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Lecture 8: SSR Mode S Uplink Formats in Mark X Standard - The Reply Message

This lecture covers the SSR downlink format and the encoding of information in the reply message for different modes, such as Mode A and Mode C. It also explains the use of the SPI pulse and the issues of garbling and garbling decoding. Additionally, the lecture introduces Mode S Super Beacon and the Mode S Uplink Formats.

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Lecture 8: SSR Mode S Uplink Formats in Mark X Standard - The Reply Message

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  1. Lecture 8SSR Mode S

  2. Uplink-formats in Mark X Standard

  3. The Reply MessageThe SSR down link format consists of a number of pulses, nominally 0.45 µs (±0.1 µs). F1 and F2 are always present and separated by 20.3 µs (±0.1 µs) – they are often referred to as a bracket or framing pair. Other pulse positions within this framing pair are spaced by 1.45 µs and are used to convey the required reply information in answer to the specific interrogation (e.g Mode A identity or Mode C flight level values).

  4. The pulses are identified to give the bits of an octal code (ABCD). The X pulse at the centre of the reply is not used. The three blank positions may not be occupied by pulses, otherwise some decoders may reject the entire answer as interference. Note that the reply information itself does not contain any information to indicate which mode it is a reply to. The interrogator will assume that the replies received are in answer to it latest mode of interrogation.

  5. SSR down link format

  6. In the case of Mode A, the octal code (ABCD) is set by a control panel in the cockpit. In the case of mode C, the flight level is encoded in a special way (by a special form of Gray code known as Gillham code - this has the characteristic of only one bit changing for each change in flight level).

  7. The SPI (Special Purpose Identification) pulse is used by air traffic controllers to confirm the identity of certain aircraft. The controller will ask the pilot to squawk ident – the pilot pressing a button on the control panel which adds the SPI pulse to SSR replies for a certain period (18±1 s).

  8. The display system will then highlight aircraft with SPI. (The SPI pulse may have been appropriate to distinguish aircraft on older display systems before fully plot extracted displays became available). The out of frame position of the SPI pulse is somewhat strange and, as will be seen later, the SPI pulse position chosen introduces rather unfortunate complications for automatic decoding purposes. According to ICAO the SPI-pulse will be added to Mode A reply only.

  9. Each answer receives its meaning only in connection with the respective question. For example:- 7700 in Mode 3/A: general air emergency- 7700 in Mode C: 20,000 ft height

  10. Garbling/DegarblingGarbling is a fundamental problem in the design of the classical SSR system and the situation is made worse by increased traffic. Aircraft are often closely spaced in range and azimuth but at different heights. Replies from two aircraft will overlap if their range separation is within the equivalent of the 20.3 µs reply length. This is approximately 1.7 Nm.

  11. The most serious garbling situations occur when the azimuth separation is very small such that replies from both aircraft are received from all interrogations across the beam. With advanced reply processing techniques and algorithms, it may sometimes be possible to extract all some or all of the replies from the received signal.

  12. At this, in principle, one distinguishes two manners of the overlapping:- Non-synchronous Garbling;- Synchronous GarblingTwo replies overlaps in time such that its time grids are not congruent, so one speaks about Non-synchronous Garbling. Such answers can be separated and one by one be decoded correctly!

  13. Synchronous garbling

  14. But if two or more replies overlap in time such that its time grids are congruent, so one speaks about Synchronous Garbling. It cannot to state in the decoding any more, whether this single impulse belongs to one or the other ones response telegrams. Through this it would come to the decoding of completely new and wrong replies and difficult from the original replies. These replies must therefore be disabled!

  15. Mode S Super Beacon • Discrete Address BeaconSystem (DABS) project,later renamed Mode S • Enable two way ground-airdata transmission • S = Select: Uses discreteaddressing to interrogatejust one aircraft

  16. reply interrogation Overview

  17. Mode S

  18. existing signal Mode S ground station Mode S equipped new signal Interoperability Issues • Transparency: Mode S must not break existing systems • Backwards-compatibility: Existing systems must still see Mode S equipped planes other aircraft existing ground station

  19. Frequency • New frequency: difficult to allocate • Same frequency as old system (1030/1090 MHz): interoperable, but may cause interference 300 MHz 1030 MHz 1090 MHz 3000 MHz VHF UHF SHF

  20. Mode S Uplink FormatsA conventional SSR interrogator may have a typical sequence of Mode A interrogation, followed by Mode C interrogation or other modes. This would be repeated continually at a high rate to ensure that a position/identity plot can be produced for all targets in line-of-sight range of the interrogator during each antenna revolution.

  21. The Mode S ground station produces a larger variety of interrogation types. These types can be roughly classified into two types:- All-call interrogations- Roll-call interrogations

  22. All-call interrogations obtain replays from all aircraft in the beam dwell, although, under certain circumstances, Mode S aircraft can be „locked out” to all interrogations so that they do not reply.Roll-call interrogations are selectively addressed to acquired Mode S equipped aircraft using the unique 24-bit address assigned to each aircraft. Only the addressed aircraft produce replies.

  23. The first problem for the Mode S system is to find the addresses of aircraft that are in radar cover so that selective addressed interactions can be made with them. This is achieved by Mode S all-call interrogator witch are made periodically from the radar.

  24. Transponders • Standard doesn’t specify what ATCRBS transponders should not do: • 549 transponders on the market • Each had unique behavior

  25. Side Lobe SuppressionIn secondary surveillance radar technology the side lobes of the antennae affect particularly unfavorably. Transponders also can be interrogated over the side lobes and then answer about these, too and a response telegram can be received also over the side lobes. This circumstance results from the fundamentally better transmitting- and reception conditions for secondary radar units.

  26. Such answers cannot be assigned obviously on the radar screen. They rather appear as several targets in the same range but in different directions. In the extreme case an airplane can be interrogated permanently during a turn of the antenna. Such an reply then appears on the PPI-scope as a „ring around”.

  27. There are two principles of Side Lobe Suppression (SLS)- Interrogation Path Side Lobe Suppression (ISLS), and- Reply Path Side Lobe Suppression (RSLS)The techniques for ISLS are very similar to those for RSLS. A supplementary so called- Improved Interrogation Path Side Lobe Suppression (IISLS)method uses the techniques of ISLS to reduce the influence of false replays caused by reflection.

  28. aircraft 1 P1 main lobe side lobe aircraft 3 aircraft 2 The Hack • Existing ATCRBS transponders used sidelobe suppression INTERFERENCE!!! ground station

  29. aircraft 1 A1 P1 P2 aircraft 2 aircraft 3 P2 A2 P1 P2 The Hack • Existing ATCRBS transponders used sidelobe suppression P1 main lobe ground station side lobe

  30. Hacking the Hack • Purposely send a small P1 and large P2 • “Disables” ATCRBS transponders • Use the time to cram in Mode S data blocks • Limited number of bits can be sent in this window P1 P2 Mode S data block 35 microseconds

  31. What Changed Things • Mid-air collision in 1986 • Congress passes a law mandating that all commercial aircraft be equipped with a Traffic Collision and Avoidance System (TCAS) by 1993 • TCAS uses Mode S • TCAS is now an international standard • Mode S technology is now commercially available

  32. Mode S Today • 108 of the U.S.’s busiest airports have Mode S ground stations • Majority of aircraft landing at these airports have Mode S transponders • Without Mode S, the 1030/1090 Mhz band would be completely overloaded • Mode S used in TCAS and many other applications

  33. Transponder Basics

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