Transponder landing system

ES 2 232 631 T3

DESCRIPTION

Transponder landing system.

Cross reference to related request

This application claims priority under 35 USC section 119 (e) to US Provisional Application No. 60 / 203,039 filed May 9, 2000 and US Patent Application No. 09 / 695,359 filed October 24, 2000.

Background of the invention

The present application relates to navigation systems. The invention has special application in aircraft landing systems that provide precision elevation guidance to a user, such as a controller or pilot, during approach and landing.

Various precision aircraft landing systems have been employed in order to make it easier for the pilot to maintain a desired glide path to a runway. The Instrument Landing System (ILS) is commonly used for precision approaches; however, ILS systems are prone to interference from nearby FM broadcasts, require extensive ground grading and property acquisition in some airport locations, and are vulnerable to guidance beam distortion when considering construction near an airport. The Microwave Landing System (MLS) is used much less frequently than ILS, and is falling out of date in response to economic issues. Precision Approach Radar (PAR) is commonly used in military environments and requires a ground operator to verbally send glide path guidance corrections to the pilot via a communications link. Landing aids based on the Global Position System have been proposed which include two systems under development, the Wide Area Augmentation System (WAAS) and the Local Area Augmentation System (LAAS) which are subject to interference and counterfeiting, and they may not be suitable as the only means of precision approximation.

Aircraft navigation systems employing the Air Traffic Control Radar Beacon System (ATCRBS) transponder are generally known in the art. Transponders are typically deployed on aircraft to facilitate the Secondary Surveillance Radar (SSR) function of verifying and controlling the aircraft en route. Most commercial aircraft are equipped with two transponder antennas, one on the top and one on the bottom of the aircraft fuselage to maintain reliable transponder responses during aircraft turns. Such transponder antenna configurations are known as diversity antennas. A transponder equipped with diversity antennas selects the antenna that received the highest amplitude interrogation signal from a ground station to transmit the encoded reply message. International Standards and Recommended Practices currently require that the horizontal distance between the upper and lower antennas be less than 7.6 meters, to control the apparent SSR range instability from response to response due to antenna diversity switching. . The vertical separation of the diversity antennas varies depending on the height of the aircraft fuselage and can be approximately between 3 and 10 meters.

Landing systems using the ATCRBS transponder must determine the position of the aircraft, compare it to a desired approach path, and transmit the required correction to the aircraft. US Patent No. 3,564,543 to Nehama describes such a system, which uses symmetry and simplified mathematics to define a conical approach path. In general, the position determination system described in Nehama and analogous systems are based on measurements of the time of arrival of the transponder response derived from the time required for the interrogation to reach the transponder, during the time for the transponder to respond, and the time required for the signals to advance between the landing aircraft and a plurality of positions on the ground. From these distances the position of the aircraft is estimated. The Nehama patent recognizes the existence of a variable transponder response time that can induce substantial errors in the navigation solution. As a compromise, Nehama arranges the transmitter and sensors in a substantially vertical geometric plane transverse to the length of the runway. This arrangement projects the error in a horizontal direction along the axis of the track. As a side effect, this arrangement requires the use of elevated antenna towers near the airport, because if all the sensors were placed at ground level, and therefore in a horizontal plane, the calculated altitude of the aircraft would contain substantial errors, which would not would be permissible in a precision landing system.

US Patent No. 5,017,930 to Stoltz describes a system that is an advance over Nehama's because, among other things, it also solves the transponder coding delay using four sensors. Unfortunately, the time of arrival measurements used by landing systems, such as those described in Nehama and Stoltz, are subject to significant multipath errors. These multipath errors are induced by terrain characteristics along the approach path to the runway and induce errors in time of arrival measurements. Errant measurements of time of arrival degrade the navigation solution, thereby reducing the accuracy of the guidance signals transmitted to the aircraft.

ES 2 232 631 T3

US-A-4 454 510 describes an aircraft guidance system that includes a ground station and a transponder mounted on an aircraft. The ground transmitter emits interrogation pulses that are coded to contain position data. The transponder decodes the interrogation pulses and transmits response pulses containing altitude and other data that are used to obtain three-dimensional position data on the ground for navigation, landing guidance, and collision avoidance effects.

It is desirable that landing systems meet the International Standards and Recommended Practice limit on signal excursion characteristics on the navigation path that includes curves, recesses, bumps and other aberrations with a two-sigma limit approximately equivalent to 3 meters at a point 1.4 km from the Runway Interception Point on the glide path. Unfortunately, switching diversity antennas, even on the smallest aircraft, can potentially produce performance outside of that window.

The present invention contemplates an improved method and apparatus that overcome the above and other problems.

Summary of the invention

According to one aspect of the present invention, a method of determining a position of an aircraft having a transponder that transmits a response signal in response to an interrogation signal includes first receiving response signals at a plurality of antennas arranged as an oriented array. vertically. The characteristics of the response signal, such as differential phase, amplitude, frequency, and the like, are measured and used to estimate the position of the aircraft. The differential phase is analyzed between at least two response signals to determine whether the respective response signals originate from different antennas on the aircraft. In case it is determined that the response signals originate from diversity antennas, the estimated position is adjusted to compensate for the distance between the respective antennas. The method can also calculate an error between the adjusted position and a desired position and transmit this error to a user such as a pilot, air traffic controller, or cockpit displays of another aircraft.

An aircraft precision landing system determines on a real-time basis the position of an aircraft by measuring the elapsed time between the interrogation signal and the response of the transponder at a plurality of predetermined positions. The system manages the effects of multipath and achieves the exact position of the aircraft by measuring the differential phase of the transponder response to calculate the angle of arrival.

The present invention has the ability to compensate for the switching of the transponder diversity antenna, and as a consequence of this compensation, to achieve an elevation estimate with the least dynamic delay.

According to another aspect of the present invention, a multipath correction is applied to selected features to compensate for induced multipath errors in the estimated position, thereby achieving the best possible detection and compensation of the diversity antenna.

An advantage of the present invention resides in the ability to accurately determine the position of the aircraft based on a cooperative response signal from the transponder coming from an aircraft.

Another advantage of the present invention resides in the ability to manage or cancel the effects of multipath returns of the transponder response signal.

Another advantage of the present invention resides in the ability to accurately determine the position of the aircraft by measuring the angle of arrival and the time of arrival of the transponder response.

Other advantages will be apparent to those skilled in the art after reading and understanding the following detailed description.

Brief description of the drawings

The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The figures are for the purpose of illustrating the preferred embodiments only and are not to be construed as limiting the invention.

Figure 1 is a perspective drawing illustrating the elements of the Transponder Landing System according to this invention.

Figure 2 is a block diagram of the AOA sensor and the front end RF array switched antenna inputs in accordance with this invention.

ES 2 232 631 T3

Figure 3 is an illustration of the error attributable to the diversity antennas of the aircraft.

Figure 4 is an illustration of the error attributable to multipath signal transmission.

And Fig. 5 is a flow chart illustrating the processing that takes place in the processors, which adequately implements the present invention.

Detailed description of the invention

Referring now to Figure 1, an airport is depicted having a runway 10. An aircraft precision landing system in accordance with the present invention guides an aircraft along a predefined approach path (not shown), leading to track 10.

The system includes an interrogation transmitter housed in base station shelter 20 that is used to generate interrogation and blanking pulses. The interrogation signal, composed of pulses modulated on a carrier frequency, emanates from an interrogation antenna 22. As will be better explained below, the base station 20 also sends a timing signal 76 coincident with the interrogation signal to receiving networks 26, 28 to synchronize the processing of the sensor measurement. A transponder 86 (Figures 3,4), for example, in an aircraft, transmits a response signal that is also composed of pulses modulated on a carrier frequency in response to receiving the interrogation signal. The response signal is received by receiver networks 26, 28 and is preferably sent to dual different processors 78a, 78b (FIG. 2) within base station 20 for processing. The response signals are processed, as explained below, with respect to various characteristics such as time, amplitude, frequency, and differential carrier phase.

Referring now to Figure 2, a functional block diagram of components is illustrated. Four antennas 26a-26d form the antenna array 26. For simplicity, the antenna array 28 is not illustrated; however, those skilled in the art will appreciate that similar processing will occur on signals received in such a network. The received response signals are received at antennas 26, pass through filter 30, and are processed by RF receiver assembly 32. By designating reference 26a to one of the antennas and measuring the differential carrier phase between reference antenna 26a and one of the three remaining antennas 26b-26d, three apertures of the antenna array are achieved and thus three measurements of different resolution, for example low, medium and high. The reference antenna 26a is inserted into a dedicated receiver path 34. Other antennas 26b-26d are multiplexed to two RF receiver paths 36, 38 using switches within the RF receiver assembly 32 although those skilled in the art can envision equivalent mechanisms for transferring signals to the assembly. Each path 34, 36, 38 receives 1090 MHz pulse modulated RF signals from antennas 26 originating from the response transponder. The Phase Amplitude Measurement (PAM) 50 receives intermediate frequency paths 34-38 from the RF receiver assembly, and provides 54 log video signals and 58A, 58B digitized phase data to the Phase Acquisition Card (PAC) 60 for signal processing.

Two sets of digitized phase data are obtained: the phase difference between the reference and IF channel A (low, medium or high channels 26b-26d) 58A; and the difference between the reference and the IF channel C (low, medium or high channels) 58B. The video log from each of the three inputs is also passed to the PAC 54A, 54B, 54C. A limited IF signal 68 from the low resolution input is sent to the frequency discriminator 70. The frequency discriminator 70 receives the limited IF signal 68 from the input of the RF receiver assembly, and provides an analog output 72 to the data acquisition section of the PAC 60 for frequency measurement. The PAC 60 thus also receives a start signal or synchronization timer coinciding with the interrogation signal via fiber optic 74 from the base 20 (figure 1), video amplitude data log 54, digital phase data 58 from the PAM 50, and frequency video 72 from frequency discriminator 70. Those skilled in the art will appreciate that the synchronization signal can be transmitted by means of data communication other than fiber optic cables such as by wireless transmission, conventional hard wiring, and the like. Alternatively, synchronization can be implemented by internal mechanisms in the various components such as by internal clocks or GPS signals.

Sensor acquisition begins processing the RF inputs from antennas 26 upon receipt of a start signal on cable 74. Response signals from the transponder are analog processed and stored. The phase and frequency data are correlated with the stop video signal acquisition data. All data passes to processors 78 at base station 20.

Kalman filtering is used to improve the accuracy of the position estimate. Filtering improves accuracy by using not only the most recent measurements from the receiver, but also the previously determined position, the statistical "reliability" of that position, and the statistical variance of current measurements. The application of Kalman filtration to navigation systems is well understood by those skilled in the art; however, the following aspect of Kalman filter tuning is special to this invention. The process noise covariance matrix Q is usually a heuristic set to accommodate expected desired maneuvers, but can be adaptively established 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 dynamics of the aircraft. Generally, an estimated Q is chosen and then "fine-tuned" by simulation by

ES 2 232 631 T3 computer or flight test. Generally, a first approximation for Q is calculated by considering the maximum acceleration that the aircraft is likely to make. The optimum value of the process noise is achieved in conjunction with applying the Diversity Antenna (DA) algorithm below.

Referring now to Figure 3, an exaggerated but exemplary illustration of the diversity antenna error is illustrated. An aircraft 80 approaching a runway is equipped with a lower antenna 86_L and a top antenna 86_or. As illustrated, at first the aircraft 80 responds to an interrogation with the lower antenna 86_L and signal 90 passes directly to antenna array 26. After the initial interrogation and response sequence, the aircraft has moved and is now illustrated at reference numeral 80 '. However, as illustrated, the aircraft 80 ', due to position, responds to a subsequent interrogation with the upper antenna 86_or. As explained above, because of the spacing between the diversity antennas 86L, ​​86u, an error is introduced into the navigation problem. The processors 78 in the base station 20 (Figure 2) are equipped with a diversity antenna algorithm that evaluates the digitized differential phase of the response signals over time to detect an aircraft transponder response coming from the diversity antennas. . The algorithm uses the two Angle of Arrival (AOA) data sets, 58A and 58B, to establish the existence of a diversity antenna configuration and calculate the diversity antenna spacing. As used herein, the term AOA is understood to imply any of several methods of finding out the angular deviation from some predetermined normal angle of incidence, and includes determining an actual angular deviation, determining a phase difference between multiple signals, or otherwise calculate an address of arrival. As explained more fully below, AOA measurements that are determined to originate from the aircraft's upper diversity antenna, compensate for the diversity antenna spacing, to produce a measurement set that would have the lower antenna as a point of reference. emanation.

In a currently preferred embodiment, the DA algorithm initializes parameters (Table 1) at the beginning of track acquisition. These parameters are tuned using simulation and field data applying a wide range of aircraft types. During an approach to a runway, detection of measurement skips, which could be due to diversity antenna switching, is carried out by examining the delta between the previous interrogation count and the current. If the interrogation delta indicates sequential measurement samples, and the TOA measurement range is within the maximum range 1, the sine of the jump angle is calculated using the difference in phase of the last interrogation and the corresponding aperture of the AOA antenna. , then a jump detected flag is set to indicate that there is data. If the sine of the jump angle is greater than the minimum 2, the jump distance is calculated from the jump angle (meters) and the direction is determined. If the jump distance is between the minimum and maximum jump limits 3, 4, the jump detected flag is set and the jump amount is set to the distance. Finally, the interrogation count and phase measurement are stored for comparison with subsequent interrogation data. An alternative embodiment of this portion of the algorithm would use angle information only (2) in a narrow range to determine when a jump occurred.

A further embodiment, a type of jump (Table 2) includes assignment according to the result of the medium and / or high jumps. The confidence of a DA configuration is calculated by weighting the number of various types of measurement jumps 10. High and medium channel jumps in the same direction have a large positive weight. In addition, the level of agreement between the high and medium channel jumps is used to increase weight. The high and medium measurement channel jumps in the opposite direction have a large negative weight. High channel hops when Medium is not available (due to AOA sensor antenna interleaving) have a low positive weight. Similarly, medium channel hops when no high channel is available have low positive weight. High or medium channel jumps that occur when a medium or high measurement is available, but do not indicate a jump, have a low negative weight. Measurements that do not indicate a jump have zero weight. A diversity configuration existence flag is set after a sufficient number of measurement updates have met the jump set criteria 9-12 indicating that the aircraft is equipped with a diversity antenna.

Once established, one DA separation track and one DA separation variance track are estimated by two monostate Kalman filters with constant gain. Tracks are calculated 5-8 based on the average of high and medium channel hops in the same direction. An alternative embodiment of this portion of the algorithm would also use individual high or medium channel hops when medium or high is not available, respectively, to calculate tracks.

Antenna state 13-27 is maintained (ie high or low diversity antenna) along with reliance on that state. A type of SAME jump has very high confidence in the indicated direction. An CONTRARY jump type sets the state to unknown. A jump type HIGH, MEDIUM, ATO NOISE, or MEDIUM NOISE has high confidence only if the jump size matches the expected jump as indicated by the DA separation track, otherwise the state confidence is lowered. A jump type of NO DATA lowers state confidence. A jump type of NONE does not change the state trust. If the state changes from higher to lower or vice versa, the state confidence is increased. In addition, the state confidence is increased based on the level of agreement between the jump and the DA separation track.

ES 2 232 631 T3

A response that is determined to come from the higher diversity antenna is regulated to an emanation point corresponding to the lower diversity antenna, based on the separation track DA. Window thresholds 2835 are set as minimum and maximum limits, biasing the window based on the speed of the measurement track 33. The window size is adjusted based on the standard deviation of the spacing estimate variance 31 and the expected measurement variance 32. A wide window is also set, which is a linear scale 36 of the normal window. The difference between the medium and / or high measurement and the expected measurement is checked against these windows. Diversity antenna status confidence 37-38 is also checked. The results of these 39-43 tests are used to determine if the measurement should be adjusted. If so, the value of the separation track DA is subtracted from the measurement. An alternative embodiment of this portion of the algorithm would calculate the relationship between the separation track DA and the difference between the expected measurement and the actual measurement. This value for the medium and / or high measurement along with the diversity antenna state and state confidence would be used to determine if the measurement should be adjusted.

TABLE 1

Diversity antenna algorithm parameters

<td></td><td>Parameter</td><td>Purpose</td>

<td colspan="3">Based on the evaluation of the measurement jumps produced by diversity antenna switching</td>

<td> 1</td><td>jump eval max mg</td><td>Maximum range to detect commutation</td>

<td> 2</td><td>min jump without theta</td><td>Minimum angle to detect switching</td>

<td> 3</td><td>min jump</td><td>Minimum DA switching</td>

<td> 4</td><td>max jump</td><td>Maximum DA switching</td>

<td colspan="3">Based on the estimate of the diversity antenna spacing</td>

<td> 5</td><td>sep gain</td><td>Filter gain to estimate DA separation</td>

<td> 6</td><td>sep var gain</td><td>Filter gain to estimate DA separation</td>

<td> 7</td><td>use medium only jumps</td><td>Control media usage</td>

ES 2 232 631 T3

<td></td><td></td><td>just to estimate the DA separation</td>

<td> 8</td><td>use high only jumps</td><td>Control the use of high just to estimate the DA separation</td>

<td colspan="3">According to the determination of the confidence of existence of diversity antenna</td>

<td> 9</td><td>exist conf limit</td><td>Limit to which it is confirmed existence DA</td>

<td> 10</td><td>conf_weights [MAX JUMP SET]</td><td>Confidence weights for different types of measurement jumps</td>

<td> 11</td><td>conf same ratio weight</td><td>Confidence weighting to apply the same direction relationship</td>

<td> 12</td><td>conf same ratio limit</td><td>Limit for the same address relationship</td>

<td colspan="3">According to the calculation of the diversity antenna status</td>

<td> 13</td><td>use high only to upper</td><td>Use only high jumps to put the state in higher</td>

<td> 14</td><td>use_medium only to upper</td><td>Use half-only jumps to put the state on top</td>

<td> 15</td><td>use high only to lower</td><td>Use only high jumps to put the state in lower</td>

<td> 16</td><td>use medium only to lower</td><td>Use jumps only means to set state in lower</td>

<td> 17</td><td>both status conf</td><td>State trust to use if both jumps</td>

<td> 18</td><td>high only status conf</td><td>State trust to use if i just jump high</td>

<td> 19</td><td>Medium only status conf</td><td>State trust to use</td>

ES 2 232 631 T3

<td></td><td></td><td>if I jump half only</td>

<td> 20</td><td>expected jump bonus conf</td><td>Confidence Bonus to be applied if there is an expected match of the jump</td>

<td> 21</td><td>expected_jump conf limit</td><td>Confidence limit at which an expected jump can be determined</td>

<td> 22</td><td>same ratio bonus conf</td><td>Confidence Bonus for high ratio between jumps</td>

<td> 23</td><td>same ratio bonus limit</td><td>Relationship to which it begins to apply the bonus</td>

<td> 24</td><td>Status conf reduce</td><td>Amount by which confidence will be reduced when miss detection opportunities (for example, Medium in Channel A and Medium in channel C when not use medium to set the state</td>

<td> 25</td><td>one meas trk ratio limit</td><td>Relationship between a jump of measurement and the DA separation track to which meas_trk_b ° begins to be appliedⁿus · In case there is only one measurement</td>

<td> 26</td><td>two meas trk ratio limit</td><td>Relationship between a jump of measurement and the separation track DA to which it begins to apply meas trk bonus. For him case there are two measurements, but only one in-</td>

ES 2 232 631 T3

<td></td><td></td><td>dock jump (i.e. medium noise or high noise)</td>

<td> 27</td><td>meas trk bonus conf</td><td>Confidence Bonus to be applied based on the relationship between a measurement unique and track</td>

<td colspan="3">According to the compensation of the AOA measurement if it comes from the upper antenna of the transponder</td>

<td> 28</td><td>smallest window min</td><td>Minimum of window plus small allowed</td>

<td> 29</td><td>largest window max</td><td>Maximum window plus large allowed</td>

<td> 30</td><td>baseline sep</td><td>Nominal amount to allow DA window</td>

<td> 31</td><td>sep weight</td><td>Weighting factor for separation uncertainty DA</td>

<td> 32</td><td>meas trk weight</td><td>Weighting factor for track uncertainty measurement</td>

<td> 33</td><td>meas vel weight</td><td>Weighting factor for measurement track speed</td>

<td> 34</td><td>upper weight</td><td>Weighting factor yes top antenna</td>

<td> 35</td><td>lower weight</td><td>Weighting factor yes lower antenna</td>

<td> 36</td><td>wide window weight</td><td>Weighting factor to determine slightly wider window</td>

<td> 37</td><td>status highest conf</td><td>Level at which state confidence is highest</td>

<td> 38</td><td>status high conf</td><td>Level to which confidence state is high</td>

<td> 39</td><td>normal window pts</td><td>Set points to sell</td>

ES 2 232 631 T3

<td></td><td></td><td>so normal</td>

<td> 40</td><td>wide window pts</td><td>Adjustment points for wide window</td>

<td> 41</td><td>status highest pts</td><td>Set points for higher confidence</td>

<td> 42</td><td>status high pts</td><td>Fit tips for high confidence</td>

<td> 43</td><td>limit pts</td><td>Points to adjust</td>

TABLE 2

Jump types and associated arrival angle measurement event

<td>Kind of jump</td><td>Measurement event associated with jump type indicated</td>

<td rowspan="2">Same</td><td>High and medium indicate a DA jump in the same</td>

<td>direction</td>

<td>Contrary</td><td>High and medium indicate a DA jump in the opposite direction</td>

<td>High</td><td>High indicates a DA jump and no medium is available for comparison</td>

<td>Half</td><td>Medium indicates a DA jump and no high is available for comparison</td>

<td>High noise</td><td>High indicates a DA jump and not medium. it implies a high noise measurement</td>

<td>None</td><td>Medium indicates a DA jump and not high. it implies an average noise measurement</td>

<td>No data</td><td>No data available for high or medium</td>

Referring now to Figure 4, an illustration of multipath returns is provided. Signals between an aircraft 80 and antennas 26a-26d can follow two or more propagation paths between respective antennas. The first path is along direct line of sight 90 and the other paths are reflections from the ground, hills, buildings, vehicles, aircraft, or other objects, depending on the objects' conductivity, size, orientation, and direction. angle of incidence of the signal. Reflections from objects that are very close to the direct path and allow the reflected signal to arrive very soon after the direct path signal are commonly called short path multipaths 92. Reflections from objects in addition to the direct path can produce what commonly referred to as long-haul multipath. Short path multipaths 92 can impact the accuracy of the AOA measurement, since the carrier phase of the signal striking the AOA antennas is the vector sum of the direct and multipath components. Other references to the short path multipath variety will be referred to herein as multipath 92.

Multipath errors in the response signals are corrected by entering a look-up table with an estimated initial position of the aircraft, which returns with the calibrated phase deviation to compensate for expected errors induced in each of the low, medium resolution channels. and high depending on the position of the plane

ES 2 232 631 T3 in range, azimuth and / or elevation. Of course, those skilled in the art will appreciate that selecting various corrections from the look-up table will require a certain amount of interpolation, thresholding, or other intermediate selection techniques to determine correction values ​​for positions between calibrated positions. Indeed, transponder diversity antenna switching (as discussed above) is more reliably detected with the multipath calibration correction applied to AOA measurements prior to employing the DA algorithm.

Remember that multiple antennas 26a-26d are arranged together to form a network 26 desirably provides multiple obvious apertures, hence resolutions, for analysis. Therefore, interleaving between various channels is desirable depending on the phase of the approximation and the confidence of the position estimate.

Initially, the antenna interleaving is initially set to the low setting according to Table 3. The low setting of the interleaving speeds provides sufficient low resolution measurements to establish a reliable estimate of the aircraft position. The low resolution channel provides a wider beamwidth than the medium or high channels and is used to select from the ambiguous cycles available on the medium and high channels. An alternative embodiment of the invention uses the Mode C response of the transponder to resolve cycle ambiguity for the medium and high channels, instead of using the low channel to resolve cycle ambiguity. During the aircraft approach to the runway the processing algorithm determines the antenna interleaving for subsequent interrogations based on the position of the aircraft with respect to the desired angle of the approach path and the desired accuracy of the aircraft position estimate, and then it sets the antenna interleaving to the low, high, or close settings. For optimal antenna diversity and offset detection, the close-up configuration is the most desirable because it provides the maximum opportunity to compare the following medium and high channel measurements. An alternative embodiment of the antenna configuration includes programmable control of a phased array to select the apparent aperture.

TABLE 3

AOA antenna interleaving rates

<td>Update n °</td><td colspan="2">Low setting</td><td colspan="2">High setting</td><td colspan="2">Approach settings</td>

<td></td><td>C. A</td><td>C. C</td><td>C. A</td><td>C. C</td><td>C. A</td><td>C. C</td>

<td> 1</td><td>Under</td><td>Under</td><td>High</td><td>High</td><td>High</td><td>High</td>

<td> 2</td><td>High</td><td>High</td><td>Half</td><td>Half</td><td>Half</td><td>Half</td>

<td> 3</td><td>High</td><td>Under</td><td>High</td><td>Half</td><td>High</td><td>Half</td>

<td> 4</td><td>Half</td><td>Half</td><td>Under</td><td>Under</td><td>High</td><td>Half</td>

<td> 5</td><td>Half</td><td>Under</td><td>High</td><td>Half</td><td>High</td><td>Half</td>

<td> 6</td><td>High</td><td>Under</td><td>High</td><td>Under</td><td>High</td><td>Half</td>

<td> 7</td><td>High</td><td>Half</td><td>High</td><td>Half</td><td>High</td><td>Half</td>

<td> 8</td><td>Half</td><td>Under</td><td>High</td><td>Under</td><td>High</td><td>Half</td>

<td> 9</td><td>High</td><td>Under</td><td>High</td><td>Half</td><td>High</td><td>Half</td>

<td> 10</td><td>High</td><td>Half</td><td>High</td><td>Under</td><td>High</td><td>Half</td>

<td> 11</td><td>Half</td><td>Under</td><td>High</td><td>Half</td><td>High</td><td>Half</td>

<td> 12</td><td>High</td><td>Under</td><td>High</td><td>Under</td><td>High</td><td>Half</td>

<td> 13</td><td>High</td><td>Half</td><td>High</td><td>Half</td><td>High</td><td>Half</td>

<td> 14</td><td>Half</td><td>Under</td><td>High</td><td>Under</td><td>High</td><td>Half</td>

<td> 15</td><td>High</td><td>Under</td><td>High</td><td>Half</td><td>High</td><td>Half</td>

ES 2 232 631 T3

TABLE 3 (continued)

<td>Update n °</td><td colspan="2">Low setting</td><td colspan="2">High setting</td><td colspan="2">Approach settings</td>

<td></td><td>C. A</td><td>C. C</td><td>C. A</td><td>C. C</td><td>C. A</td><td>C. C</td>

<td> 16</td><td>High</td><td>Half</td><td>High</td><td>Under</td><td>High</td><td>Half</td>

<td> 17</td><td>Half</td><td>Under</td><td>High</td><td>Half</td><td>High</td><td>Half</td>

<td> 18</td><td>High</td><td>Under</td><td>High</td><td>Under</td><td>High</td><td>Half</td>

<td> 19</td><td>High</td><td>Half</td><td>High</td><td>Half</td><td>High</td><td>Half</td>

<td> 20</td><td>Half</td><td>Under</td><td>High</td><td>Under</td><td>High</td><td>Half</td>

Referring now to Figure 5, a general flow chart of the steps that properly practice the present invention is illustrated. An aircraft antenna 86 transmits a response signal 90 that is received on a ground-based antenna array 26, as seen in step 100. The received signals are sent to processors 78 at base 20 to generate a position estimate from the time of arrival and angle of arrival of the response signal 90, as seen in step 104. Central processors 78 at the base 20 then make corrections to the home position estimate to account for multipath returns of the response signals 90, as seen in step 108. After multipath correction, central processors 78, executing a DA algorithm employing antenna interleaving over successive updates, determine if the approaching aircraft 80 responds via diversity antennas 86_L, 86_or and applies a correction to the position estimate based on said determination, as seen in step 110.

Central processors 78 calculate a position error by comparing the adjusted position of the aircraft with a desired position, such as an approach path, as seen in step 114. Those skilled in the art can appreciate that any type of path can be employed. approach consisting of a plurality of interconnected positions that can be compared to a desired position of the aircraft. In other words, unlike conventional straight approaches now used at many airports, approach corridors can be defined that avoid noise sensitive areas, and terrain features, and / or circumnavigation areas where air traffic is undesirable. Processors 78 convert the determined position to a user-usable format, such as an air traffic controller or airplane pilot, as seen in step 118.

The invention has been described with reference to preferred embodiments. Modifications and alterations will obviously be thought by other people after reading and understanding the detailed description above. The invention is intended to be construed to include such modifications and alterations insofar as they fall within the scope of the appended claims.