System for processing directional signals

A system for calculating the bearing of a signal source, with a directional antenna, provides corrections for distortion, such as due to a small fuselage of the monitoring aircraft and the elevation angle of an intruder aircraft with respect to the monitoring aircraft. A correction is applied to the bearing estimate that is based on relevant factors, such as the fuselage size and the elevation angle of the intruder aircraft. The correction can be calculated or applied through the use of a look-up table, which may be either pre-selected or selected after calculation of the elevation angle of the intruder aircraft.

BACKGROUND
 1. Field of the Invention
 The present invention generally relates to processing signals received with
 a directional antenna, and more particularly, to identifying the bearing
 of a signal source based on signals received with a directional antenna.
 2. Description of the Related Art
 As technology in air transportation has evolved, the demands on the members
 of the flight deck have become increasingly severe. To avoid flight path
 conflicts, the flight deck crew monitors considerable aircraft status
 information for multiple surrounding aircraft at a time when air traffic
 is dramatically increasing. Higher aircraft speeds magnify the burden by
 reducing the time in which the flight deck crew can respond to threatening
 situations.
 To assist the flight deck crew and enhance safety, several systems have
 been and are being developed. Many aircraft carry transponders (e.g., mode
 S, mode C, mode A) by which one aircraft can communicate to a second
 aircraft both its identity and various flight parameters. Typically, a
 monitoring aircraft transmits a signal in a predetermined format which,
 upon receipt by an intruding aircraft, causes the intruding aircraft to
 respond with a transmission which includes information in a predetermined
 format. Systems generally referred to as traffic alert and collision
 avoidance systems (TCAS) process information received from intruder
 aircraft along with the status parameters of the receiving aircraft to
 identify potential collision situations. A TCAS also typically provides
 the flight deck crew with advisory information suggesting an action to
 avoid the collision situation.
 A TCAS typically includes a directional antenna. The TCAS uses the
 directional antenna to determine the bearing of an intruder aircraft
 relative to the TCAS equipped monitoring aircraft. When receiving signals
 from the intruder aircraft, TCAS processes the signals to calculate an
 estimated bearing for the intruder aircraft, and this information is
 displayed to the flight deck crew to assist them in obtaining visual
 contact with the intruder aircraft.
 One approach used by TCAS systems is to estimate an intruder aircraft's
 bearing by comparing magnitudes of signals received by the components of
 the directional antenna. FIG. 1 illustrates the radiation pattern of
 signals received by a typical directional antenna having four antenna
 elements measured on a test four foot diameter flat ground plane. This
 radiation pattern simulates the performance of the antenna on a large
 transport aircraft, such as an aircraft having a fuselage curvature radius
 greater than 80 inches. The performance of the antenna beams in each of
 the four quadrants representing port, starboard, fore and aft, is
 virtually identical. To estimate the bearing of an intruder aircraft,
 conventional TCAS uses a model based on the radiation pattern measured on
 the test ground plane, such as the radiation pattern illustrated in FIG.
 1. An exemplary model used by a conventional TCAS signal processing scheme
 is illustrated in FIG. 2. When an intruder aircraft is detected, the
 bearing of the intruder aircraft is calculated by determining which beam
 (from the four beams representing each of the four quadrants of the polar
 coordinate system) has the largest amplitude and which beam has the second
 largest amplitude and then taking the difference: between the two. Based
 on this difference, a bearing estimate can be generated using a
 conventional TCAS model, such as the model illustrated in FIG. 2.
 Various factors, however, may degrade the accuracy of the bearing estimate.
 For example, monitoring aircraft having small fuselages may detect
 transponder signals differently than larger aircraft, which may degrade
 the bearing estimate accuracy. Consequently, the model illustrated in FIG.
 2 is not as accurate with aircraft that have small fuselages, such as
 those aircraft with a radius of fuselage curvature smaller than 64 inches.
 The smaller fuselage causes a distortion such that the beam peak in the
 port and starboard directions occurs at a lower elevation angle than the
 beams of the model illustrated in FIG. 2. This displacement may degrade
 the accuracy of the bearing estimate. The degradation tends to be more
 pronounced in aircraft with smaller fuselages than those with a larger
 fuselage. The amount of the error is also dependent on the elevation angle
 of the intruder aircraft.
 SUMMARY OF THE INVENTION
 In accordance with the present invention, a system for calculating a
 bearing of a signal source, such as an intruder aircraft, using an
 antenna, such as a directional antenna having a plurality of receiving
 elements, may include selecting a correction model from a plurality of
 correction models, wherein the correction model selection may be based on
 the fuselage radius of the monitoring aircraft or the estimated elevation
 angle of the intruder aircraft. The monitoring aircraft receives a
 plurality of incoming signals with the receiving elements and processes
 the incoming signals to produce a plurality of electrical connection
 signals, wherein each of the electrical connection signals corresponds to
 a different quadrant of a polar coordinate system and each of the
 electrical connection signals has an amplitude. The system selects the
 electrical connection signals with the strongest amplitude and the second
 strongest amplitude, calculates an amplitude difference between the two
 selected signals and applies a correction model to the amplitude
 difference in order to obtain the bearing of the signal source. The
 correction model may be pre-selected by the supplier or the operator, or
 alternatively, the correction model may be automatically selected by the
 system.
 In accordance with an embodiment of the present invention, the correction
 model may comprise a look-up table.
 The correction model is applied to improve the bearing estimate accuracy of
 the intruder aircraft by, for example, minimizing the distortion caused by
 the curvature of the monitoring aircraft's fuselage.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
 A signal processing system according to various aspects of the present
 invention provides a system for calculating the bearing of a signal
 source, such as an intruder aircraft, which is adjusted according to
 various criteria, such as the size of the fuselage of the monitoring
 aircraft or the relative elevation angle of an intruder aircraft. Although
 various aspects of the invention may be used in conjunction with a variety
 of systems that have a directional antenna with a plurality of receiving
 elements, the present invention is conveniently described below in
 connection with a TCAS. This exemplary implementation, however, should in
 no way be construed to limit the applicability of various aspects of the
 invention in other environments or otherwise limit the claims.
 FIG. 3 is a block diagram of a conventional TCAS comprising a TCAS
 directional antenna 300, a TCAS omnidirectional antenna 302, and a TCAS
 computer unit 305 which includes a receiver 310, a transmitter 320, and a
 processor 330. The TCAS also includes an aural annuciator 340, a traffic
 advisory (TA) display 350, and resolution advisory displays 360. A
 transponder is also shown comprising a transponder unit 370, a control
 panel 380, and transponder antennas 390 and 395. The TCAS and transponder
 operate together to function as a collision avoidance system. The present
 embodiment is merely illustrative of a typical TCAS and many other
 configurations are possible, such as adding a second directional antenna
 or utilizing a transceiver.
 The operations of the TCAS and each component illustrated in FIG. 3 are
 well known and therefore will not be described in detail. A general
 description of TCAS technology, however, is provided in Introduction to
 TCAS II published by the United States Department of
 Transportation--Federal Aviation Administration.
 Referring to FIG. 4, a typical radiation pattern of a TCAS directional
 antenna having four receiving elements is shown in polar coordinates. The
 TCAS directional antenna is mounted on a monitoring aircraft 400. The
 directional antenna of monitoring aircraft 400 receives a plurality of
 incoming signals on its four antenna elements substantially
 simultaneously. The incoming signals are processed inside the antenna to
 produce a plurality of electrical connector signals 505 (shown in FIG. 5),
 such that each electrical signal 505 represents a unique quadrant of the
 polar coordinate system. The electrical connector signals 505 in the
 present embodiment correspond to the four quadrants representing fore 410,
 starboard 420, aft 430, and port 440. FIG. 4 illustrates the signal
 amplitude received by each of the four elements from various angles for
 receiving signals.
 Referring to FIG. 5, the TCAS computer unit 305 calculates an estimated
 intruder aircraft bearing based on the intruder's transponder signal. As
 described above, directional antenna 300, having a plurality of receiving
 elements, receives the incoming transponder signals. Directional antenna
 300 processes the incoming signals and produces electrical connector
 signals 505 that are routed to TCAS computer unit 305. TCAS computer unit
 305 processes signals 505 and produces the bearing estimate of intruder
 aircraft 450 for display on TA display 350.
 More particularly, the signals received by antenna 300 are routed to a
 selector 510. Selector 510 selects the signal with the strongest amplitude
 and the signal with the next strongest amplitude. The selected signals
 from selector 510 are applied to an identifier 520 and to a comparator
 530. Identifier 520 identifies the corresponding quadrants (i.e., fore,
 starboard, aft, or port) for the strongest signal and the second strongest
 signal selected by selector 510. Comparator 530 compares the amplitudes
 for each of the two selected signals and calculates an amplitude
 difference that is the difference between the two amplitude values. The
 quadrant identification and the amplitude difference of the two selected
 signals are supplied to correction system 540, which then suitably
 generates a corrected bearing estimate of intruder aircraft 450. In
 accordance with various aspects of the present invention, correction
 system 540 applies a correction to an initial bearing estimate, which is
 suitably generated in any manner. Typically, the corrected bearing
 estimate of the intruder aircraft (i.e., intruder aircraft icon 360 on TA
 display 350) is then shown relative to the monitoring aircraft (i.e.,
 monitoring aircraft icon 370) on TA display 350.
 As illustrated in FIG. 6, the radiation pattern of a directional antenna
 mounted on an aircraft with a small fuselage, such as a fuselage having a
 radius of curvature of approximately 64 inches or less, has beam peaks of
 different amplitudes for each of the four quadrants for a specified
 elevation angle. The radiation pattern illustrated in FIG. 6 is for an
 elevation angle of 90 degrees (i.e., horizon). As stated previously, the
 rounding of the port and starboard sides of the aircraft fuselage has
 little effect on the fore and aft beams of the directional antenna.
 However, the beam peaks of the port and starboard beams occur at a lower
 elevation angle for an antenna mounted on the fuselage than for an antenna
 mounted on the flat ground plane. Thus, for directional antenna azimuth
 radiation patterns at lower elevation angles (i.e., near or below the
 horizon), the port and starboard beams are stronger than the fore and aft
 beams, creating a radiation pattern similar to that shown in FIG. 6. The
 reciprocal effect is that at higher elevation angles, the fore and aft
 beams are stronger than the port and starboard beams. There is a
 particular elevation angle at which the fore and aft beam peak amplitudes
 are equal to the port and starboard beam peak amplitudes. At this unique
 elevation angle, the antenna radiation approximates FIG. 1 very well, but
 for all other elevation angles, the system based on FIG. 1 inherently
 contains errors. Since the amplitudes of the beam peaks affect the
 calculation of the bearing of the intruding aircraft, the correction
 system 540, according to various aspects of the present invention performs
 a correction that accounts for the different beam peaks.
 In accordance with the present invention, a correction model, such as the
 correction model 700 illustrated in FIG. 7, is applied by the correction
 system 540 to improve the bearing estimate accuracy of the intruder
 aircraft based on, for example, the size of the monitoring aircraft's
 fuselage and/or the elevation angle of the intruder aircraft. The specific
 values of the model suitably vary depending on fuselage size and elevation
 angle, but the application of the model may remain the same.
 In the present embodiment, correction model 700 approximates, in a
 piece-wise linear pattern, the radiation pattern illustrated in FIG. 6.
 The model is divided into four quadrants--fore 410, port 440, aft 430, and
 starboard 420. Each quadrant has a cardinal angle that corresponds to that
 quadrant's position in the polar coordinate system. Fore 410 has a
 cardinal angle 415 of 0 degrees. Port 440 has a cardinal angle 445 of 270
 degrees. Aft 430 has a cardinal angle 435 of 180 degrees and starboard 420
 has a cardinal angle 425 of 90 degrees. In addition, each quadrant has a
 primary crossover spacing 702A, B that varies depending on any relevant
 factors, such as the fuselage size and the elevation angle of the intruder
 aircraft in the present embodiment. The primary crossover spacing for a
 quadrant is the spacing between the crossovers of the beam for the current
 quadrant and the beams for the adjacent quadrants. For example, primary
 crossover spacing 702A for port quadrant 440 illustrated in FIG. 7 is 100
 degrees, and primary crossover spacing 702B for aft quadrant 430 is 80
 degrees. Similarly, the primary crossover spacing for starboard quadrant
 420 is 100 degrees and the primary crossover spacing for fore quadrant 410
 is 80 degrees.
 Each quadrant of the correction model also has a depth of a secondary
 crossover. The depth of the secondary crossover for a quadrant is the
 depth or difference, such as in decibels (dB), between the beam peak of
 the quadrant and the point at which the beams for the two adjacent
 quadrants intersect. For example, the depth of the secondary crossover for
 port quadrant 440 illustrated in FIG. 7 is 13.0 dB, and the depth of aft
 quadrant 430 is 11.0 dB. Similarly, the depth of starboard quadrant 420 is
 13.0 dB, and the depth of fore quadrant 410 is 11.0 dB.
 The correction model 700 applied to the bearing estimate of an intruder
 aircraft, in accordance with various aspects of the present invention,
 uses the cardinal angle, the primary crossover spacing, and the depth of
 the secondary crossover, as well as any other suitable criteria, to
 correct for distortion of the signal. More specifically, in the present
 embodiment, the correction may be applied in accordance with the following
 formula:
 (cardinal angle)+((sign)*(primary crossover spacing/2))-((amplitude
 delta)*(sign)*(ratio))=bearing estimate of intruder aircraft in degrees
 where,
 cardinal angle=cardinal angle, in degrees, of the quadrant containing the
 strongest beam (i.e., highest amplitude), received from the intruder
 aircraft;
 sign=multiplier that determines whether the offset due to the amplitude
 delta is added or subtracted from the cardinal angle as discussed below;
 primary crossover spacing=primary crossover spacing, in degrees, of the
 quadrant containing the strongest beam;
 amplitude delta=difference, in dB, between the amplitudes of the strongest
 beam, and the next strongest beam of the signals received from the
 intruder aircraft; and
 ratio=(primary crossover spacing/2)/(depth of the secondary crossover),
 where the primary crossover spacing and the depth of the secondary
 crossover are for the quadrant containing the strongest beam.
 In accordance with one embodiment of the present invention, the correction
 model can be implemented by a look-up table, such as the look-up table
 shown in FIG. 8 for the correction model illustrated in FIG. 7. The
 look-up table is suitably organized by the quadrant of the strongest beam
 and the quadrant of the next strongest beam. The cardinal angle, primary
 crossover spacing, sign, and ratio may be stored in the look-up table.
 The value for sign is determined in the present embodiment based on an
 initial bearing estimate using the quadrants of the strongest and the next
 strongest beam. The only positions on the antenna pattern where the
 difference between the amplitudes of the strongest and the next strongest
 beam is known occurs at the primary crossover points where the differences
 are zero (e.g., primary crossover points 610, 620, 630 and 640 in FIG. 6).
 The formula uses these points as anchor points. Depending on which half of
 the quadrant in which the intruder is located according to the initially
 estimate bearing, the formula either subtracts a fixed offset (i.e.,
 primary crossover spacing divided by 2) while adding the variable offset
 (i.e., amplitude delta*ratio) or the formula adds the fixed offset while
 subtracting the variable offset.
 The look-up table is suitably pre-calculated to provide cardinal angle,
 sign, primary spacing, and ratio, for various elevation angles and various
 fuselage sizes. The operator of the monitoring aircraft may preselect the
 look-up table, from a plurality of look-up tables, based on the fuselage
 size of the monitoring aircraft. Alternatively, the TCAS may automatically
 select an appropriate look-up table, for example upon entry of the
 aircraft model number. In addition, since air traffic is heaviest at the
 horizon, the supplier or operator may pre-select a look-up table for the
 horizon (i.e., elevation angle of 90 degrees), so that the bearing
 estimate would be more accurate for the greatest number of intruder
 aircraft. Alternatively, the look-up table may be selected for any
 suitable elevation angle. In this embodiment, the correction applied would
 be dependent only on the fuselage size.
 The radiation pattern illustrated in FIG. 6 is for an elevation angle of 90
 degrees (i.e., horizon). The values of the beam peaks, and therefore the
 characteristics of an appropriate correction model 700, tend to differ
 depending on the elevation angle of the intruder aircraft as well as the
 radius of curvature of the monitoring aircraft's fuselage.
 In accordance with another embodiment of the present invention, the
 correction model may be applied by automatically selecting a look-up table
 based on the elevation angle of the intruder aircraft. A plurality of
 look-up tables for different fuselage sizes and for different elevation
 angles may be pre-calculated. The correct look-up table is suitably
 selected after calculating the elevation angle of the intruder aircraft.
 This selection may also be based on the fuselage size of the monitoring
 aircraft. For example, the elevation angle of the intruder aircraft may be
 calculated using altitude information available in the incoming signals
 from the intruder aircraft and the distance to the intruder aircraft. This
 distance may be determined in any manner, such as the duration of the
 delay between a transmitted signal and response from the intruder
 aircraft.
 The present invention has been described above with reference to a
 preferred embodiment. However, those skilled in the art having read this
 disclosure will recognize that changes and modifications may be made to
 the preferred embodiment without departing from the scope of the present
 invention. For example, instead of applying the correction model by a
 look-up table, the correction model could be applied by a mathematical
 formula that varies the correction depending on stored correction values
 for different fuselage sizes and elevation angles. These and other changes
 or modifications are intended to be included within the scope of the
 present invention, as expressed in the following claims.