(12) United States Patent
`Kuntman et al.
`
`USOO6313783B1
`(10) Patent No.:
`US 6,313,783 B1
`(45) Date of Patent:
`Nov. 6, 2001
`
`(54)
`
`(75)
`
`(73)
`
`(*)
`
`(21)
`(22)
`
`(60)
`
`(51)
`(52)
`
`(58)
`
`(56)
`
`TRANSPONDER HAVING DIRECTIONAL
`ANTENNAS
`
`ASSignee:
`
`Inventors: Daryal Kuntman, Bellevue, WA (US);
`Ruy L. Brandao, Fort Lauderdale, FL
`(US); Ruy C. P. Brandao, Redmond,
`WA (US)
`Honeywell International, Inc.,
`Morristown, NJ (US)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`Notice:
`
`Appl. No.
`: 09/369,958
`Aug. 6, 1999
`Filed:
`Related U.S. Application Data
`Provisional application No. 60/125,994, filed on Mar. 24,
`1999.
`Int. Cl." ........................... G01S 13/74; G01S 13/93
`U.S. Cl. ................................. 342/32: 342/29; 342/30;
`342/36; 342/37; 342/42: 342/.51
`Field of Search ............................ 342/29-32, 36–40,
`342/42–51, 156, 351, 417-424
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`3,375,516 *
`3,792.472 *
`4,638,320
`4,855,748
`5,041,800
`5,077,673
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`3/1968
`2/1974
`1/1987
`8/1989
`8/1991
`12/1991
`
`Hart et al. .............................. 342/.51
`Payne et al. ........................... 342/32
`Eggert et al. ........................ 342/442
`Brandao et al. ..................... 342/455
`Long et al. .
`Brodegard et al. .
`
`
`
`5,157,615
`5,182.563
`5,185,606
`5,191,340
`5,191,349
`5,200,928
`5,235,336
`5,280.285
`5,302,953
`5,317.316
`5,455,586
`5,528,244
`5,552,788
`5,604,504
`5,619,206
`5,629,692
`5,636,123
`5,721,628
`5,805,111
`
`10/1992 Brodegard et al..
`1/1993 Blinchikoff et al. ................... 342/32
`2/1993 Verbaarschot et al. .
`3/1993 Brandao et al. .
`3/1993 Dinsmore et al. ................... 343/751
`4/1993 MaCleod.
`8/1993 Sturm et al. ........................... 342/30
`1/1994 Curtis.
`4/1994 Pierre et al. ........................... 342/37
`5/1994 Sturm.
`10/1995 Barbier et al. ......................... 342/37
`6/1996 Schwab ........
`... 342/37
`9/1996 Ryan et al.
`... 342/30
`2/1997 Nail ..............
`... 342/417
`4/1997 Cole, Jr. et al. ....................... 342/37
`5/1997 Stayton et al. .
`6/1997 Rich et al. .
`2/1998 Phillips et al. .
`9/1998 Brettner, III et al. ............... 342/455
`OTHER PUBLICATIONS
`AlliedSignal Product Brochure: Directional Antenna ANT
`81A, Copyright 2000.
`* cited by examiner
`Primary Examiner Bernarr E. Gregory
`(57)
`ABSTRACT
`A method and apparatus for transmitting a directional reply
`Signals in response to Air Traffic Control Radar Beacon
`System and Mode Select Signals interrogation Signals. Two
`directional antennas independently acquire an interrogation
`Signal and are connected by means of a configurable Switch
`to a transponder. The antennas may include multiple direc
`tional antenna elements.
`
`31 Claims, 12 Drawing Sheets
`
`Y
`-
`
`OO
`
`f
`A1
`
`-4
`
`102
`
`118
`
`110
`
`CAS facie-Stransmitter
`
`Transrand Switch
`witty
`
`116
`
`|
`
`
`
`Moe-STranspodner
`Receiver Top and Bottom
`Arnna)
`
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`

`

`U.S. Patent
`
`Nov. 6, 2001
`
`Sheet 1 of 12
`
`US 6,313,783 B1
`
`
`
`Figure. 1
`(PRIOR ART)
`
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`

`U.S. Patent
`
`Nov. 6, 2001
`
`Sheet 2 of 12
`
`US 6,313,783 B1
`
`30
`
`N -m-...-.
`
`33
`
`34A
`
`10
`
`?
`
`l4
`A
`
`16
`
`DISPLAY
`
`42
`
`32
`
`2A
`
`-1
`
`18
`
`TRANSMITTER
`
`
`
`36
`/7
`
`PRIMARY AND
`SECONDARY
`TRANSPONDERS
`
`E.
`PROCESSOR
`
`MESSAGE
`PROCESSER
`
`ALTITUDE
`SOURCE
`
`20
`
`CONTROL
`PANEL
`
`33
`
`34B
`
`38
`
`2E
`
`16
`
`22
`
`- N.
`4B
`
`Figure 2
`(PRIOR ART)
`
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`U.S. Patent
`
`Nov. 6, 2001
`
`Sheet 3 of 12
`
`US 6,313,783 B1
`
`
`
`Figure 3
`(PRIOR ART)
`
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`U.S. Patent
`
`Nov.6, 2001
`
`Sheet 4 of 12
`
`US 6,313,783 B1
`
`(LUVYOMd)pemeory
`
`8
`
`re2a=ZZ"AO})w@~440vex06OLAV
`
`SVaULV
`
`or
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`
` S¥LSGESYzit40YddXJS3LSVOLS1dyiW
`
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`

`

`U.S. Patent
`
`Nov. 6, 2001
`
`Sheet 5 of 12
`
`US 6,313,783 B1
`
`
`
`Figure 5
`(PRIOR ART)
`
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`U.S. Patent
`
`Nov. 6, 2001
`
`Sheet 6 of 12
`
`US 6,313,783 B1
`
`INTRUDER AIRCRAFT
`
`
`
`FIGURE 6
`(PRIOR ART)
`
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`U.S. Patent
`
`Nov. 6, 2001
`
`Sheet 7 of 12
`
`US 6,313,783 B1
`
`
`
`104A
`
`1 O2
`1N1
`
`Combined
`TCAS
`Transponder
`Unit
`
`104B
`
`FIGURE 7
`
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`U.S. Patent
`
`Nov. 6, 2001
`
`Sheet 8 of 12
`
`US 6,313,783 B1
`
`Transm and Switch
`Corstso
`
`CAS Mode-STransmitter
`
`Mode-STranspodner
`Receiver (Top and Bottom
`Antenna)
`
`
`
`
`
`
`
`
`
`
`
`? CONTROLPANEL
`122
`
`DISPLAY
`
`FIGURE 8
`
`106
`
`104B
`
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`U.S. Patent
`
`Nov. 6, 2001
`
`Sheet 9 of 12
`
`US 6,313,783 B1
`
`
`
`110
`
`Switch
`
`12
`
`Mcte-STranspodner
`Receiver (Top and Bottom
`Antering)
`
`to
`
`124
`
`114
`
`122
`
`120
`
`104B
`
`FIGURE 9
`
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`U.S. Patent
`
`Nov. 6, 2001
`
`Sheet 10 of 12
`
`US 6,313,783 B1
`
`
`
`FIGURE 10
`(PRIOR ART)
`
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`U.S. Patent
`
`Nov. 6, 2001
`
`Sheet 11 of 12
`
`US 6,313,783 B1
`
`FIGURE II
`(PRIOR ART)
`
`250
`
`POWER NPUT
`
`
`
`A2 POWER OUTPU
`-90 RELATIVE TO POWER NPUT)
`
`a^ 254 M
`NO POWER OUTPUT
`
`266
`
`\
`M2 POWER OUTPUT
`to BO RELATIVE TO PoweR INPUT)
`
`FIGURE 12
`(PRIOR ART)
`
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`U.S. Patent
`
`Nov. 6, 2001
`
`Sheet 12 of 12
`
`US 6,313,783 B1
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`
`
`
`
`
`
`
`
`
`
`
`
`JO NO1105T3S
`
`19:49, VfT
`
`FIGURE 13
`(PRIOR ART)
`
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`

`US 6,313,783 B1
`
`1
`TRANSPONDER HAVING DIRECTIONAL
`ANTENNAS
`
`This application claims the benefit of U.S. Provisional
`Application Ser. No. 60/125,994, filed in the names of
`Daryal Kuntman, Ruy L. Brandao, and Ruy C. P. Brandao on
`Mar. 24, 1999, the entirety of which is incorporated herein
`by reference.
`
`5
`
`FIELD OF THE INVENTION
`The present invention relates generally to aircraft Surveil
`lance and collision avoidance Systems, and particularly, to
`combined Air Traffic Control Radar Beacon System/Mode
`Select (ATCRBS/Mode-S) and Air Traffic Alert and Colli
`Sion Avoidance System (TCAS) Systems utilizing common
`antennaS.
`
`15
`
`2
`a predetermined frequency and are Separated by a predeter
`mined interval. The time interval between the first and third
`pulses defines what information the interrogator is request
`ing: eight (8) microSeconds for identification and twenty-one
`(21) microSeconds for altitude. The operator of the ground
`based SSR Sets the radar interrogation code to request either
`Mode-A or Mode-C replies from the aircraft transponder.
`Typically, the radar is Set to request a sequence of two
`Mode-A replies followed by a single Mode-C reply. This
`Sequence is repeated So that a radar operator continuously
`receives both the Mode-A identification code and the
`Mode-C altitude information. Upon receipt of the interro
`gation signal, the aircraft transponder develops and trans
`mits a reply signal which includes the identification or
`altitude information. The ground based SSR receives and
`processes the transponder reply Signal, together with time of
`arrival range information, to develop a measurement of
`position for each responding aircraft. Under Such a System,
`the air traffic controller uses this information to contact
`involve the aircraft by radio, usually with voice
`communication, to maintain or restore Safe Separations
`between aircraft. The system is inherently limited because
`each aircraft needs be dealt with individually which requires
`a share of the air traffic controller's time and attention. When
`traffic is heavy, or visibility is low, collision potential
`CCSCS.
`During the 1960s the increases in the number of aircraft,
`the percentage of aircraft quipped with transponders, and the
`number of ATCRBS radar installations began to overload the
`ATCRBS system. This system overload caused a significant
`amount of interference and garble in the Mode-A and
`Mode-C transmissions because of replies from many Simul
`taneously interrogated aircraft. Furthermore, the Mode-A
`and Mode-C systems are unable to relay additional infor
`mation or messages between the ground based SSR and the
`interrogated aircraft, other than the aforementioned identi
`fication and altitude information. The Mode Select, or
`Mode-S, was the response to this overload and other defi
`ciencies in ATCRBS. Mode-S is a combined secondary
`Surveillance radar and a ground-air-ground data link System
`which provides aircraft Surveillance and communication
`necessary to Support automated ATC in the dense air traffic
`environments of today.
`Mode-S incorporates various techniques for Substantially
`reducing transmission interference and provides active
`transmission of messages or additional information by the
`ground based SSR. The Mode-S sensor includes all the
`essential features of ATCRBS, and additionally includes
`individually timed and addressed interrogations to Mode-S
`transponders carried by aircraft. Additionally, the ground
`based rotating directive antenna is of monopulse design
`which improves position determination of ATCRBS target
`aircraft while reducing the number of required interrogations
`and responses, thereby improving the radio frequency (RF)
`interference environment. Mode-S is capable of common
`channel interoperation with the ATC beacon system. The
`Mode-SSystem uses the same frequencies for interrogations
`and replies as the ATCRBS. Furthermore, the waveforms, or
`modulation techniques, used in the Mode-S interrogation
`Signal were chosen Such that, with proper demodulation, the
`information content is detectable in the presence of Overlaid
`ATCRBS signals and the modulation of the downlink or
`reply transmission from the transponder is pulse position
`modulation (PPM) which is inherently resistant to ATCRBS
`random pulses. Thus, the Mode-S system allows full Sur
`veillance in an integrated ATCRBS/Mode-S environment.
`The Radio Technical Commission for Aeronautics
`(RTCA) has promulgated a specification for the Mode-S
`
`BACKGROUND OF THE INVENTION
`Aircraft pilots are expected to visually identify collision
`threats and avoid them. This “see and avoid” technique
`based on the pilot's visual Sense remains the most basic
`method of aircraft collision avoidance. However, Since the
`1950's electronic techniques based on radio frequency and
`optical transmissions have been developed to Supplement
`the pilot's Visual Sense. The government has developed and
`implemented a System of ground based and aircraft carried
`equipment designated the Air Traffic Control Radar Beacon
`System (ATCRBS). This system includes two different types
`of ground based radar emitters located at each of a plurality
`of Air Traffic Control (ATC) stations. One type of radar is
`referred to as the Primary Surveillance Radar (PSR), or
`Simply as the primary radar. The primary radar operates by
`Sending out microwave energy which is reflected back by the
`aircraft's metallic Surfaces. This reflected Signal is received
`back at the ground radar Site and displayed as location
`information for use by an air traffic controller. The second
`type of radar is referred to as the Secondary Surveillance
`Radar (SSR), or simply secondary radar. Unlike the primary
`radar, the SSR is a cooperative System in that it does not rely
`on reflected energy from the aircraft. Instead, the ground
`based SSR antenna transmits a coded 1030 MHz microwave
`interrogation Signal. A transponder, i.e., a transmitter/
`receiver, carried on the aircraft receives and interprets the
`interrogation signal and transmits a 1090 MHz microwave
`reply signal back to the SSR ground site. This receive and
`reply capability greatly increases the Surveillance range of
`the radar and enables an aircraft identification function,
`referred to as Mode-A, wherein the aircraft transponder
`includes an identification code as part of its reply signal.
`This identification code causes the aircraft's image or blip
`on the ATC operator's radar screen to stand out from the
`other targets for a short time, usually about 20 Seconds.
`Thus, Mode-A provides an rudimentary identification func
`tion.
`In addition to the identification function provided by
`Mode-A, the aircraft altimeter is typically coupled to the
`transponder Such that a reply Signal includes altitude
`information, referred to as Mode-C.
`A ground based SSR sequentially transmits both Made A
`60
`and Mode-C interrogation signals to aircraft in the area.
`Accordingly, the interrogation signal transmitted by the SSR
`contains three pulses. The Second pulse is a Side-lobe
`Suppression Signal transmitted from an omnidirectional
`antenna co-located with a mechanically rotating antenna
`which provides a highly directive antenna beam. The first
`and third pulses are transmitted by the directive antenna at
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`3
`system, RTCA/DO-181A, Minimum Operational Perfor
`mance Standards for Air Trafic Control Radar Beacon
`System/Mode Select (ATCRBS/Mode-S) Airborne
`Equipment, issued Jan. 1992, and incorporated herein by
`reference. According to RTCA specification DO-181A, the
`airborne portion of the Mode-S system includes in one form
`or another at least a dedicated transponder, a cockpit
`mounted control panel, two dedicated antennas and cables
`interconnecting the other elements. Shadowing is attenua
`tion of the received transponder Signals by the airframe
`blocking the antenna from the SSR ground Station transmit
`ter when a single antenna is used. The Shadowing problem
`is overcome by locating a first antenna on a top Surface of
`the aircraft and a Second antenna on a bottom Surface of the
`aircraft. AS discussed more fully below, each aircraft may be
`within range of more than one SSR ground Station at any
`time and must respond to interrogation signals broadcast
`from multiple directions. Therefore, the Mode-S system uses
`two Single element omnidirectional antennas to receive
`interrogation Signals from any quarter and reply in kind.
`In operation, a unique 24-bit address code, or identity tag,
`is assigned to each aircraft in a Surveillance area by one of
`two techniques. One technique is a Mode-S"Squitter pre
`formed by the airborne transponder. Once per Second, the
`Mode-S transponder Spontaneously and pseudo-randomly
`transmits (Squits) an unsolicited broadcast, including a spe
`cific address code unique to the aircraft carrying the
`transponder, via first one and then the other of its two
`dedicated antennas which produce an omnidirectional
`pattern, discussed below. The transponder's transmit and
`receive modes are mutually exclusive to avoid damage to the
`equipment. Whenever the Mode-S transponder is not
`broadcasting, it is monitoring, or “listening,” for transmis
`Sions simultaneously on both of its dedicated omnidirec
`tional antennas. According to the Second technique, each
`ground based Mode-S interrogator broadcasts an ATCRBS/
`Mode-S “ All-Call” interrogation signal which has a wave
`form that can be understood by both ATCRBS and Mode-S
`transponders. When an aircraft equipped with a Standard
`ATCRBS transponder enters the airspace served by an ATC
`Mode-S interrogator, the transponder responds to the with a
`standard ATCRBS reply format, while the transponder of a
`Mode-Sequipped aircraft replies with a Mode-S format that
`includes a unique 24-bit address code, or identity tag. This
`address, together with the aircraft's range and azimuth
`location, is entered into a file, commonly known as putting
`the aircraft on roll-call, and the aircraft is thereafter dis
`cretely addressed. The aircraft is tracked by the ATC inter
`rogator throughout its assigned airspace and, during Subse
`quent interrogations, the Mode-Stransponder reports in its
`replies either its altitude or its ATCRBS 4096 code, depend
`ing upon the type of discrete interrogation received. AS the
`Mode-S equipped aircraft moves from the airspace Served
`by one ATC Mode-S interrogator into that airspace served by
`another Mode-S interrogator, the aircraft's location infor
`mation and discrete address code are passed on via landlines,
`else either the ground based SSR station picks up the
`Mode-Stransponder's “squitter” or the Mode-Stransponder
`responds to the All-Call interrogation Signal broadcast by the
`next ATC Mode-S interrogator.
`The unique 24-bit address code, or identity tag, assigned
`to each aircraft is the primary difference between the
`Mode-S system and ATCRBS. The unique 24-bit address
`code allows a very large number of aircraft to operate in the
`air traffic control environment without an occurrence of
`redundant address codes. Parity check bits overlaid on the
`address code assure that a message is accepted only by the
`
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`US 6,313,783 B1
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`4
`intended aircraft. Thus, interrogations are directed to a
`particular aircraft using this unique address code and the
`replies are unambiguously identified. The unique address
`coded into each interrogation and reply also permits inclu
`Sion of data link messages to and/or from a particular
`aircraft. To date, these data link messages are limited to
`coordination messages between TCAS equipped aircraft, as
`discussed below. In future, these data link messages are
`expected to include Aircraft Operational Command (AOC)
`information consisting of two to three pages of text data with
`flight arrival information, Such as gates, passenger lists,
`meals on board, and Similar information, as well as Flight
`Critical Data (FCD). However, the primary function of
`Mode-S is Surveillance and the primary purpose of Surveil
`lance remains collision avoidance.
`Collision avoidance Systems which depend on aircraft
`carried transponders are usually divided into two classes:
`passive and active. The ATCRBS, including Mode-S,
`described above are passive Systems because the transpon
`der reply emissions alone provide the only information for
`locating and identifying potential threats. While passive
`Systems tend to be simple and low cost when compared to
`active Systems and do not crowd the Spectrum with addi
`tional RF transmissions, detection of transponder emissions
`from other aircraft is difficult. A passive collision threat
`detector is essentially a receiver having Sufficient intelli
`gence to first detect and then locate the existence of potential
`collision threats represented by nearby aircraft. The air
`craft's receiver is of necessity operating in close proximity
`to the host aircraft's ATCRBS transponder. Government
`regulations require the ATCRBS transponder to emit RF
`energy at 125-500 watts in response to interrogation signals
`from a ground based SSR. The transponder aboard any
`potential collision threat aircraft flying along a radial from
`the directional SSR antenna, usually about 3 to 4 wide,
`will respond at about the same time as the host aircraft's
`transponder. The host aircraft's transponder is So much
`closer, usually no more than a few feet, to any receiver that
`the host aircraft's own response to the interrogation Signal
`will Swamp the response from any other aircraft in its
`vicinity. Thus, the host aircraft flies in a “blind” region
`wherein any potential threat aircraft is not "seen, unless
`other provisions are made. This blind region expands as the
`target approaches the host. Furthermore, typically each
`aircraft is within range of more than one SSR site and a blind
`region is associated with each SSR site. Because wholly
`passive Systems are generally believed insufficient for reli
`able collision avoidance, the government and aviation indus
`try have cooperated in developing Operational Performance
`Standards for a Traffic Alert and Collision Avoidance or
`TCAS system, separate from the ATCRBS/Mode-S tran
`sponder system. The standards are set forth in the RTCA
`specifications DO-185, Minimum Operational Performance
`Standards for Air Trafic Alert and Collision Avoidance
`System (TCAS) Airborne Equipment, issued Sep. 23, 1983,
`consolidated Sep. 6, 1990, and DO-185A, Minimum Opera
`tional Performance Standards for Air Trafic Alert and
`Collision Avoidance System II (TCAS II) Airborne
`Equipment, issued Dec. 1997, both of which are incorpo
`rated herein by reference.
`TCAS is a well-known active collision avoidance system
`that relies upon reply Signals from airborne transponders in
`response to interrogation signals from an aircraft equipped
`with a ATCRBS Mode-A/Mode-C or Mode-S transponder.
`The TCAS antenna is driven to produce an omnidirectional
`microwave transmission, or radiation, pattern carrying a
`transponder generated coded interrogation signal at 1030
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`US 6,313,783 B1
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`15
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`35
`
`25
`
`S
`MHz, the same frequency used by ground based SSR
`stations to interrogate Mode-S transponders. Whenever the
`TCAS transponder is not broadcasting, it is “listening” for
`Mode-S “squitters” and reply transmissions at 1090 MHz,
`the same frequency used by Mode-Stransponders to reply to
`interrogation Signals. Thus, a TCAS equipped aircraft can
`“See' other aircraft carrying a transponder. Once a transpon
`der equipped target has been “seen,” the target is tracked and
`the threat potential is determined. Altitude information is
`essential in determining a target's threat potential. Compari
`Son between the altitude information encoded in the reply
`transmission from the threat aircraft and the host aircraft's
`altimeter is made in the TCAS processor and the pilot is
`directed obtain a Safe altitude Separation, by descending,
`ascending or maintaining current altitude.
`Collision avoidance is enhanced by including range infor
`mation during threat determination. The approximate range,
`or distance between the host aircraft and the target, is based
`on the Strength of the received transponder Signal in
`response to an interrogation Signal from the host aircraft.
`Modern TCAS systems obtain more accurate range infor
`mation by measuring the time lapse between transmission of
`the interrogation signal and reception of the reply signal,
`commonly known as “turn around time.” The time to closest
`approach as determined by the TCAS processor is the
`primary consideration in threat determination.
`Knowledge of the direction, or bearing, of the target
`aircraft relative to the host aircraft's heading greatly
`enhances a pilot's ability to visually acquire the threat
`aircraft and provides a better spatial perspective of the threat
`aircraft relative to the host aircraft. The TCAS processor can
`display bearing information if it is available. Bearing infor
`mation is also used by the TCAS processor to better deter
`mine threat potential presented by an intruder aircraft.
`Directional antennas are used in Some TCAS Systems for
`determining angle of arrival data which is converted into
`relative bearing to a threat aircraft by the TCAS processor.
`Several methods exist for determining angle of arrival data.
`One common arrangement uses a phase matched quadrapole
`antenna array with output signals being combined Such that
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`the phase difference between two output ports of the com
`bining circuitry indicates the bearing of a received transpon
`der Signal. Another method for determining angle of arrival
`data include a method based on Signal phase, commonly
`known as phase interferometry. Still another commonly
`known method is based on Signal amplitude. Attenuation of
`the received transponder Signals by the airframe blocking
`the antenna from the transmitter is often overcome by
`locating a primary directional antenna on a top Surface of the
`aircraft and a Second antenna on a bottom Surface of the
`aircraft. The Second or bottom antenna is Sometimes omni
`directional which reduces cost at the expense of reduced
`directional coverage. Other TCAS systems provide dupli
`cate directional antennas top and bottom. U.S. Pat. No.
`5,552,788, Antenna Arrangement And Aircraft Collision
`Avoidance System, issued Sep. 3, 1996, the complete dis
`closure of which is incorporated herein by reference, teaches
`an arrangement of four Standard monopole antenna
`elements, for example, /4 wavelength transponder antennas,
`arranged on opposing Surfaces of one axis of the aircraft at
`the extremes of two mutually orthogonal axes to avoid
`Shadowing and provide directional information about the
`received reply signal. For example, two monopole antennas
`are preferably mounted on a longitudinal axis of the aircraft
`and two additional monopole antennas are preferably
`mounted on a lateral axis of the aircraft orthogonal to the
`longitudinal axis passing through the first two antennas.
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`Directionality is determined by comparing the power levels
`of the received signals. Additionally, the 788 patent teaches
`a TCAS System which can transmit transponder interroga
`tion Signals directionally using predetermined ones of the
`monopole antennas, thus eliminating dependence upon
`ground based radar Systems for interrogating threat aircraft
`transponders.
`Other antennas for directionally transmitting TCAS sys
`tem transponder interrogation Signals are also commercially
`available. For example, a TCAS system-compatible direc
`tional antenna is commercially available from AlliedSignal
`Incorporated of Redmond, Wash., under the part number
`ANT 81A
`Although the ATCRBS/Mode-S Surveillance system and
`the TCAS collision avoidance System are separate, the
`TCAS processor accounts for the data provided by the
`intruder aircraft to determine what evasive maneuver to
`recommend to the host aircraft's pilot, i.e., whether to
`recommend that the pilot maintain current altitude, ascend
`or descend. The TCAS system also uses the inter-aircraft
`data link provided by the addressable Mode-Stransponder to
`coordinate the recommended evasive maneuver with a
`TCAS equipped intruder aircraft. Furthermore, a connection
`between the TCAS and Mode-S transponters and other
`avionics on an aircraft allows coordination between the
`TCAS and Mode-S transponders. This interSystem connec
`tion is often used to prevent Simultaneous transmissions
`which could interfere with the systems independent func
`tions or cause equipment damage.
`AS briefly described above and described in detail in the
`respective RTCA specifications, DO-181A and DO-185A,
`the ATCRBS/Mode-S Surveillance and TCAS collision
`avoidance Systems are separate. The most basic installations
`require at least a TCAS processor, a Mode-S transponder,
`and two Sets of independent and dedicated antennas. For
`example, U.S. Pat. No. 5,077,673, Aircraft Traffic Alert And
`Collision Avoidance Device, issued Dec. 31, 1991, describes
`a host aircraft having both an ATCRBS Surveillance device
`and an aircraft traffic alert and collision avoidance device
`installed thereon, each of the ATCRBS Surveillance device
`and an aircraft traffic alert and collision avoidance device
`having an antenna dedicated to Supporting the respective
`independent function. U.S. Pat. No. 5,552,788 suggests
`using four dedicated monopole antennas to Support just the
`an aircraft traffic alert and collision avoidance device. These
`redundant antennas are costly and add unnecessary weight to
`the aircraft. The omnidirectional nature of each of the
`Mode-S “squitter” and the Mode-S reply transmission
`require large amounts of transmission power and crowd the
`Spectrum with additional RF transmissions, thereby degrad
`ing the RF interference environment. Although RTCA docu
`ments have suggested the possibility of a combined TCAS/
`Mode-S system, to date no enabling disclosure has been
`made and no product embodying Such a combined TCAS/
`Mode-S system has been either used or offered for sale.
`Furthermore, no publication to date has Suggested a com
`bined TCAS/Mode-S system wherein both functions share
`COmmon antennaS.
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`SUMMARY OF THE INVENTION
`The present invention overcomes the limitations of the
`prior art by providing a method for transmitting a directional
`reply signals in response to Air Traffic Control Radar Beacon
`System and Mode Select Signals interrogation signals. The
`method includes configuring two directional antennas to
`independently acquire an interrogation Signal; receiving an
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`APPL-1035 / IPR2018-00394
`Apple v. Uniloc / Page 16 of 27
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`acquired interrogation Signal; and determining a directional
`Source of the received interrogation Signal. The method then
`provides for configuring a configurable Switch to couple a
`transponder transmitter to one of the two directional anten
`nas and transmitting a directional radiation pattern toward
`the determined vertical and the determined azimuthal direc
`tional Source of the received interrogation signal. The direc
`tional radiation pattern is preferably a Mode Select wave
`form and further includes a Mode-Aidentification signal and
`a Mode-Caltitude Signal. The method alternately configures
`the configurable Switch to acquire the interrogation Signal on
`one and then the other of the two directional antennas,
`whereby the method determines the vertical directional
`Source of the received interrogation signal by determining
`the relative Strength of the Signals received on each of the
`tWO antennaS.
`According to one aspect of the invention, the two direc
`tional antennas each include multiple directional antenna
`elements, and the method determines the azimuthal Source
`of the received interrogation Signal by determining the
`relative Strength of the received interrogation Signal
`acquired by different ones of multiple directional antenna
`elements. The relative Strength of the received interrogation
`Signal acquired by different ones of multiple directional
`antenna elements is determined by determining the relative
`induced Signal amplitudes at each of the directional antenna
`elements. Alternatively, the azimuthal Source is determined
`using phase interferometry.
`According to another aspect of the invention, the method
`receives the acquired interrogation Signal into a Mode Select
`transponder receiver coupled to the configurable Switch. The
`Mode Select transponder receiver provides a signal to a
`transmit and Switch control function which, in response to
`that Signal, provides a configuration signal to the config
`urable Switch to transmit the reply signal the one antenna
`element having the Strongest reception. Thus, the reply
`Signal is transmitted on the antenna element most exposed to
`the interrogation Signal Source and, therefore, the reply
`Signal is transmitted toward the Source. Alternatively, the
`directional beam or transmission pattern is formed by deliv
`ering power to all of the elements in the antenna and
`modifying the phase at each element with respect to the
`phases of each other antenna element. The phase is shifted
`to combine the Signals from all of the antenna elements in
`the direction of the Source and cancel the Signals in other
`directions. Because the reply signal is transmitted only
`toward the Source of the interrogation Signal, energy is not
`wasted on transmitting Signals in other directions and inter
`ference in the ATC environment is reduced.
`The reply signal is, preferably, a Mode Select waveform,
`including a unique address code.
`According to other aspects of the present invention, an
`airborne directional transponder device is provided for
`detecting and responding to Air Traffic Control Radar Bea
`con System format interrogation Signals using a pair of
`directional antennas. The device of the invention includes
`two first and second individually addressable directional
`antennas with a Switch coupled to of each the antennas. A
`transponder receiver is coupled to the Switch for receiving
`and decoding an interrogation signal. The transponder
`receiver includes a circuit for determining the direction of
`the Source of a received interrogation signal relative to the
`interrogated host aircraft. A transmitter, coupled to the
`Switch, transmits a Signal in response to the received and
`decoded interrogation signal. A transmit and Switch control
`circuit couple the Switch to a one of the two individually
`addressable directional antennas for transmitting the
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