Secondary Surveillance Radar - Mode S

Mode S

A more detailed description of Mode S is given in the Eurocontrol publication Principles of Mode S and Interrogator Codes and the ICAO circular 174-AN/110 Secondary Surveillance Radar Mode S Advisory Circular. The 16 million permutations of the 24 bit aircraft address codes have been allocated in blocks to individual states and the assignment is given in ICAO Annex 10, Volume III, Chapter 9.

A mode S interrogation comprises two 0.8 µs wide pulses, which are interpreted by a mode A & C transponder as coming from an antenna sidelobe and therefore a reply is not required. The following long P6 pulse is phase modulated with the first phase reversal, after 1.25 µs, synchronising the transponder's phase detector. Subsequent phase reversals indicate a data bit of 1, with no phase reversal indicating a bit of value 0. This form of modulation provides some resistance to corruption by a chance overlapping pulse from another ground interrogator. The interrogation may be short with P6 = 16.125 µs, mainly used to obtain a position update, or long, P6 = 30.25 µs, if an additional 56 data bits are included. The final 24 bits contain both the parity and address of the aircraft. On receiving an interrogation, an aircraft will decode the data and calculate the parity. If the remainder is not the address of the aircraft then either the interrogation was not intended for it or it was corrupted. In either case it will not reply. If the ground station was expecting a reply and did not receive one then it will re-interrogate.

The aircraft reply consists of a preamble of four pulses spaced so that they cannot be erroneously formed from overlapping mode A or C replies. The remaining pulses contain data using pulse position amplitude modulation. Each 1 µs interval is divided into two parts. If a 0.5 µs pulse occupies the first half and there is no pulse in the second half then a binary 1 is indicated. If it is the other way round then it represents a binary 0. In effect the data is transmitted twice, the second time in inverted form. This format is very resistant to error due to a garbling reply from another aircraft. To cause a hard error one pulse has to be cancelled and a second pulse inserted in the other half of the bit period. Much more likely is that both halves are confused and the decoded bit is flagged as "low confidence".

The reply also has parity and address in the final 24 bits. The ground station tracks the aircraft and uses the predicted position to indicate the range and bearing of the aircraft so it can interrogate again and get an update of its position. If it is expecting a reply and if it receives one then it checks the remainder from the parity check against the address of the expected aircraft. If it is not the same then either it is the wrong aircraft and a re-interrogation is necessary, or the reply has been corrupted by interference by being garbled by another reply. The parity system has the power to correct errors as long as they do not exceed 24 µs, which embraces the duration of a mode A or C reply, the most expected source of interference in the early days of Mode S. The pulses in the reply have individual monopulse angle measurements available, and in some implementations also signal strength measurements, which can indicate bits that are inconsistent with the majority of the other bits, thereby indicating possible corruption. A test is made by inverting the state of some or all of these bits (a 0 changed to a 1 or vice versa) and if the parity check now succeeds the changes are made permanent and the reply accepted. If it fails then a re-interrogation is required.

Mode S operates on the principle that interrogations are directed to a specific aircraft using that aircraft's unique address. This results in a single reply with aircraft range determined by the time taken to receive the reply and monopulse providing an accurate bearing measurement. In order to interrogate an aircraft its address must be known. To meet this requirement the ground interrogator also broadcasts All-Call interrogations, which are in two forms.

In one form, the Mode A/C/S All-Call looks like a conventional Mode A or C interrogation at first and a transponder will start the reply process on receipt of pulse P3. However a Mode S transponder will abort this procedure upon the detection of pulse P4, and instead respond with a short Mode S reply containing its 24 bit address. This form of All-Call interrogation is now not much used as it will continue to obtain replies from aircraft already known and give rise to unnecessary interference. The alternative form of All-Call uses short Mode S interrogation a with a 16.125 µs data block. This can include an indication of the interrogator transmitting the All-Call with the request that if the aircraft has already replied to this interrogator then do not reply again as aircraft is already known and a reply unnecessary.

The Mode S interrogation can take three forms:

The first five bits, known as the uplink field (UF) in the data block indicate the type of interrogation. The final 24 bits in each case is combined aircraft address and parity. Not all permutations have yet been allocated but those that have are shown:

Similarly the Mode S reply can take three forms:

The first five bits, known as the downlink field (DF) in the data block indicate the type of reply. The final 24 bits in each case is combined aircraft address and parity. Eleven permutations have been allocated.

A transponder equipped to transmit Comm-B replies is fitted with 256 data registers each of 56 bits. The contents of these registers are filled and maintained from on-board data sources. If the ground system requires this data then it requests it by a Surveillance or Comm-A interrogation.

ICAO Annex 10 Volume III, Chapter 5 lists the contents of all those currently allocated. A reduced number are required for current operational use. Other registers are intended for use with TCAS and ADS-B. The BDS numbers are in hexadecimal notation.