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These reports were filed to the Federal Aviation Administration under Air Force Contract F19628-70-C-0230 between 1970-1975. All the documents are unclassified and can be obtained from the MIT Retrospective Collection (RSC) or the MIT Lincoln Laboratory Archives.
  • Abstract from ATC-42 "DABS: A System Description" (P.R Drouilhet)
    November 18, 1974

    The Discrete Address Beacon System (DABS) is a cooperative surveillance and communication system for air traffic control. It employs ground-based sensors (interrogators) and airborne transponders. Ground-to-air and air-to-ground data-link communications are accommodated integrally with the surveillance interrogations and replies. DABS has been designed as an evolutionary replacement for the current Air Traffic Control Radar Beacon System (ATCRBS) to provide the enhanced surveillance and communication capability required for air traffic control in the 1980s and 1990s. Compatibility with ATCRBS has been emphasized to permit an extended, economical transition.

    A principle feature of DABS is that each aircraft is assigned a unique address code. using this unique code, interrogations can be directed to a particular aircraft, and replies umambiguously identified. Channel interference is minimized because a sensor can limit its interrogation to targets of interest. In addition, by proper timing of interrogations, replies from closely-spaced aircraft can be received without mutual interference. The unique address in each interrogation and reply also permits the inclusion of data-link messages to or from a particular aircraft. DABS uses the same frequencies for interrogations and replies as ATCRBS (1030 and 1090 MHz, respectively). The DABS interrogation is transmitted using DPSK at a 4 Mbps rate, and comprises 56 or 112 bits including the 24-bit discrete address. The reply also comprises56 or 112 bits including address, and is transmitted at 1 Mbps using binary pulse-position modulation. Coding is used on both interrogations and replies to protect against errors.

  • Abstract from ATC-12 "A Comparison of Immunity to Garbling for Three Candidate Modulation Schemes for DABS" (D. A. Schnidman)
    August 14, 1972

    The performance of three candidate modulation schemes for DABS is analyzed in this report and a comparison on the basis of probability of error per bit is made. The three types of modulation are PAM, DPSK, and FSK. The results show that, at a given level of interference and signal-to-noise ratio, DPSK and FSK have a lower probability of error per bit than PAM and this difference is significant in most cases. In addition to the probability of error per bit, however, the choice of modulation and mesage format depends on the capacity required, bandwidth occupancy, and cost of implementation. This list consideration is especially important with regard to the transponder.

  • Abstract from ATC-52 "DABS Modulation and Coding Design: A Summary" (T. J. Goblick)
    March 12, 1976

    This report presents the rationale for the selection of the DABS signalling waveforms and error control techniques. The main issue in arriving at the fina link design were (1) affordable transponder cost, (2) electromagnetic compatability with ATCRBS and TACAN, and (3) adequate performance in the link design uses the ATCRBS frequencies, achieves surveillance reliability and accuracy superior to that of ATCRBS, and transmits ground-to-air data link message with high reliability. This is accomplished without noticeably affecting ATCRBS performance, with less channel occupancy per target report than ATCRBS, and with transponders projected to cost approximately 160% of the cost of ATCRBS transponders.
  • Abstract from ATC-49 "DABS Uplink coding" (J. T. Barrows)
    July 25, 1975

    This report details the coding techniques incorporated into the DABS uplink design. Justification is given for the error control method selected in terms of the link characteristics and design constraints. Performance results, including extensive evaluation by simulation and bench test, are presented for the selected code.

    A binary shortened cyclic code having 24 redundant bits was selected. The overhead due to the code redundancy is minimized by a scheme in which the parity check bits are overloaded on the discrete address field in the encoded message. This code is shown to have the capability of protecting a DABS transponder from accepting an erroneous uplink message with an error probability of less than 10-7 in a severe interference environment.

    This same code will be used in the DABS downlink in a burst erasure correction mode. Results of the analysis of the downlink will appear in a separate report.

  • Abstract from ATC-48 "DABS Downlink coding" (J. T. Barrows)
    September 12, 1975

    This report concentrates on the error correction techniques including specific implementations. Evaluation of the performance of the code by simulation and/or bench test is presently being carried out and will be reported in a future document.
Transponder testing
  • Abstract from ATC-9 "Transponder Test Program (G.V. Colby and E.A. Crocker)
    April 2, 1972

    Performance parameters of transponders installed in aircraft were measured to determine their degree of compliance with current specifications. A mobile van was outfitted with electronic test equipment which simulated the transmitter and receiver sections of a ground interrogator and which allowed measurement of transponder parameters. A horn antenna located near the aircraft under test was used to couple signals to and from the transponder.

    The results of measurements on 504 transponders installed in general aviation aircraft, 17 transponders installed in military aircraft, and 28 transponders installed in air carrier aircraft are reported. Of these, 31 general aviation, 2 military, and one air carrier transponders were inoperative. The results of measurements of reply frequency, squitter, delay, Mode C operation, dead time, P2/P1 ratio required for suppression, suppression time, framing pulse spacing, power output and sensitivity are included.

  • Abstract from ATC-25 "Final Report: DABS/ATCRBS Transponder Bench Testing Program" (J. R. Samson, J. D. Welch, E. R. Becotte, E. A. Crocker, H. D. Schofield)
    November 28, 1973

    This report discussed the effects on ATCRBS transponders of different modulation schemes in DABS uplink transmissions. Bench tests were conducted on twenty-two different transponder samples, including representative air carrier and general aviation transponders and a military transponder. The test procedures were outlined, data presented, and conclusions reached regarding the selection of the DABS uplink waveforms. The basis of the conclusions is a DABS design goal of generating the least possible interference to the existing ATCRBS system.

    The conclusions were that DABS signals modulated by either PSK or FSK would be demodulated by a significant fraction of ATCRBS transponders and that there are no feasible PAM pulse widths or spacings which would not be decoded by a significant fraction of ATCRBS transponders. ATCRBS transponder reply probability drops nearly to zero when the DABS message (of arbitrary modulation bype) has a duration less than 30 microseconds and is preceded by ATCRBS suppressing preamble.

  • Abstract from ATC-27 "A Summary of the DABS Transponder Design/Cost Studies" (T. J. Goblick, P. H. Robeck)
    March 1, 1974

    One of the major concerns in the DABS development program has been the cost of the DABS transponder. In order to realistically assess the impact on transponder cost of the many alternative techniques and design choices being considered for DABS, four study contracts were awarded to avionics manufacturers to design and estimate costs of special circuitry and complete transponders. This report summarizes the major results of these design/cost studies, which cover general aviation, military, and air carrier transponder designs.

    The transponder design/cost studies have had a marked influence on the design of the DABS signal and message formats. Since the cost studies were basically intended for comparing link options for DABS, the transponder specifications used in these cost studies do not correspond in detail to current DABS transponder specifications. Therefore the cost data contained in this report cannot be taken to be completely representative of the cost of the finally specified DABS transponders.

  • Abstract from ATC-31 "Report on DABS/ATCRBS Field Testing Program" (J. R. Samson, E. A. Crocker)
    February 13, 1974

    This experiment verified laboratory measurements of the effect of DABS uplink signal formats on ATCRBS transponders. The DABS interrogations tested were: a combined ATCRBS Mode A/DABS All-Call consisting of a standard Mode A interrogation with an additional pulse following the P3 pulse; a DABS uplink message transmission consisting of a P1-P2 pulse pair followed by 25 microseconds of 4 megabit per second DPSK data modulation. The timing aspects of these waveforms were selected on the basis of a previous series of laboratory bench tests made in a small cross-section of the ATCRBS transponder population. The results of these field measurements performed on a larger cross-section of field operational ATCRBS transponders agree with the results of the bench testing program: The recommended DABS uplink transmissions result in an effective combined ATCRBS/DABS All-Call mode and minimum triggering of ATCRBS transponders by DABS message transmissions.

The Story of Mode S: An Air Traffic Control Data Link Technology last modified: 12.06.2000