Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
2000-07-12
2004-09-14
Fan, Chieh M. (Department: 2634)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S267000, C375S349000
Reexamination Certificate
active
06792058
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns a digital receiving system for an ADS-Mode-S system which is capable of effectively functioning and operating competently within a highly congested airspace having a dense environment of aircraft.
In an ADS (Automatic Dependent Surveillance) system, an aircraft periodically broadcasts its state vector (horizontal and vertical position, horizontal and vertical velocity). The aircraft relies upon on-board navigation sources such as an on-board GPS (Global Positioning System) receiver and its broadcast transmission systems to provide surveillance information to other users.
The ADS-Mode-S concept is based upon the use of a Mode S squitter which provides a periodic broadcast (i.e., “squitter”) of the aircraft's position, altitude, identification, and other information.
At the present time, a Mode-S 56-bit squitter is used by the Traffic Alert and Collision Avoidance System (TCAS) to detect the presence of Mode-S equipped aircraft. The TCAS listens for Mode-S squitters and extracts a 24-bit Mode-S address, and uses this address for discrete interrogation. The longer 112 bit squitter replies contain, in addition to other data, 56-bits referred to as an ADS message field. The bits in the ADS message field convey data including (1) barometric altitude, (2) latitude, and (3) longitude. Aircraft equipped with a Mode-S transponder and a GPS receiver determine their position once every second, and this positional information is inserted into the 56-bit ADS message field of the long squitter reply, and broadcast twice every second to increase the probability of a successful reception. The current 56-bit short squitter continues to be broadcast once every second for compatibility with the TACS. Thus, potential interference is caused by both short and long squitter transmissions.
The present invention enhances the capability of a basic ADS-Mode-S system to receive ADS-Mode-S messages error free by using an array of basic Mode-S antenna elements, and processes the ensemble of signals electronically. The subject invention provides a signal processing/receiver architecture which enhances the capability of a basic ADS-Mode-S system to receive ADS-Mode-S messages error free, so that it is capable of effectively functioning within a highly congested airspace.
The information provided by an ADS-Mode-S message can assist pilots of nearby aircraft in the implementation of free flight plans, and is also monitored by ATS (Air Traffic Services) to ensure that an aircraft maintains conformance to its intended trajectory.
Free Flight is a concept which is being developed, tested, and implemented incrementally by the Federal Aviation Administration (FAA) and the aviation community. Free Flight is designed to enhance the safety and efficiency of the National Airspace System (NAS). The concept moves the NAS from a centralized command-and-control system between pilots and air traffic controllers to a distributed system that allows pilots, whenever practical, to choose their own route and file a flight plan that follows the most efficient and economical route.
The realization of free flight will require cooperative decision making among aircraft. One of the key system considerations for implementing such realization is an adequate surveillance architecture. Today navigation and surveillance is accomplished primarily by ground equipments, i.e., beacon radars and primary radars that do not operate in concert.
Radars are used today in two ways in air traffic control. The first, primary radar usage is an equipment which measures distance to an aircraft by measuring the round-trip time of a pulse from the radar to the plane which scatters the pulse. Some of the scattered or reradiated energy is detected by the radar receiver which is usually collocated with the radar transmitter. The second type of radar which is used in air traffic control is termed secondary radar. This genre of radar requires a transponder to be installed aboard an aircraft. When the aircraft is illuminated by an interrogation pulse of a secondary radar, the transponder broadcasts a digital signal which may convey aircraft identification and altitude.
ADS-B (Automatic Dependent Surveillance-B) is a function on an aircraft or a surface vehicle operating within the surface movement area which periodically broadcasts its state vector (horizontal and vertical position, horizontal and vertical velocity) and other information. ADS-B is automatic because no external stimulus is required to elicit a transmission; it is dependent because it relies on on-board navigation sources and on-board broadcast transmission systems to provide surveillance information to other users. The aircraft or vehicle originating the broadcast may or may not have knowledge of which users are receiving its broadcast; any user, either aircraft or ground-based, within range of the broadcast, may choose to receive and process ADS-B surveillance information. With ADS-B, ATS (Air Traffic Services) would monitor the ADS-B messages ensuring that an aircraft maintains conformance to its intended trajectory. The increased accuracy and additional information directly provided by the aircraft (via ADS-B), in comparison to radar-based monitoring, will result in quicker blunder detection and reduce false alarms.
The National Airspace System (NAS) Architecture (Dec. 1997), section 7.9, notes that ADS-B is initially intended as a surveillance system, not an avoidance system. Broader application will depend upon the creation of an ADS-B ground surveillance capability. As restrictions are relaxed and flexibility enhanced, it is expected that ADS-B will be a key component leading to both free flight and IFR (Instrument Flight Rules) cooperative-separation.
The ADS-B system is required to operate competently within a peak traffic environment. This requirement is based upon the Los Angeles Basin traffic model. The ADS-B network must be designed to accommodate expected future peak airborne traffic levels, as well as any airport surface units within range. The expected peak instantaneous airborne count (IAC) in the US has been given by the Los Angeles Basin traffic model. Traffic distributions for the Los Angeles Basin, as well as for a number of other measured terminal area distributions, are closely approximated by a uniform density function (cumulative number increasing as the square of the range from the center) out to about 15 nmi. From this point to 60 nmi, the cumulative number of aircraft is proportional to the range. The approximate IAC distribution for the Los Angeles Basin model has a peak count of 750 aircraft. This does not include aircraft or vehicles which are operating on the airport surface. Estimates for these additional traffic elements are 100 vehicles in motion and 150 surface units at rest. Thus, the total traffic density may total to 1,000 units within a radius of 60 nmi. This does not include any adjacent sector en route traffic.
Mode S was originally developed as a requisite surveillance improvement for Mode A/C secondary surveillance radar (Air Traffic Control Radar Beacon System or ATCRBS). In these modes, an interrogation at 1030 MHz triggers a response at 1090 MHz from an aircraft equipped with a Mode S transponder. Mode A consists of an 8 microsecond interrogatory which is answered with a 20.3 microsecond replay conveying the aircraft identification code. In Mode C, the interrogatory is 21 microseconds and the reply is 20.3 microseconds and yields the aircraft altitude. Typically an aircraft is illuminated with Mode A and then Mode C.
In Mode S, using the same frequency plan, an interrogatory consists of a preamble of duration 3.5 microseconds followed by a data block of either 16.26 microseconds or 30.26 microseconds conveying 56 or 112 data bits. (One and a quarter microseconds are used for a synchronization phase reversal training.) The interrogatory is DPSK at a rate of 4 megabits per second. The Mode S reply comprises a 6 microsecond preamble followed by a data block of 56 or 112 microseconds. The sig
Hershey John Erik
Hoctor Ralph Thomas
Korkosz Richard August
Puckette, IV Charles McDonald
Fan Chieh M.
Lockheed Martin Corporation
Scully Scott Murphy & Presser
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