WO1996021164A1 - Microwave energy implemented aircraft landing system - Google Patents

Microwave energy implemented aircraft landing system Download PDF

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Publication number
WO1996021164A1
WO1996021164A1 PCT/US1995/016501 US9516501W WO9621164A1 WO 1996021164 A1 WO1996021164 A1 WO 1996021164A1 US 9516501 W US9516501 W US 9516501W WO 9621164 A1 WO9621164 A1 WO 9621164A1
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WO
WIPO (PCT)
Prior art keywords
signal
unit
aircraft
phase
airborne
Prior art date
Application number
PCT/US1995/016501
Other languages
French (fr)
Inventor
Aubrey I. Chapman
George F. Ridpath
Original Assignee
Focused Energy Technologies, Inc.
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by Focused Energy Technologies, Inc. filed Critical Focused Energy Technologies, Inc.
Priority to AU47405/96A priority Critical patent/AU4740596A/en
Publication of WO1996021164A1 publication Critical patent/WO1996021164A1/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q25/00—Antennas or antenna systems providing at least two radiating patterns
    • H01Q25/007—Antennas or antenna systems providing at least two radiating patterns using two or more primary active elements in the focal region of a focusing device
    • H01Q25/008—Antennas or antenna systems providing at least two radiating patterns using two or more primary active elements in the focal region of a focusing device lens fed multibeam arrays
    • 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
    • G01S1/00—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith
    • G01S1/02—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith using radio waves
    • G01S1/04—Details
    • G01S1/042—Transmitters
    • 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
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/74—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems
    • G01S13/76—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems wherein pulse-type signals are transmitted
    • G01S13/765—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems wherein pulse-type signals are transmitted with exchange of information between interrogator and responder
    • 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
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/74—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems
    • G01S13/76—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems wherein pulse-type signals are transmitted
    • G01S13/767—Responders; Transponders
    • 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
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/87—Combinations of radar systems, e.g. primary radar and secondary radar
    • G01S13/876—Combination of several spaced transponders or reflectors of known location for determining the position of a receiver
    • 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
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/91—Radar or analogous systems specially adapted for specific applications for traffic control
    • G01S13/913—Radar or analogous systems specially adapted for specific applications for traffic control for landing purposes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q15/00—Devices for reflection, refraction, diffraction or polarisation of waves radiated from an antenna, e.g. quasi-optical devices
    • H01Q15/02—Refracting or diffracting devices, e.g. lens, prism
    • H01Q15/06—Refracting or diffracting devices, e.g. lens, prism comprising plurality of wave-guiding channels of different length
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q15/00—Devices for reflection, refraction, diffraction or polarisation of waves radiated from an antenna, e.g. quasi-optical devices
    • H01Q15/02—Refracting or diffracting devices, e.g. lens, prism
    • H01Q15/08—Refracting or diffracting devices, e.g. lens, prism formed of solid dielectric material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q19/00—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic
    • H01Q19/06—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic using refracting or diffracting devices, e.g. lens
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q19/00—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic
    • H01Q19/06—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic using refracting or diffracting devices, e.g. lens
    • H01Q19/062—Combinations of primary active antenna elements and units with secondary devices, e.g. with quasi-optical devices, for giving the antenna a desired directional characteristic using refracting or diffracting devices, e.g. lens for focusing
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q3/00—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system
    • H01Q3/24—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system varying the orientation by switching energy from one active radiating element to another, e.g. for beam switching
    • H01Q3/242—Circumferential scanning
    • 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
    • G01S1/00—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith
    • G01S1/02—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith using radio waves
    • G01S1/08—Systems for determining direction or position line
    • G01S1/14—Systems for determining direction or position line using amplitude comparison of signals transmitted simultaneously from antennas or antenna systems having differently oriented overlapping directivity-characteristics
    • G01S1/16—Azimuthal guidance systems, e.g. system for defining aircraft approach path, localiser system
    • 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
    • G01S1/00—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith
    • G01S1/02—Beacons or beacon systems transmitting signals having a characteristic or characteristics capable of being detected by non-directional receivers and defining directions, positions, or position lines fixed relatively to the beacon transmitters; Receivers co-operating therewith using radio waves
    • G01S1/08—Systems for determining direction or position line
    • G01S1/14—Systems for determining direction or position line using amplitude comparison of signals transmitted simultaneously from antennas or antenna systems having differently oriented overlapping directivity-characteristics
    • G01S1/18—Elevational guidance systems, e.g. system for defining aircraft glide path
    • 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
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/74—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems
    • G01S13/82—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems wherein continuous-type signals are transmitted
    • G01S13/84—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems wherein continuous-type signals are transmitted for distance determination by phase measurement

Definitions

  • DESCRIPTION TITLE MICROWAVE ENERGY IMPLEMENTED AIRCRAFT LANDING SYSTEM BACKGROUND OF THE INVENTION 1.
  • Field of Use This invention relates generally to aircraft landing systems, and, more particularly, to aircraft landing systems using microwave radio frequency (RF) energy to transmit audio frequency guidance tones.
  • RF microwave radio frequency
  • Patent No. 3,295,132 which issued Dec. 27, 1966, to Aubrey I. Chapman, Jr., for Modulating Radar Reflector.
  • Patent No. 4,698,636 which issued Oct. 6, 1987, to Raymond Marlow, et al, for Ground Speed Determining Radar System
  • Patent No. 4,723,123 which issued Feb. 2, 1988, to Raymond Marlow, et al, for Radar System
  • Patent No. 4,806,932 that issued Feb. 21, 1989, to Bartow Bechtel, for Radar-Optical Transponding system.
  • This invention utilizes a ground unit non-rotating uneberg : .
  • lens antenna having an array of low power, independently controlled microwave radio frequency transponder modules to facilitate phase- referenced signals necessary for improved landing operations and an aircraft-borne interrogator/receiver unit to request and process the signals from the ground unit.
  • the ground unit responds by transmitting a continuous wave (CW) signal containing guidance information back to the aircraft.
  • Instrumentation on the aircraft receives the ground unit signal and extracts the guidance information to facilitate landing of the aircraft. It is therefore an object of the present invention to improve microwave energy implemented aircraft landing methods and systems.
  • Another object is to utilize a ground-based transponder array unit and an airborne transmitter/receiver unit as part of an improved landing system, wherein the airborne unit transmits a pulsed interrogation signal to the ground unit and the ground unit responds by transmitting a continuous wave guidance signal to the airborne unit.
  • a further object is to convey aircraft azimuth and glide slope guidance information by transmitting phase-referenced audio frequency tones on a ground unit transmitted continuous wave signal.
  • Another object is to convey aircraft range, velocity, and acceleration guidance information by superimposing an airborne unit pulsed interrogation signal onto a ground unit transmitted continuous wave signal as a phase-reversal. Still another object is for the airborne transmitter/receiver unit to receive the ground unit transmitted continuous wave signal and extract the azimuth, glide slope, range, velocity, and acceleration guidance information for use in automatic, semi ⁇ automatic, or manual guidance of the aircraft. Additional objects, advantages, and novel features of the invention will be set forth in the description which follows, and will become apparent to those skilled in the art upon examination of the following description and drawings. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in this specification. BRIEF DESCRIPTION OF THE DRAWINGS
  • Figure 1 is an elevation view of a prior art Luneberg lens
  • Figure IA is a plan view of the Luneberg lens of Fig. 1 operating as a collector of electromagnetic energy;
  • Figure IB is a plan view of the Luneberg lens of Figs. 1 & 2 operating as both a collector and reflector of electromagnetic energy;
  • Figure 2A is an elevation view of a prior art Luneberg lens using reflective elements to establish coded, narrow beamwidth electromagnetic beams in elevation for use as descent path (glide slope) references;
  • Figure 2B is a plan view of another prior art Luneberg lens using reflective elements to establish coded, narrow beamwidth electromagnetic beams in azimuth for use as a localizer
  • Figure 3 is a conceptual view of another prior art Luneberg lens used as a focusing means and having an array of transponder modules;
  • Figures 4A and 4B are elevation and plan views, respectively, of a prior art embodiment of a transponder unit using an array of transponder modules arranged in close proximity to the surface of a radio frequency Luneberg lens to generate a matrix of high gain, narrow beamwidth beams;
  • Figure 5A shows a block diagram of a prior art embodiment of a ground transponder unit implemented with a single Luneberg lens focusing means
  • Figure 5B shows an arbitrary selection of four horns: two horns in azimuth and two horns in elevation;
  • Figure 6 shows a prior art embodiment of an airborne interrogator/receiver used in conjunction with a ground transponder unit
  • Figure 7A shows an azimuth pattern of phase-referenced microwave beams for one embodiment of the present invention having a 10 degree beam spacing covering 360 degrees;
  • Figure 7B shows an elevation (glide slope) pattern of phase- referenced microwave beams for one embodiment of the present invention having a 5 degree beam spacing covering 30 degrees;
  • Figure 7C is a cross section of a small sector of a phase- referenced microwave beam pattern emanating from one embodiment of a ground-based transponder unit of the present invention showing beam azimuth/elevation values;
  • Figure 8 is a block diagram of an airborne radio frequency pulsed interrogator transmitter and continuous wave receiver unit for use with a ground-based transponder unit in one embodiment of the present invention
  • Figure 9 is a block diagram of a ground-based transponder unit utilizing a pulsed receiver and continuous wave transmitter for use with an airborne interrogator/receiver unit in one embodiment of the present invention.
  • a Luneberg lens 10 is rotated by a motor 12 at a constant angular velocity.
  • Lens 10 focuses incoming radio frequency energy to a small spot near the lens surface 14 opposite the side where the RF energy entered. If no reflective element 16 is present where the spot is formed, as shown in Figure IA, the RF energy passes through the surface and is dissipated, and essentially none of the focused RF energy is returned to the transmitting vehicle. If an RF reflective element 16, which is, for example, a piece of aluminum foil, is present at the focal point, as shown in Figure IB, the RF energy is reflected precisely back in a very narrow beam along the path from whence it came.
  • each row 17 and 18 has a different number of reflective elements 16 for producing different modulation frequencies at different elevation angles.
  • the focused spot of RF energy has a definite size depending, in part, on the frequency of the RF energy used. At X-band, the spot is about 0.75 inches in diameter.
  • the modulating frequencies in space establish 360 degree conical patterns of descent paths.
  • transponding transmit/receive, or beacon
  • a transponder system comprises a stationary Luneberg lens 10 and an array 19 of audio frequency amplitude modulated pulse forming modules 20.
  • a platform 22 supports the Luneberg lens 10.
  • FIG. 4A an elevation view of a stationary Luneberg lens 10 focusing means is shown with several transponding modules 242 arranged in a vertical column. The number of modules 242 in a column is determined by the size of the lens, the size of the modules, and the total coverage desired in elevation angle. A column that is six modules high has been arbitrarily chosen as an example.
  • the elevation view of Figure 4A shows only one column, but a matrix pattern is actually used with a few or many columns mounted around the lens.
  • FIG. 5A and 5B A plan view of the lens 10 (Figure 4B) and module matrix are shown with several columns mounted side by side. Additional columns could be used to cover the complete circumference of the lens to provide guidance throughout 360 degrees in azimuth if desired for great flexibility in choosing an azimuth heading, for example, for use with helicopters that do not have a runway they have to line up with.
  • a single Luneberg lens 10 focuses incoming energy into four horns 122, 124, 126, and 128.
  • the horns are arranged in a cluster, as shown in Figure 5B, i.e.
  • This four horn cluster in conjunction with the focusing means, generates a pattern of relatively high gain pencil beams that crossover at their half power points. These crossover points define an aircraft's approach path to the landing site.
  • the position of the aircraft (high-low, left-right) relative to the defined approach path is determined by amplitude comparison of the audio tone modulation which identifies each beam.
  • the four transmit/receive modules 130 use a common RF source 133 to minimize output amplitude fluctuation, a potential source of approach path position error.
  • the components of the transmit/receive modules 130 include, besides the horns 122, 124, 126, and 128, bandpass filters (BPF) 134, 136, 138, and 140 connected to the horns, respectively, receive and transmit circulators 142, 144, 146, and 148 connected to the band-pass filters, respectively, and to superheterodyne circuits including limiters 150, 152, 154, and 156 connected through the circulators to the BPF's, respectively, mixers 158, 160, 162, and 164 connected, respectively, to the limiters and to a common local oscillator (LO) 182 for producing IF signals, and detectors 174, 176, 178, and 180, connected respectively, to the IF amplifiers 166, 168, 170, and 172, for detecting the envelope of the video pulses.
  • BPF bandpass filters
  • the interrogating signal from an aircraft interrogator transmitter located within the locus of four beams may be intercepted by the horns or predominantly by a single horn.
  • the outputs of the detectors 174, 176, 178, and 180 are logically OR'd in OR gates 184 and 186 connected to the detectors with the output thereof exiting a final OR gate 188; a high output triggers an RF reply pulse from the RF source 133.
  • the driver 190 connected to the OR gate 188 is triggered to produce a pulse to the pulse modulator 194 through the encoder 192.
  • the encoder may produce a pulse train to the pulse modulator to encode data from the data source 196 corresponding to the interrogator signals but modified to contain the intelligence of the external data source.
  • the pulse modulator 194 is connected to the encoder 192 and the RF (9075 MHz) source 195 and converts the video pulses to RF pulses.
  • a four way power divider 197 is connected to the pulse modulator for dividing the RF pulse power equally through phase trimmers 198, 200, 202, and 204 connected to the divider, to audio tone modulators 206, 208, 210, and 212.
  • An audio tone generator 214 producing frequencies f j _, f2 # £ 1 and f 4 is connected in common to the audio tone modulators.
  • the tone modulators impose the individual tone modulations on the RF pulses, which identify each of the four beams and define the approach path for the aircraft's interrogator/receiver tracker to generate azimuth and glide slope guidance signals.
  • Port (c) of the circulators 142, 144, 146, and 148 is connected to the tone modulators for passing the modulated RF pulses through the band-pass filters 134, 136, 138, and 140, respectively, horns 122, 124, 126, and 128, respectively, and lens 10 to the aircraft's interrogator/receiver unit.
  • the ground transponder information is transmitted to the aircraft's guidance receiver for processing the beam's modulation tones into up-down, left-right guidance signals to guide the aircraft along the approach path.
  • a built- in-test (BIT) circuit 216 is provided.
  • the test circuit includes a single horn 218 for illuminating the transmit/receive horns 122, 124, 126, and 128 from the opposite (forward) side of the Luneberg lens 120.
  • the test signal is a continuous pulse train at the interrogating frequency (9375 MHz) obtained by directional coupler 220 operatively connected to the RF (9075 MHz) source 195, mixing the sample with a 300 MHz offset frequency from offset oscillator 222 in mixer 224.
  • a BPF 226 connected to the mixer eliminates the unwanted mixer outputs, and an amplifier 228 connected to the BPF amplifies the remaining carrier (9375 MHz) .
  • a pulse modulator 230 is connected to the amplifier 228 and to a test pulse generator 233 to produce the train of RF pulses for the horn 218.
  • Horns 122, 124, 126, and 128 intercept a portion of the test train energy, which is amplified and processed the same way as an aircraft interrogation signal.
  • the RF reply pulse train generated in response to the test signal is sampled by four directional couplers and detectors 232, 234, 236, and 238.
  • a performance monitor 240 is connected to the detectors. Any discrepancies will trigger a control/alarm.
  • the airborne interrogator/receiver is basically a range-only interrogator/receiver with a pulse transmitter (interrogator) 244 and receiver 246.
  • the interrogator/receiver includes a fixed, non- scanning reflector, flat-plate, horn, or stub antenna 248 which both transmits and receives.
  • the interrogate transmit pulse is generated in a pulse transmitter 250 and is fed into port (c) of a conventional RF circulator 252, which directs it out of port (a), through BPF 254 and out the antenna 248.
  • This interrogation pulse triggers one or more of the ground transponder modules, which then transmits a pulse in a very narrow microwave beam back to the antenna 248, now used as a receive antenna.
  • the receive pulse passes through BPF 254 and into port (a) of the circulator 252, which directs the receive pulse out of port (b) into the signal level limiter 256 of the receiver 246. From the limiter 256 it passes through BPF 258 and is fed to an RF mixer 260 where it mixes with a signal from LO 262 to produce an IF signal, which is filtered in BPF 263 before being applied to an IF amplifier 264 with automatic gain control (AGO 266. The pulse signal is amplitude detected in the detector 268 to produce a video signal, which is fed in two directions.
  • one path is through a tracker 269, which includes circuitry used to determine range, velocity, and acceleration relative to the ground unit, and the second path is through a filter-bank 287 which includes circuitry for determining azimuth and glide slope errors relative to the ground unit.
  • the present invention overcomes the tone and filter problems associated with the above prior art landing system when wide area aircraft guidance coverage is needed.
  • the present invention requires only three tones to provide 360 degree azimuth coverage with extended elevation coverage: one tone for azimuth, one tone for elevation, and one tone for a reference. Phase shift, as a function of azimuth, is applied to the azimuth tone. Similarly, phase shift, as a function of elevation, is applied to the elevation tone.
  • the reference tone is a tone that is frequency modulated by both the azimuth and the elevation tones.
  • prior art landing systems utilizing pulsed signals from the ground unit required RF spectrum bandwidth accommodation by the airborne receiver determined, in part, by the duration of the pulses; shorter pulse durations required greater bandwidth.
  • the present invention overcomes the bandwidth problems associated with such prior art ground units by utilizing continuous wave (CW) signal transmission.
  • Continuous wave transmission of guidance information results in a significant reduction in bandwidth of an airborne unit's receiver, which extends the range from which an aircraft can be guided to a landing site.
  • CW continuous wave
  • An incremental phase delay from a zero degree reference is applied separately to the azimuth tone and the elevation tone for uniquely designating the array location, and, thus, the beam orientation, of each transponder module on the ground transponder unit.
  • These two tones are used to frequency modulate the 20 kHz reference tone, resulting in a complex waveform that contains all the information needed to determine the azimuth and glide slope of an aircraft.
  • the assignment of azimuth and glide slope information is as follows. Referring to Figure 7A, transponder modules are arbitrarily located, for example, every 10 degrees around the ground unit's Luneberg lens 350. Module number 36 is pointed to the south.
  • the phase shift of its transmitted 100 Hz azimuth tone frequency is zero degrees.
  • transponder modules are arbitrarily located on the equator of the ground unit's Luneberg lens 350 and every five degrees therebelow to cover a range of elevation from zero through 30 degrees.
  • Module number 3 is pointed upward at an angle of ten degrees.
  • the phase shift of its transmitted 180 Hz elevation tone is ten degrees.
  • an airborne interrogator/receiver unit decodes the phase of this module, a glide slope of ten degrees is indicated.
  • the 180 Hz tone decoder will process the vectorial sum of the phases from both modules and indicate a glide slope that is between the two modules, in this case, 22.5 degrees. Therefore, glide slope vectoring is also obtainable from any point within the transmitting and receiving range of the ground and airborne units.
  • FIG. 7C is a cross section of an arbitrarily chosen sector of a beam pattern emanating from a ground transponder unit located at a landing site. It shows the beam pattern as viewed from an aircraft north of the ground unit.
  • Modules number 16 through 20 provide azimuth coverage from 160 degrees (south-southeast) through 200 degrees (south-southwest) in 10 degree angular increments, and a glide slope angle of 10 degrees.
  • An airborne interrogator/receiver unit intercepting energy from module number 18 would, by detecting and decoding the phase-shift of both the 100 Hz azimuth tone and the 180 Hz elevation tone, output an azimuth of 180 degrees (due south) and a glide slope angle of 10 degrees, giving a flight path to the landing site.
  • Figure 8 shows one implementation of a phase-referenced, three tone embodiment for an airborne interrogator/receiver unit of this embodiment of the invention. It transmits a 9200 MHz pulsed interrogation signal to the ground transponder unit and receives the ground unit's 8900 MHz CW signal. In addition to azimuth and glide slope information, the ground unit's signal also includes a phase- reversal return of the airborne unit's pulsed 9200 MHz interrogation signal, which the airborne unit utilizes to determine range, velocity, and acceleration with respect to the ground unit.
  • the 8900 MHz RF energy is collected by an antenna 300, which may be a fixed, non-scanning reflector, a horn, a flat-plate, or a stub design antenna.
  • the signal is then routed through BPF 301 to port (a) of a circulator 302.
  • the circulator routes the signal to a limiter 303 from port (b) of the circulator and then from the limiter through BPF 304 to a mixer 307 where it is mixed with the 9200 MHz signal of LO 306 to provide a 300 MHz IF signal.
  • the IF signal is then routed to two different paths. One path is to a BPF 308 for subsequent determination of range, velocity, and acceleration. The other path is to BPF 309 for subsequent determination of azimuth and glide slope.
  • the IF signal from BPF 308 is routed through a limiter/amplifier 311 to an amplitude detector 314. From the amplitude detector the demodulated signal is routed to a tracker 313, where range, velocity, and acceleration are determined by the time delay between the output of a pulse repetition frequency (PRF) /pulse modulator control 310, which initiates the transmission of a signal pulse to the ground unit, and the arrival of the return of the signal pulse from the ground unit at tracker 313.
  • PRF pulse repetition frequency
  • the second path for the IF signal from mixer 307 is through BPF 309 to an amplitude detector 312.
  • the signal from amplitude detector 312 is divided into three paths.
  • the first path is through a 20 kHz filter 316 to a frequency discriminator/filter 319 where the 100 Hz and 180 Hz phase-referenced signals are recovered and used as the phase references by a 100 Hz azimuth tone phase detector 318 and a 180 Hz elevation tone phase detector 320.
  • the second path for the signal from amplitude detector 312 is through a 100 Hz tone filter 315 to the 100 Hz azimuth tone phase detector 318.
  • the output of the 100 Hz azimuth tone phase detector 318 is the azimuth to the landing site.
  • the third path for the AM detected signal from amplitude detector 312 is through a 180 Hz tone filter 317 to the 180 Hz elevation tone phase detector 320.
  • the output of the 180 Hz elevation tone phase detector 320 is the glide slope to the landing site.
  • the airborne interrogator/receiver pulsed 9200 MHz interrogation signal to the ground unit is generated by a PRF/pulse modulator control 310, the 9200 MHz LO 306, and a gate switch/amplifier 305.
  • the pulsed signal from gate switch/amplifier 305 is routed to port (c) of circulator 302 and on to BPF 301 via port (a) of circulator 302. From BPF 301, the 9200 MHz pulsed signal is routed to antenna 300, which directs the interrogation pulse to the ground station.
  • Figure 9 shows one implementation of the phase-referenced, three tone embodiment of a ground unit transponder module that receives a pulsed 9200 MHz interrogation signal from an airborne interrogator/receiver and returns an 8900 MHz CW signal.
  • the pulsed 9200 MHz signal is received by the ground station Luneberg lens 350 and is focused into a transponder module antenna 351, which routes a signal to a coupler 352.
  • the signal passes through coupler 352 to a BPF 354 and then to port (a) of a circulator 356, which directs the signal out through port (b) to a BPF 358.
  • the signal is routed through a signal level limiter 361 to a mixer 363 where it mixes with the 8900 MHz signal from a LO 362 to produce a 300 MHz IF signal.
  • the signal is routed to BPF 365 to remove the unwanted mixer frequency products before being applied to a limiter/amplifier 366.
  • the signal is routed from limiter/amplifier 366 to amplitude detector 367 where the AM detected signal is passed to the modulation control 364 where signals are generated to determine the correct phase modulation for the 100 MHz azimuth tone and the 180 Hz glide slope tone.
  • the modulation control 364 also includes an encoder that allows external data to be added to the 8900 MHz CW output for transmission to the airborne interrogator/receiver unit if dictated by system requirements.
  • the 8900 MHz CW output signal is generated by amplifying the signal from the 8900 MHz LO 362 using an amplifier 360 to a power level that meets system requirements (typically 1 to 5 watts for a range of 25 miles) .
  • the amplified signal from amplifier 360 is routed through a switch 359 to an amplitude and phase modulator 357, into which is also routed the 100 Hz azimuth tone, the 180 Hz elevation tone, and the 20 kHz reference tone from a tone generator 355.
  • Control signals for switch 359 and amplitude and phase modulator 357 are routed from modulation control 364, providing the correct phase and amplitude modulation for the three tones contained in the AM modulation and which includes phase-reversal keying of the signal in synchrony with the airborne unit's pulsed interrogation signal.
  • modulation control 364 provides the correct phase and amplitude modulation for the three tones contained in the AM modulation and which includes phase-reversal keying of the signal in synchrony with the airborne unit's pulsed interrogation signal.
  • the signal is routed to port (c) of circulator 356, which directs the signal out through port (a) to BPF 354.
  • BPF 354 From BPF 354, the 8900 MHz CW signal is routed to antenna 351 and Luneberg lens 350, which direct the signal to the airborne unit.
  • a BIT/automatic level control (ALC) 353 is implemented by coupling a small percentage of the output CW signal via coupler 352 and comparing signal content with commanded content.
  • a performance monitor in the BIT detects any discrepancies and triggers and alarm when problems are encountered.
  • the sampled output power level is compared to a set reference level and used to control the gain of amplifier 360, thereby maintaining a relatively stable transmitted output power level.
  • the airborne transmitter/receiver unit transmits a pulsed interrogation signal toward the ground-based transponder unit.
  • the ground unit receives the pulsed signal through its non- rotating Luneberg lens antenna, which collects and focuses the RF energy into one or more transponder modules arranged in a fixed azimuth/elevation array in close proximity to the outside surface of the Luneberg lens.
  • the ground unit responds by transmitting a CW signal to the aircraft using the transponder module or modules irradiated by the interrogation signal.
  • the CW signal transmitted by each module in the array conveys three discrete audio frequency tones: a phase-referenced azimuth tone, a phase-referenced elevation (glide slope) tone, and a reference tone.
  • the CW return signal further conveys a phase- reversal corresponding to the pulsed interrogation signal.
  • the three tones produce a complex audio frequency waveform that encodes the azimuth and glide slope of the flight path toward the module or modules that transmitted the signal.
  • the CW signal receiver in the airborne unit decodes the information contained in the complex waveform and determines the aircraft's azimuth, glide slope, distance, velocity, and acceleration with respect to the ground unit. This information is then used for automatic, semi-automatic, or manual guidance of the aircraft.
  • CW transmission of guidance information by the present invention ground unit results in narrower bandwidth receiver design in the airborne transmitter/receiver unit and, therefore, increased range of use, as compared to prior art aircraft landing systems using pulsed signals from a ground unit.
  • Phase-referenced encoding of azimuth and glide slope guidance information tones, together with a reference tone, by the present invention ground unit results in the use of only three tones and, thus, only three filters, to provide 360 coverage in azimuth and extended coverage in elevation.

Abstract

A microwave RF landing system having an airborne transmitter/receiver unit which transmits a pulsed interrogation signal toward a ground based transponder unit which receives the pulsed signal through its non-rotating luneberg lens antenna (350, 351) and then transmits a continuous wave (CW) return signal to the aircraft using a transponder module or modules (fig. 7A) irradiated by the interrogation signal. The CW signal transmitted by each module in the array conveys three discrete audio frequency tones: a phase-shifted azimuth tone, a phase-shifted elevation (glide slope) tone, and a reference tone and further conveys a phase-inversion corresponding to the interrogation signal. The three tones produce a complex audio frequency waveform that encodes the azimuth and glide slope of the flight path toward the module or modules that transmitted the signal. The CW signal receiver in the airborne unit decodes the information contained in the complex waveform and determines the aircraft's azimuth, glide slope, distance, velocity and acceleration relative to the ground unit.

Description

DESCRIPTION TITLE: MICROWAVE ENERGY IMPLEMENTED AIRCRAFT LANDING SYSTEM BACKGROUND OF THE INVENTION 1. Field of Use This invention relates generally to aircraft landing systems, and, more particularly, to aircraft landing systems using microwave radio frequency (RF) energy to transmit audio frequency guidance tones. The invention further relates to a method of using such audio frequency guidance tones. 2. Prior Art
A principal prior art patent is considered to be one of the inventors' own, Patent No. 3,295,132, which issued Dec. 27, 1966, to Aubrey I. Chapman, Jr., for Modulating Radar Reflector.
Other known patents in related fields comprise: Patent No. 4,698,636, which issued Oct. 6, 1987, to Raymond Marlow, et al, for Ground Speed Determining Radar System; Patent No. 4,723,123, which issued Feb. 2, 1988, to Raymond Marlow, et al, for Radar System; and Patent No. 4,806,932, that issued Feb. 21, 1989, to Bartow Bechtel, for Radar-Optical Transponding system. The above, and any other U.S. Patents referenced in this Specification, are incorporated herein by reference. SUMMARY OF THE INVENTION
This invention utilizes a ground unit non-rotating uneberg :. lens antenna having an array of low power, independently controlled microwave radio frequency transponder modules to facilitate phase- referenced signals necessary for improved landing operations and an aircraft-borne interrogator/receiver unit to request and process the signals from the ground unit. When an approaching aircraft transmits a pulsed interrogation signal to the ground unit, the ground unit responds by transmitting a continuous wave (CW) signal containing guidance information back to the aircraft. Instrumentation on the aircraft receives the ground unit signal and extracts the guidance information to facilitate landing of the aircraft. It is therefore an object of the present invention to improve microwave energy implemented aircraft landing methods and systems. Another object is to utilize a ground-based transponder array unit and an airborne transmitter/receiver unit as part of an improved landing system, wherein the airborne unit transmits a pulsed interrogation signal to the ground unit and the ground unit responds by transmitting a continuous wave guidance signal to the airborne unit.
A further object is to convey aircraft azimuth and glide slope guidance information by transmitting phase-referenced audio frequency tones on a ground unit transmitted continuous wave signal.
Another object is to convey aircraft range, velocity, and acceleration guidance information by superimposing an airborne unit pulsed interrogation signal onto a ground unit transmitted continuous wave signal as a phase-reversal. Still another object is for the airborne transmitter/receiver unit to receive the ground unit transmitted continuous wave signal and extract the azimuth, glide slope, range, velocity, and acceleration guidance information for use in automatic, semi¬ automatic, or manual guidance of the aircraft. Additional objects, advantages, and novel features of the invention will be set forth in the description which follows, and will become apparent to those skilled in the art upon examination of the following description and drawings. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in this specification. BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is an elevation view of a prior art Luneberg lens;
Figure IA is a plan view of the Luneberg lens of Fig. 1 operating as a collector of electromagnetic energy;
Figure IB is a plan view of the Luneberg lens of Figs. 1 & 2 operating as both a collector and reflector of electromagnetic energy;
Figure 2A is an elevation view of a prior art Luneberg lens using reflective elements to establish coded, narrow beamwidth electromagnetic beams in elevation for use as descent path (glide slope) references;
Figure 2B is a plan view of another prior art Luneberg lens using reflective elements to establish coded, narrow beamwidth electromagnetic beams in azimuth for use as a localizer; Figure 3 is a conceptual view of another prior art Luneberg lens used as a focusing means and having an array of transponder modules;
Figures 4A and 4B are elevation and plan views, respectively, of a prior art embodiment of a transponder unit using an array of transponder modules arranged in close proximity to the surface of a radio frequency Luneberg lens to generate a matrix of high gain, narrow beamwidth beams;
Figure 5A shows a block diagram of a prior art embodiment of a ground transponder unit implemented with a single Luneberg lens focusing means;
Figure 5B shows an arbitrary selection of four horns: two horns in azimuth and two horns in elevation;
Figure 6 shows a prior art embodiment of an airborne interrogator/receiver used in conjunction with a ground transponder unit;
Figure 7A shows an azimuth pattern of phase-referenced microwave beams for one embodiment of the present invention having a 10 degree beam spacing covering 360 degrees;
Figure 7B shows an elevation (glide slope) pattern of phase- referenced microwave beams for one embodiment of the present invention having a 5 degree beam spacing covering 30 degrees;
Figure 7C is a cross section of a small sector of a phase- referenced microwave beam pattern emanating from one embodiment of a ground-based transponder unit of the present invention showing beam azimuth/elevation values;
Figure 8 is a block diagram of an airborne radio frequency pulsed interrogator transmitter and continuous wave receiver unit for use with a ground-based transponder unit in one embodiment of the present invention; Figure 9 is a block diagram of a ground-based transponder unit utilizing a pulsed receiver and continuous wave transmitter for use with an airborne interrogator/receiver unit in one embodiment of the present invention. BACKGROUND ART
Referring now to the prior art of Figure 1, a Luneberg lens 10 is rotated by a motor 12 at a constant angular velocity. Lens 10 focuses incoming radio frequency energy to a small spot near the lens surface 14 opposite the side where the RF energy entered. If no reflective element 16 is present where the spot is formed, as shown in Figure IA, the RF energy passes through the surface and is dissipated, and essentially none of the focused RF energy is returned to the transmitting vehicle. If an RF reflective element 16, which is, for example, a piece of aluminum foil, is present at the focal point, as shown in Figure IB, the RF energy is reflected precisely back in a very narrow beam along the path from whence it came.
Referring now to the prior art of Figure 2A, in which is shown a Luneberg lens 10 rotated about a vertical axis by motor 12 and having two horizontal rows 17 and 18 of reflective elements 16 for establishing a descent path, it will be noted that each row 17 and 18 has a different number of reflective elements 16 for producing different modulation frequencies at different elevation angles. It will be appreciated that the focused spot of RF energy has a definite size depending, in part, on the frequency of the RF energy used. At X-band, the spot is about 0.75 inches in diameter. Thus, if the vertical dimension of the reflective elements 16 is properly sized, as the spot moves when the elevation angle of the incoming RF energy is changed, there is a smooth transition of the percentage of intermediate frequency (IF) of the two modulation frequencies f^ and f2 in the return signals. This mixing action is such that with a 15 inch radius Luneberg lens, a precision of about 0.01 degrees of elevation angle is attainable.
In a Luneberg lens 10 in Figure 2A, the modulating frequencies in space establish 360 degree conical patterns of descent paths.
Referring now to the prior art of Figure 2B, if another Luneberg lens 10 is mounted with its axis of rotation horizontal and uses an arrangement of reflective elements 16 different from those used on the elevation lens, a localizer (azimuth) pattern is produced.
To alleviate the problems in rotating a purely reflective lens used in the prior art, various configurations of transponding (transmit/receive, or beacon) modules are used.
Referring now to the prior art of Figure 3, a transponder system comprises a stationary Luneberg lens 10 and an array 19 of audio frequency amplitude modulated pulse forming modules 20. A platform 22 supports the Luneberg lens 10. Referring to the prior art of Figure 4A, an elevation view of a stationary Luneberg lens 10 focusing means is shown with several transponding modules 242 arranged in a vertical column. The number of modules 242 in a column is determined by the size of the lens, the size of the modules, and the total coverage desired in elevation angle. A column that is six modules high has been arbitrarily chosen as an example. The elevation view of Figure 4A shows only one column, but a matrix pattern is actually used with a few or many columns mounted around the lens. A plan view of the lens 10 (Figure 4B) and module matrix are shown with several columns mounted side by side. Additional columns could be used to cover the complete circumference of the lens to provide guidance throughout 360 degrees in azimuth if desired for great flexibility in choosing an azimuth heading, for example, for use with helicopters that do not have a runway they have to line up with. Referring to the prior art of Figures 5A and 5B showing an embodiment for a ground-based transponder unit used in an aircraft landing system, a single Luneberg lens 10 focuses incoming energy into four horns 122, 124, 126, and 128. The horns are arranged in a cluster, as shown in Figure 5B, i.e. there are two horns in a vertical plane and two horns in a perpendicular plane. This four horn cluster, in conjunction with the focusing means, generates a pattern of relatively high gain pencil beams that crossover at their half power points. These crossover points define an aircraft's approach path to the landing site. The position of the aircraft (high-low, left-right) relative to the defined approach path is determined by amplitude comparison of the audio tone modulation which identifies each beam. The four transmit/receive modules 130 use a common RF source 133 to minimize output amplitude fluctuation, a potential source of approach path position error.
The components of the transmit/receive modules 130 include, besides the horns 122, 124, 126, and 128, bandpass filters (BPF) 134, 136, 138, and 140 connected to the horns, respectively, receive and transmit circulators 142, 144, 146, and 148 connected to the band-pass filters, respectively, and to superheterodyne circuits including limiters 150, 152, 154, and 156 connected through the circulators to the BPF's, respectively, mixers 158, 160, 162, and 164 connected, respectively, to the limiters and to a common local oscillator (LO) 182 for producing IF signals, and detectors 174, 176, 178, and 180, connected respectively, to the IF amplifiers 166, 168, 170, and 172, for detecting the envelope of the video pulses. The interrogating signal from an aircraft interrogator transmitter located within the locus of four beams may be intercepted by the horns or predominantly by a single horn. The outputs of the detectors 174, 176, 178, and 180 are logically OR'd in OR gates 184 and 186 connected to the detectors with the output thereof exiting a final OR gate 188; a high output triggers an RF reply pulse from the RF source 133.
The driver 190 connected to the OR gate 188 is triggered to produce a pulse to the pulse modulator 194 through the encoder 192. Depending on system requirements, the encoder may produce a pulse train to the pulse modulator to encode data from the data source 196 corresponding to the interrogator signals but modified to contain the intelligence of the external data source.
The pulse modulator 194 is connected to the encoder 192 and the RF (9075 MHz) source 195 and converts the video pulses to RF pulses. A four way power divider 197 is connected to the pulse modulator for dividing the RF pulse power equally through phase trimmers 198, 200, 202, and 204 connected to the divider, to audio tone modulators 206, 208, 210, and 212. An audio tone generator 214 producing frequencies fj_, f2# £31 and f4 is connected in common to the audio tone modulators. The tone modulators impose the individual tone modulations on the RF pulses, which identify each of the four beams and define the approach path for the aircraft's interrogator/receiver tracker to generate azimuth and glide slope guidance signals. Port (c) of the circulators 142, 144, 146, and 148 is connected to the tone modulators for passing the modulated RF pulses through the band-pass filters 134, 136, 138, and 140, respectively, horns 122, 124, 126, and 128, respectively, and lens 10 to the aircraft's interrogator/receiver unit. In this fashion, the ground transponder information is transmitted to the aircraft's guidance receiver for processing the beam's modulation tones into up-down, left-right guidance signals to guide the aircraft along the approach path.
To assure proper system operation of the ground unit, a built- in-test (BIT) circuit 216 is provided. The test circuit includes a single horn 218 for illuminating the transmit/receive horns 122, 124, 126, and 128 from the opposite (forward) side of the Luneberg lens 120. The test signal is a continuous pulse train at the interrogating frequency (9375 MHz) obtained by directional coupler 220 operatively connected to the RF (9075 MHz) source 195, mixing the sample with a 300 MHz offset frequency from offset oscillator 222 in mixer 224. A BPF 226 connected to the mixer eliminates the unwanted mixer outputs, and an amplifier 228 connected to the BPF amplifies the remaining carrier (9375 MHz) .
A pulse modulator 230 is connected to the amplifier 228 and to a test pulse generator 233 to produce the train of RF pulses for the horn 218. Horns 122, 124, 126, and 128 intercept a portion of the test train energy, which is amplified and processed the same way as an aircraft interrogation signal. The RF reply pulse train generated in response to the test signal is sampled by four directional couplers and detectors 232, 234, 236, and 238. A performance monitor 240 is connected to the detectors. Any discrepancies will trigger a control/alarm.
Referring to the prior art of Figure 6, the airborne interrogator/receiver is basically a range-only interrogator/receiver with a pulse transmitter (interrogator) 244 and receiver 246. The interrogator/receiver includes a fixed, non- scanning reflector, flat-plate, horn, or stub antenna 248 which both transmits and receives. The interrogate transmit pulse is generated in a pulse transmitter 250 and is fed into port (c) of a conventional RF circulator 252, which directs it out of port (a), through BPF 254 and out the antenna 248. This interrogation pulse triggers one or more of the ground transponder modules, which then transmits a pulse in a very narrow microwave beam back to the antenna 248, now used as a receive antenna.
The receive pulse passes through BPF 254 and into port (a) of the circulator 252, which directs the receive pulse out of port (b) into the signal level limiter 256 of the receiver 246. From the limiter 256 it passes through BPF 258 and is fed to an RF mixer 260 where it mixes with a signal from LO 262 to produce an IF signal, which is filtered in BPF 263 before being applied to an IF amplifier 264 with automatic gain control (AGO 266. The pulse signal is amplitude detected in the detector 268 to produce a video signal, which is fed in two directions. Basically, one path is through a tracker 269, which includes circuitry used to determine range, velocity, and acceleration relative to the ground unit, and the second path is through a filter-bank 287 which includes circuitry for determining azimuth and glide slope errors relative to the ground unit.
In the prior art embodiment landing system described above in which a ground unit transmitted unique tones to designate individual locations within an azimuth/elevation matrix of microwave pencil beams, a filter was required for each tone, both in the ground unit and in the airborne interrogator/receiver unit. This technique was well suited for installations using a limited number of tones. However, increasing the azimuth/elevation coverage of the system increased the number of tones and/or decreased the frequency spacing between the tones. Providing 360 degree azimuth coverage with extended elevation coverage made both the ground unit and the airborne unit more complicated and expensive because of the necessity for more filters. It especially complicated the airborne unit because high Q, narrow bandpass filters required by close tone spacing are not readily available in a small, compact, lightweight form suitable for airborne use. The present invention overcomes the tone and filter problems associated with the above prior art landing system when wide area aircraft guidance coverage is needed. The present invention requires only three tones to provide 360 degree azimuth coverage with extended elevation coverage: one tone for azimuth, one tone for elevation, and one tone for a reference. Phase shift, as a function of azimuth, is applied to the azimuth tone. Similarly, phase shift, as a function of elevation, is applied to the elevation tone. The reference tone is a tone that is frequency modulated by both the azimuth and the elevation tones. As a result, the present invention requires only three filters to decode azimuth and glide slope information and it eliminates the necessity for filters of unreasonably high Q design.
Further, prior art landing systems utilizing pulsed signals from the ground unit required RF spectrum bandwidth accommodation by the airborne receiver determined, in part, by the duration of the pulses; shorter pulse durations required greater bandwidth. The present invention overcomes the bandwidth problems associated with such prior art ground units by utilizing continuous wave (CW) signal transmission. Continuous wave transmission of guidance information results in a significant reduction in bandwidth of an airborne unit's receiver, which extends the range from which an aircraft can be guided to a landing site. BRIEF DESCRIPTION OF THE PREFERRED EMBODIMEN (S) In the three tone, phase-referenced embodiment of the present invention, consider arbitrarily chosen frequencies, for example, of 100 Hz for the azimuth tone, 180 Hz for the elevation tone, and 20 kHz for the reference tone. An incremental phase delay from a zero degree reference is applied separately to the azimuth tone and the elevation tone for uniquely designating the array location, and, thus, the beam orientation, of each transponder module on the ground transponder unit. These two tones are used to frequency modulate the 20 kHz reference tone, resulting in a complex waveform that contains all the information needed to determine the azimuth and glide slope of an aircraft. The assignment of azimuth and glide slope information is as follows. Referring to Figure 7A, transponder modules are arbitrarily located, for example, every 10 degrees around the ground unit's Luneberg lens 350. Module number 36 is pointed to the south. The phase shift of its transmitted 100 Hz azimuth tone frequency is zero degrees. This means that when an airborne interrogator/receiver unit decodes the phase of this module, a heading of zero degrees is indicated, which is a flight vector to the north. Module number 9 is pointed west and the phase shift of its azimuth tone is 90 degrees. When an airborne interrogator/receiver unit phase decodes this module's 100 Hz azimuth tone, a heading of 90 degrees is indicated, which is a flight vector to the east. When an airborne unit receives energy from any two adjacent ground unit transponder modules, the airborne unit's 100 Hz tone phase decoder will process the vectorial sum of the phases from both modules, indicating an azimuth between that of the two adjacent modules. Thus, azimuth vectoring is available from any point within the transmitting and receiving range of the ground and airborne units. Glide slope capability is implemented in a similar manner.
Referring to Figure 7B, transponder modules are arbitrarily located on the equator of the ground unit's Luneberg lens 350 and every five degrees therebelow to cover a range of elevation from zero through 30 degrees. Module number 3 is pointed upward at an angle of ten degrees. The phase shift of its transmitted 180 Hz elevation tone is ten degrees. When an airborne interrogator/receiver unit decodes the phase of this module, a glide slope of ten degrees is indicated. When an airborne unit receives energy from adjacent transponder modules, for example, module number 5 and module number 6, the 180 Hz tone decoder will process the vectorial sum of the phases from both modules and indicate a glide slope that is between the two modules, in this case, 22.5 degrees. Therefore, glide slope vectoring is also obtainable from any point within the transmitting and receiving range of the ground and airborne units.
Although the azimuth and elevation functions of a ground unit transponder module in this embodiment of the present invention are described independently, above, note that in each module these functions actually occur concurrently. Figure 7C is a cross section of an arbitrarily chosen sector of a beam pattern emanating from a ground transponder unit located at a landing site. It shows the beam pattern as viewed from an aircraft north of the ground unit. Modules number 16 through 20 provide azimuth coverage from 160 degrees (south-southeast) through 200 degrees (south-southwest) in 10 degree angular increments, and a glide slope angle of 10 degrees. An airborne interrogator/receiver unit intercepting energy from module number 18 would, by detecting and decoding the phase-shift of both the 100 Hz azimuth tone and the 180 Hz elevation tone, output an azimuth of 180 degrees (due south) and a glide slope angle of 10 degrees, giving a flight path to the landing site.
Figure 8 shows one implementation of a phase-referenced, three tone embodiment for an airborne interrogator/receiver unit of this embodiment of the invention. It transmits a 9200 MHz pulsed interrogation signal to the ground transponder unit and receives the ground unit's 8900 MHz CW signal. In addition to azimuth and glide slope information, the ground unit's signal also includes a phase- reversal return of the airborne unit's pulsed 9200 MHz interrogation signal, which the airborne unit utilizes to determine range, velocity, and acceleration with respect to the ground unit.
The 8900 MHz RF energy is collected by an antenna 300, which may be a fixed, non-scanning reflector, a horn, a flat-plate, or a stub design antenna. The signal is then routed through BPF 301 to port (a) of a circulator 302. The circulator routes the signal to a limiter 303 from port (b) of the circulator and then from the limiter through BPF 304 to a mixer 307 where it is mixed with the 9200 MHz signal of LO 306 to provide a 300 MHz IF signal. The IF signal is then routed to two different paths. One path is to a BPF 308 for subsequent determination of range, velocity, and acceleration. The other path is to BPF 309 for subsequent determination of azimuth and glide slope.
The IF signal from BPF 308 is routed through a limiter/amplifier 311 to an amplitude detector 314. From the amplitude detector the demodulated signal is routed to a tracker 313, where range, velocity, and acceleration are determined by the time delay between the output of a pulse repetition frequency (PRF) /pulse modulator control 310, which initiates the transmission of a signal pulse to the ground unit, and the arrival of the return of the signal pulse from the ground unit at tracker 313. The second path for the IF signal from mixer 307, is through BPF 309 to an amplitude detector 312. The signal from amplitude detector 312 is divided into three paths. The first path is through a 20 kHz filter 316 to a frequency discriminator/filter 319 where the 100 Hz and 180 Hz phase-referenced signals are recovered and used as the phase references by a 100 Hz azimuth tone phase detector 318 and a 180 Hz elevation tone phase detector 320. The second path for the signal from amplitude detector 312 is through a 100 Hz tone filter 315 to the 100 Hz azimuth tone phase detector 318. The output of the 100 Hz azimuth tone phase detector 318 is the azimuth to the landing site. The third path for the AM detected signal from amplitude detector 312 is through a 180 Hz tone filter 317 to the 180 Hz elevation tone phase detector 320. The output of the 180 Hz elevation tone phase detector 320 is the glide slope to the landing site. The airborne interrogator/receiver pulsed 9200 MHz interrogation signal to the ground unit is generated by a PRF/pulse modulator control 310, the 9200 MHz LO 306, and a gate switch/amplifier 305. The pulsed signal from gate switch/amplifier 305 is routed to port (c) of circulator 302 and on to BPF 301 via port (a) of circulator 302. From BPF 301, the 9200 MHz pulsed signal is routed to antenna 300, which directs the interrogation pulse to the ground station.
Figure 9 shows one implementation of the phase-referenced, three tone embodiment of a ground unit transponder module that receives a pulsed 9200 MHz interrogation signal from an airborne interrogator/receiver and returns an 8900 MHz CW signal. The pulsed 9200 MHz signal is received by the ground station Luneberg lens 350 and is focused into a transponder module antenna 351, which routes a signal to a coupler 352. The signal passes through coupler 352 to a BPF 354 and then to port (a) of a circulator 356, which directs the signal out through port (b) to a BPF 358. From BPF 358, the signal is routed through a signal level limiter 361 to a mixer 363 where it mixes with the 8900 MHz signal from a LO 362 to produce a 300 MHz IF signal. From mixer 363, the signal is routed to BPF 365 to remove the unwanted mixer frequency products before being applied to a limiter/amplifier 366. The signal is routed from limiter/amplifier 366 to amplitude detector 367 where the AM detected signal is passed to the modulation control 364 where signals are generated to determine the correct phase modulation for the 100 MHz azimuth tone and the 180 Hz glide slope tone. The modulation control 364 also includes an encoder that allows external data to be added to the 8900 MHz CW output for transmission to the airborne interrogator/receiver unit if dictated by system requirements.
The 8900 MHz CW output signal is generated by amplifying the signal from the 8900 MHz LO 362 using an amplifier 360 to a power level that meets system requirements (typically 1 to 5 watts for a range of 25 miles) . The amplified signal from amplifier 360 is routed through a switch 359 to an amplitude and phase modulator 357, into which is also routed the 100 Hz azimuth tone, the 180 Hz elevation tone, and the 20 kHz reference tone from a tone generator 355. Control signals for switch 359 and amplitude and phase modulator 357 are routed from modulation control 364, providing the correct phase and amplitude modulation for the three tones contained in the AM modulation and which includes phase-reversal keying of the signal in synchrony with the airborne unit's pulsed interrogation signal. From amplitude and phase modulator 357, the signal is routed to port (c) of circulator 356, which directs the signal out through port (a) to BPF 354. From BPF 354, the 8900 MHz CW signal is routed to antenna 351 and Luneberg lens 350, which direct the signal to the airborne unit. A BIT/automatic level control (ALC) 353 is implemented by coupling a small percentage of the output CW signal via coupler 352 and comparing signal content with commanded content. A performance monitor in the BIT detects any discrepancies and triggers and alarm when problems are encountered. The sampled output power level is compared to a set reference level and used to control the gain of amplifier 360, thereby maintaining a relatively stable transmitted output power level.
In summary, the airborne transmitter/receiver unit transmits a pulsed interrogation signal toward the ground-based transponder unit. The ground unit receives the pulsed signal through its non- rotating Luneberg lens antenna, which collects and focuses the RF energy into one or more transponder modules arranged in a fixed azimuth/elevation array in close proximity to the outside surface of the Luneberg lens. The ground unit responds by transmitting a CW signal to the aircraft using the transponder module or modules irradiated by the interrogation signal.
The CW signal transmitted by each module in the array conveys three discrete audio frequency tones: a phase-referenced azimuth tone, a phase-referenced elevation (glide slope) tone, and a reference tone. The CW return signal further conveys a phase- reversal corresponding to the pulsed interrogation signal. The three tones produce a complex audio frequency waveform that encodes the azimuth and glide slope of the flight path toward the module or modules that transmitted the signal.
The CW signal receiver in the airborne unit decodes the information contained in the complex waveform and determines the aircraft's azimuth, glide slope, distance, velocity, and acceleration with respect to the ground unit. This information is then used for automatic, semi-automatic, or manual guidance of the aircraft. CW transmission of guidance information by the present invention ground unit results in narrower bandwidth receiver design in the airborne transmitter/receiver unit and, therefore, increased range of use, as compared to prior art aircraft landing systems using pulsed signals from a ground unit. Phase-referenced encoding of azimuth and glide slope guidance information tones, together with a reference tone, by the present invention ground unit, results in the use of only three tones and, thus, only three filters, to provide 360 coverage in azimuth and extended coverage in elevation. Prior art aircraft landing systems using audio frequency tones to designate individual azimuth and elevation values required a filter for each tone, both in the ground unit and in the airborne unit. Thus, the utilization of phase-referenced azimuth and elevation tones by the present invention results in reduced complexity and cost over prior art aircraft landing systems when providing wide coverage guidance. From the foregoing description and examples, it will be seen that there has been produced a method and system that substantially fulfills the object of this invention as set forth herein. This invention is not limited to the examples shown herein, but may be made and used in many ways within the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:
1. A microwave radio frequency landing method for aircraft utilizing an aircraft-borne transmitter/receiver unit and a ground- based transponder unit comprising a non-rotating Luneberg lens antenna with an array of transponder modules, wherein: a. said airborne unit transmits a pulsed interrogation signal toward said ground unit, and b. said ground unit receives said pulsed interrogation signal through its non-rotating Luneberg lens antenna, and c. said Luneberg lens antenna collects and focuses said received pulsed interrogation signal into at least one of its said transponder modules, and d. said ground unit responds to said received pulsed interrogation signal by transmitting a continuous wave return signal to said aircraft from said transponder module or modules irradiated by said pulsed interrogation signal, wherein: e. said continuous wave return signal conveys phase-shifted audio frequency guidance tones, and f. said continuous wave return signal is phase-inverted in response to, and for the duration of each pulse of said received pulsed interrogation signal, and g. said airborne transmitter/receiver unit receives said continuous wave return signal and interprets said phase-shifted audio frequency guidance tones and said phase-inversion of said continuous wave signal for guidance information to facilitate landing of said aircraft.
2. A microwave radio frequency landing method as in claim 1, wherein said transponder modules are arranged in a fixed azimuth/elevation array in close proximity to the outside surface of said Luneberg lens.
3. A microwave radio frequency landing method as in claim 2, wherein each said transponder module, when interrogated by said airborne transmitter/receiver unit, transmits a guidance information signal comprising a complex waveform to said airborne unit.
4. A microwave radio frequency landing method as in claim 3, wherein each said guidance information signal comprises three discrete audio frequency tones.
5. A microwave radio frequency landing method as in claim 4, wherein said three discrete audio frequency tones comprise a phase- shifted azimuth tone, a phase-shifted elevation (glide slope) tone, and a reference tone.
6. A microwave radio frequency landing method as in claim 4, wherein each said guidance information signal is phase-inverted in response to, and for the duration of, each pulse of said pulsed interrogation signal.
7. A microwave radio frequency landing method as in claim 3, wherein said receiver in said airborne unit decodes said guidance information contained in said complex waveform and determines the aircraft's azimuth, glide slope, distance, velocity, and acceleration relative to said ground unit.
8. A microwave radio frequency landing method as in claim 7, wherein said complex waveform may be used for automatic, semi¬ automatic, or manual guidance of the aircraft.
9. A microwave radio frequency landing system for aircraft utilizing an aircraft-borne transmitter/receiver unit and a ground- based transponder unit comprising a non-rotating Luneberg lens antenna with an array of transponder modules, wherein: a. said airborne unit is capable of transmitting a pulsed interrogation signal toward said ground unit, and b. said ground unit non-rotating Luneberg lens antenna is capable of receiving said pulsed interrogation signal, and c. said Luneberg lens antenna is further capable of collecting and focusing said received pulsed interrogation signal into at least one of its said transponder modules, and d. said ground unit is capable of responding to said pulsed interrogation signals by transmitting a continuous wave return signal to said aircraft from said transponder module or modules irradiated by said pulsed interrogation signal, and e. said continuous wave return signal is capable of conveying phase-shifted audio frequency guidance tones, and f. said continuous wave return signal is further capable of being phase-inverted in response to, and for the duration of, each pulse of said received pulsed interrogation signal, and g. said airborne transmitter/receiver unit is capable of receiving said continuous wave return signal and interpreting said phase-shifted audio frequency guidance tones and said phase- inversion of said continuous wave signal for guidance information to facilitate landing of said aircraft.
10. A microwave radio frequency landing system as in claim 9, wherein said transponder modules are arranged in a fixed azimuth/elevation array in close proximity to the outside surface of said Luneberg lens.
11. A microwave radio frequency landing system as in claim 10, wherein each transponder module, when interrogated by said airborne transmitter/receiver unit, transmits a guidance information signal comprising a complex waveform to said airborne unit.
12. A microwave radio frequency landing system as in claim 11, wherein each said guidance information signal comprises three discrete audio frequency tones.
13. A microwave radio frequency landing system as in claim 12, wherein said three discrete audio frequency tones comprise a phase- shifted azimuth tone, a phase-shifted elevation (glide slope) tone, and a reference tone.
14. A microwave radio frequency landing system as in claim 12, wherein each said guidance information signal is phase-inverted in response to, and for the duration of, each pulse of said pulsed interrogation signal.
15. A microwave radio frequency landing system as in claim 11, wherein said receiver in said airborne unit decodes said guidance information contained in said complex waveform and determines the aircraft's azimuth, glide slope, distance, velocity, and acceleration relative to said ground unit.
16. A microwave radio frequency landing system as in claim 15, wherein said complex waveform may be used for automatic, semi¬ automatic, or manual guidance of the aircraft.
17. A microwave radio frequency landing method for aircraft utilizing an airborne transmitter/receiver unit and a ground-based transponder unit comprising a stationary spherical lens antenna with an array of transponder modules, wherein: a. said airborne unit transmits a pulsed interrogation signal to said ground unit, and b. said ground unit receives said signal through said antenna, and c. said antenna collects and focuses said signal into at least one of its transponder modules, and d. said transponder module then transmits a continuous wave return signal to said airborne unit with guidance information to facilitate landing said aircraft.
18. A microwave radio frequency landing system for aircraft utilizing an airborne transmitter/receiver unit and a ground-based transponder unit comprising a stationary spherical lens antenna with an array of transponder modules, wherein: a. said airborne unit is capable of transmitting a pulsed interrogation signal to said ground unit, and b. said ground unit is capable of receiving said signal through said antenna, and c. said antenna collects and focuses said signal into at least one of its transponder modules, and d. said transponder modules then transmits a continuous wave return signal to said airborne unit with guidance information to facilitate landing said aircraft.
PCT/US1995/016501 1994-12-30 1995-12-20 Microwave energy implemented aircraft landing system WO1996021164A1 (en)

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