Mode S Technology





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Tom Goblick interview - 11/17/00

Summary: ATCAC formed in 1969 to look at ATC systems. Suggested an upgrade to ATCRBS. In 1972, DABS/Super Beacon was developed. Compatiblity was an important issue in its development. Hard to find a new frequency for DABS. 1030/1090 MHz band was overloaded. The challenge for DABS was to overlay a new system onto an overloaded channel. ATCRBS was ripped off from IFF. Before ATCRBS only radar was used for civilian systems. Planes were just blips on a screen with no identity. This caused an accident in which a plane crashed into a mountain because an air traffic controller started giving directions to one plane and then unknowingly continued giving the directions to a different plane. Sidelobe suppression was built into ATCRBS. Lincoln Lab solves a problem. But it solves it within cost limitations and keeping in mind the requirements of the general aviation community and compatibiliy issues. General aviation community is cost sensitive.

[History of ATC]

  • 1969 ATCAC (Air Traffic Control Advisory Committee) looked at all kinds of options for collision avoidance, including satellites. Finally they settled on an upgrade to the existing ATCRBS system.

  • 1972 DABS/superbeacon, called ADSEL (address selective) by the British was created. In some ways it was inspired by SAGE, which used radar and beacons.

  • The compatibility issue was especially important because ground stations were all over the place, and each needed its own access road, electricity, phone, and other utilities. If location was determined by triangulation, coverage by multiple sites was required.

  • If a new frequency was chosen, a new, clear frequency would have to be found, and there were none. Instead, they chose to use the DME frequency band.

  • Frequencies:

    VHF 108-138 MHz
    military 225-400 MHz
    DME stations 960-1218 MHz

  • 1030 and 1090 were chosen as the interrogation and reply frequencies. It was a "de facto standard" that there be at least 60 MHz between them, because of modulation done on the waveforms.

  • DABS/Mode S was developed was that the 1030/1090 bands were getting overloaded. The reason for this was that a given transponder would be interrogated several times within the sweep of a beacon, about 50 times per transponder, generating many replies. The issue was scalability.

  • Also there was the "synchronous garble" problem. If the communication between ground station and transponder could be reduced to ONE interrogation and reply, traffic in the channel would be greatly reduced. This requires a tracking system, because specific aircraft are interrogated (addressed). "Monopulse" allows the ground station to hit the aircraft with only one interrogation.

  • Especially with aircraft in holding patterns, it was impossible to distinguish one aircraft from another because the pulses from their replies would overlap.

  • The design challenge for DABS was to overlay a new system onto an overloaded channel, so that the system would work together with the old system. In addition, as people moved to using the new system, the traffic would be reduced. DABS also should be compatible with IFF which is the military ATC system, which they are still using.

  • Essentially ATCRBS was ripped off from IFF. In the 1950s IFF was using the 1030-1090 band. In civil ATC only radar was used, there was no way to identify which blip on the screen corresponded with which aircraft. This caused a collision when a student pilot got on the channel and started following the instructions that were being given to another aircraft, and the air traffic controller unknowingly switched to giving the student directions instead of the other plane, and the other plane crashed into a mountain. After this the FAA decided they needed to be able to identify planes, and adopted IFF, which became the first generation of ATCRBS. The military had modes 1, 2, 3, and 4. Mode 3 transmits identification info, and in ATCRBS this became Mode A. Mode 4 is encrypted, the military uses that today. Mode C which deals with altitude came later. Now the FAA owns the 1030/1090 channel, but IFF still uses it, so sometimes there are problems.

  • Mode S was "really thoroughly wrung out here [at LL]." One time LL had to prove to a bunch of people from Britain that it really worked, so they put a camera on the tracking system and used optics and a mirror so that the mirror would actually reflect the plane that they were tracking. They then tried out the system on a 727 that happened to be taking off from Logan. The system was so accurate that the mirror was pinpointed to within 3 feet of the antenna on the 727, not just the plane itself. This kind of visual presentation was really important because it was convincing; it was a very powerful demonstration.

  • "It's all about the application of technology to engineering." LL has to appreciate where the technology is, and where it's heading, and use that to solve problems. In designing Mode S, LL proposed that the transponder act like a modem, with lots of modes of data transmission. Possibilities for data were the next waypoints of a flight path, location data (like GPS), and other stuff. Mode S was designed for "future compatibility" so it was very flexible. In fact, LL proposed an air to air mode for DABS, but an FAA official said "absolutely not" because they were focused on low cost at the time, "not 25 cents more than necessary for the ATC job" not air to air.

  • Ironically, during this development there was a collision in Indianapolis between a DC-9 (using IFR) and a Cherokee (using VFR) on a cloudy day. One of the planes ducked into a cloud and came out and the other couldn't avoid it, and the DC-9 lost its tail and crashed. After this the FAA became concerned with air to air data transmission, and one FAA official tried to design a system of his own that was "truly garbage." In fact, there's a version of the DABS spec that has this system in it, it's called "synchro DABS" and doesn't use squitter but relies on precisely synchronized clocks, etc. and would have been really expensive. This system was completely removed from the DABS spec, but DABS msg formats were designed flexibly to accomodate new data links. TCAS developed instead, using Mode S to do collision avoidance.

  • At this time, Dr. Goblick was an Assistant Group Leader in the ATC group. There were two groups, one that dealt with sensors and one that dealt with "systems" and knitting together the data and making it useful.

  • Testing that was done with DABS was very complete, to the point of pouring concrete to make a field to test on. BCAS was the predecessor to TCAS. [Something about ACAS then BCAS?]

  • NATO sponsored an effort to replace IFF at one point, in fact there existed a NATO standard, but when the USSR collapsed the funding dried up.

  • When LL was trying to make DABS compatible with ATCRBS transponders, they ran into a lot of problems with electromagnetic compatibility. Ideally, the transponders should not respond to DABS signals. LL thought they had designed DABS appropriately but they went out and bought some $500 transponders that were popular with GA aircraft owners and discovered a fatal flaw in the FAA's specs for ATCRBS transponders; they said "lots of stuff about what they *should* do and nothing about what they shouldn't do" and so some of the transponders were responding to stuff like simple sine waves. At this point LL was very concerned about making DABS so that it was usable by everyone so "it went out and bought a whole bunch of GA transponders as well as some commercial ones" and tested DABS with them all and concluded that there was no way to be transparent to them all except to use sidelobe suppression.

  • Sidelobe suppression was built into the original ATCRBS system. What happened was that sidelobes could also trigger transponder responses, as well as the main beam, so in order to suppress responses to the sidelobes [this part really needs the diagram he drew] ATCRBS transponders were designed so that they would only respond if the first pulse were at least 3 db greater than the second pulse. If not, ATCRBS would ignore 25-35 microseconds of the transmission starting with the first pulse. DABS used this by making the second pulse *always* greater in magnitude than the first pulse, and using the remaining 20 microseconds to transmit information. This was what dictated the 56/112 bits. The fastest flipping frequency was 4 MHz because really fast flipping back and forth generates out of band spectra, and with the 4 MHz set, 112 bits was as many bits as they dared to push through. The question was how many bits could be transmitted in the 20 microseconds? Originally the devices were analog circuits and so 20 microseconds was an upper limit. Now that they are digital more data could probably be transmitted because the curve is cleaner; in retrospect 64/128 bits would have made more sense for the short/long replies for DABS, but at the time there was no way to know that 128 bits would be safe in the future. It's all based on "what information people have when they're making design decisions." After finishing up with this design they tested DABS with automated tests in the normal transponder environment.

  • FAA wanted to use amplitude modulation instead of phase modulation for both interrogation and reply, because they were convinced it was difficult to demodulate phase modulation. Once LL proved to them it could be done they were okay with it. In choosing the modulation scheme, LL was basically limited to phase, amplitude, and frequency modulation, because of cost limitations they couldn't get too complicated. LL wanted to use phase modulation because it was less susceptible to multipath effects; pulses that were phase shifted would not be amplified. In the end, different modulation schemes were used for uplink and downlink.

  • The transponder was "cleverly designed for its time." Basically what was needed was a high powered transmitter. Demodulation used DPSK and modulation used PAM - binary PPM = energy for every bit. They could not anticipate solid state transmitters that had enough power, so they gave in to the FAA.

  • [About how the 1030/1090 band was secured.] DME which originally owned the band cleared out some frequencies around both for ATC, and the military was more stubborn but eventually FAA pressure forced them to reassign.

  • When designing the format of the data, they had to anticipate how many modes there would be, because they marked a certain number of bits for control - to specify which mode/kind of message was being sent. All of these kinds of considerations lead to a very thick and detailed spec for DABS.

  • LL solves a problem. They design and evolve a set of requirements, looking at advanced technology and trying to take advantage, within the cost limitations. They have to keep in mind the cost, the requirements of the GA pilots, EM compatibility issues, etc. "What's put together here is not what's going to go out in the field and be used" so LL has to always keep in mind the audience. Dr. Goblick flew in order to determine the pilot workload during a flight, as part of the official research on DABS.

  • It's a good deal for the gov't, LL doesn't have any patents, they all belong to the gov't. They own everything, get a very detailed spec, and get unbiased information. Commercial companies always have their own interests in mind. Because LL is non-profit, there's no incentive to use proprietary technology, they can *only* work for the government, unless there's a special exception, anyway.

  • Now there are lots of Mode S transponders and sensors deployed. Industry always argue that they have R&D resources, but they have bias. "To some people, a satellite is the answer, before they even hear the question." Mode S was well designed and went on to become the international standard; countries could hire contractors w/o worrying about proprietary technology. "I give the FAA credit for coming to LL" but they had a very narrow goal; "if we wanted to go and look at an air to air mode we'd get beaten down." The FAA is technologically conservative, and didn't understand error control coding at all, so they accepted it as "magic" and didn't ask questions. [Some good stuff about the error code here but I didn't get it.]

  • It wasn't an LL idea to overlap the parity and address bits to save 24 bits, but it was a good idea. "Erasure" channel was used in downlink coding. There were three options: 0, 1, X. The goal was the make the reply channel into an erasure channel. If there was an error or uncertainty an X would be transmitted. If there were not too many X's, correction code can fix the signal. The code was designed to correct 24 microseconds or shorter, so it could deal with overlapping ATCRBS replies which were 22 microseconds, including the framing pulses. These were turned into X's with an occasional 1 or 0. If an ATCRBS reply overlaps a pulse, even at a higher magnitude, the system can correct it, but if there are two overlapping ATCRBS replies, the code is saturated and the signal is not handled. However, the chances of there being two ATCRBS replies is much lower than the chance of there being one.

  • Beacon compared to radar: radar is accurate up to a fraction of a mile, but beacon uses active transmitters, so the signal levels are much higher. Reflections for radar lose energy along the path, and they lose more on reflection because not all the energy reflects. Beacon is a cooperative system.

  • Lesson from SAGE (1950s) is that there is difficulty in doing computation on data, but a slight increase in data quality makes everything easier; it's a non-linear relationship between data quality and computation jobs (metaphorically speaking).

  • GA requirements on Mode S were mostly cost related - private owners wanted the transponder to be affordable, like the $500 ATCRBS transponders they were using before. LL did cost studies (Collins, Bendix?) with 3-4 contractors, and in the end Mode S cost no more than 50% more than ATCRBS - it was 1.5x the cost of the King transponder which was popular among GA pilots. However, even though LL thought Mode S could be done so that GA would accept it (~$5000) GA has not accepted it. GPS Squitter is now free with the transponder so if they would install Mode S they would have the benefit of GPS as well.

  • TCAS, etc. were all designed within Mode S capabilities, Mode S was kind of like an infrastructure element that other things could be put into.

  • Airlines need two transponders (IPC) so there is that cost issue, but there aren't a lot of engineering requirements from the commercial airliners. If a new frequency was used the system requirements would double; more transponder, more everything. This would be very traumatic for the system, so in doing design LL would have had to prove that the problem couldn't have been solved using the old frequency. Instead, they used the old frequency. Bill Harman worked on modeling the traffic that would occur during the transition period. They had to prove it was possible to transition to the new system, Mode S.

  • Currently 100s of ground stations have transitioned, as well as commercial aircraft. The military has not moved to mode S but is coming to realize that Mode S is useful and good. They're starting to install TCAS, they like GPS squitter, etc. The next generation of IFF will incorporate Mode S. FAA is now trying to make Mode S appealing rather than mandating it, but this incentive based approach has been largely a failure. TIS/TIS-B (broadcast) and weather information are being offered to AOPA/GA as enticement to install Mode S, but it's not really working. European organizations are much more successful in mandating stuff; they've mandated 8.33 MHz for voice (instead of old 25 MHz system) because they're running out of room, and it's the same with TCAS; they have the strongest Mode S mandate in the world.

  • Commercial airlines install things because if there is a collision and the technology exists to avoid it, the company can be held liable. All aircraft with over 30 seats have TCAS. New TCAS transponders all have Mode S, and in the future may have GPS squitter.

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