Vehicle surveillance system

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority under 35 U.S.C. section 119(e) from U.S. Provisional Application No. 60/203,039 filed on May 9, 2000.

BACKGROUND OF THE INVENTION

[0002] The present application relates to the detection and tracking of aircraft. The invention finds particular application in detecting and tracking suitably equipped aircraft on an airport surface, aircraft on closely spaced parallel approaches and aircraft in proximity of an airport and will be described with reference thereto. It is to be appreciated however, that the teachings disclosed herein are also amenable to the detection and tracking of suitably equipped vehicles in a variety of environments such as highway, maritime and other applications.

[0003] The position of aircraft near airports and other aircraft is of crucial importance to the safe movement of aircraft. Multiple techniques which make use of the Air Traffic Control Radar Beacon System (ATCRBS) transponders have previously been employed to track and monitor airborne aircraft enroute to an airport, provide guidance or monitor aircraft on final approach to airports and monitor aircraft movement within the airports runways and taxiways. The tracking techniques have been realized in the following systems.

[0004] Airborne aircraft enroute to an airport are monitored using Air Traffic Control Beacon Interrogator (ATCBI-6) ground systems and associated displays. These systems interrogate Mode A/C and are fully compliant with the new Mode S (for selective) transponder formats that include P4 suppression to reduce synchronous garble.

[0005] Traffic Collision And Avoidance System (TCAS) is operated onboard aircraft to interrogate aircraft transponders and measure time of arrival (TOA) and in some cases angle of arrival (AOA), then track, display and issue resolution advisories when aircraft closure criteria exceed certain thresholds, as taught in U.S. Pat. No. 5,387,915 issued on Feb. 7, 1995 to Moussa et el.

[0006] The Transponder Landing System (TLS) provide means for the pilot to monitor aircraft position with respect to the desired course and glide path, on final approach to an airport. The system relies on Mode A/C interrogations to track the transponder.

[0007] Systems are known which monitor aircraft on the surface of an airport based only on time of arrival multilateration of the aircraft transponder response, and some prototype multilateration systems have been fielded which rely heavily on the new Mode S transponder, as taught in U.S. Pat. No. 5,262,784 issued on Nov. 16, 1993 to Drobnicki et el.

[0008] The fundamental design constraint of all of the above systems for which the transponder reply is the basis of positive identification is the desire for the system to operate during periods of synchronous garble. Garbling of transponder replies occurs where positive identification of the aircraft transponder response is thwarted by overlapping transponder replies from aircraft that are near the same slant distance from the ground sensor. To increase the capacity of the transponder based systems and minimize synchronous garble, the new Mode S has been defined and standardized. All aircraft with more than 30 seats are now equipped with Mode S transponders and TCAS. In contrast to normal Secondary Surveillance Radar (SSR), some features of the SSR Mode S make it very suitable for ground traffic control as well. The problems in cooperative ground detection tracking methods are due to the lack of Mode S transponders in general aviation aircraft. This problem is likely to persist for many years. For reliably detecting aircraft within dense RF environments which include Mode A/C transponders, new methods must be developed.

[0009] The present invention provides an improved method and apparatus for measuring and processing aircraft transponder replies degraded by synchronous garble which overcomes problems with the current techniques used by the above-referenced systems and others.

SUMMARY OF THE INVENTION

[0010] In accordance with one embodiment of the present invention, a method of tracking aircraft in a surveillance area includes a conventional IFF transponder responsive to interrogation signals at a first frequency, receiving reply signals from aircraft in the surveillance area at a second frequency, the signals being received on a plurality of antenna arrays. An angle of the received reply signals is determined relative to each array from the differential carrier phase. A range is determined based on the time of arrival (TOA) measurement, which is the time from signal transmission to reception. These and other characteristics of the reply signals, such as amplitude and frequency, are used to correlate pulses with each other over time. A position is calculated for each reply pulse indicative of an origin or source point of the reply signals from data including the determined angle and range. This Pulse Track data is conveyed to a central processing location. Pulse tracks are then correlated between multiple ground sensors using all track states which include reply amplitude, position (point of reply origin), velocity, acceleration and reply frequency (waveform cycles per second), to yield an aircraft position and ID. A single array may not be able to calculate a unique angle, but TOA measurements from multiple antenna arrays can be used to calculate a unique angle. This is accomplished by using the TOA data from 2 non-co-located arrays to calculate an initial position and then selecting the angle with the lowest residual. Further processing of the data allows the ultimate system accuracy to be achieved by solving for aircraft position estimates based on intersecting two or more lines of bearing. This final step in the transponder reply processing eliminates the transponder encoding delay. Data representative of a plurality of calculated positions is periodically provided to vehicles in the surveillance area or other users such as ground or air traffic controllers.

[0011] In accordance with another aspect of the present invention, a system for locating an object within a monitored area which receives and measures at least two angles from at least two separate sensor locations without external interrogation of the transponder, commonly referred to as a transponder Mode S squitter.

[0012] In accordance with another aspect of the present invention, Angle of arrival data is measured on every reply pulse to improve pulse to pulse correlation. A position can be calculated for each pulse and combined with velocity, acceleration, reply amplitude and reply signal frequency, can be used to associate pulses over time. This provides synchronous garble mitigation, as the entire reply is not needed to provide position and ID.

[0013] In accordance with an aspect of the present invention, the angle is determined by determining at least one of an elevation angle and an azimuth angle.

[0014] In accordance with an aspect of the present invention, the method further includes receiving the provided data representative of a plurality of calculated positions in a particular vehicle, for example, an aircraft either airborne or taxiing, or airport service vehicles. From the received data a position corresponding to the particular vehicle is extracted and compared with another position from onboard sensors.

[0015] In accordance with an aspect of the present invention, the angle of each reply pulse is determined by receiving the reply signal on a reference antenna in the array and receiving the reply signal on another antenna in the array. A difference in phase between the signal received on the reference antenna and the signal received on the other antenna is determined.

[0016] In accordance with an aspect of the present invention, the method of tracking reply pulses in a surveillance area further includes calculating an elapsed time between transmitting an interrogation signal and receiving the reply signal and determining a range based on the elapsed time.

[0017] In accordance with an embodiment of the present invention, an apparatus which detects aircraft in an area and includes an angle determining apparatus, a range determining apparatus, a position processor and a transmitter. The angle determining apparatus includes a plurality of antennas disposed as an array, and a phase calculator which calculates a difference in phase of the reply signal between a first receive channel including a first antenna, and a second receive channel including a second antenna. The range determining apparatus includes a synchronized timer which determines a time between the interrogation signal and receipt of the transponder reply signal at each of the plurality of antennas, and a range estimator which estimates a range based on the determined time between the interrogation signal and receipt of the transponder reply signal. The position processor determines a position based on the calculated difference in phase and the estimated range. The transmitter transmits data including positions representative of a plurality of aircraft in the area.

[0018] In accordance with an aspect of the present invention, the apparatus also includes a second angle determining apparatus in data communication with the position processor which determines a position based on the calculated difference in phase and the second calculated difference in phase.

[0019] In accordance with an embodiment of the present invention, a system for locating a cooperative object within a monitored area includes an interrogator which transmits an interrogation signal at a frequency. First and second arrays receive a reply signal transmitted by a transponder. An angle of arrival processor calculates an angle of the received reply signal relative to the first and second arrays, and a position processor calculates a position based on data including the calculated angle of the received reply signal.

[0020] In accordance with an aspect of the present invention, the system further includes a display in communication with the position processor which graphically depicts the calculated position.

[0021] In accordance with an aspect of the present invention, the system further includes an identification processor in communication with the sensor which determines an identification associated with the object based upon information encoded in the transponder reply signal.

[0022] In accordance with an aspect of the present invention, the system further includes a range processor which determines a range of the transponder from the ground station based on the received reply signal.

[0023] In accordance with an aspect of the present invention, the system further includes means for otherwise detecting objects in the monitored area and displaying detected objects simultaneously.

[0024] In accordance with an aspect of the present invention, the number of arrays is at least two, and the arrays are non-linearly disposed.

[0025] In accordance with an aspect of the present invention, the system further includes a broadcaster which broadcasts a plurality of calculated positions throughout the monitored area.

[0026] In accordance with another embodiment of the present invention, a system for locating a vehicle within a monitored area, includes at least one receiver array which receives a reply signal transmitted by a transponder associated with the vehicle. A range processor calculates a range based on the received reply signal and an angle of arrival processor calculates an angle of the received reply signal. A position processor calculates a vehicle position based upon the range and the angle.

[0027] In accordance with an aspect of the present invention, the range processor includes a time of arrival processor which calculates an elapsed time between a reference time and a time the reply signal is received at the receiver array, where the range is determined from the elapsed time at the receiver.

[0028] In accordance with an aspect of the present invention, the reference time includes receipt of a timing signal at the receiver.

[0029] In accordance with an aspect of the present invention, the angle of arrival processor extracts an in phase (I) component and a quadrature (Q) component of the signal, and further calculates an azimuth based on the in phase and quadrature components.

[0030] In accordance with an aspect of the present invention, the receiver array includes two adjacent closely spaced antenna elements to provide a coarse azimuth calculation, and two non-adjacent antenna elements linearly disposed with the adjacent elements to provide a fine azimuth calculation.

[0031] In accordance with another embodiment of the present invention, a method of determining a position of a cooperative object in a monitored area, includes receiving an encoded reply signal at a frequency from the cooperative object transmitted in response to an interrogation signal at another frequency. A line of bearing is calculated from characteristics of the received reply signal, the line of bearing relative to a plurality of antenna elements forming an array. A second solution is calculated based on receipt of the reply signal, and a position is determined based on the line of bearing and the second solution.

[0032] In accordance with another aspect of the present invention, the characteristics of the received reply signal from which the line of bearing is calculated include a spatial phase distribution of the reply signal.

[0033] One advantage of the present invention resides in the use of legacy equipment to provide a more reliable and accurate navigation solution during synchronous garble events.

[0034] Another advantage of the present invention resides in the use of AOA data to provide the most efficient use of each interrogation and or transponder reply in an environment where it is extremely important to minimize interrogations, and as a consequence minimize the occurrence of garble.

[0035] Another advantage of the present invention resides in the increased situational awareness of both pilots and ground-based controllers.

[0036] Another advantage of the present invention resides in the application of the present teachings to any transponder based navigation applications such as TCAS, Parallel Runway Monitoring, TLS, or airborne and ground surveillance.

[0037] Still further advantages will become apparent to those of ordinary skill in the art upon reading and understanding and following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention.

[0039]FIG. 1 is a block diagram of a system suitable to practice an embodiment of the present invention;

[0040]FIG. 2 is a functional block diagram of a system which suitably practices the present invention;

[0041]FIG. 3 is a flowchart illustrating a process which suitably practices the present invention;

[0042]FIG. 4 illustrates an exemplary application of the transponder position determination in a ground based environment; and

[0043]FIGS. 5<i>a, </i><b>5</b><i>b </i>and <b>5</b><i>c </i>illustrate an exemplary application of the transponder position determination in an airborne environment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] With reference to FIG. 1, an aircraft <b>10</b> or other vehicle with an associated transponder (not shown) is interrogated by interrogator <b>12</b>. In compliance with Federal Aviation Administration (FAA) regulations, this interrogation signal <b>14</b> is preferably transmitted at 1030 MHZ, although other frequencies may also be used and may be desirable in environments other than tracking airborne aircraft. Interrogation strategies to reduce synchronous garble may include those well known in the art such as directional antenna lobbing using either phased array or fixed directional parabolic antenna or the like, and or whisper shout interrogation amplitude sequences, well known in Traffic Collision Avoidance System (TCAS) applications. Other interrogation beam shapes including a wide area beam, phased array and the like are also contemplated.

[0045] In response, the transponder sends a transponder reply <b>16</b> which preferably complies with FAA regulations specifying 1090 MHZ. This signal is depicted in FIG. 1 as two separate beams, but the transponder preferably acts as an approximate point source with signal preferably emitting omni-directionally and extending throughout the monitored area.

[0046] Transponder signal <b>16</b> is received by a plurality of receiver arrays, in FIG. 1 shown as two arrays, one to determine azimuth <b>18</b>_aand one to determine elevation <b>18</b>_e. While these arrays are shown separately, those skilled in the art will appreciate that they may be combined on a frame or support in a “L” configuration or alternately, a phased array type panel may employed with no loss of functionality. The discrete arrays <b>18</b><i>a, </i><b>18</b><i>e </i>are illustrated for clarity only. As used herein, the term “array” should include receivers capable of determining azimuth only, elevation only, range only or combinations thereof. The received signal of each antenna is processed by an angle of arrival processor <b>20</b>_a, <b>20</b>_eto determine various characteristics such as time (for example, between transmission and reception), amplitude, frequency, differential carrier phase, and/or angle of the reply signal <b>16</b> from the source transponder with respect to the array <b>18</b>.

[0047] Referring to FIG. 2, each array <b>18</b> includes several individual antennas <b>30</b>, <b>32</b>, <b>34</b> and <b>36</b>, a coherent reference demodulation signal, 90° phase shifting elements, and other circuitry necessary to extract the in-phase (I) component which is in phase with the reference, and the quadrature (Q) component which is 90° out of phase. Signal decomposition into in-phase and quadrature components, and the electronics required for implementation thereof, are well known to those skilled in the art. Knowledge of the in phase and quadrature signal components at each antenna in turn yields the phase of the signal <b>16</b> at each antenna <b>30</b>, <b>32</b>, <b>34</b> and <b>36</b>.

[0048] The angle of arrival is determined from the differences in phase among pairs of antenna <b>30</b>, <b>32</b>, <b>34</b>, <b>36</b>. Arbitrarily defining the phase at antenna <b>30</b> as 0°, the phase at antenna <b>32</b> will be 2π(ΔL/λ), where λ is the wavelength of the transponder reply (˜0.27 m for 1090 MHZ), and ΔL is the difference in transponder signal path length with respect to antennas <b>30</b> and <b>32</b>. ΔL is thus obtained from the signal phase measurements at antennas <b>30</b> and <b>32</b>. The azimuth θ is given by: <maths id="MATH-US-00001" num="1"><math overflow="scroll"><mrow><mrow><mi>sin</mi><mo></mo><mrow><mo>(</mo><mi>θ</mi><mo>)</mo></mrow></mrow><mo>=</mo><mfrac><mrow><mi>Δ</mi><mo></mo><mstyle><mtext> </mtext></mstyle><mo></mo><mi>L</mi></mrow><mi>d</mi></mfrac></mrow></math><img file="US20030142002A1-20030731-M00001.TIF" id="EMI-M00001" he="17.03835" wi="216.027" img-format="tif" img-content="mf" /><attachments><attachment idref="MATHEMATICA-00001" attachment-type="nb" file="US20030142002A1-20030731-M00001.NB" /></attachments></maths>

[0049] where θ is the azimuth angle, d is the individual antenna spacing, and ΔL is the path length difference.

[0050] The resolution of the angle of arrival method will improve with increasing spacing d. However, the angle of arrival method also yields multiple solutions of θ=sin⁻¹(nλ/d), where n is an integer. This may be resolved by using differential range measurements from two antenna arrays. The range measurement is derived from TOA or amplitude measurements.

[0051] Referring back now to FIG. 1, the characteristics including azimuth angle and others as discussed above, are forwarded to two processors <b>60</b>, <b>62</b> which independently calculate a position of the source transponder from the characteristics, thereby locating the responding vehicle <b>10</b>. The calculated positions are provided to comparator <b>66</b> and tested to determine if the independently calculated solutions lie within allowable limits. If so, a position estimate of the vehicle is formatted and transmitted or broadcast from transmitter <b>68</b> to users such as other aircraft in flight or vehicles on the ground, and air traffic or ground controllers within operable range. While the illustrated implementation shows RF transmission <b>70</b>, providing the position estimate to users may be accomplished by a variety of means, such as fiber optic cabling, Ethernet or other network protocols and the like without adversely affecting performance. It is now evident that the calculated positions may also be usefully combined or integrated with other detection means or systems <b>76</b> such as active radar surveillance systems, airport surface detection equipment, tracking subroutines, chart or map displays, operator-inserted overlays or data and the like for display <b>78</b>. In one embodiment, position data from the present system is registered or overlaid with position data from other systems by selecting a particular target of interest with a known position, as such aligning position data from the plurality of systems.

[0052] With reference now to FIG. 2, a functional block diagram of components is illustrated. Four antennas <b>30</b>-<b>36</b> comprise the antenna array <b>18</b>_a. For simplicity antenna array <b>18</b>_eis not depicted however those skilled in the art will appreciate that similar processing will occur on signals received on that array. Received reply signals from antennas <b>30</b>-<b>36</b>, pass through filter <b>80</b> and are processed by the RF receiver assembly <b>82</b>. By designating one of the antennas a reference <b>30</b> and measuring differential carrier phase between the reference antenna <b>30</b> and one of the remaining antennas <b>32</b>, <b>34</b>, <b>36</b> three different antenna array apertures are achieved and therefore, three different resolution measurements are possible, for example low, medium and high. An intermediate frequency from the reference antenna <b>30</b> is output from the receiver assembly <b>80</b> on a dedicated receiver path <b>84</b>. Signals from antennas <b>32</b>, <b>34</b>, <b>36</b> are multiplexed onto two RF receiver paths <b>86</b>, <b>88</b> using switches within the RF receiver assembly <b>82</b> although those skilled in the art can envision equivalent mechanisms to transfer signals to the assembly. The Phase, Amplitude, and Frequency Measurement (PFM) <b>90</b> receives the intermediate frequency paths <b>84</b>-<b>88</b> from the RF receiver assembly <b>82</b>, and provides log video signals <b>94</b><i>a, </i><b>94</b><i>b, </i><b>94</b><i>c, </i>and digitized phase data <b>98</b><i>a, </i><b>98</b><i>b </i>to the Phase Acquisition Card (PAC) <b>100</b> for signal processing.

[0053] Two sets of digitized phase data are provided: the phase difference <b>98</b><i>a </i>between the reference and IF channel A (low, medium, or high channels <b>32</b>, <b>34</b>, <b>36</b>); and the difference <b>98</b><i>b </i>between the reference and IF channel C (low, medium, or high channels). Log video of each of the three inputs <b>94</b><i>a, </i><b>94</b><i>b, </i><b>94</b><i>c </i>is also passed to the PAC <b>100</b>. The PAC <b>100</b> additionally receives a start signal or synchronization timer <b>102</b> indicating an interrogation sequence <b>14</b> for use in making range calculations. Suitable mechanisms and methods for calculating range and even position based on range alone are known as exemplified in U.S. Pat. No. 5,017,930 issued to Stoltz et al. and incorporated herein by reference. Those skilled in the art will appreciate that the synchronization signal <b>102</b> may be provided by various forms of data communication such as fiber optic cabling, wireless transmission, conventional hard wiring and the like. Alternately, the synchronization may be implemented through internal mechanisms on the several components such as by internal clocks or GPS signals.

[0054] Sensor acquisition begins processing RF inputs from the antennas <b>30</b>, <b>32</b>, <b>34</b>, <b>36</b> upon receipt of a start signal <b>102</b>. Transponder reply signals <b>16</b> are analog processed and stored. Phase and frequency data is correlated with stop video signal acquisition data. All collected signal characteristic data passes to processors <b>60</b>, <b>62</b> which calculate independent position solutions as more fully discussed below.

[0055] With reference now to FIG. 3, an overview flowchart of steps which suitably practice the present invention are illustrated. An aircraft antenna transmits a reply signal <b>16</b> which is received on a ground based antenna arrays, <b>18</b>_a, <b>18</b>_e, and sent to processors <b>20</b> (FIGS. 1, 2). The processors receive the reply data and identify pulses as seen in step <b>110</b>. The processor identifies pulses based on amplitude characteristics to detect a leading edge (LE) and trailing edge (TE) of the pulse as seen in step <b>111</b>. If it is successful, then Pulse Data is extracted as seen in step <b>118</b>. In addition to measurement data (AOA, TOA, Amplitude and Frequency), this also includes pulse information such as width and shape. Note that for Mode S squitter operation TOA measurements are not available because a reference or timing signal is not available. Tracks are maintained for the Pulse Data as seen in step <b>120</b>. The processors then associate pulses over time using measurement and pulse information data as seen in step <b>122</b>. This occurs for data from an individual antenna array along with data from other antenna arrays. The processors then make a final ID and position calculation, as seen in step <b>124</b>, if there is enough data to resolve ID and position (i.e. multiple cycle) ambiguities. Environmental considerations can also be used to resolve position (i.e. eliminating a solution that clearly places an aircraft off airport property). Finally, the available data is tested against various limits (step <b>126</b>) such as within accuracy requirements or identification confidence levels. If the data is acceptable, information is then forwarded to the transmitter <b>68</b> for broadcast to, and display within other vehicles in the monitored area A or other users, as seen in step <b>126</b>.

[0056] Kalman filtering is used to improve the accuracy of the position estimate. The filtering improves the accuracy by using not only the most recent receiver measurements, but also the previously determined position, the statistical “reliability” of that position, and the statistical variance of the current measurements. The application of Kalman filtering to navigation systems is well understood by someone skilled in the art however, the following aspect of Kalman filter tuning is specialized to this invention. The process noise covariance matrix Q is usually a heuristic set to accommodate expected target maneuvers, but it may be set adaptively by more advanced formulations of the Kalman filter. The process noise covariance matrix Q is an “aging” matrix that allows acceleration events to occur in the state vector representing the aircraft dynamics. Usually one chooses an estimated Q and then “fine-tunes” it through computer simulation or flight test. A first approximation for Q is usually computed by considering the maximum acceleration that the aircraft is likely to perform.

[0057] Referring back to FIG. 2, the position information in one embodiment of the present invention is incorporate, aligned and displayed with other position data. Position information is received on a track of interest such as a particular aircraft or vehicle, a reference point and the like. Typically the track data will be extracted from the plurality of position data received by code, or other identifying means. Other position data is then aligned or registered with the transponder data and the results are displayed to a user.

[0058] With reference to FIG. 4 an embodiment for airport or other surface detection and tracking environment is illustrated. An aircraft <b>10</b> or other vehicle with an associated transponder (not shown) is interrogated by interrogator <b>12</b>. Depending upon the environment, the interrogation rate may be fairly low, especially as compared with interrogation rates used for monitoring airborne aircraft. For example, ground vehicles at an airport are usually moving relatively slowly over fairly open areas, so relatively long interrogation intervals, or reply intervals may be used. Additionally, the above discussed frequencies of interrogation and reply may be different in the surface environment to de-conflict airborne or other signals. The reply transmission <b>16</b> is received on receiver array <b>18</b>. In the illustrated embodiment a single azimuth only array may suffice to provide an unambiguous angle of bearing to the vehicle. The unambiguous angle of bearing coupled with range estimation processing, as discussed above are sufficient to roughly position the vehicle <b>10</b>. In the illustrated example, a hard wired cable <b>130</b> links the array to displays in the tower <b>132</b> which broadcasts <b>70</b> the calculated position over an RF or optical link across the surveillance area A to other vehicles in range. Those skilled in the art can now appreciate that additional angle and range processing arrays may be located at other locations around the surveillance area A to provide a more robust, fault tolerant system with increased precision. Moreover, those skilled in the art can appreciate that various combinations of arrays may also be and desirable in other situations. For example, a single range finder coupled with several angle determining systems may be desirable in some locales while a positioning system comprising only range determination (or alternately angle determination) may be desirable in others.

[0059] It is also envisioned that other detecting systems <b>76</b> (FIG. 1) such as surface detection equipment be incorporated into the system, for example, by registering both surface detection radar and transponder position for display <b>78</b> or transmission to surrounding craft. Alternately, the radar display may be processed by a radar object processor as is known, to extract the location of detected objects. The detected objects may then be compared with transponder locations to determine if the object is unidentified or associated with an existing or new transponder track. The radar objects may then also be appropriately marked in the display <b>78</b>.

[0060] With reference now to FIGS. 5<i>a</i>-<b>5</b><i>c, </i>an embodiment for an in-flight surveillance system is illustrated. Referring first to FIG. 5<i>a, </i>interrogator <b>12</b> transmits an interrogation signal <b>14</b> which is received by the cooperatively equipped aircraft <b>10</b> transiting surveillance area A. Referring now to FIG. 5<i>b, </i>a single reply transmission <b>16</b> from aircraft <b>10</b> is received by a plurality of geographically spaced receivers <b>18</b>. In the present example, the four illustrated receiver arrays <b>18</b> in range of the transponder reply <b>16</b> each calculate angle of arrival in both azimuth and elevation. The elevation calculation may involve Mode C interrogations, which return the aircraft's altitude instead of ID. Additionally, the originating interrogating station <b>12</b> coupled with its local receiver array <b>18</b> determines range. The stations are in data communication with position processors <b>60</b>, <b>62</b> which combine the respective data or signal characteristics to determine a position from which the reply signals <b>16</b> originated. Referring now to FIG. 5<i>c, </i>this calculated position is forwarded to transmitters <b>68</b> associated with each receiver array <b>18</b> in the illustrated example although they could be elsewhere located. The transmitters <b>68</b> then broadcast <b>70</b> the calculated position throughout the surveillance area A to other aircraft as well as the replying aircraft <b>10</b>, as illustrated. Again, those skilled in the art can now envision various combinations of range and angle processors usable to determine unambiguous positions. Moreover, the calculated position may then be converted into a format for display <b>78</b>, integration with other systems <b>76</b> and the like for use also by air traffic controllers or others.

[0061] As can now be appreciated, the position broadcasts include position data on all cooperatively responding aircraft in the surveillance area. Thus, if such a cooperatively responding aircraft also has the capability to receive and process the broadcast position data, a navigational backup or integrity monitor is provided by the transponder surveillance system. In other words, such an aircraft can compare or register the position information from organic navigation equipment such as INS, GPS, and the like, to the position information provided by the broadcast position data. As such, overall confidence of the navigational accuracy is increased, and extra redundancy is provided.

[0062] Receipt and decoding of the transponder reply may further provide additional useful content to improve the situational awareness of both controllers and flight crew. For example, many aircraft transponders include identification codes in the transponder reply. These codes may be advantageously used by a vehicle identification processor to derive additional vehicle information, such as aircraft type, flight plan, maintenance requirements, and the like, which may be included in display <b>78</b>.

[0063] The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.