Publications

The principal function of the Mode S sensor (1), an evolutionary upgrade to the current ATCRBS (Air Traffic Control Radar Beacon System) sensor, is the output of one reportper aircraft per antenna scan. This report contains the current aircraft position (range and azimuth), the identity code of its transponder, and the altitude code as supplied by its encoding altimeter. This information is derived from the aircraft transponder replies received at the sensor in response to interrogations transmitted by the sensor. For aircraft equipped with Mode S transponders, a single scheduled interrogation, directed only to that aircraft, elicits a single coding-protected reply containing both identity code and altitude code. For aircraft equipped with ATCRSS transponders, a sequence of interrogations alternately eliclt replies containing un-protected identity code or altitude code from all aircraft in the antenna mainbeam. From this description, it is clear that a Mode S aircraft report can be constructed directly fron the single reply. Surveillance processing, defined as functions that perform scan-to-scan correlation and tracking, are required in general only to predict the next scan position of the aircraft. This information is needed for the proper scheduling of the next interrogation. ATCRBS reports constructed from the aircraft replies, on the other hand, can have a number of deficiencies. The more common such problems are: 1. Either the identity code or altitude code or both can have bits declared either in error or with low confidence by the reply processor due to garbling of overlapping replies. 2. False alarm reports not corresponding to aircraft can be generated from fruit replies (responses to other sensors' interrogations) or reflection replies. 3. Multiple reports for an aircraft can be generited due to incorrect correlation of replies caused by errors in range, azimutn, or code determination. Surveillance processing for ATCRBS aircraft is tasked with correcting these problems prior to report output to the controllers or other users. It does this by correlating raw target reports with, existing track files, and using the information in these files derived from prior scan reports to correct, complete, or reject erroneous reports. This paper presents the major algorithms contained within the Mode S sensor ATCRBS surveillance processing function. It then presents experimental results that demonstrate their effectiveness. Full details of surveillance processing can be obtained by reference to (2) or [3).

This report presents the details and basic theory of the coding scheme employed on Mode S uplink and downlink transmissions. Since ATCRBS interference is the main source of error for these signals, a cyclic burst detection code was chosen for Mode S. This code permits simple error detection at the transponder and more complex error correction at the sensor. The theory behind cyclic encoding and decoding as used for Mode S is presented first. Then, since polynomial multiplication and division are required for these processes, circuits for these operations are described. Finally, the last chapter describes the actual implementations specified for encoding and decoding in both the transponder and sensor.

The surveillance performance of a single Mode S Sensor is degraded by several factors, including: poor crossrange accuracy at long range, diffraction-induced azimuth errors, missing of incomplete reports, and extraneous reports. The surveillance netting project reported here sought to overcome these difficulties by employing information from a secondary (and perhaps also a tertiary) sensor. The project was performed to determine what auxiliary information is most useful, how this information could be used for maximum effect, when help should be sought from other sensors, what form this inter-sensor communication should take, and where the netting algorithms should be implemented. It was also planned to include the construction of a real-time netting demonstration system to exercise and test the concepts developed. The central issue in this project was the approach to be used for multi-sensor azimuth determination. In particular, a new form of incremental bilateration, employing a flat earth model, is shown to be both accurate and bias-resistant. Altitude estimation methods and multi-sensor tracker design are also addressed, with new algorithms developed in each case. Finally, the deisgn of the netting subsystem for a Mode S sensor is presented.

The Lincoln Laboratory Air Traffic Control Radar Beacon System (ATCRBS) Monopulse Processing System (AMPS) is a mobile, stand-alone, ATCRBS surveillance sensor for processing and disseminating target reports from transponder-equipped aircraft. AMPS is essentially the ATCRBS portion of the Mode Select Beacon System (Mode S), a system designed to be an evolutionary replacement for the present third generation ATCRBS. AMPS utilizes several new features introduced by the Mode S sensor concept. In particular, the use of monopulse angle estimation permits more accurate aircraft azimuth estimation with fewer replies per scan, and improved decoding (identification) performance when garble is present. This report provides a description of the details and philosophy of the AMPS computer system implementation and operation. In particular, specific and detailed descriptions of the interrelations between AMPS's several subsystems and subtasks are provided as well as a guide on how to run them.

The Discrete Address Beacon System (DABS) has been designed to be an evolutionary replacement oth the third generation Air Traffic Control Radar Beacon System (ATCRBS). Although the ATCRBS returns processed by DABS will be identical to those currently being employed, the DABS processing system will not merely mimic the present system. Instead, it has been designed to surpass current performance levels even while reducing the number of interrogations transmitted per scan. This will be made possible by utilizing the availability of several new features introduced by the DABS sensor. In particular, the employment of monopulse antenna will permit both more accurate azimuth estimation with fewer replies per scan and improved decoding performance when garble is present. The ATCRBS portion of the DABS sensor has been designed to be a complete, self-contained package that performs all ATCRBS functions required for aircraft surveillance. The major tasks it implements are: 1. Determining the range, azimuth, and code of each received ATCRBS reply 2. Grouping replies from the same aircraft into target reports and discarding fruit replies 3. Identifying all false alarm target reports due to reflections, coincident fruit, splitting, or ringaround 4. Initiating and maintaining a track on all aircraft in the covered airspace The first function has been implemented in hardware while the remaining ones are performed in software. This report will discuss in detail only the software subsystems. The ATCRBS system described in this report has been implemented in the ATCRBS Monopulse Processing System (AMPS) built at Lincoln Laboratory. Although the AMPS design is based upon the specifications contained in the DABS Engineering Requirements (ER), there are two major differences between AMPS and the ER system. First, the design described here is for a standalone ATCRBS system; no capabilities are built in to send, receive, or employ information from other sensors, and no formal interfaces to other ATC functions are defined. Second, this system was not intended to be a production prototype, so no reliability features have been included.