Patent Publication Number: US-6222480-B1

Title: Multifunction aircraft transponder

Description:
Priority claim is made to 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 in incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to aircraft surveillance and collision avoidance systems, and particularly, to combined Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode-S) and Air Traffic Alert and Collision Avoidance System (TCAS) systems utilizing common antennas. 
     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&#39;s visual sense remains the most basic method of aircraft collision avoidance. However, since the 1950&#39;s electronic techniques based on radio frequency and optical transmissions have been developed to supplement the pilot&#39;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&#39;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&#39;s image or blip on the ATC operator&#39;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 function. 
     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 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 a predetermined frequency and are separated by a predetermined interval. The time interval between the first and third pulses defines what information the interrogator is requesting: 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 interrogation signal, the aircraft transponder develops and transmits 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&#39;s time and attention. When traffic is heavy, or visibility is low, collision potential increases. 
     During the 1960&#39;s the increases in the number of aircraft, the percentage of aircraft equipped 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 simultaneously interrogated aircraft. Furthermore, the Mode-A and Mode-C systems are unable to relay additional information or messages between the ground based SSR and the interrogated aircraft, other than the aforementioned identification and altitude information. The Mode Select, or Mode-S, was the response to this overload and other deficiencies 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-S system 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 surveillance in an integrated ATCRBS/Mode-S environment. 
     The Radio Technical Commission for Aeronautics (RTCA) has promulgated a specification for the Mode-S system, RTCA/DO-181A,  Minimum Operational Performance Standards for Air Traffic Control Radar Beacon System/Mode Select  ( ATCRBS/Mode - S )  Airborne Equipment , issued January 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 attenuation of the received transponder signals by the airframe blocking the antenna from the SSR ground station transmitter 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” preformed by the airborne transponder. Once per second, the Mode-S transponder spontaneously and pseudo-randomly transmits (squits) an unsolicited broadcast, including a specific 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&#39;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 transmissions simultaneously on both of its dedicated omnidirectional antennas. According to the second technique, each ground based Mode-S interrogator broadcasts an ATCRBS/Mode-S “All-Call” interrogation signal which has a waveform 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-S equipped aircraft replies with a Mode-S format that includes a unique 24-bit address code, or identity tag. This address, together with the aircraft&#39;s range and azimuth location, is entered into a file, commonly known as putting the aircraft on roll-call, and the aircraft is thereafter discretely addressed. The aircraft is tracked by the ATC interrogator throughout its assigned airspace and, during subsequent interrogations, the Mode-S transponder reports in its replies either its altitude or its ATCRBS 4096 code, depending 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&#39;s location information and discrete address code are passed on via landlines, else either the ground based SSR station picks up the Mode-S transponder&#39;s “squitter” or the Mode-S transponder 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 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 inclusion 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 surveillance 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 transponder 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 additional RF transmissions, detection of transponder emissions from other aircraft is difficult. A passive collision threat detector is essentially a receiver having sufficient intelligence to first detect and then locate the existence of potential collision threats represented by nearby aircraft. The aircraft&#39;s receiver is of necessity operating in close proximity to the host aircraft&#39;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&#39;s transponder. The host aircraft&#39;s transponder is so much closer, usually no more than a few feet, to any receiver that the host aircraft&#39;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 reliable collision avoidance, the government and aviation industry have cooperated in developing Operational Performance Standards for a Traffic Alert and Collision Avoidance or TCAS system, separate from the ATCRBS/Mode-S transponder system. The standards are set forth in the RTCA specifications DO-185 , Minimum Operational Performance Standards for Air Traffic Alert and Collision Avoidance System  ( TCAS )  Airborne Equipment , issued Sep. 23, 1983, consolidated Sep. 6, 1990, and DO-185A,  Minimum Operational Performance Standards for Air Traffic Alert and Collision Avoidance System II  ( TCAS II )  Airborne Equipment , issued December 1997, both of which are incorporated 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 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-S transponders to reply to interrogation signals. Thus, a TCAS equipped aircraft can “see” other aircraft carrying a transponder. Once a transponder equipped target has been “seen,” the target is tracked and the threat potential is determined. Altitude information is essential in determining a target&#39;s threat potential. Comparison between the altitude information encoded in the reply transmission from the threat aircraft and the host aircraft&#39;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 information 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 information 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&#39;s heading greatly enhances a pilot&#39;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 information is also used by the TCAS processor to better determine 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 the phase difference between two output ports of the combining circuitry indicates the bearing of a received transponder 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 omnidirectional which reduces cost at the expense of reduced directional coverage. Other TCAS systems provide duplicate 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 disclosure of which is incorporated herein by reference, teaches an arrangement of four standard monopole antenna elements, for example, ¼ 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. Directionality is determined by comparing the power levels of the received signals. Additionally, the &#39;788 patent teaches a TCAS system which can transmit transponder interrogation 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 system transponder interrogation signals are also commercially available. For example, a TCAS system-compatible directional 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&#39;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-S transponder to coordinate the recommended evasive maneuver with a TCAS equipped intruder aircraft. Furthermore, a connection between the TCAS and Mode-S transponders and other avionics on an aircraft allows coordination between the TCAS and Mode-S transponders. This intersystem connection is often used to prevent simultaneous transmissions which could interfere with the system&#39;s independent functions 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 degrading the RF interference environment. Although RTCA documents 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 combined TCAS/Mode-S system wherein both functions share common antennas. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the limitations of the prior art by providing a combined airborne Air Traffic Control Radar Beacon System/Mode-Select (ATCRBS/Mode-S) surveillance system and Traffic Alert and Collision Avoidance (TCAS) collision avoidance system device, including a switch coupling common antennas to the relevant functions of the combined airborne Air Traffic Control Radar Beacon System/Mode-Select (ATCRBS/Mode-S) surveillance system and Traffic Alert and Collision Avoidance (TCAS) collision avoidance system. 
     According to one aspect of the invention, the present invention provides a combined TCAS transponder device having two common-use antennas and a switch coupled to each of the two antennas. A transponder receiver is coupled to the switch for receiving and decoding ATCRBS/Mode-S format interrogation signals. A transponder transmitter is coupled to the switch for transmitting an ATCRBS/Mode-S format reply signal in response to the received interrogation signals. An air traffic alert and collision avoidance system receiver is coupled to the switch for receiving and decoding both unsolicited squitters and reply signals transmitted in response to an interrogation signal transmitted by an air traffic alert and collision avoidance system transmitter coupled to the switch. A transmit and switch control circuit is coupled the air traffic alert and collision avoidance system transmitter to drive the transmitter to generate the ATCRBS/Mode-S format interrogation signals. The transmit and switch control circuit is also coupled to the ATCRBS/Mode-S transponder transmitter to drive the transmitter to generate reply signals. The transmit and switch control circuit is further coupled to the switch to drive the switch to relay the generated interrogation and reply signals for transmission by at least one of the two common-use antennas. 
     According to one aspect of the invention, the ATCRBS/Mode-S transponder transmitter and the air traffic alert and collision avoidance system transmitter are combined in a combined air traffic alert and collision avoidance system and transponder transmitter. 
     According to another aspect of the invention, the transponder receiver is configured to detect and decode at least one of standard Air Traffic Control Radar Beacon System (ATCRBS) interrogation signals, Mode Select interrogation signals, Mode-A interrogation signals, and Mode-C interrogation signals. Also, the air traffic alert and collision avoidance system receiver is configured to detect and decode at least one of standard transponder squitters and reply signals, i.e., Mode Select squitters, Mode Select reply signals, Mode-A reply signals, and Mode-C reply signals. 
     According to another aspect of the invention, the combined TCAS transponder device is alternately configured in one of two receive modes. In the first receive mode both antennas are coupled to the transponder receiver and one antenna is coupled to the air traffic alert and collision avoidance system receiver. According to the second receive mode, both antennas are again coupled to the transponder receiver while the other one of the antennas is coupled to the air traffic alert and collision avoidance system receiver. The switch is alternately configured according to the first receive mode and the second receive mode such that the transponder receiver receives and decodes interrogation signals received on each of the two common-use antennas and the air traffic alert and collision avoidance system receiver is alternately coupled to first receive a reply signal on one of the two antennas and then coupled to receive a reply signal on the other of one of the antennas. 
     According to yet another aspect of the invention, the switch is configured either according to a first transmit mode which couples one of the two common-use antennas to the combined air traffic alert and collision avoidance system and transponder transmitter or according to a second transmit mode which couples the other one of the two common-use antennas to the combined air traffic alert and collision avoidance system and transponder transmitter. 
     According to another aspect of the invention, the combined TCAS transponder device determines air traffic alert and collision avoidance information determined from the decoded reply signal. The air traffic alert and collision avoidance system receiver is coupled to output a second control signal to the transmit and switch control circuit which drives the transmit and switch control circuit to signal the combined air traffic alert and collision avoidance system and transponder transmitter to generate an interrogation signal and/or a reply signal, and also drives the transmit and switch control circuit to drive the switch to relay the generated interrogation signal and/or a reply signal for transmission by one or both the antennas. 
     The combined TCAS transponder device preferably also includes a display coupled to receive the air traffic alert information and said collision avoidance information determined by the air traffic alert and collision avoidance system receiver and to display the information. 
     The combined TCAS transponder device preferably also includes a control function coupled to air traffic alert and collision avoidance system receiver. The control function provides an interface through which an operator inputs control signals to the air traffic alert and collision avoidance system receiver. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 illustrates the warning zones typically used by a modern Traffic Alert and Collision Avoidance or TCAS system; 
     FIG. 2 illustrates the separate airborne Air Traffic Control Radar Beacon System/Mode-Select (ATCRBS/Mode-S) surveillance system and Traffic Alert and Collision Avoidance (TCAS) collision avoidance systems of the prior art and the prior art interconnection between the two separate systems; 
     FIG. 3 shows one configuration of a display used with the prior art Traffic Alert and Collision Avoidance (TCAS) system; 
     FIG. 4 illustrates one configuration of a control panel for use with the dual transponder airborne Air Traffic Control Radar Beacon System/Mode-Select (ATCRBS/Mode-S) surveillance system of the prior art; 
     FIG. 5 illustrates the omnidirectional transmission pattern produced by the omnidirectional antenna utilized by the airborne Air Traffic Control Radar Beacon System/Mode-Select (ATCRBS/Mode-S) system of the prior art. 
     FIG. 6 illustrates both the omnidirectional transmission pattern and one of the four directional transmission patterns produced by the prior art Traffic Alert and Collision Avoidance (TCAS) system; 
     FIG. 7 illustrates a high level block diagram of the combined airborne Air Traffic Control Radar Beacon System/Mode-Select (ATCRBS/Mode-S) surveillance system and Traffic Alert and Collision Avoidance (TCAS) collision avoidance system device of the present invention; 
     FIG. 8 illustrates a more detailed block diagram of the combined airborne Air Traffic Control Radar Beacon System/Mode-Select (ATCRBS/Mode-S) surveillance system and Traffic Alert and Collision Avoidance (TCAS) collision avoidance system device of the present invention shown in FIG. 7, including a switch coupling the common antennas to the relevant functions of the combined airborne Air Traffic Control Radar Beacon System/Mode-Select (ATCRBS/Mode-S) surveillance system and Traffic Alert and Collision Avoidance (TCAS) collision avoidance system of the present invention; 
     FIG. 9 illustrates an alternative detailed block diagram of the combined airborne Air Traffic Control Radar Beacon System/Mode-Select (ATCRBS/Mode-S) surveillance system and Traffic Alert and Collision Avoidance (TCAS) collision avoidance system device of the present invention shown in FIG. 7; 
     FIG. 10 illustrates an exploded view of one embodiment of the directional antenna capable of simultaneously receiving and monitoring both ground based Air Traffic Control Radar Beacon System/Mode-Select (ATCRBS/Mode-S) surveillance system and airborne Traffic Alert and Collision Avoidance (TCAS) collision avoidance system interrogation signals and capable of transmitting such interrogation signals and of transmitting reply signals in response to such interrogation signals; 
     FIG. 11 illustrates components of a beam forming network used in the directional antenna of FIG. 10; 
     FIG. 12 illustrates the operation of the power dividing component used in the beam forming network illustrated in FIG. 11; and 
     FIG. 13 illustrates the conversion of signals from the directional antenna of FIG. 10 used in the combined TCAS transponder device of the present invention as shown in FIGS.  8  and  9 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     FIG. 1 illustrates the warning zones typically used by a modern Traffic Alert and Collision Avoidance or TCAS system, as described in U.S. Pat. No. 5,629,692, Method And Apparatus For Alerting Pilot To Transponder Antenna Failure In A Traffic Alert And Collision Avoidance System, issued May 13, 1997, the complete disclosure of which is incorporated herein by reference. FIG. 1 depicts a host aircraft  1  moving along a flight path indicated by vector  2 . A first large volume of 3-dimensional airspace or “caution area,” surrounding but biased primarily in front of host aircraft  1  along flight path  2 , is identified by generally oblong cylinder  3  labeled as the “traffic alert” or TA zone. TA zone  3  defines a time zone prior to which an intruder aircraft is predicted to enter a second, smaller volume of 3-dimensional airspace around host aircraft  1 , identified by second inner generally oblong cylinder  4  labeled as the “collision area” or CA zone. TA zone  3  begins approximately 20 to 48 seconds, depending upon the speed and trajectory of the intruder aircraft relative to host aircraft  1 , before the intruder aircraft enters CA zone  4  of host aircraft  1 . The TCAS system provides visual and/or aural warnings and indication when an intruder aircraft is about to enter or has entered TA zone  3 , as will be explained in detail below. A third volume of 3-dimensional airspace or “warning area,” smaller than TA zone  3  and larger than CA zone  4  is identified by another generally oblong cylinder  5  referred to as the “resolution advisory” or RA zone. RA zone  5  defines a second time zone beginning approximately 15 to 35 seconds prior to which an intruder aircraft is predicted to enter CA zone  4 . The TCAS system provides visual and/or aural warnings and indication when an intruder aircraft is about to enter or has entered RA zone  5 , as will also be explained in detail below. 
     FIG. 2 illustrates the separate airborne Air Traffic Control Radar Beacon System/Mode-Select or ATCRBS/Mode-S surveillance system and Traffic Alert and Collision Avoidance or TCAS collision avoidance system of the prior art, and the prior art interconnection between the two separate systems. In FIG. 2, ATCRBS/Mode-S system  10  includes a primary transponder  12 A and a secondary or back-up transponder  12 B and two dedicated transponder antennas  14  coupled to transponders  12  by cables  16 . Each transponder antenna  14  is a single element or omni-blade omnidirectional antenna operating in the L-band and broadcasting an omnidirectional signal. Attenuation of the received transponder signals by the airframe shadowing the antenna from the SSR ground station transmitter is usually avoided by locating the two antennas  14  in spaced apart locations on the aircraft, usually on a top surface and a bottom surface of the aircraft. Complementary to the ground based Secondary Surveillance Radar or SSR (not shown) of the Air Traffic Control Radar Beacon System (ATCRBS) located at each of a plurality of Air Traffic Control (ATC) stations, transponder  12  drives one of transponder antennas  14  to spontaneously and pseudo-randomly transmit (squit) an unsolicited broadcast, including a specific address code unique to the aircraft carrying the transponder, commonly termed a “squitter,” in an omnidirectional pattern, as described in detail in connection with FIG. 5 below. Whenever ATCRBS/Mode-S transponder  12  is not broadcasting, it is monitoring, or “listening,” for transmissions simultaneously on both dedicated omnidirectional antennas  14 . The transmit and receive modes of transponder  12  are mutually exclusive to avoid damaging the equipment. Each ground based Mode-S interrogator broadcasts a coded 1030 MHz microwave interrogation signal “All-Call” interrogation signal which has a waveform that can be understood by both ATCRBS and Mode-S transponders. Transponder  12  is capable of receiving interrogation signals broadcast at 1030 MHz. When an aircraft equipped with ATCRBS/Mode-S transponder  12  enters the airspace served by an ATC Mode-S interrogator, transponder  12  receives and interprets the interrogation signal. 
     Transponder  12  is optionally coupled to a message processor  18 . Message processor  18  generates a “confidence string” to represent the quality of the received interrogation signal, wherein “quality” refers to the precision of the bit-by-bit decisions provided by message processor  18  as described in detail in U.S. Pat. No. 5,528,244, Processing For Mode S Signals Suffering Multipath Distortion, issued Jun. 18, 1996, the complete disclosure of which is incorporated herein by reference. Briefly, the Mode-S interrogation signal transmitted in a series of data bit pulses one microsecond long, including a ½ microsecond message pulse followed by a ½ microsecond space. A confidence bit is produced for each bit pulse in the received microwave interrogation signal. A confidence count of “1” represents a high quality transmission and is assigned when the amplitude of the ½ microsecond message pulse is within a specified range and the ½ microsecond space has no energy greater than a threshold reference value. The confidence count for each bit in the message is grouped in serial fashion to develop a “confidence count string” which is further processed to determine a confidence value. A message with a low confidence value is discarded while a message with a high confidence value is decoded, corrected and interpreted. Transponder  12  replies to the Mode-S interrogation signal by driving one of antennas  14  to transmit a 1090 MHz microwave Mode-S format reply signal that includes its unique 24-bit address code, or identity tag, back to the SSR ground site. In order to respond to Mode-C interrogations requesting altitude information, transponder  12  is coupled to an altitude source  20 , for example, a blind encoding altimeter, as described in U.S. Pat. No. 5,077,673, Aircraft Traffic Alert And Collision Avoidance Device, issued Dec. 31, 1991, the complete disclosure of which is incorporated herein by reference. Blind encoding altimeter  20  measures the barometric pressure and provides a digital signal to transponder  12  representing the pressure altitude of the aircraft. This pressure altitude information is subsequently encoded and transmitted in response to a Mode-C interrogation as a Mode-C reply. A control panel  22  coupled to transponder  12  provides means for the aircraft&#39;s crew to configure the functions of transponder  12 . Control panel  22  also provides means to prepare and transmit additional information or messages to the ground based SSR, other than the aforementioned identification and altitude information, and to receive such messages transmitted by the ground based SSR via the downlink provided by the ATCRBS/Mode-S system. ATCRBS/Mode-S system  10  is described in greater detail in publication  Mode Select Beacon System  ( Mode - S )  Sensor , available from the U.S. Department of Transportation, Federal Aviation Administration, Specification Number FAA-E-2716, amendment 2, dated Mar. 2, 1983, which is incorporated herein by reference in its entirety. 
     FIG. 2 also illustrates TCAS collision avoidance system  30  which includes a TCAS processor  32  coupled via cables  33  to two dedicated 4-element directional TCAS antennas  34 A and  34 B. While TCAS antennas  34  are separate and independent from transponder antennas  14  of ATCRBS/Mode-S system  10 , TCAS antennas  34 A and  34 B are also usually mounted on top and bottom surfaces of the aircraft to avoid attenuation of the received transponder signals by the airframe blocking the antenna from the transmitter. Thus, upper antenna  34 A is mounted on an upper surface of host aircraft  1  (shown in FIG. 1) and lower antenna  34 B is mounted on a lower surface of host aircraft  1 . A connection via a standard ARINC  429  communication link  36  between TCAS  30  and transponders  12 A and  12 B of ATCRBS/Mode-S system  10  and other avionics on host aircraft  1  that transmit in the L-band exists to allow coordination between the TCAS  30  and transponders  12 A and  12 B. This intersystem connection is also used to prevent simultaneous transmissions which could interfere with the system&#39;s independent functions or cause equipment damage. Most modern transponders respond to suppression signals in accordance with published standards. When a proper suppression pulse is supplied to transponder  12  over suppression line  38  connected between TCAS system  30  and ATCRBS/Mode-S system  10 , the receiver portion of transponder  12  is disabled so that transponder  12  does not generate reply signals to, except the aforementioned “squitters.” This suppression feature prevents interference by other equipment, such as distance measuring equipment (DME). Typically, the suppression input is AC coupled to transponder  12 , with a time constant of about 5 milliseconds. This time constant limits the effective suppression period to about 2 milliseconds, as described in above incorporated U.S. Pat. No. 5,077,673. Suppression input is DC coupled to some transponders, and some transponders do not have provisions for suppression. TCAS system  30  typically requires the on-board transponder to have some means for suppression. The newer TCAS II system, for example the TCAS II system sold by Honeywell, Incorporated, typically has several modes of operation selectable via a control panel  40  coupled to TCAS processor  32 . Control panel  40  is also coupled to transmit control signals to TCAS processor  32 . The function of control panel  40  is described in detail in connection with FIG. 4 below. processor  32  is also coupled provide an output signal to one or more displays  42 . The function of display  42  is described in detail in connection with FIG. 3 below. 
     In the TCAS II block diagram shown in FIG. 2, a receiver  44  is coupled to receive signals from each of TCAS antennas  34 A and  34 B by cables  33 . Receiver  44  is couple to relay the received signals to TCAS processor  32 . TCAS processor  32  is also coupled to drive a transmitter  46  which is in turn coupled via additional cables  33  to each of TCAS antennas  34 A and  34 B. TCAS processor  32  causes transmitter  46  to drive one of top and bottom TCAS antennas  34 A,  34 B to produce and broadcast an omnidirectional 1030 MHz microwave transmission interrogation signal at 1030 MHz, the same frequency used by ground based SSR stations to interrogate ATCRBS/Mode-S transponders. Whenever transponder  12  of ATCRBS/Mode-S system  10  is not broadcasting, TCAS system  30  is “listening” on either one of the two directional TCAS antennas  34 A and  34 B to intercept transponder “squitters” and reply transmissions at 1090 MHz, the same frequency used by airborne ATCRBS/Mode-S transponders to reply to ground based SSR generated interrogation signals. ATCRBS/Mode-S transponders carried by target aircraft reply to the TCAS 1030 MHz interrogation signal as if to an interrogation signal generated by a ground based SSR, supplying the target aircraft&#39;s identification and altitude information. Typically, TCAS collision avoidance system  30  also detects target aircraft equipped only with a Mode-A transponder, but will lack altitude information for the target aircraft. 
     TCAS processor  32  receives and decodes the 1090 MHz microwave reply signals from each of the one or more interrogated transponders via its connection to receiver  44 , the reply signals generally include Mode-A identification information, Mode-C altitude information and Mode-S reply format that includes a unique 24-bit address code, or identity tag, if available. TCAS processor  32  determines threat potential of responding aircraft using range, bearing and altitude information. Altitude information is supplied in the target aircraft&#39;s Mode-C reply signal. Range is either estimated approximately based on the strength of the received transponder signal or calculated more accurately based on the time delay between transmission of the interrogation signal and reception of the reply signal. Relative bearing information is generally based on angle of arrival information provided by 4-element directional TCAS antenna  34 . TCAS processor  32  determines evasive action, i.e., whether to maintain current altitude, ascend or descend, and recommends the proper maneuver to the host aircraft&#39;s pilot. TCAS processor  32  also uses the inter-aircraft data link provided by the addressable Mode-S transponder to coordinate the recommended evasive maneuver with a TCAS equipped intruder aircraft. 
     Display 
     FIG. 3 shows one configuration of display  42  used with TCAS collision avoidance system  30 . Display  42  includes an aircraft symbol  48  to depict the position of host aircraft  1  of FIG. 1. A circle, formed by multiple dots  50  surrounding host aircraft position symbol  48 , indicates a 2 nautical mile range from host aircraft  1 . Generally, semi circular indicia  52  around the periphery of indicator display  42  an a rotatable pointer  54  together provide an indication of the rate of change of altitude of host aircraft  1 . Indicia  52  are typically marked in hundreds of feet per minute. The portion indicia  52  above the inscriptions “0” and “6” indicates rate of ascent while the portion below indicates rate of descent. 
     Other target aircraft or “intruders” are identified on display  42  by indicia or “tags”  56 ,  58  and  60 . Tags  56 ,  58 ,  60  are shaped as circles, diamonds or squares and are color coded (not shown) to provide additional information. Square  58  colored red represent an intruder entering warning or RA zone  5  of FIG.  1  and suggests an immediate threat to host aircraft  1  with prompt action being required to avoid the intruder. Circle  56  colored amber represents an intruder entering caution or TA zone  3  of FIG.  1  and suggests a moderate threat to host aircraft  1  recommending preparation for intruder avoidance. Diamond  60  represents near or “proximate traffic” when colored solid blue or white and represents more remote traffic or “other traffic” when represented as an open blue or white diamond. Air traffic represented by either solid or open diamond  60  is “on file” and being tracked by TCAS processor  32 . 
     Each indicia or tag  56 ,  58 ,  60  is accompanied by a two digit number preceded by a plus or minus sign. In the illustration of FIG. 1 for example, a “+05” is adjacent circle tag  56 , a “−03” is adjacent square tag  58  and a “−12” is adjacent diamond tag  60 . Each tag may also have an vertical arrow pointing either up or down relative to the display. The two digit number represents the relative altitude difference between host aircraft  1  and the intruder aircraft, the plus and minus signs indicating whether the intruder is above or below host aircraft  1 . Additionally, the two digit number appears positioned above or below the associated tag to provide a visual cue as to the intruder aircraft&#39;s relative position: the number positioned above the tag indicates that the intruder is above host aircraft  1  and the number positioned below the tag indicates that the intruder is below host aircraft  1 . The associated vertical arrow indicates the intruder aircraft&#39;s altitude is changing at a rate in excess of 500 feet per minute in the direction the arrow is pointing. The absence of an arrow indicates that the intruder is not changing altitude at a rate greater than 500 feet per minute. 
     Display  42  includes several areas represented by rectangular boxes  62 ,  64 ,  66 ,  68 ,  70  which are areas reserved for word displays wherein conditions of TCAS system  30  are reported to the host aircraft pilot. For example, if a portion or component of TCAS system  30  fails, a concise textual report describing the failure appears in one of rectangular boxes  62 ,  64 ,  66 ,  68 ,  70 . In another example, if the operator uses control panel  40  to select one of a limited number of operational modes, a concise textual message indicating the choice of operational mode appears in another of rectangular boxes  62 ,  64 ,  66 ,  68 ,  70 . Selectable operational modes typically include a “standby” mode in which both host aircraft transponder systems  12  are inactive, a “transponder on” mode in which a selected one of primary transponder  12 A and secondary transponder  12 B is active, a “traffic alert” mode in which an alert is transmitted to the host aircraft pilot if any Mode-C or Mode-S transponder equipped aircraft are entering a first predetermined cautionary envelope of airspace, and a “traffic alert/resolution advisory” mode in which an alert is issued if any Mode-C or Mode-S transponder equipped aircraft are entering a second predetermined warning envelope of airspace. The various operational modes described above are selectable using control panel  40 . 
     Control Panel 
     FIG. 4 illustrates one configuration of control panel  40  for use with the dual transponder system described herein. Control panel  40  includes a traffic avoidance display switch  72  in the upper left corner which activates the display by switching from an “off” condition to either an “auto on” or an “on” condition. A push to activate-type switch  74  controls transponders  12  and allows the operator to specifically identify host aircraft  1  to a receiving SSR ground station when requested. A center display  76  shows the host aircraft&#39;s identification code which is operator selectable utilizing four knob-type switches  78 ,  80 ,  82  and  84  located below display  76 . The selected code is automatically broadcast and permits ground based SSR receivers and other TCAS equipped aircraft in the vicinity to identify host aircraft  1  on their display screens. Three positions switch  86  in the lower left corner of control panel  40 , when switched from an “off” position, provides the operator with a selectable choice of two sources of altitude information to be broadcast to ground based SSR receivers and other TCAS equipped aircraft. The selectable choices include, for example, the pilot&#39;s altimeter (not shown) or the co-pilot&#39;s altimeter (not shown). The “off” position on three positions switch  86  permits the operator to stop broadcasting this information if so requested to reduce clutter under crowded conditions or to eliminate erroneous altitude reports when the information supplied is incorrect. A TCAS TEST push-type switch  87  causes display  42 , shown in FIG. 3, to produce predetermined symbols similar to symbols  56 ,  58  and  60  which permits the operator to determine that TCAS system  30  is operating to produce proper symbolism for intruder aircraft. Two position switch  88  in the lower right corner of control panel  40  permits the operator to select either primary transponder  12 A or secondary transponder  12 B to be coupled for transmission on both TCAS antennas  34 A and  34 B. If a transponder system fails, either transponder  12  or TCAS antenna  34 , a small light  90  positioned to the upper right of display  76  is illuminated which provides an indication of failure to the operator. An aural announcement, which is optionally among the various displays of FIG. 2 but not shown, also indicates such failure of TCAS system  30  at the conclusion of a system self-test initiated by depression of TCAS TEST switch  86 . If such failure is reported, the operator may position switch  88  to connect the other of transponders  12 A and  12 B to TCAS antennas  34 A and  34 B. If selecting the other of transponders  12 A and  12 B removes the failure, indicator light  90  is extinguished. Else, indicator light  90  remains illuminated until certain conditions are satisfied. Such conditions and results are described in detail in above incorporated U.S. Pat. No. 5,629,692 and are not relevant to the present invention. Briefly, if selecting the other of transponders  12 A and  12 B does not remove the failure, the operator selects among various TCAS system modes using switch  92  in the upper right corner of control panel  40 . The operator can select a “standby” switch position, labeled “STBY,” wherein TCAS system  30  is sleeping. The operator can select a “transponder on” switch position, labeled “XPDR ON,” wherein TCAS system  30  is activated but not currently monitoring the airspace around host aircraft  1 . The operator can select a “traffic alert” switch position, labeled “TA,” wherein TCAS system  30  monitors only TA zone  3  (shown in FIG. 1) or a ordinary operation switch position, labeled “TA/RA,” wherein TCAS system  30  functions normally and monitors both TA zone  3  and RA zone  5 . Thus, TCAS system  30  of the prior art can be configured to operate under various circumstance and equipment conditions. 
     FIG. 5 illustrates the omnidirectional transmission, or radiation, pattern  94  produced by a modern ATCRBS/Mode-S single element antenna  14 , shown in FIG.  2 . 
     FIG. 6 illustrates the omnidirectional  96  transmission, or radiation, pattern produced by a modem TCAS system  30 , shown in FIG.  2 . FIG. 6 also illustrates one of the four directional TCAS transmission, or radiation, patterns  98 , a forwardly broadcast transmission, or radiation, pattern is shown as an example. 
     Combined TCAS Transponder Device 
     FIGS. 7 and 8 illustrate the combined TCAS transponder device of the present invention. FIG. 7 is a high level block diagram of the combined TCAS transponder device  100  of the present invention. In FIG. 7, according to one preferred embodiment of the present invention, the ATCRBS/Mode-S function and the TCAS function are co-located in a combined TCAS/transponder processor  102 . Combined TCAS transponder device  100  utilizes a single pair of antennas  104  coupled to combined TCAS/transponder processor  102  using cables  106 . Attenuation of transponder signals, i.e., either interrogation signals from SSR ground station transmitters and TCAS equipped aircraft or reply signals from a TCAS interrogated aircraft, is usually avoided by locating the two antennas  104  in spaced apart locations on the aircraft, thereby eliminating shadowing the antenna by the airframe. In a preferred embodiment, a first antenna  104 A is located on a top surface of the aircraft and a second antenna  104 A is located on a bottom surface of the aircraft. Other alternative configurations are known and the present invention contemplates these alternative configurations without limitation. For example, first and second antennas  104 A and  104 B are sometimes located fore and aft on the aircraft, rather than top and bottom. Antennas  104  are four element, or four blade, directional antenna. According to the present invention, the ATCRBS/Mode-S omnidirectional, or omni-blade, antennas of the prior art are eliminated in the present invention. Directional antennas  104  of the present invention broadcast both directional and omnidirectional 1030 MHz interrogation signals. Antennas  104  of the present invention are configured to receive and transmit on both 1030 MHz and 1090 MHz, i.e., ATCRBS interrogation and reply frequencies. Antennas  104  are capable of simultaneously receiving and monitoring both ground based SSR and airborne TCAS interrogation signals. Antennas  104  are capable of transmitting reply signals in response to such interrogation signals and of transmitting interrogation signals from host aircraft  1 , shown in FIG.  1 . Accordingly, antennas  104  are capable of both receiving interrogation signals transmitted at 1030 MHz and transmitting reply signals at 1090 MHz which is standard operational mode of the ATCRBS/Mode-S surveillance system. Antennas  104  are also capable of transmitting interrogation signals at 1030 MHz to target aircraft and receiving the reply signals at 1090 MHz which is the standard operational mode of the TCAS collision avoidance system. Such capability is available in a conventional four-element directional antenna as is used in current TCAS collision avoidance systems as described in detail below. 
     The transponder receive function of a combined TCAS processor/transponder processor  102  utilizes the directional and locational information provided by antennas  104  to determine the directional source of an interrogation signal. Accordingly, the transponder receive function determines which of top antenna  104 A and bottom antenna  104 B received the interrogation signal and from which direction, i.e., left, right, fore or aft, the strongest interrogation signal is received. The host aircraft  1  transponder transmits a directionally oriented reply signal toward the source of the interrogation signal using top antenna  104 A or bottom antenna  104 B on which the interrogation signal was received. Thus, in contrast to the transponder of the prior art, the transponder of the present invention reduces the transmission power by focusing the reply signal in a single direction. Such focuses signal reduces the amount of transmission power the host aircraft must generate and reduces the amount of interference in the 1090 MHz reply signal bandwidth resulting from replies being transmitted in an omnidirectional pattern. 
     FIG. 8 illustrates a more detailed block diagram of a combined TCAS processor/transponder processor  102  of the present invention. In FIG. 8, a switch  110  connects a dual Mode-S transponder signal receiver  112 , a TCAS receiver  114 , and a combination TCAS/Mode-S transmitter  116  to antennas  104 . Switch  110  is coupled to receive an input signal from antennas  104  and output the received signal to each of Mode-S transponder signal receiver  112  and TCAS receiver  114 . Switch  110  is also coupled to relay a TCAS drive signal from combination TCAS/Mode-S transmitter  116  to drive top and bottom antennas  104 A and  104 B to output the TCAS signal. Switch  110  is further coupled to relay a Mode-S transponder drive signal generated by combination TCAS/Mode-S transmitter  116  to drive top and bottom antennas  104 A and  104 B to output the transponder signal. Transmit and switch control circuit or function  118  is coupled to control each of switch  110  and combination TCAS/Mode-S transmitter  116 . Transmit and switch control function  118  is coupled to receive a control signal from Mode-S transponder signal receiver  112 . 
     Transmit and switch control  118  configures switch  110  to couple each of top and bottom antennas  104 A and  104 B to each of Mode-S transponder signal receiver  112 , TCAS receiver  114 , and combination TCAS/Mode-S transmitter  116  in various transmit and receive modes. In a first receive mode, switch  110  is configured to couple both top and bottom antennas  104 A and  104 B to Mode-S transponder signal receiver  112  while coupling top antenna  104 A to TCAS receiver  114 . In a second receive mode, switch  110  is configured to couple both top and bottom antenna  104 A and  104 B to Mode-S transponder signal receiver  112  while coupling bottom antenna  104 B to TCAS receiver  114 . In each of the first and second receive modes, Mode-S transponder signal receiver  112  processes signals from both top and bottom antenna  104 A and  104 B simultaneously, while TCAS receiver  114  processes the signals from each of top and bottom antenna  104 A and  104 B alternately. In a first transmit mode, switch  110  is configured to couple top antenna  104 A to combination TCAS/Mode-S transmitter  116 . In a second transmit mode, switch  110  is configured to couple bottom antenna  104 B to combination TCAS/Mode-S transmitter  116 . 
     According to the first receive mode, both top and bottom antennas  104 A and  104 B simultaneously receive, or “listen,” omnidirectionally utilizing all four elements of each antenna  104 A and  104 B. Transmit and switch control  118  configures switch  110  to couple the output of both top and bottom antennas  104  to Mode-S transponder signal receiver  112 , whereby combined TCAS transponder device  100  of the present invention receives and monitors ATCRBS/Mode-S 1030 MHz interrogation signals. In the first receive mode, transmit and switch control  118  also configures switch  110  to couple the output of top antenna  104 A to TCAS receiver  114 , whereby TCAS receiver  114  receives and monitors responses to TCAS interrogation signals broadcast by target aircraft at a frequency of 1090 MHz. According to the second receive mode, both top and bottom antennas  104 A and  104 B again simultaneously receive, or “listen,” omnidirectionally utilizing all four elements of each antenna  104 . Transmit and switch control  118  again configures switch  110  to couple the output of both top and bottom antennas  104  to Mode-S transponder signal receiver  112  such that combined TCAS transponder device  100  again receives and monitors ATCRBS/Mode-S 1030 MHz interrogation signals. However, in the second receive mode, transmit and switch control  118  configures switch  110  to couple the output of bottom antenna  104 B, rather than the output of top antenna  104 A, to TCAS receiver  114  such that TCAS receiver  114  receives and monitors TCAS responses utilizing bottom antenna  104 B, rather than the output of top antenna  104 A. 
     The first receive mode alternates with the second receive mode such that Mode-S transponder signal receiver  112  is receiving and monitoring ATCRBS/Mode-S 1030 MHz interrogation signals on both top and bottom antennas  104 A and  104 B while TCAS receiver  114  alternately monitors 1090 MHz response signals on first one and then the other of top and bottom antennas  104 A and  104 B. Mode-S transponder signal receiver  112  is configured to detect and decode standard interrogation signals, including Mode-S, Mode-A and Mode-C interrogation signals. Signal attenuation due to shadowing of top and bottom antennas  104  by the host aircraft&#39;s airframe blocking the antenna from the transmitter provides a difference in signal strength between the interrogation signals received at each of top and bottom antennas  104 A and  104 B. Mode-S transponder signal receiver  112  determines the relative vertical direction of a received interrogation signal based on this difference in signal strength of and determines azimuth bearing from comparison of the relative signal strength at the four elements of top or bottom antenna  104 , as described in detail below in connection with FIG.  13 . 
     Mode-S transponder signal receiver  112  provides an input signal, including directional information, to transmit and switch control  118 . The input signal drives transmit and switch control  118  to send a transmit signal to TCAS/Mode-S transmitter  116  and a switch control signal to switch  110 . In response to the received transmit signal, TCAS/Mode-S transmitter  116  generates a reply drive signal which includes the requested information. In response to the switch control signal, switch  110  is configured in one of the first and second transmit modes described above, wherein switch  110  is configured to couple either top antenna  104 A or bottom antenna  104 B to combination TCAS/Mode-S transmitter  116 , depending upon whether the received interrogation signal originated above or below host aircraft  1 . Switch  110  is further configured to drive coupled antenna  104 A or  104 B to transmit either an omnidirectional reply signal, or, preferably, a directed reply signal, or directional radiation pattern, toward the directional source of the received interrogation signal using one of either top antenna  104 A or bottom antenna  104 B. Alternatively, switch  110  is configured to drive coupled top antenna  104 A or bottom antenna  104 B to transmit either an omnidirectional interrogation signal, or, preferably, a directed interrogation signal toward the directional source of the received squitter or an earlier received reply signal. The optional directional transmission, or radiation, patterns thus generated by such directed transmissions are similar to directional TCAS transmission, or radiation, transmission patterns  98  illustrated in above described FIG.  6 . Transmitting on a predetermined one of top and bottom antenna  104 A and  104 B reduces by one-half the amount of power which must be generated to provide a reply signal and also reduces the amount of RF interference generated by each reply signal. The optional directional transmissions utilizing only one blade or element of top or bottom directional antenna  104  further reduce both the amount of power needed for transmission and the amount of RF interference generated by the transmission by a factor equivalent to the ratio of the directional transmission beamwidth to the beamwidth of an omnidirectional transmission, i.e., 360 degrees. 
     In compliance with aforementioned RTCA ATCRBS/Mode-S specification DO-181A, once per second, switch  110  is configured to drive first one and then the other of coupled top antenna  104 A and bottom antenna  104 B to transmit an unsolicited omnidirectional pattern Mode-S “squitter,” including the host aircraft&#39;s unique address code, using all the elements, or blades, of each directional antenna  104 . 
     As mentioned above, TCAS receiver  114  alternately monitors 1090 MHz response signals on first one and then the other of top and bottom antennas  104 A and  104 B concurrently with Mode-S transponder signal receiver  112  while combined TCAS transponder device  100  alternates between the first and second receive modes, described above. TCAS receiver  114  is configured to detect and decode standard transponder “squitters” and reply signals. TCAS receiver  114  determines vertical direction and relative bearing, or azimuthal direction, of a target aircraft in the manner mentioned above in connection with Mode-S transponder signal receiver  112  and described in detail below. TCAS receiver  114  is further configured to determine altitude, range and bearing of one or more target aircraft according to known methods based on the transponder reply signal received from each target aircraft. TCAS receiver  114  tracks each detected target aircraft and determines the current and potential threat represented by each of the one or more detected target aircraft. Track data, including range, range rate, relative bearing, relative altitude and rate of change of altitude of a target aircraft, together with the currently specified protected volume around the host aircraft, commonly referred to as TCAS sensitivity, are used by TCAS receiver  114  to determine whether the intruder aircraft is a threat. Each threat aircraft is processed individually to permit selection of the minimum safe resolution advisory based on the track data and coordination with other TCASequipped aircraft. TCAS receiver  114  determines evasive action necessary to ensure the safe vertical separation of the host aircraft, i.e., whether to maintain current altitude, ascend or descend, and recommends the proper maneuver to the host aircraft&#39;s pilot. The appropriate maneuver is one that ensures adequate vertical separation while causing the least deviation of the host aircraft from its current vertical rate. Currently, the resolution advisories are not intended to increase horizontal separation and therefore do not indicate horizontal escape maneuvers. TCAS receiver  114  optionally uses the inter-aircraft data link provided by the addressable Mode-S transponder to coordinate the recommended evasive maneuver with TCAS equipped intruder aircraft. This coordination procedure ensures that the aircraft resolution advisories are compatible. This coordination procedure is performed before displaying the advisory to the pilot/operator to avoid confusion. TCAS transponder device  100  further includes the capability to communicate with the ground based air traffic control system when a ground based Mode-S sensor is available. TCAS transponder device  100  can provide the Mode-S ground system with the resolution advisories that are displayed to the pilot/operator. These resolution advisories can be displayed to the air traffic control if desired. TCAS transponder device  100  can also receive sensitivity level commands from ground based Mode-S sensors. 
     As mentioned above, switch  110  is configurable in various transmit modes. In a first transmit mode, switch  110  is configured by transmit and switch control  118  to couple combination TCAS/Mode-S transmitter  116  to top antenna  104 A. In a second transmit mode, switch  110  is configured by transmit and switch control  118  to couple bottom antenna  104 B to combination TCAS/Mode-S transmitter  116 . Combination TCAS/Mode-S transmitter  116  is coupled in the first transmit mode or the second transmit depends upon which of top and bottom antennas  104 A and  104 B received the stronger radiation signal. Combination TCAS/Mode-S transmitter  116  is coupled to the one of top and bottom antennas  104 A and  104 B which received the stronger radiation signal. Each of the first and second transmit modes are further configured by switch  110  depending upon the azimuthal direction, or relative bearing, of the strongest received signal, the determination of which is described in detail below. Accordingly, switch  110  is configured to relay a generated interrogation signal to a predetermined one of the multiple directionally transmitting antenna elements of top antenna  104 A (in the first transmit mode) or bottom antenna  104 B (in the second transmit mode), and a directional transmission, or radiation, pattern is transmitted in the direction of the strongest received signal. 
     Combined TCAS transponder device  100  preferably includes a display  120  coupled to TCAS receiver  114 . Traffic advisories indicating range, range rate, bearing, and when available altitude and altitude rate, are displayed. Traffic advisories without altitude are provided for non-altitude reporting, transponder equipped aircraft. The traffic advisories displayed to the pilot/operator preferably describe the relative positions of proximate aircraft that are, or may become, collision threats. The display of traffic advisories alerts the flight crew to the presence of threat and potential threat aircraft and generally improve the crew&#39;s ability to respond to subsequent resolution advisories. Traffic advisories may also improve the crew&#39;s ability to visually acquire the traffic. Display  120  is generally similar to prior art display  42  shown in FIG.  3  and functions similarly to provide similar information. Combined TCAS transponder device  100  also includes a control panel  122  coupled to TCAS receiver  114 . Control panel  122  is similar to prior art control panel  40  shown in FIG.  4  and functions similarly to input similar information and instructions. 
     Switch  110  and transmit and switch control  118  also provide coordination between the TCAS/Mode-S transmitter  116  and each of Mode-S transponder receiver  112  and TCAS receiver  114  to prevent transmissions which could interfere with the system&#39;s independent functions or cause equipment damage. Switch  110  functions as an isolator between functions, while a control signal from transmit and switch control  118  configures switch  110 . 
     FIG. 9 illustrates one alternative configuration of the present invention, including the same functions and configuration as the above described configuration, like numbering indicating like functions. Accordingly, combined TCAS transponder device  100 ′ includes independent TCAS transmitter  124  and independent ATCRBS/Mode-S transmitter  126 , each of which are well known in the art. Each of independent TCAS transmitter  124  and independent ATCRBS/Mode-S transmitter  126  are independently coupled to switch  110  for driving one or both of common antennas  104 A and  104 B to transmit a signal. Transmit and switch control circuit  118  is coupled to each of independent TCAS transmitter  124  and independent ATCRBS/Mode-S transmitter  126  individually. Accordingly, transmit and switch control circuit  118  drives TCAS transmitter  124  to generate an interrogation signal while configuring switch  110  to relay the interrogation signal to a predetermined one of top antenna  104 A and bottom antenna  104 B for transmission. Similarly, transmit and switch control circuit  118  drives independent ATCRBS/Mode-S transmitter  126  to generate a reply signal in response to a received interrogation signal while configuring switch  110  to relay the reply signal to a predetermined one of top antenna  104 A and bottom antenna  104 B for transmission. Thus, in a first TCAS transmitter mode, independent TCAS transmitter  124  is coupled by switch  110  to one of top and bottom antennas  104 A and  104 B and in a second TCAS transmitter mode, independent TCAS transmitter  124  is coupled by switch  110  to the other one of top and bottom antennas  104 A and  104 B. Similarly, in a first ATCRBS/Mode-S transmitter mode, independent ATCRBS/Mode-S transmitter  126  is coupled by switch  110  to one of top and bottom antennas  104 A and  104 B and in a second ATCRBS/Mode-S transmitter mode, independent ATCRBS/Mode-S transmitter  126  is coupled by switch  110  to the other one of top and bottom antennas  104 A and  104 B. Transmit and switch control function  118  includes conventional circuitry for separating a TCAS transmission from a ATCRBS/Mode-S transmission. In other words, transmit and switch control function  118  ensures that simultaneous TCAS and ATCRBS/Mode-S transmissions do not interfere with one another or damage the equipment. 
     Common Directional Antenna 
     The ATCRBS/Mode-S transponder and the air traffic alert and collision avoidance system functions of combined TCAS transponder device  100  and  100 ′ share common antennas  104 A and  104 B. As discussed above, common antennas  104  are directional antennas which can be driven to transmit omnidirectional signals. Each of common antennas  104  are capable of receiving and transmitting 1030 MHz interrogation signals and receiving and transmitting 1090 MHz reply signals. Such antennas are known in the art and are described at least in U.S. Pat. No. 5,191,349, Apparatus And Method For An Amplitude Monopulse Directional Antenna, issued Mar. 2, 1993, the entire disclosure of which is incorporated herein by reference. Common antennas  104  are multi-element directional antennas capable of determining the azimuthal direction from which radiation is being transmitted by the relative induced signal amplitudes at each of the antenna elements. Common antennas  104  are suitable both for an ATCRBS/Mode-S transponder system and for air traffic alert and collision avoidance system inter-aircraft communications. In preferred embodiments, common antennas  104  provide a minimum profile to reduce drag, are relatively simple to manufacture, and are relatively impervious to environmental hazards while precise positional relationships between the components are maintained. 
     FIG. 10 illustrates just one possible embodiment of directional antenna  104 . Any of several commercially available directional antennas are suitable in practicing the present invention. For example, one preferred embodiment of the present invention incorporated the aforementioned AlliedSignal antenna part number ANT 81A. The following description of a direction antenna as taught in above incorporated U.S. Pat. No. 5,191,349 is provided for illustrative purposes only and is not intended to limit the scope of the present invention in any way. FIG. 10 illustrates an exploded view of directional antenna  104 , including a radome assembly  200 , a ground plate assembly  202 , a base plate  204 , and adapter plate  206 . A radome  208  is manufactured of a polyethersufone resin having various structures formed on an interior surface, including fastening posts  210 , internally threaded grounded portions  212  of the monopole antenna elements, and free portions  214  of the monopole antenna elements. Fastening posts  210  are provided with surfaces, recessed relative to the exterior of radome  208 , for engaging fasteners which pass through apertures in fastening posts  210  to couple either to adapter plate  206  or host aircraft  1 , shown in FIG.  1 . Monopole antenna portions  212  and  214  are coated with copper directly on the surfaces thereof. Capacitors  216  are formed directly on the interior surface of radome  208 . Upon assembly, copper coated antenna portions  212  and  214  contact capacitors  216  to form folded monopole antenna elements. Structural decoupling elements, for example, copper coated fastening posts  210 , between the folded monopole antenna elements decouple the individual antenna elements. 
     Ground plate  204  includes a conducting plate  217  provided with passages  218  for fasteners coupling antenna  104  to adapter plate  206  or to host aircraft  1 . A beam forming circuit is formed on circuit assembly  220 , which is described in detail below and in above incorporated U.S. Pat. No. 5,191,349. Circuit assembly  220  includes passages  221  aligned with coordinated passages formed in ground plate  204 . Passages  221  provide clearance for free antenna elements  214  to extend therethrough conducting plate  217  and through beam forming circuit card assembly  220 . Connectors  222  electrically couple the processing and signal generating apparatus of host aircraft  1  to the beam forming circuit on circuit card assembly  220 . 
     Base plate  204  provides structural support for antenna  104 . Base plate  204  includes passages  224  for the fasteners (not shown) coupling antenna  104  either to adapter plate  206  or host aircraft  1 . Base plate  204  also includes passages  226  through which pass electrical connectors  222 . Electrical connectors  222  couple antenna  104  to combined TCAS transponder  100  mounted on host aircraft  1 . 
     Optional adapter plate  206  adapts antenna  104  to any aircraft surface configuration with formed passages  228  providing multiple securing points. A central passage  230  provide clearance for electrical connectors  222 . 
     FIG. 11 illustrates components of a beam forming network  250  formed on circuit assembly  220 . Terminals  252  are each coupled to one of electrical connectors  222 . Two power dividing components  254 A are positioned on opposite sides of the center of beam forming circuit network  250  and are coupled to two terminals  252 . Each of two power dividing components  254 A are coupled to another two power dividing components  254 B. Each of two power dividing components  254 B are coupled through a ¼ wave transformer  256  to one free antenna element portion  214  extending through passage  221 . ¼ wave transformer  256  is coupled to antenna elements  214  by a contact (not shown). A conducting strip, described in detail below and in above incorporated U.S. Pat. No. 5,191,349, is positioned between each side of each power dividing component  254  and includes a capacitor  260 . Capacitor  260  is essentially a short circuit at operational frequencies and is used for test purposes. Components  262  are each a resistor and a capacitor, coupled in parallel, which are used for test purposes. 
     FIG. 12 illustrates the operation of power dividing component  254 . Power dividing component  254  includes two parallel conducting strips  264  and  266  coupled at their ends by conducting strips  268 . Conducting strips  268  include capacitors  260 , discussed above. When input power P with 0° phase is applied to one end of one conducting strip  268 , the second end of conducting strip  268  provides an output power ½ P with −90° phase relative to input power P. The end of conducting strip  266  proximate the end of conducting strip  264  to which power P was applied provides no power output. The end of conducting strip  266 , opposite to the end providing no power output, provides an output power of ½ P with −180° phase relative to the input power. In operation, antenna  104  outputs directional radiation signals  98 , shown in FIG.  6  and omnidirectional radiation signals  94  and/or  96 , shown in FIGS. 5 and 6, respectively. 
     In contrast to the method of directional transmission described above and in above incorporated U.S. Pat. No. 5,191,349, the aforementioned AlliedSignal antenna forms a directional beam or transmission pattern  94 , as shown in FIG. 5, by delivering power to all four antenna elements 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 one direction and cancel the signals in other directions. Thus, proper phasing of the individual antenna elements enables the AlliedSignal directional antenna to transmit omnidirectional pattern  94 ,  96 , as shown in FIGS. 5 and 6, respectively. Such omnidirectional transmission patterns are useful for transmitting any of the various TCAS and ATCRBS/Mode-S transmissions described herein, including omnidirectional unsolicited Mode-S broadcasts or squitters, replies to ATCRBS/Mode-S interrogation signals, and transmission of ATCRBS/Mode-S interrogation signals. 
     FIG. 13 illustrates one possible signal conversion function  275  for converting signals from antenna  104  to display  120  of the combined TCAS transponder device  100  and  100 ′ of the present invention, shown in FIGS. 8 and 9, respectively. In FIG. 13, the signals from antennas  104  are converted to display the direction of the intruder aircraft relative to host aircraft  1 , shown in FIG.  1 . The directional signals from antennas  104  are compared for selection of the two strongest signals at an amplitude comparitor  280 . The two selected strongest signals are applied to identifier  282 , wherein the electrical connectors  222 , shown in FIG. 10, having the strongest signals are identified. The two selected strongest signals are also applied to a value-of-two comparitor  284 , wherein the relative strength of the two selected signals is compared. Signals identifying the two electrical connectors  222  having the strongest signals as identified by identifier  282  and the value of the comparison of the two strongest signals as provided by value-of-two comparitor  284  are applied to look-up table  286 . Look-up table  286  provides to display  120  a bearing or direction relative to host aircraft  1 , whereby the information is displayed as described above. The signals from electrical connectors are converted to digital signals by conventional methods commonly known to those of ordinary skill in the relevant art and are processed by combined TCAS transponder device  100  to determine threat potential and evasive action as well as to determine the appropriate direction in which to transmit a reply signal to a received interrogation signal and the appropriate direction in which to transmit an interrogation signal to a detected target aircraft. 
     Preferably, the circuitry implementing signal conversion function  275  is contained in transmit and switch control function  118 , described above and shown in FIGS. 8 and 9. Alternatively, the circuitry implementing signal conversion function  275  is contained in each of dual Mode-S transponder signal receiver  112  and TCAS receiver  114 . Both dual Mode-S transponder signal receiver  112  and TCAS receiver  114  are coupled to send an appropriate control signal to transmit and switch control function  118  to configure switch  110  to transmit a directional interrogation and/or reply signal via one of top and bottom antennas  104 A and  104 B. 
     Furthermore, although disclosed in combination with combined TCAS transponder system  100 , directional antennas  104  are used in combination with stand-alone Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode-S) to form a low-cost and reduced power system, wherein the power generate is only sufficient to transmit a directional 1090 MHz transponder reply signal using a single antenna  104 . The generated transponder reply signal is coupled to one of top and bottom antennas  104 A and  104 B such that the signal is transmitted in the direction of the strongest received interrogation signal. Optionally, if the tracking information determined by TCAS receiver  114  indicates that the interrogating aircraft has moved relative to host aircraft  1  sufficiently that a more effective reply signal is possible by transmission through a different one of the antenna elements, then transmit and switch control function  118  sends a control signal to switch  110  to transmit using that different one of the antenna elements, as appropriate. 
     Those of ordinary skill in the relevant art recognize that the present invention is not limited to the combined TCAS transponder device described above and shown in the FIGURES. Although the foregoing invention has been described in detail for purposes of clarity, it will be obvious to those of ordinary skill in the relevant art that certain modifications may be practiced within the scope of the appended claims. For example, combined TCAS transponder device is alternatively modified to work with a display  120  and or/control panel  122  different from those shown in the prior art. Display  120  alternatively provides more or less or different textual messaging, provides different altitude rate-of-change reporting, provides display brightness and/or contrast control, or provides one or more other unspecified distinctive display features. Control panel  122  alternatively provides variable displayed range limits, i.e. zoom in/out control, provides multiple transponder failure indicators reporting both primary transponder  12 A and secondary transponder  12 B, provides different nature, function and/or location for one or more control switches, or provides one or more other unspecified distinctive control features. 
     While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.