# Patent

## Title
Traffic alert and collision avoidance system enhanced surveillance system and method

## Abstract

A traffic alert and collision and avoidance system (TCAS) is disclosed. The TCAS comprises a first transponder and a first interrogator associated with a first aircraft. The first transponder sends an interrogation request. The TCAS also comprises a second transponder associated with a second aircraft. The second transponder responds to the interrogation request. The response comprises enhanced surveillance data that was asked for in the request.

## Background
BACKGROUND
Conventionally, avionics, engineers and certification bodies are improving and providing pilots and flight crews with information necessary to identify in-flight traffic. An approach has been to provide the flight crew with information on the location of other aircraft in the area. One exemplary system which is capable of providing this information, is known as a traffic alert and collision avoidance system (TCAS). The systems are required for all airliners flying the United States air space today. TCAS devices have been designed to interrogate transponders of other aircrafts, sometimes referred to as intruder aircraft. The TCAS system evaluates the threat of collision with other aircraft and coordinates an avoidance maneuver for the aircraft.
Recently, the aviation industry has worked to define a new standard of performance for Mode S transponders which are the communication devices used in most TCAS systems. The extension of the function of Mode S transponders is to provide more information about the state and plans of aircraft which are under surveillance. This extended information may collectively be called “enhanced surveillance”. The feature is designed to provide ground-based aircraft control and additional information to enable improved management of air traffic. Although this information is provided from Mode S transponders to ground-based air traffic control, it may be desirable and useful for a first aircraft to be able to interrogate a Mode S transponder and receive enhanced surveillance information from a Mode S transponder on a second aircraft.
Accordingly, there is a need for a system and method in which the first aircraft may request and receive enhanced surveillance information from other airborne Mode S transponders. Further, there is a need for a system and method in which the first aircraft may request and receive enhanced surveillance information from a second aircraft Mode S transponder and provide integrity checking of the position of the aircraft. Further still, there is a need for a airborne system onboard a first aircraft which may interrogate a second aircraft's Mode S transponder at a variable rate, the rate dependent upon the determined dynamics of particular requested data.
It would be desirable to provide a system and/or method that provides one or more of these or other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the aforementioned needs.
SUMMARY
What is provided is a traffic alert and collision and avoidance system (TCAS). The TCAS comprises a first interrogator associated with a first aircraft, the first transponder sending an interrogation request. The TCAS system also comprises a second transponder associated with a second aircraft, the second transponder responding to the interrogation request. The response comprises enhanced surveillance data that was asked for in the request.
What is also provided is a method of providing air traffic information from a first aircraft to a second aircraft. The method comprises receiving an interrogation request from a first aircraft. The request includes a request for enhanced surveillance data. The method also comprises responding by a second aircraft transponder a message comprising enhanced surveillance data that was asked for in the request.
Further, what is provided is a method of integrity checking the position of a second aircraft. The method comprises interrogating an aircraft transponder including a request for enhanced surveillance data including the position of the second aircraft. The method also comprises receiving a reply to the interrogation request by the first aircraft. Further, the method comprises calculating the position of the second aircraft based on the message response time and its bearing. Further still the method comprises comparing the calculated position with the received position. Yet further still, the method comprises determining the integrity of the received position.
Further still, what is provided is a method of interrogating the traffic alert and collision avoidance system transponder. The method comprises interrogating an aircraft transponder including a request for enhanced surveillance data. The method also comprises determining based on the enhanced surveillance data received, whether the rate of interrogation should be changed. Further, the method comprises changing the rate of interrogation if it is determined that the rate should be changed.
Alternative examples and other exemplary embodiments relate to other features and combinations of features as may be generally recited in the claims.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will become more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like elements, in which:
FIG. 1 is an exemplary block diagram of two aircraft, each having transponders and one aircraft providing enhanced surveillance data to the other aircraft in response to a request.
FIG. 2 is an exemplary embodiment of the data registers for a second aircraft transponder in which enhanced surveillance data is available.
FIG. 3 is an exemplary embodiment of a process which retrieves requested enhanced surveillance data from a second aircraft transponder by a first aircraft.
FIG. 4 is an exemplary embodiment of a process in which the integrity of a communicated aircraft's position is checked by a first aircraft using enhanced surveillance data.
FIG. 5 is an exemplary embodiment of a variable rate interrogation transponder.



DETAILED DESCRIPTION OF PREFERRED AND EXEMPLARY EMBODIMENTS
Before describing in detail the particular improved system and method, it should be observed that the invention includes, but is not limited to a novel structural combination of conventional data/signal processing elements and communication circuits, and not in the particular detailed configurations thereof. Accordingly, the structure, methods, functions, control and arrangement of conventional components and circuits have, for the most part, been illustrated in the drawings by readily understandable block representations and schematic diagrams, in order not to obscure the disclosure with the structural details which will be readily apparent to those skilled in the art, having the benefit of the description herein. Further, the invention is not limited to the particular embodiments depicted in the exemplary diagrams, but should be construed in accordance with the language in the claims.
Referring now to FIG. 1, a first aircraft 100 is depicted. First aircraft 100 may be any of a variety of aircrafts including but not limited to airplanes and helicopters. Aircraft 100 may have an onboard TCAS system 110 which may include a transponder 120 and an interrogator. As depicted in FIG. 1, the interrogator may be a part of TCAS block 110 or may alternatively be a separate interrogator unit. Transponder 120 may be a Mode S transponder or other type of applicable transponder. Similarly, a second aircraft 130 is in the vicinity of aircraft 100. Aircraft 130 also has a TCAS system 150 including a transponder 140 which may be a Mode S or other transponder. In operation, as aircraft 130 comes into the local vicinity of aircraft 100, aircraft 100 transmits an interrogation signal 160 to aircraft 130. Transponder 140 interprets the interrogation signal and provides the requested information over the air to aircraft 100. In an exemplary embodiment, the requested information includes enhanced surveillance information, that is information which is beyond the typical position and identification information used in conventional TCAS systems.
Table 1 is an exemplary embodiment of the registers identified in a Mode S transponder and the register parameter contents that may be extracted from the transponder by an interrogator according to the ground-initiated COMM-B protocol (GICB). The information shown corresponds with enhanced surveillance data.







TABLE 1







Register Allocation and Parameters as per ICAO Annex 10,


Volume III, Chapter 5, Appendix.









GICB (BDS)

Register Parameter


Register #
Register Name
Contents





0.5
Extended Squitter
Format Type Code



Airborne Position
Surveillance Status




Single Antenna Flag Bit




Altitude




Time Tag Bit




Compact Position




Reporting Format Bit




Encoded Latitude




Encoded Longitude


0.6
Extended Squitter
Format Type Code



Surface Position
Movement




Ground Track Status Bit




Ground Track (True)




Time Tag Bit




Compact Position




Reporting Format Bit




Encoded Latitude




Encoded Longitude


0.7
Extended Squitter Status
Transmission Rate Sub-




field (TRS)




Altitude Type Subfield




(ATS)


0.8
Extended Squitter Air-
Format Type Code



craft Identification and
Aircraft Category



Category
Aircraft Identification




Character 1 through 8


0.9
Extended Squitter Airborne
Format Type Code



Velocity - Subtype 1 and 2
Subtype



(Velocity over Ground)
Intent Change Flag Bit




IFR Capability Flag Bit




Navigation Uncertainty




Category - Velocity




East/West Velocity




North/South Velocity




Vertical Rate




GNSS Altitude




Difference from




Barometric Alt.


0.9
Extended Squitter Airborne
Format Type Code



Velocity - Subtype 3 and 4
Subtype



(Airspeed and Heading)
Intent Change Flag Bit




IFR Capability Flag Bit




Navigation Uncertainty




Category - Velocity




Magnetic Heading




Airspeed (True or




Indicated)




Vertical Rate




GNSS Altitude




Difference from




Barometric Alt.


0.A
Extended Squitter Event
Used to multiplex



Driven Messages
transmission of messages




contained in registers




6.1–through- 6.f


0.B
Aircraft State Information 1
True Airspeed



(Aircraft State)
Heading




True Track Angle




Ground Speed


0.C
Aircraft State Information 2
Level Off Altitude



(Aircraft Intent)
Next Course (True




Ground Track)




Time to Next Waypoint




Vertical Velocity




Roll Angle


0.D–0.E
Reserved for Air/Air State



Information


0.F
Reserved for ACAS


1.0
Data Link Capability
Various Datalink and



Report
TCAS Capability




Annunciations


1.7
Common Usage GICB
Used to declare common



Capability Report
usage GICB register




services currently being




supported


1.8–through- 1.C
Mode-S Specific Services
Used to declare GICB



GICB Capability Reports
services that are installed


1.D–through- 1.F
Mode-S Specific Services
Used to indicate Modes



MSP Capability Reports
S Specific Protocol




services that are installed




and require servicing.


2.0
Aircraft Identification
Aircraft Identification




Character 1–through- 8


2.1
Aircraft and Airline
Aircraft Registration



Registration
Number, Character 1–




through- 7




ICAO Airline Registra-




tion Marking, Character




1 and 2


2.2
Antenna Positions
Antenna 1 Position




Antenna 2 Position




Antenna 3 Position




Antenna 4 Position


2.5
Aircraft Type
Aircraft Type




Number of Engines




Engine Type




Model Designation,




Character 1–through- 5




Wake Turbulence




Category


3.0
ACAS Active Resolution
Active Resolution



Advisory
Advisories




Resolution Advisory




Complement Record




Resolution Advisory




Terminated Bit




Multiple Threat




Encounter Bit




Threat-Type Indicator




Threat Identity Data


4.0
Selected Vertical Intention
MCP/FCU Selected




Altitude




FMS Selected Altitude




Barometric Pressure




Setting - minus 800 mb




Status of MCP/FCU




Mode Bits




VNAV Mode Bit




Altitude Hold Bit




Approach Mode Bit




Status of Target Altitude




Source Bits




Target Altitude Source




Bits (2)


4.1
Next Waypoint Details -
Characters 1–through- 9



Identity


4.2
Next Waypoint Details -
Waypoint Latitude



Position
Waypoint Longitude




Waypoint Crossing




Altitude


4.3
Next Waypoint Details -
Bearing to Waypoint



Nav
Time-to-Go




Distance-to-Go


4.4
Meteorological Routine Air
FOM/Source



Report
Wind Speed




Wind Direction (True)




Static Air Temperature




Average Static Pressure




Turbulence




Humidity


4.5
Meteorological Hazard
Turbulence



Report
Wind Shear




Microburst




Icing




Wake Vortex




Static Air Temperature




Average Static Pressure




Radio Height


4.6
Reserved for Flight



Management System



Mode 1


4.7
Reserved for Flight



Management System



Mode 2


4.8
VHF Channel Report
VHF 1




VHF 1 Audio Status




VHF 2




VHF Audio Status




VHF 3




VHF 3 Audio Status


5.0
Track and Turn Report
Roll Angle




True Track Angle




Ground Speed




Track Angle Rate




True Airspeed


5.1
Position Report - Course
Latitude - Coarse




Longitude - Coarse




Pressure Altitude


5.2
Position Report - Fine
Latitude - Fine




Longitude - Fine




Pressure Altitude or




GNSS Height (HAE)


5.3
Air Referenced State
Magnetic Heading



Vector
Indicated Airspeed (IAS)




Mach Number




True Airspeed (TAS)




Altitude Rate


5.4
Waypoint 1 Information
Status




Waypoint Identifier,




Character 1–through- 5




Estimated Time of




Arrival (Normal Flight)




Estimated Flight Level




(Normal Flight)




Time-to-Go (Direct




Route)


5.5
Waypoint 2 Information
Status




Waypoint Identifier,




Character 1–through- 5




Estimated Time of




Arrival (Normal Flight)




Estimated Flight Level




(Normal Flight)




Time-to-Go (Direct




Route)


5.6
Waypoint 3 Information
Status




Waypoint Identifier,




Character 1–through- 5




Estimated Time of




Arrival (Normal Flight)




Estimated Flight Level




(Normal Flight)




Time-to-Go (Direct




Route)


5.F
Quasi-Static Parameter
Selected Altitude



Monitoring
Monitor_2 bits




Selected Heading




Monitor_2 bits




Selected Mach Number




Monitor_2 bits




Selected Altitude Rate




Monitor_2 bits




Next Waypoint




Monitor_2 bits




FMS Horizontal Mode




Monitor_2 bits




VHF Channel Report




Monitor_2 bits




Meteorological Hazards




Monitor_2 bits




Target Altitude




Monitor_2 bits


6.0
Heading and Speed
Magnetic Heading



Report
Indicated Airspeed




Mach




Barometric Altitude Rate




Inertial Vertical Velocity


6.1
Emergency/Priority Status
Format Type Code = 28




Subtype Code = 1




Emergency/Priority




Status


6.2
Reserved for Trajectory



Change Point


6.3
Reserved for Trajectory



Change Point


6.4
Reserved for Aircraft



Operational Coordination



message


6.5
Reserved for Aircraft



Operational Status


6.6–6.F
Reserved for Extended



Squitter


7.0–7.5
Reserved for Future



Aircraft Downlink



Parameters


E.1–E.2
Reserved for Mode-S Byte


F.1
Reserved for Military



Applications


F.2
Reserved for Military



Applications









Referring to FIG. 2, aircraft 200 which includes TCAS system 220, including a transponder 215 and an interrogator 210 sends an interrogation signal 230 to a transponder 235 which has an interrogation interpreter and a plurality of data registers 245 including, but not limited to registers having altitude information 250, identification information 260, heading information 270, roll attitude information 280, pitch attitude information 290, and trajectory information 295. Registers 235 are simply illustrative and are not limited by the types of information depicted. As interrogation interpreter receives the interrogation signal 230, interrogation interpreter 240 determines what data registers should be reported in a reply to aircraft 200's request. For example, in the figure, heading information 270 is reported via a communication 275. Pitch attitude 290 is reported in a communication 285, and trajectory information is communicated in a communication 298 back to interrogator transponder 210. Accordingly, the benefits of enhanced surveillance to airborne interrogators is extended. Currently, only ground-based air traffic control interrogators may utilize this capability. An airborne interrogator makes the benefits of enhanced surveillance available to aircraft. In one exemplary embodiment, the airborne interrogator may be a TCAS. In current applications, TCAS simply interrogates the transponder of another intruder aircraft to obtain its range and altitude. Given the small amount of information, the TCAS system decides if the intruder is a collision threat. Utilizing enhanced surveillance information, a TCAS can make interrogation of the intruder's transponder. This provides TCAS with significantly more information (enhanced surveillance data) than range and altitude. This includes items like intruder's aircraft's heading, pitch and roll angles, the aircraft's velocities and position, and many other items including intended flight path trajectories (see Table 1).
As an example, an operational enhancement that may occur using enhanced surveillance data is an opportunity to avoid collisions and/or reduce the number of false alarms by utilizing information of the intended trajectory of a second aircraft. In past applications of TCAS, the inability to obtain the intended trajectory information has been a significant limitation to TCAS applications.
TCAS must provide decisions to avoid collisions with enough warning to allow the pilot of the aircraft to react. The period is typically 30 to 45 seconds. Without intent information, TCAS may only be used to project the trajectory of the intruder based on the past behavior of that aircraft. If the intruder does not intend to continue “doing what it's been doing”, this projection may be in error. This may result in unnecessary alarms or alarms that come without comfortable warning. In the case that enhanced surveillance information is available to the aircraft and the intended trajectory is known, alarms can be deferred without the potential for a collision. One such situation, for example, is called a “bump up.” This occurs when a first aircraft is level at a given altitude and an intruder aircraft is climbing to level off at an altitude 1,000 feet below the first aircraft. In today's TCAS systems, TCAS does not know that the intruder intends to level off and issues an alarm, advising the pilot of the first aircraft to climb to avoid the intruder. However, the alarm, in this case, is unnecessary and could be avoided if TCAS had known that the intruder was going to level off.
The implementation of the system utilizes existing hardware on aircraft. TCAS interrogates Mode S transponders for the purpose of obtaining range and altitude. Enhanced surveillance transponders have the capability to respond to an interrogation requesting the enhanced information discussed above. The Mode S transponder responds with the enhanced surveillance information requested by an interrogator by processing the content of the interrogation. TCAS needs only to change the data content of its existing interrogation to request the data information in this way. Only minor changes to the content of the TCAS interrogation and no changes to the enhanced surveillance Mode S transponder are necessary to make the new information available for use by TCAS. The airborne interrogator may use the ground-initiated comm-B protocol (GICB) to extract the information from other aircraft. The protocol has already been standardized internationally in the Mode S minimum operational performance standard (ICAO annex 10).
Referring now to FIG. 3, an exemplary process of the interrogation for enhanced surveillance information is depicted. Process 300 begins with the information determined (step 310). The first aircraft interrogator then interrogates the second aircraft's transponder (step 320). The second aircraft's transponder interprets the interrogation request (step 330). The second aircraft's transponder retrieves the requested information (step 340). The second aircraft's transponder then transmits the requested information via the transponder to the first aircraft (step 350).
As a result of the use of an interrogation to obtain enhanced surveillance information from an intruder aircraft, a further benefit may be obtained. If the information requested is the other aircraft's position based on its on-board navigation system (such as a GPS system or other on-board navigation systems), the result may be verified by comparing it to its own aircraft's position. The difference in position should agree within some tolerance level with the range and bearing measured directly as a result of the interrogation. This check provides a level of integrity to the position information that cannot be achieved by a system where the position of the other aircraft is merely broadcast.
An exemplary process in which integrity of another aircraft's position is checked by a first aircraft is depicted by process 400 of FIG. 4. A second aircraft's transponder is interrogated, including a request for position information (step 410). A reply is received from the second aircraft by the first aircraft (step 420). The range and bearing are determined based on the time travel of the reply and request (step 430). Position based on the range and bearing are then calculated (step 440). A comparison is then made of the communicated position with the calculated position (step 450). At that stage then a determination of whether the integrity of the broadcast position has been compromised is made.
Additionally, a system where interrogation is used to obtain enhanced surveillance data from another aircraft provides benefits in that the interrogator may vary its rate of interrogation to satisfy the needs of a dynamic situation. For example, if the data is changing fast and the situation demands attention, the interrogator can increase the rate of interrogation, allowing the data to closely follow the situation of the other aircraft. If the situation is changing slowly and does not demand close attention, the interrogator can reduce its interrogation rate to minimize channel loading. A further benefit of the interrogation system may be that if the data is not received the first time, a retry can occur to insure the information is obtained.
Having the enhanced surveillance data available to an aircraft may allow an aircraft to maintain spacing with another aircraft very precisely in addition to all of the other benefits described above with regards to the communication of enhanced surveillance data.
While the detailed drawings, specific examples, and particular formulations given describe preferred and exemplary embodiments, they serve the purpose of illustration only. The inventions disclosed are not limited to the specific forms shown. For example, the methods may be performed in any of a variety of sequence of steps. The hardware and software configurations shown and described may differ depending on the chosen performance characteristics and physical characteristics of the computing and communication devices. For example, the type of computing and communications devices, communications bus, or processor used may differ. The systems and methods depicted and described are not limited to the precise details and conditions disclosed. Furthermore, other substitutions, modifications, changes, and omissions may be made in the design, operating conditions and arrangement of the exemplary embodiments without departing from the scope of the invention as expressed in the appended claims.

## Claims

1. Traffic alert and collision avoidance system (TCAS), comprising:
first interrogator positioned onboard a first aircraft, the first interrogator sending an interrogation;
second transponder positioned onboard a second aircraft, the second transponder responding to the interrogation request, the response comprising enhanced surveillance data that was asked for in the request.

2. The system of claim 1, wherein the first interrogator is a TCAS.

3. The system of claim 1, wherein the second transponder is a Mode S transponder.

4. The system of claim 1, wherein the interrogation request requests aircraft intent information.

5. The system of claim 1, wherein the enhanced surveillance data received allows the first aircraft to maintain a precise spacing with the second aircraft.

6. A method of providing air traffic information from a first aircraft to a second aircraft, comprising:
receiving an interrogation request from a first aircraft transponder, the request including a request for enhanced surveillance data; and
responding by a second aircraft transponder a message comprising enhanced surveillance data, that was asked for in the request.

7. The method of claim 6, wherein the enhanced surveillance data comprises aircraft intent.

8. The method of claim 6, further comprising:
providing an alert to the pilot of the first aircraft based on the received enhanced surveillance information.

9. The method of claim 6, further comprising:
generating an aircraft maneuver command for the first aircraft based on the received enhanced surveillance information.

10. The method of claim 6, wherein the fist aircraft transponder is a Mode S transponder.

11. The method of claim 6, wherein the second aircraft transponder is a Mode S transponder.

12. A method of integrity checking the position of a second aircraft, comprising:
interrogating an aircraft transponder onboard the second aircraft including a request for enhanced surveillance data including the position of the second aircraft;
receiving a reply to the interrogation request by a first aircraft;
calculating the position of the second aircraft based on the message response time and bearing;
comparing the calculated position with the received position; and
determining the integrity of the received position.

13. The method of claim 12, wherein the position of the second aircraft is derived from a global positioning system (GPS) receiver.

14. The method of claim 12, wherein the position of the second aircraft is derived from an on-board navigation system of the second aircraft.

15. The method of claim 12, further comprising:
providing an alert to a crew member of the first aircraft that the integrity of the received position has been compromised.

16. A method of interrogating a traffic alert and collision avoidance system transponder, comprising:
interrogating an aircraft transponder including a request for enhanced surveillance data;
determining based on the enhanced surveillance data received, whether the rate of interrogation should be changed; and
changing the rate of interrogation if it is determined that the rate should be changed.

17. The method of claim 16, further comprising:
retrying the interrogation if a reply is not received.

18. The method of claim 16, further comprising:
retrying the interrogation if a reply is not correct.

19. The method of claim 16, wherein the rate is changed to less than every 30 seconds.

20. The method of claim 16, further comprising:
providing an alert if the interrogation rate crosses a predetermined threshold.