for like components~ However, the receiver unit 200 has plural mixers 207Al 2~7B~ 207C
so that the baseband products from the three receivers are kept separate. Additionally~ the master reference oscillator is offset from that of the mobile unit receiver so that the baseband signals VA~ vB. VC rotate with respect the baseband signal vD
from the mobile unit by about lOOHz when the mobile unit is stationary.
15The base station 2 also includes a link antenna 209 via which the signal from the mobile unit 1 is received in a receiver 210. A phase corrector 211 removes the differential~ phase shifts inherently introduced by the link transmitter and receiver circuits and outputs the baseband sign~l VD. The : link transmitter and receiver can be one of many types, but preferably transmit and receive in the VHF
band for example. The phase correc~or 211 is of appropriate type to correct for the type~of phase respon~e o~ the transmitter and receiver circuits.
For example, if the phase response were like that of an R-C lag then the phase corrector would have and R-C lead configuration. In practice more complex phase responses are present, but can be corrected by conventional phase corrector circuits.
To process the baseband signals the base station 2 has a correlator section 220 ~ee Figure 9) for correlating the baseband signals and a microcomputer processor 230 for receiving the correlator products and generating a display of the position and track of the mobile unit.
~ 3 ~
The correlator section 220 has three correlators 221A, 221B~ 221~ for correlating, respectively, the baseband signals vA & VD~ vB & VD, and VC & vD. These produce quasi-sinusoidal outputs at about 100 ~z which reflect the offset between the master reference oscillators 104, 204. The quasi-sinusoidal outputs are passed through identical low-pass filters 222 to phase locked loops ~PLL) 223 where 'clean' versions of the quasi-sinusoids are produced either as sine- or as ~quare-waveB. The : PLL's 223 employ narrow loop bandwidths of say less than lHz in order to remove the effects of noise in the outputs of the low-pa~s filters 222. PLL's 224 receive the outputs of ~LL's 223 to produce outputs offset by 90 with those of PLL's 223 and these may have wider bandwidths - of the order of 10 Hz.
The sine and cosine products thus produced are fed to sample and hold circuits 225 on a conventional interface board in a microcomputer 230 which has a microprocessor 231 and timer 232, by means of which the data representing the products is manipulated to form a video output sent to a visual display unit 233~ In the prototype the display is used to represent a track of the path of the mobile unit as shown in Figure 11 r the scale being adjustable under computer control to one of a number o~ desired scales.
Fig. lOA illustrates, by way of a flow chart, the main elements of the computer's operation under program control~ in order to display on the VDU 233 a : 30 representation of the track of the mobile unit.
After the program is entered at step S01 the tlmer 232 and internal data buffers in the computer are initialized in step S02 and the interrupt timing ~see description below~ set going in step S03. In ~tep S04 the program checks to see whether or not a flag ~set by a separate interrupt routine - see Fig. lOB) is set or not. If it is then data in the current ~ 3 ~
DESCRI PTION
NAVIGATION AND T~CKING SYSTBM ~
This lnvention relates to a navigation and tra~king system using radio or other broadcast transmissions.
A navigation system is a means by which the position of a navigator can be determined, either by the navigator himself, or at another location from which the information is relayed to the navigator or is used ~o direct his progress. A trackîng system is a means by which the position of a remote roving point may be tracked at one or more base stations, the coordinates deining the position of the rover being continuously available at the or each base station. There are many different systems.
For example, conventional radio navigation systems are known which use the signals from two or more pairs of special radLo transmitters, and fall into two distinct cateyories: (1) time referencing, and (2) phase referencing.
Systems in the first category include the Long Range Navigation System (LORA~ - C1 set up by the US
Coastguard. The radio transmissions take ~he form of radio pulses whose times of arrival a~ the roving point can be accurateiy measured. The navigator uses a special receiver to determine the time difference between the reception of pulses from each of a pair of such transmitters. This in~ormation defines a curve on which he must lie, along which all points have the same difference in distance from the two transmitters. The navigator then measureæ the time difference between the reception of pulses from a second similar pair of transmitters. The information defines a second curve and the point of intersection o~ the two curves is the position of the navigator.
1 3 ~
In general, there is more than one point of intersection, but the navigatvr usually knows roughly where he i9 and can select the correct one, or he can use signals from a third pair of transmitters to determine his position uniquely.
Systems in the second category include the Decca Navigation System. In such systems the radio transmissions from a pair of transmitters are coherent with each other, that is, they maintain a predictable phase relationship at all times. In the simplest case, they may be continuous sinusoidal transmissions of exactly the same frequency. The transmissions then form an interference, or 'fringe', pattern in which curves of minimum and maximum intensity may be identified~ The navigator measures the fluctuations in intensity as he moves around. He also measures the fluctuations in intensity of signals from another pair of coherent signal transmitters. If he knows his position at the start, he can determine his position at any subsequent tlme by counting the numbers of fringes of each pair that he has crossed, thereby establish1ng a point of intersection of ~he fringes and hence his location within the intersecting fringe patterns. A similar system is described in US-A-3889264 which shows the use of pairs o~ transmitters to create plural grids of isophase hyperbolic lines, two or more pairs being required for tracking purposes.
Other systems rely on signals transmitte~ by a roving object whose position is to be measured.
US-A-4651156 discloæes a system in which the position of a roving object is measured by comparing the phase of a range tone transmitted by the rover with that transmitted by a fixed transmitter. US-A-4169245 discloses a system for tracking a roving transmitter by estimating the differences in times of arrival o~
signals at two pairs of spaced receiver~ r using the .
1 3 ~
spread of frequences associated with modulation of the carrier transmission wave.
In US-A-3774215 the system ~hown uses OMEGA
~very low frequency) transmitters transmitting to a roving object r the roving object receiver detecting differences in phase between a reference signal and the received transmissions. Phase difference data is transmitted to a communication ~tation where it is analysed in a computer and positional data determined therein is transmitted back to the roving receiver.
US-A-4054880 discloses the use of three transmitters r the phases of which are compared with a local oscillator reference signal, measured phase difference data being passed to a central station for correction of phase drifts in the transmitters and reference oscillators.
The present invention is intended to overcome problems of the prior systems, particularly with regard to the necessity for dedicated transmitters and the need to provide these in pairsL
According to the present invention a navigation and tracking system, for monitoring the movement of a rovinq object, comprises a pair of receiving stations for receiving the signal~ transmitted by a number of independent transmis~ion sources equal at least to the number of dimensions in which movement is to be monitored r in use, one o the receiving stations being at a known position and the other being located on the roving objact; means for passing a representation of the signal or ~ignals received at one of the receiving stations to the other receiving station; means at the other receiving station for comparing the signal representation~ and for determining the pbase differellce or time delay between the received signals and hence the change in phase difference or time delay of the signals as the objec~ moves, in order to determine the movement o~
``` ~3~L~9~
the object relative to the transmission source or sources and hence its position.
When phase differences are measured the position of the roving object is not determined ~absolutely", but only relative to a known starting position. Furthermore, the phases must be monitored continuously so tha~ they may be tracked through ambiguous cycles of 360~ Each error of one cycle introduces an error in position at leas~ as large as one wavelength. A particular advantage of the system described is that the transmitters used may have any spectral characteristics provided that the signals are continuous (in the sense that there are no long gaps - pulsed transmissions are allowed if the repetition rate is high enough). Continuous wave unmodulated transmissions will therefore suffice. In principle, two transmitters are needed for navigation and tracking over a plane, but in practice three are required since the unknown and variable offset between the reference oscil]ators at the receiving stations must also be measurecl.
When the signals from the transmitters are modulated, the spread of frequencies within the bandwidth of the modulation enables timing delays ~etween the transmitted signals received at the two receiving stations to be determined and used instead of or in addition to phase difference measurements.
If the system is primarily a navigation system then the comparison and position determining means will preferably be located in the receiving station on the roving object; but if it is located at the base station then the position o~ the roving object can be signalled back to the roving object. If the system is primarily a tracking system then signalling back the position of the roving object to the roving object receiving station may not be required.
Although the transmission sources will ~c~9~
preferably be radio sources, particularly where the system is to operate say for tracking vehicles, transmission ak any frequency is envisaged, using waves of any type, such as sound signals, which might be used~ for example in laboratory systems.
ln a land based system for determining location over a localised area of the Earth's surface, two transmission sources at least will be required, but the system may be used to determine the distance moved by an object that is constrained to move along a fixed path, in which case a single transmission source only may be required.
Preferably, when usinq measurements of phase differences, the two receiving stations convert the signals received to baseband signals by the use of local oscillators which are phase-locked to local frequency standards or reference oscillators and the baseband signal from one of the receiving stations is transmi~ted to the other. ~n this case it i5 an advantaye if the system includes an additional transmis~ion source for measuring the drift of the reference oscillators or frequency standards, but an alternative is to lock the reference oscillators to one another using the transmission link between the two receiving stations.
The system may also include the transmission sources themselves, but since there does not need o be any specific phase relationship between them, the sources used by the system may be any exi6ting radio transmitters.
The system o the invention differs from conventional sy~tems such as those described above in ~he opening paragrapbs in that ~i) a single transmitter or transmitters are used rather than pairs o~ transmitters, (ii) the sîgnals are received at two points rather than one, (iii) the signals received at one point are relayed to the other and 6 ~
compared there to determine their phase or time delay difference, (iY) almost any radio transmitters may be used for this purpose provided that their positions are known.
Time delay measurements, for example, may use amplitude modulated signals in the long, medium or short wavebands which usually o~cupy bands of about lOk~z width each, frequency modulated VHF broadcasts which have widths of about 150k~z, or TV signals in the UHF band which occupy several M~z each~ The time delays between the arrivals of the signals at the two receiving stations from a modulated transmitter may be measured with an accuracy which i5 inversely related to the bandwidth (all other things being equal). The time delays are unambiguous quantities ln the sense that there are no recurrences of 360 as there are when determining phase differences and thus the time delays measured may b~ used to determine the pos~tion of the roving object without reference to a starting positionO
In either system there may be advantages ln placing the non-roving r~eceiving station (base station) at the same location as one of the transmitters. This is especially so when the system is used for nav~gation rather than tracking and correlation and processing wo~ld preferably take place on the roving object, the link being provided in the direction from the base station to the rovin~
object. If the base station which transmits the link signals to the roving object has an omni-directional radiation pattern then any number of separate roving objects can use the same link signals. This is equivalent to setting, say ~ = 0 in equation lC below~
Additionally, whether or not the base sta~ion is at the same location as one of the transmitters, the transmissions from the transmitter may be used to provide the link, by suitable modification, for ~3~9~
example, in a system in which the ba~e station receives the signals from all three transmitters and mixes tham to baseband and inserts them into the modulation o~ the transmissions from the transmitter at which it is located. It may be possible to do this without interference with the normal use o~ the transmissions; for examplej when the transmitter is a domestic broadcast FM repeater, the link signals may be inserted between the side band signals so that normal reception of the FM signals will not be affected, but the receiver on the or each roving object is able to extract the link signals from the modulation and use them to measure the phases or time delays as previously described.
There may also be advantages, in some applications, in using the same reference oscillator for the transmitter and co-located base station.
~here may also be advantages in modiying every transmitter in the way described in the immediately preceding paragraph. In that case, every roving receiving station then has a link built ln to every transmission it receives, thus giving considerable redundancy in the system and hence increased freedom from errors. When the system is being used to track the position of a roving object~ there may also be advantages in using the link transmitter itself on the roving object as one of the three triangulation transmitters. Thus, the roving object receives transmissions from two independent fixed transmitters, and transmits baseband representations of their signals over the link. The base station also receives signals from the two fixed transmi~ters; but additionally measures the link signals themselves as if they had come ~rom the third fixed transmitter~ This is equivalent to setting, say, ~ = L in equation lC below, and is also equivalent to using the link to lock ~he local oscillators.
In the examples given below, conventional broadcasting VHF transmitters are used, but naturally occurring transmission sources, such as radio stars, might also be used especially for space navigation.
Two examples of systems illustrating the principles of the invention and one example of a prototype system constructed in accordance with the invention will now be described with reference to the accompanying drawings in which:-Figure 1 shows a typical arrangement of~ransmitters and receiving pointsS
Figure 2 illustrates how the signals received from the transmitters are processed;
Figure 3 shows an arrangement for measuring the phase difference between the two signalE~
Figure 4 is a vector d:iagram of the system;
Figure 5 illustrates an arrangement for determining time delays;
Figure 6 illustrate~ the use of an interpolation function for determining time offsets.
Figures 7 and 8 i:llustrate, respectively a mobile receiver unit and a base station receiver unit of a prototype;
: 25 Figure 9 illustrates a correlation/computer system for analysis of the signals received by the roving and base station receiver units Figures lOA and lOB illustrate~ by way of flow charts, the signal processing which takes place in the base station computer to determine the path of the roving recelver; and Figure 11 shows a plot of a track of a mobile unit~
The particular systems shown are land based systems and~ rely on using transmitters A and B and, optionally a third transmitter C.
The first examplP relates to a system in which ~3~4~
phase differences are used to track the posi~ion of a receiver, The signals from the ~ransmitter A are received at a base statio~ D (which i6 a known length lAD away S from the transmitter A) and at roving point ~, whose position iæ to be tracked or determined (and which is an unknown distance lAE rom A). The receivers convert the incoming signals, within a reception band of finite bandwidth, to baseband Isee below) and those from the roving point E are then relayed to the base station D by means of a link F. This link F
might, for example, take the form of a dedicated telecommunications link, or non-dedicated one such aæ
a public cellular radio telephone nPtwork. At the base station D, the signals received directly from A
are compared with those received at E to determine their phase difference PA~ The phase difference PA, measured in degrees, i8 equal to the value:
(1AD--lAE).360/n-A~
where lAD is as beore, lAE is the distance fro~ A to E, ~A is the centre wavelength of the reception band, and n is an :integer. This process of determining PA is one of multiplication, averaging r and mathematical analysis (see below). ~he measured phase difference may be converted directly into a path length difference (modulo one wavelength~ as the centre wavelength ~A f the emissions received from A
can be determined by any conventional mean~.
Now, because the path length difference (lAD-lAE) can be determined, and as lAD i8 known, the possible values of the distanc* lAE can be calculated. Thus E lies on one of a number of spheres ~on a ground based system these can be considered to be circles) of radius lAD - PA.n.~/36~.
~ he signals received at D and E from the ~31~
transmitter B (in a band whose centre wavelength is different from that of A) are treated in the same fashion to determine their phase difference P~, and this defines a second set of circles of radiuR
lBD - PB.m.AB/360 centred on B on one o which E must also lie. m i an integer and AB is the centre wavelength of the transmission received from B. The location of E is then one of the points of intersection of two circles, one centred on ~ and one on B.
In order to determine the position of uniquely, the tracking or navi~ation process begins by calibration~ The rover E must know his initial position with respect to the base station D since the procedure outlined above produces an a~biguous result both because (i) there are two points of intersection of each pair of circles, and (ii) the phase differences PA and PB det~rmine the radii of the circles modulo one wavelength, and not uniquely. PA
and Pg may initially be any values and preferably are set to zero by starting E at D for example.
Thereaf~er, as the rover E moves away from the starting position at D, the values of PA and Pg are continuously monitored, and their values at any time define the current position of E uniquely~
For example, suppose that the transmissions from A and B are both centred on wavelengths close to 3 metres. Initially, PA and PB are both zero (the rover is at the base station) and after some movement of E they may take tbe values 1620 and -3240~
respectively, corresponding to differences of distance of 13.5 and -~7 metres. Thus we know tbat E
has moved from its (known) starting position in such a way as to increase its distance from A by 13 5 ; 35 metres, and to decrease its distance from B by 27 metres. Its new position is thereore determined.
During movement of E the phases must be monitored sufficiently often to ensure that no changes of more than 180 occur between one signal sample and the ne~t. If interruptions occur, either in the transmissions from A and B or in the link F~
the immediate histories of the phases might be used to interpolate over the interruption or an error~correcting transmission protocol may be used to overcome the problem if the error is in the transmission link. Otherwise, each 3609 of phase introduced, and not accounted for, results in an error in position of at least one wavelength (3 metres in this example~, The process of signal conversion to baseband at D and ~ is illustrated in Figure 2. The signals received from A by the antennas G and H are multiplied in mixers I and J with sinusoid~ generated by local oscillators R ancl L. Baseband products are output at P and M. Those from the roving station are then transferred to the base station via the link F
using transmitter ~ and receiver O, and appear at Q.
: The signals at P and Q can be compared (see below) to determine their phase difference. This will reflect the movement of E described above.
The signals received from B are treated in the same manner using a different set of mixers and local os~illators (not shown).
There is one combination of mixer and local oscillator for each transmitter to be monitored. All the local oscillators at each station are phase-locked to common reference oscillators W and X. However, as technology has not yet advanced sufficiently far for the construction of rèference oscillators W and X of sufficiently high stabili~y, any drift in either or ~oth of their frequencies results in a corresponding change in the calculated phase difference between P and Q. Thus it may appear that E is moving~ when in realit~ it is stationary, ~ 3 ~
This problem may be overcome using a third transmitter C (Fiyure 1). Its signals are received at D and E and are used to correct drifts ~n the reference oscillators W and X. In effect, the reference oscillators are both locked to C. If the frequency of C varies, it does not matter since both reference oscillator~ W and X are corrected by the same amount, their frequency difference remaining zero.
Note that the (changing) path of the link F
does not have a significant effect on the phase difference as the signals are converted to baseband (near zero frequency) before conveyance over the link. Furthermore, use of a double sideband transmission system offers immunity to errors of this sort.
An alternative is to lock one reference oscillator to the o~her using the transmiss~on link F.
In practice, the signals from A, Br and C are all treated in exactly the same manner to produce phases P~, PB, and Pc. Three non-linear equations involving the two unknown coordinates of E, and the unknown phase drift of W relative to X, can then be solved to determinQ the unknown qu ntitieæ. This can appreciated from tbe following, which ~hould be read in conjunction with Figure 4~
If A, B, and C are at vector positions ~, ~r and ~ relative to the base station D and the rover is :at a vector position ~ and if the three transmitting stations operate at centre wavelengths of AA~ AB~ and Ac~ then as PA, PB, and Pc are measured, r and P0 (the varying phase difference between the reference oscillators W and X) can be determined from the following equations:-, .
~ 3 ~
IL ~l l~l ~(qa ~qo) ~ ..... ~,............. lA
IL ~ (qb ~qo) = ~.. O.. ~............ lB
~ Icl ~(qc ~qo) ~ .......................... lC
where:
qa ~ ~APA~360l qb ~ ABPg/360~ qc ~ ACPC/360~ and qO = ~APo/360.
The phase diference between the signalsreceived by D and E can be measured using a process similar to that illustrated in Figure 3. The signals P and Q, which may have significant bandwidthsi are multiplied in a multiplier R. This could, for example, be a digltal correlator or a linear multiplier. Two products in phase ~uadrature are output and averaged in low-pass fil~ers ~ and T. It should be noted that, in practice, it may be simpler to produce two signal~, P and P', by mixing with separate o~cillators in phase-quadrature before correlating with Q in two separate multipliers R and R' to produce phase-quadrature product~ S and T. We can conveniently think o~these a~eraged products as 'sine' and Icosine'. A computer ~ performs the mathematical process of taking the inverse tangent of the fraction sine/cosina, giving the phase difference output at V. Thiæ will always be returned in the range -18Q to +180', and hence it i8 necessary to sample sufficiently often :to ensure that the change between ~uccessiYe values i8 less than 1805.
The example described above relates to ~he two-dimensional case where the transmitters~ base ; 35 station, and rover all lie roughly in one plane (eg~
the Earth's surface over sufficien~ly small ranges)O
With per~ect reference oscillators, two transmitters ' ' ~ `
1 3149~
would suffice to determine the position of the rover with respect to its starting point; in practice, three are needed as explained above. Greater accuracy and freedom from interruptions may be achieved by using more than this number. ~he same technique may be applied to the three dimensional case (eg. navigation in space) when a theoretical minimum of three transmitters, but four in practice, must be used.
The second example relates to a system that uses time delay measurements to track position of a receiver.
If the timing delay between the signal~
received at. the base station D directly from transmitter A and via ~he roving station E is determined as t~ and if the delay between signals received at base station D directly from transmitter B and via the roving st2ation E is tB then two non-linear equations can be solved once these ; 20 quantities have been measured, to determine the unknown coordinates of the roving station E~ This can be appreciated from the following which should be ; read in conjunction with Figure 4.
; If A and B are at a vector positions a and ` 25 relativ~ to the base station D and the rover is at vector position Lf then if t~ and tB are measured, can be found from the equations:
ct~ = 0 ..... ~........... 2A
Ll - ILI - ctg = 0 ................. 2 30 where c represents the speed of light.
In particular it should be note~ that the position of the rover can now be determined uniquely without reference to a known starting position so that, in principle at least, the method can be uæed : 35 to find the starting position for subsequent phase tracking D
Timing delays may be measured at any point in ~ 3 ~
time without any reference to any previous measurement and tracking is not required7 Furthermore, the transmitters are likely to have a broad range of spectral characteristics, but they must be modulated, as continuous wave unmodulated transmissions do not provide time delay ; information. The signals do not have to be continuous, and the wider the bandwidth o~ the transmissions the more accurately the timing delay may be measured. If there is more than one independent transmitter at the same transmission site, ~he total bandwidth occupied by the transmissions may be used as if the signals had all come rom one source.
In th~ method of measuring time delays described further below, the maximum bandwidth is used to obtain the highest accuracyO
In practice~ two transmitters are needed for navigation and tracking over a plane as previously described, the offset between the oscillators at the two receiYing stations being of almost no consequence. However, if three stations are being monitored for phase tracking purposes for greater accuracyJ then~ the accuracy of the position determined from timing delays alone can be improved by averaging. The accuracy with which a position may be determined in practice using time delays will be inferior to that determined by phase tracking by at least one order of magnitude.
; 30 Referring now to Figure 5, the baseband signals P, P' and Q are extracted as described in the first example (Figure ~ - it qhould be noted that in practice P as previously referred to ~omprises two signals Cnow P and P'] whlch have been mixed by separate oscillators in phase quadrature) and are digitized in one bit digitizers a, b and b' and the outputs of the digitizers are digital representations .
~L31~9~
1~
of the analog input signals. The sampling interval for digitization should be less than half the inverse of the bandwidth BW of the radio signals 7 i.e.
~ < (2BW)-l (the Ny~uist criterion) in order that minimal information is lost in the sampling process.
The bit streams from a, b and b' are delayed in shift registers c, d and d' respectively. The register c has parallel outputs e each corresponding to a delay of one unit of ts. The shift registers d and d' each have one output corresponding to half the maximum delay from c so that the outputs from d and d' correspond in time with the output from e which is half way along the register c, assumlng ~here is no further delay between the signals. Each of the outputs e is correlated (exclusive-OR operations) wlth the streams from d and d', two such correlations being shown by ~ and f'. The correlator X has two outputs corresponding to correlations in phase-quadrature. These are integrated in the low-pass digital filters g and h and the results passed to a computer U.
The computer ~ receives para]lel correlations, as above, corresponding to steps in the delay between the two signals in units of ts. A plot of the amplitude of the cross-correlation function verses time delay might be as shown for example in Figure 6. The large dots represent the numbers supplied to the computer and it can be seen clearly that the correlation is largest where the total time offset between the data streams is zero. In Figure 6 this corresponds to a time delay of ts + T~
It is unlikely that any of the discrete steps in delay which are actually measured will lie exactly on top of the required delay. Instead, the computer must fit an interpolation function (curve shown in Figure 6) to the measured points to determine the additional offset T. The accuracy with which the peak of the interpolation function can be found limits the accuracy of the method. The interpolation function may be calculated, or measured from the received signals.
Pigures 7 to 10 illustrates various components and features of a prototype system comprising a mobile unit 1 and a base station 2.
Figure 7 shows a rovlng or mobile unit rece~ver 100, which has a receiving antenna 101 which receives signals from three FM transmitters (A,B,C say - see Figure l)o A three-way splitter 102 supplies the signals from the an~enna to three substantially identical receivers 103A, 103B, 103C, each of which is tuned to receive the signals of one of the transmitters~ In this example the receivers 103A, 1O3B~ 103C are shown tuned to receive signals FA~ FB-FC of 96 M~z, 92.3MHz and 89.7 M~z respectively, corresponding to the nominal centre frequencies of three FM transmitters in the East Anglian region of England. Suitable receivers are Sony ICF 2001D~
Each of the receivers i8 phase locked to a master reference oscillator 104 via a local o~cillator 105 for frequency synthesis, the master oscilla~or being, for example a high-stability crystal oscillator or rubidium frequency standard, In ~he example the master oscillator outputs a 10 MHz signal and the phase-locked local oscillator has an output ~requency of 6.275 MHz. ~he receivers output signals at an intermediate frequency, in this case 10.7 MHz, to a 3-way combiner 106, and the combined signal, is mixed to baseband in a mixer 107 using a second local oscillator 105' at 10.7015 MHz which is also phase locked to the master reference oscillator 104. The baseband signal, which has an approximate band width of 10 K~z,i~ passed to a link transmitter 108 which thus transmitæ a signal containing representations of the three received signalæ, via a lB
link transmitter antenna lG9, to the base station 2 (see Figure 8), where analysis takes place.
The base station 2 has a receiver unit 200 (see Figure 8) which in most respectfi is identical with the mobile unit, reference numerals increased by 100 be~ng used for like components~ However, the receiver unit 200 has plural mixers 207Al 2~7B~ 207C
so that the baseband products from the three receivers are kept separate. Additionally~ the master reference oscillator is offset from that of the mobile unit receiver so that the baseband signals VA~ vB. VC rotate with respect the baseband signal vD
from the mobile unit by about lOOHz when the mobile unit is stationary.
15The base station 2 also includes a link antenna 209 via which the signal from the mobile unit 1 is received in a receiver 210. A phase corrector 211 removes the differential~ phase shifts inherently introduced by the link transmitter and receiver circuits and outputs the baseband sign~l VD. The : link transmitter and receiver can be one of many types, but preferably transmit and receive in the VHF
band for example. The phase correc~or 211 is of appropriate type to correct for the type~of phase respon~e o~ the transmitter and receiver circuits.
For example, if the phase response were like that of an R-C lag then the phase corrector would have and R-C lead configuration. In practice more complex phase responses are present, but can be corrected by conventional phase corrector circuits.
To process the baseband signals the base station 2 has a correlator section 220 ~ee Figure 9) for correlating the baseband signals and a microcomputer processor 230 for receiving the correlator products and generating a display of the position and track of the mobile unit.
~ 3 ~
The correlator section 220 has three correlators 221A, 221B~ 221~ for correlating, respectively, the baseband signals vA & VD~ vB & VD, and VC & vD. These produce quasi-sinusoidal outputs at about 100 ~z which reflect the offset between the master reference oscillators 104, 204. The quasi-sinusoidal outputs are passed through identical low-pass filters 222 to phase locked loops ~PLL) 223 where 'clean' versions of the quasi-sinusoids are produced either as sine- or as ~quare-waveB. The : PLL's 223 employ narrow loop bandwidths of say less than lHz in order to remove the effects of noise in the outputs of the low-pa~s filters 222. PLL's 224 receive the outputs of ~LL's 223 to produce outputs offset by 90 with those of PLL's 223 and these may have wider bandwidths - of the order of 10 Hz.
The sine and cosine products thus produced are fed to sample and hold circuits 225 on a conventional interface board in a microcomputer 230 which has a microprocessor 231 and timer 232, by means of which the data representing the products is manipulated to form a video output sent to a visual display unit 233~ In the prototype the display is used to represent a track of the path of the mobile unit as shown in Figure 11 r the scale being adjustable under computer control to one of a number o~ desired scales.
Fig. lOA illustrates, by way of a flow chart, the main elements of the computer's operation under program control~ in order to display on the VDU 233 a : 30 representation of the track of the mobile unit.
After the program is entered at step S01 the tlmer 232 and internal data buffers in the computer are initialized in step S02 and the interrupt timing ~see description below~ set going in step S03. In ~tep S04 the program checks to see whether or not a flag ~set by a separate interrupt routine - see Fig. lOB) is set or not. If it is then data in the current ~ 3 ~
2~
buffer is valid and is converted to phase increments in step S05 and in step S06 the phase increments are added to the totals (for each of the A,B and C
channel ) to produce new total~. The po~ition of the mobile unit is calculated in step S07 and displayed on the VDU 233 in step S08. The current-bufer-fuql flag is reset in step S09 and the program loops back to step S04. If the buffer-full ~lag is not detected in step SQ4 then the program loops around S04 to check again.
In connection with the flow chart of Figure lOa, the following should be noted:-(a~ The current-buffer-full flag is set by the interrupt routine as will be described below in connection with Figure lOB.
(b~ The phase difference of each channel is calculated by means of the relation:
phase = arctan (S/C~
where S and C are the values of the voltages from the sine and cosine P~L's 223,224~
(c) The total phase elapsed since the start must be tracked, i.e" the algorith~m used by the program needs to carry the phases over the principal range -1809 to ~180'.
(d) Calculation of present position is by means of the three equations lA to ~C specified earlier. ~hese may be solved ~n the program by any one of a number of well-known methods, ~he presently preferred method being to use the Newton-Raphson method (see "Numerical ~ecipes, the ar~ of Scien~ific Computingn, by W. H~
Press et al, Cambridge University Press 1986) to calculate the current position using the previous position as a starting point for the iteration.
(e) The phase tracking algorithm used for each channel is as follows. If ~ is the total phase ~ 3 ~
tracked so far and the new data samples are S
and C (for the sine and cosine channels respectively), then the phase increment ~0 is given by:-~an ~0 = (S cos0 - C sin 0)/~5 sin0 ~ C cos ~) and the new value of the tracked phase is ~ + ~0. As long as no phase increments greater than 180 or less than -180 occur between samples this will hold true. For this to be so see description below with regard to the interrupt service routine.
The interrupt service routine, by means of which data from the sample and hold circui~s 225 ~s stored in one of two buffers, and which is shown in : 15 flow chart form in Fig. 10B, will now be described~
The timer 232, which is resettable, causes interrupts to occur in the operating program (Fig.
10A) at regular intervals (say 500 per secona) as : determined by the programO On occurence of an interrupt the interrupt routine is entered ~step I01) after which the timer 232 is reset ~step I02). A
buffer pointer which poinl:s to the address in the current buffer to which data should next be stored ~s incremented or changed over ~step I03) and data i8 then read from the sample and hold circuits 225 ~step : I04) and put into the current buffer (step I~5)~ In . step I06 the routine checks to see if the current : buffer is full and if it is sets the flag (step I07) previously mentioned in connection with the operations program. The buffer to which data is to be stored is then changed over (step I08). I~ the buffer is not full, or once the buffer has been changed, the interrupts are reset ~step I09) and the interrupt routine ends (step I10), allowing control of the computer to return to the main program : (Fig. 10A)~
There are two buffers so as to allow one to be written to during an interrupt routine while the other is being read rom by the main program that has been interrupted. In the present example the buffer size and rate of reading the sample and hold circuits are chosen so that each buffer f~lls ~n about 1 second, and the reading rate i8 chosen so as to sample the data completely in order to ensure that no phase increments greater than +180 or less than -180 are 108t between samples.
Figure 11 is a printout or screen dump of a track of a mobile system carried by a man~ show~ng the path followed (in the direction of the arrow)~
It can be seen that using equipment according to the invention a highly accurate track can be plot~ed in real time. The irregularities result from movement, side-to-side of the path being walked by the man, of the aerial held by him. It can be seen that resolution as good as half a metre or less can be achieved in measurements made every 0.1 second.
3n
buffer is valid and is converted to phase increments in step S05 and in step S06 the phase increments are added to the totals (for each of the A,B and C
channel ) to produce new total~. The po~ition of the mobile unit is calculated in step S07 and displayed on the VDU 233 in step S08. The current-bufer-fuql flag is reset in step S09 and the program loops back to step S04. If the buffer-full ~lag is not detected in step SQ4 then the program loops around S04 to check again.
In connection with the flow chart of Figure lOa, the following should be noted:-(a~ The current-buffer-full flag is set by the interrupt routine as will be described below in connection with Figure lOB.
(b~ The phase difference of each channel is calculated by means of the relation:
phase = arctan (S/C~
where S and C are the values of the voltages from the sine and cosine P~L's 223,224~
(c) The total phase elapsed since the start must be tracked, i.e" the algorith~m used by the program needs to carry the phases over the principal range -1809 to ~180'.
(d) Calculation of present position is by means of the three equations lA to ~C specified earlier. ~hese may be solved ~n the program by any one of a number of well-known methods, ~he presently preferred method being to use the Newton-Raphson method (see "Numerical ~ecipes, the ar~ of Scien~ific Computingn, by W. H~
Press et al, Cambridge University Press 1986) to calculate the current position using the previous position as a starting point for the iteration.
(e) The phase tracking algorithm used for each channel is as follows. If ~ is the total phase ~ 3 ~
tracked so far and the new data samples are S
and C (for the sine and cosine channels respectively), then the phase increment ~0 is given by:-~an ~0 = (S cos0 - C sin 0)/~5 sin0 ~ C cos ~) and the new value of the tracked phase is ~ + ~0. As long as no phase increments greater than 180 or less than -180 occur between samples this will hold true. For this to be so see description below with regard to the interrupt service routine.
The interrupt service routine, by means of which data from the sample and hold circui~s 225 ~s stored in one of two buffers, and which is shown in : 15 flow chart form in Fig. 10B, will now be described~
The timer 232, which is resettable, causes interrupts to occur in the operating program (Fig.
10A) at regular intervals (say 500 per secona) as : determined by the programO On occurence of an interrupt the interrupt routine is entered ~step I01) after which the timer 232 is reset ~step I02). A
buffer pointer which poinl:s to the address in the current buffer to which data should next be stored ~s incremented or changed over ~step I03) and data i8 then read from the sample and hold circuits 225 ~step : I04) and put into the current buffer (step I~5)~ In . step I06 the routine checks to see if the current : buffer is full and if it is sets the flag (step I07) previously mentioned in connection with the operations program. The buffer to which data is to be stored is then changed over (step I08). I~ the buffer is not full, or once the buffer has been changed, the interrupts are reset ~step I09) and the interrupt routine ends (step I10), allowing control of the computer to return to the main program : (Fig. 10A)~
There are two buffers so as to allow one to be written to during an interrupt routine while the other is being read rom by the main program that has been interrupted. In the present example the buffer size and rate of reading the sample and hold circuits are chosen so that each buffer f~lls ~n about 1 second, and the reading rate i8 chosen so as to sample the data completely in order to ensure that no phase increments greater than +180 or less than -180 are 108t between samples.
Figure 11 is a printout or screen dump of a track of a mobile system carried by a man~ show~ng the path followed (in the direction of the arrow)~
It can be seen that using equipment according to the invention a highly accurate track can be plot~ed in real time. The irregularities result from movement, side-to-side of the path being walked by the man, of the aerial held by him. It can be seen that resolution as good as half a metre or less can be achieved in measurements made every 0.1 second.
3n
Claims (17)
1. A navigation and tracking system for receiving signals transmitted by a number of transmission sources (A,B,C) equal at least to the number of dimensions in which the movement of a roving object is to be monitored, the system comprising:
(a) a pair of receiving stations (D,E) in use, the first of the receiving stations (D) being at a known position and the second (E) being located on the roving object, each receiving station having:
(i) a reference oscillator;
(ii) means for deriving from the reference oscillator a local reference signal;
(iii) means for generating a signal representative of the difference in frequency and hence the difference in phase between the local reference signal and the signal received from the transmission source;
(b) means (F) for passing, as a link signal from one of the receiving stations to the other receiving station, the signal, generated at the one receiving station, representative of the frequency or phase difference between the local reference signal and the signal received from the at least one transmission source from which the phase difference or time delay between the respective signals received at the receiving stations is determined; and (c) means (R,S,T,U) at the other receiving station for comparing the information received from the one receiving station with information about the respective signal received directly from the respective transmitter and for determining the phase difference or time delay between the received signals and hence the change in phase difference or time delay of the signals, in order to determine the change in position or position of the roving object.
(a) a pair of receiving stations (D,E) in use, the first of the receiving stations (D) being at a known position and the second (E) being located on the roving object, each receiving station having:
(i) a reference oscillator;
(ii) means for deriving from the reference oscillator a local reference signal;
(iii) means for generating a signal representative of the difference in frequency and hence the difference in phase between the local reference signal and the signal received from the transmission source;
(b) means (F) for passing, as a link signal from one of the receiving stations to the other receiving station, the signal, generated at the one receiving station, representative of the frequency or phase difference between the local reference signal and the signal received from the at least one transmission source from which the phase difference or time delay between the respective signals received at the receiving stations is determined; and (c) means (R,S,T,U) at the other receiving station for comparing the information received from the one receiving station with information about the respective signal received directly from the respective transmitter and for determining the phase difference or time delay between the received signals and hence the change in phase difference or time delay of the signals, in order to determine the change in position or position of the roving object.
2. A system according to Claim 1, further including 2 number of transmission sources equal at least to the number of dimensions in which movement is to be monitored plus one.
3. A system according to Claim 1 or Claim 2, wherein the transmissions are modulated and the phase difference is determined.
4. A system according to Claim 1 or Claim 2, wherein the transmissions are continuous wave, unmodulated transmissions and the phase difference is determined.
5. A system according to Claim 1 or Claim 2, wherein the transmissions are modulated and the timing delays are determined.
6. A system according to Claim 1 or Claim 2, wherein both phase difference and timing delays are determined.
7. A system according to Claim 3, wherein the receiving stations convert the signals received to baseband signals by the use of local oscillators which are phase-locked to local frequency standards or reference oscillators (K,L) and the baseband signal from one of the receiving stations is transmitted to the other receiving station.
8. A system according to Claim 7, further including an additional transmission source (C) for measuring the drift of the reference oscillators or frequency standards (K,L).
9. A system according to Claim 7, wherein the reference oscillators (K,L) are locked to one another using the transmission link between the receiving stations.
10. A system according to Claims 2, 7, 8 or 9, wherein a non-roving receiving station (base station) is located at the same location as one of the transmitters.
11. A system according to Claim 10, wherein the link is provided in the direction from the base station to the roving object.
12. A system according to Claim 11, wherein the base station has an omni-directional radiation pattern, whereby any number of separate roving objects can navigate using the same link signals.
13. A system according to Claim 2, wherein the transmissions from at least one of the transmitters are used to provide the link between the receiving stations.
14. A system according to Claim 10, wherein the same reference oscillator is used for the transmitter and co-located receiving station.
15. A system according to Claim 5, wherein the means for comparing the signals received by the plural receiving stations includes means (a, b, b') for digitizing digitized signals and means (X) for correlating the digitized signals to produce point estimates of a cross-correlation function thereof.
16. A system according to Claim 15, wherein an interpolation function is fitted to the point estimates of the cross-correlation function to determine the time delay between the signals.
17. A system according to Claim 1, wherein the signals received by each receiving station are mixed by multiplication with a signal from a respective local oscillator.
B938:926:
B938:926:
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8718881 | 1987-08-10 | ||
| GB878718881A GB8718881D0 (en) | 1987-08-10 | 1987-08-10 | Navigation & tracking system |
| GB888803754A GB8803754D0 (en) | 1988-02-18 | 1988-02-18 | Navigation & tracking system |
| GB8803754 | 1988-02-18 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1314964C true CA1314964C (en) | 1993-03-23 |
Family
ID=26292588
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000574251A Expired - Fee Related CA1314964C (en) | 1987-08-10 | 1988-08-09 | Navigation and tracking system |
Country Status (11)
| Country | Link |
|---|---|
| US (1) | US5045861A (en) |
| EP (2) | EP0303371B1 (en) |
| JP (1) | JPH02504673A (en) |
| BR (1) | BR8807655A (en) |
| CA (1) | CA1314964C (en) |
| DE (1) | DE3879032T2 (en) |
| DK (1) | DK174254B1 (en) |
| ES (1) | ES2038758T3 (en) |
| HK (1) | HK38994A (en) |
| NO (1) | NO177944C (en) |
| WO (1) | WO1989001637A1 (en) |
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- 1988-07-28 EP EP88306951A patent/EP0303371B1/en not_active Expired - Lifetime
- 1988-07-28 ES ES198888306951T patent/ES2038758T3/en not_active Expired - Lifetime
- 1988-07-28 BR BR888807655A patent/BR8807655A/en not_active IP Right Cessation
- 1988-07-28 EP EP88906941A patent/EP0377592A1/en active Pending
- 1988-07-28 JP JP63506238A patent/JPH02504673A/en active Pending
- 1988-07-28 DE DE8888306951T patent/DE3879032T2/en not_active Expired - Fee Related
- 1988-07-28 US US07/458,615 patent/US5045861A/en not_active Expired - Lifetime
- 1988-07-28 WO PCT/GB1988/000617 patent/WO1989001637A1/en not_active Application Discontinuation
- 1988-08-09 CA CA000574251A patent/CA1314964C/en not_active Expired - Fee Related
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1990
- 1990-02-09 DK DK199000346A patent/DK174254B1/en not_active IP Right Cessation
- 1990-02-09 NO NO900632A patent/NO177944C/en unknown
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1994
- 1994-04-21 HK HK38994A patent/HK38994A/en not_active IP Right Cessation
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|---|---|
| NO177944B (en) | 1995-09-11 |
| DE3879032D1 (en) | 1993-04-15 |
| US5045861A (en) | 1991-09-03 |
| ES2038758T3 (en) | 1993-08-01 |
| BR8807655A (en) | 1990-06-05 |
| HK38994A (en) | 1994-04-29 |
| NO177944C (en) | 1995-12-20 |
| NO900632L (en) | 1990-04-09 |
| EP0377592A1 (en) | 1990-07-18 |
| DK34690D0 (en) | 1990-02-09 |
| EP0303371A1 (en) | 1989-02-15 |
| DK174254B1 (en) | 2002-10-21 |
| DK34690A (en) | 1990-02-09 |
| NO900632D0 (en) | 1990-02-09 |
| JPH02504673A (en) | 1990-12-27 |
| DE3879032T2 (en) | 1993-06-24 |
| WO1989001637A1 (en) | 1989-02-23 |
| EP0303371B1 (en) | 1993-03-10 |
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