Communications: directive radio wave systems and devices (e.g. – Aircraft collision avoidance system
Reexamination Certificate
2000-07-27
2003-05-13
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Aircraft collision avoidance system
C342S030000, C342S032000, C342S046000, C701S301000
Reexamination Certificate
active
06563453
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a Traffic Alert and Collision Avoidance System (TCAS) used in aircraft, and more particularly to an improved system and method for the TCAS units of locally clustered TCAS-equipped aircraft to transmit a signal, specifically a TCAS Broadcast Interrogation Message (Uplink Format
16
, or UF-
16
). This message provides identification information to other aircraft with collision avoidance systems in the area.
2. Description of the Related Art
A Traffic Alert and Collision Avoidance System provides a flight crew with information necessary for safe flight. More specifically, a TCAS provides the flight crew with information on the location of other aircraft in the vicinity, thereby actively preventing a collision by alerting the flight crew to the threat of a collision with another aircraft in time for evasive action.
In operation, a TCAS interrogates the transponders of other aircraft, sometimes referred to as “intruder” aircraft, and the transponders reply with information about the location of the intruder aircraft. (The aircraft on which the TCAS is installed is referred to as the “own” aircraft.) The TCAS then evaluates the threat of collision with the intruder aircraft and may provide a suggested vertical maneuver for the own aircraft.
TCAS systems operate in conjunction with Mode A, Mode C, and Mode S type transponders. When a TCAS-equipped aircraft, i.e., the own aircraft, approaches an intruder aircraft, the own aircraft determines the location of the intruder aircraft by interrogating the intruder's transponder every second. If the intruder aircraft is equipped with a Mode A transponder, the transponder receives an interrogation signal and responds with a reply signal which allows the own aircraft to calculate the range and bearing of the intruder aircraft based on the reply time and direction of the signal. If the intruder aircraft has a Mode C transponder, the reply also includes altitude information.
On the other hand, if the intruder aircraft is equipped with a Mode S transponder, the method of interrogation by the own aircraft is slightly different. Mode S transponders differ from Mode A and C transponders in that a Mode S transponder transmits a spontaneous transmission (known as a squitter) once every second, even when no interrogation signal is received, i.e., the Mode S transponder transmits an unsolicited reply. Furthermore, the Mode S unsolicited reply contains the Mode S “address” (assigned by aviation authorities) of the transmitting aircraft. Once a squitter is received, the TCAS-equipped aircraft sends a direct interrogation to the intruder aircraft, based on the known address of the intruder aircraft. The interrogation is transmitted in Uplink Format 0 (UF-0). The message is received by the transponder of the intruder aircraft, which transmits a reply signal in Downlink Format 0 (DF-0). The DF-0 reply contains information about the intruder aircraft. For example, the DF-0 reply contains information about the capability of TCAS on the intruder aircraft. The Mode S transponder of the intruder aircraft then responds with its reply signal, which includes altitude information.
Regardless of the type of transponder contained in the interrogated target aircraft, the TCAS equipped aircraft uses the received signal to determine whether the flight path of the target aircraft is a potential threat. Depending upon the complexity of the TCAS equipment, the system either generates a warning signal to the pilot indicating the existence of a potential collision, or generates a warning signal and a conflict resolution advisory, which advises the pilot of vertical maneuvers necessary to avoid a collision.
A conventional TCAS currently transmits a TCAS Broadcast Interrogation Message with a Mode S Address (MID) Subfield set to the Mode S address assigned to the own aircraft, and these Broadcast Interrogation Messages are received by other TCAS-equipped aircraft in the area. Thus, the Mode S address contained in the MID Subfield of the TCAS Broadcast Interrogation Message is the same Mode S address used in the reply is message of a Mode S transponder. Furthermore, TCAS-equipped aircraft that transmit a TCAS Broadcast Interrogation Message and have onboard Mode S transponders are tracked by other TCAS-equipped aircraft through Mode S surveillance.
A TCAS also keeps track of other TCAS-equipped aircraft in the area (nominally 30 nautical miles) and maintains a count of the unique TCAS Broadcast Interrogation Messages received within the past 20 seconds. Using this information, it determines the number of TCAS-equipped aircraft (NTA) in the area. In a process known as Interference Limiting, the TCAS reduces the Radio Frequency (RF) emissions when necessary due to the number of TCAS-equipped aircraft in the area.
Change 7 to TCAS, required by several foreign countries, has been implemented as TCAS II, Version 7 (known as Airborne Collision Avoidance System, or ACAS II, internationally). ACAS II includes the additional concepts of NTA3 and NTA6 for the purpose of Interference Limiting. NTA3 and NTA6 refer to the number of TCAS-equipped aircraft within three and six nautical miles of own aircraft, respectively, and ACAS II reduces the RF emissions when necessary due to the number of TCAS-equipped aircraft within three and six nautical miles.
To determine NTA3 and NTA6, ACAS II associates the MID Subfields of received TCAS Broadcast Interrogation Messages with the addresses of aircraft that the ACAS II currently has under Mode S surveillance. In other words, ACAS II uses Mode S addresses to determine the range to TCAS-equipped aircraft in the area.
However, a problem occurs with ACAS II when TCAS-equipped aircraft (transmitting TCAS Broadcast Interrogation Messages) are locally “clustered” in a manner not anticipated during the development of ACAS II. Such unanticipated local clusters of TCAS-equipped aircraft include helicopters flying in close proximity to one another, such as helicopters covering news and sporting events, and military formations consisting of aircraft equipped with TCAS. These local clusters of TCAS-equipped aircraft may cause the ACAS II Interference Limiting algorithms to unnecessarily reduce RF transmissions, and thus the surveillance range, of nearby ACAS II-equipped aircraft. Clusters involving as few as 3 TCAS-equipped aircraft can significantly reduce the surveillance range of proximate ACAS II-equipped aircraft flying below 18,000 feet MSL (mean sea level). For example, these three-aircraft clusters can cause a six dB reduction in power in ACAS II units within three nautical miles and below 18,000 feet MSL (effectively halving the surveillance range).
Several solutions to this problem have been proposed and subsequently rejected as not technically or politically feasible. For example, one solution is to suppress TCAS/ACAS broadcasts when the TCAS units are clustered together. However, this solution is undesirable because ACAS II-equipped aircraft will not have an accurate count of all TCAS-equipped aircraft in the area, i.e., NTA will be too low. Another solution is to modify ACAS II to accommodate the clustered TCAS units. However, modifications to ACAS II will not be approved by the International Civil Aviation Organization (ICAO) or the Federal Aviation Administration (FAA).
SUMMARY OF THE INVENTION
It is a general object of the present invention to provide a system and method for TCAS Broadcast Interrogation Messages (UF-
16
) that overcome the disadvantages of the conventional systems.
It is another object of the present invention to provide a system and method for locally clustered TCAS-equipped aircraft to transmit TCAS Broadcast Interrogation Messages that do not cause the Interference Limiting algorithms of ACAS II units to unnecessarily restrict their RF emissions and surveillance range.
In accordance with the objects described above, one aspect of the present invention includes a method of a Traffic Alert and Collision Avoidance System that includes the
Honeywell International , Inc.
Honeywell International Inc.
Sotomayor John B.
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