Reports

• General
• Modulation
• Coding
• Transponder Testing

• 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.