The present invention relates to a method of aircraft traffic indication, and more particularly, to a method of aircraft traffic surveillance for analysis to avoid potential collision with other aircraft.
The increased demands placed on the aircraft flight deck as a result of more complex technology, ever increasing aircraft traffic, and increased demands for safety has brought about a requirement for monitoring of aircraft traffic in a vicinity of an aircraft that includes automatic identification of potential threats to the monitoring aircraft. As a result, aircraft can have transponders associated therewith that, in response to appropriate electromagnetic interrogation signals induce responding electromagnetic signals that provide information with respect to the range, altitude, and bearing of the interrogated aircraft. Certain traffic control system transponders, e.g., the Mode S system, include target identification as part of the information imposed on the responding electromagnetic signals. For these Mode S systems, the identification of the path or track of the responding aircraft is relatively simple, involving time dependent positions and altitudes of an identified aircraft. Similarly, extrapolations or extensions of aircraft tracks can be relatively simple. The operation of aircraft collision avoidance apparatus is governed by the Radio Technical Commission for Aeronautics (RTCA) Minimum Operational Performance Standards (MOPS) for Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment manual document number RTCA/D0185.
In systems (such as the Air Traffic Control Radar Beacon System, ATCRBS) which do not include unique aircraft identification information, the determination of the aircraft tracks is more complicated. The information obtained by periodic interrogation of a multiplicity of unidentified targets, with associated range, altitude and bearing information being provided or determined as a result of the interrogation, can be subjected to well known algorithms to provide a target aircraft track. Once the track is identified, then the extension thereof can be computed to determine if the target aircraft is a threat to the monitoring aircraft.
The track determination is complicated for several reasons generally involving spurious target images. For example, a monitoring aircraft can transmit an interrogation signal to a target aircraft (sometimes referred to herein as an intruder or intruder aircraft since the target aircraft is "intruding" on the monitoring aircraft's airspace), whereupon a transponder in the target aircraft provides a first response signal (direct reply), the delay between the transmission of the interrogation signal and the reception of the first response signal providing the range information. However, the interrogation signal can result in a second response signal that is reflected from the earth's surface. The second response signal, reflected once from the earth, is generally referred to as a single reflection multipath (or type I) reply. Because the length of time for the travel of the second response signal is longer than that for the first response signal, the second response signal can be interpreted as a separate target aircraft at a greater range from the monitoring aircraft. Type I or single reflection multipath also can be generated by an interrogation which reflects off the ground combined with a direct reply. Since the path length is the same as the previous case, the range is the same. Similarly, an interrogation signal can reflect off the surface of the earth, activate the transponder of the target aircraft which provides a response signal that also reflects off the earth's surface. In this instance, since both the interrogation signal and the second response signal are each reflected once from the earth's surface, this reply is referred to as a double reflection multipath (or type II) reply. This response signal will be interpreted by the monitoring aircraft as a target aircraft at an even greater range than indicated by the direct or type I reply. In this situation, a single target aircraft is providing the monitoring aircraft with a plurality of target responses during each interrogation period. Thus, from a single interrogation cycle (consisting of more than one interrogation), up to three responses can be received from a single target aircraft; namely, a direct reply, a type I reply, and a type II reply. These three tracks are referred to as the normal (direct reply) track, and the image tracks (from type I and II multipath replies). A method for reducing the multipath tracks is more fully described in U.S. Pat. No. 5,107,268, entitled "Method of Multipath Track Reduction in an Aircraft Tracking System", by P. Sturm et al, issued 21 Apr. 1992, and assigned to Honeywell Inc., the assignee of the present application.
False tracks are also caused by spurious replies caused by a target (i.e., an intruder) answering the wrong pair of pulses of the interrogation signal. The interrogation signal from a directional antenna of the target aircraft, for the ATCRBS system, consists of a predefined series of pulses using a whisper-shout sequence. The series of pulses includes some suppression pulses, the pulses varying in amplitude and direction in which they are output. The purpose of the suppression pulses is to separate intruders based generally on range and based on quadrant, suppress Mode S type intruders (there being separate interrogations for Mode S intruders), and to cause the intruder not to respond under predefined conditions. Although it is desirable for each intruder to answer only one interrogation, it is common for a singe intruder to answer several interrogations of the Whisper-Shout ATCRBS interrogation sequence. The intruder should answer a predetermined interrogation pulse pair. In practice, however, intruder aircraft sometimes answer an early or late suppression pulse pair. In some cases, it appears that some intruders do not suppress properly and answer multiple interrogation pulse pairs resulting in false tracks.
False tracks are a distraction to the pilot when displayed. Thus, there is a need to eliminate the false tracks caused by the replies to the suppression pulses of the interrogation sequence.