Publications

This report explains the operation of the DABS uplink encoder and provides information useful in diagnosing its performance. Several techniques which may be useful in analyzing encoder operation are presented. One technique involves only the use of address-parity tables and modulo-2 addition. The address-parity tables included are based upon the encoding polynomial prescribed in "Provisional Signal Formats for the Discrete Address Beacon System," Lincoln Laboratory Project Report ATC-30 Rev. 1, dated 25 April 1974. The error detection and correction properties of polynomial encoding schemes are not the subject of this report.

The purpose of this field test program was to verify laboratory measurements of the effect of DABS uplink signal formats on ATCRBS transponders. The DABS interrogations tested were: (i) a combined ATCRBS Mode A/DABS All-Call consisting of a standard Mode A interragation with an additional pulse following the P(3) pulse; (ii) a DABS uplink message transmission consisting of a P(1)-P(2) 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 objective of this program was the determination of the effect on ATCRBS transponders of candidate modulation schemes in DABS uplink transmissions. Bench test were conducted on twenty-two different transponder samples, including representative air carrier and general aviation transponders and a military transponder. In this report, individual test procedures are outlined, the reduced data are presented, and some conclusions regarding the selection of the DABS uplink waveforms based upon the data are given. The basis of these conclusions is a DABS design goal of generating the least possible interference to the existing ATCRBS system. In addition to performing tests whose purpose was the direct measurement of response of ATCRBS transponders to candidate DABS uplink waveforms, tests were also run to obtain other link-related characteristic, such as response to non-standard pulses. Conclusions, based upon the test performed, were the 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 type) has a duration less than 30 microseconds and is preceded by ATCRBS suppressing preamble. It was further shown that the reliability of suppression al interrogation levels near MTL can be improved by increasing the peak power of the preamble relative to the message.