Mode S Technology
|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
- 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
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.
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
- 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.
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
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
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
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