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I understand that the radar sends a 1030 MHz interrogation signal to an aircraft and the aircraft's transpoder responds using 1090 MHz. When it comes to Mode S squitter, the transmissions are unsolicited (does not require interrogation). Considering that the airspace is highly populated, there would be multiple aircrafts transmitting data on the 1090 MHz to the radar. Won't these signals interfere with each other and become a huge mess at the radar side? (Similar is the case of ADS-B, where data would be continuously broadcasted between aircraft-aircraft, aircraft-ATC and vice versa on the 1090 MHz Extended Squitter, how are the signals prevented from mixing up with each other?)
Also how is the interrogation signal transmitter from ATC to aircraft (and also back) sent? Is Frequency modulation used here? What is the bandwidth of the 1090 Mhz channel? Does it have sidebands?

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    $\begingroup$ The transponder antenna is more like a primary radar dish and is highly directional. It does indeed get confused if two targets are merging (regardless of altitude). Mode S is still interrogated, only ADS-B does autonomous broadcasts, and I'm not sure how that works. I suppose it's some kind of multiple access scheme just like have with cell phones. $\endgroup$ – falstro Sep 12 '14 at 19:24
  • $\begingroup$ According to en.wikipedia.org/wiki/Secondary_surveillance_radar the Mode S seems to have 1Mb/s rate (each bit takes 1μs). $\endgroup$ – Jan Hudec Sep 12 '14 at 21:06
  • $\begingroup$ @falstro There is no fancy multiple access scheme in 1090ES ADS-B. It just sends out the message often enough so that is will come across. Position 2x per second, velocity 2x per second. $\endgroup$ – DeltaLima Sep 12 '14 at 22:11
  • $\begingroup$ @Jan Hudec Say there are 100 aircrafts in a 100 mile radius (ADS-B boasts greater precision which would allow aircrafts to fly closer). All these 100 aircrafts would send ADS-B OUT every second. If atleast 2 aircrafts transmit data at the exact same time intervals, then wouldn't a proper reception be impossible? Is some form of Time Division Multiplexing used in 1090 ES (or in 978 UAT?). Note that initially ADS-B was supposed to use the VDL MODE 4 which employed STDMA (Self Organized Time Division Multiple Access). Definitely something would be used here as well $\endgroup$ – Clive Sep 13 '14 at 2:45
  • $\begingroup$ A very similar scenario: how does the cell phone tower not get confused when many people are using the cells at the same time? $\endgroup$ – kevin Sep 15 '14 at 20:01
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The class of ATC radars that you are talking about is secondary radar.

Secondary radar works by transmitting an interrogation signal on 1030 MHz to which the transponder replies on 1090MHz. There are several modes of interrogation. For civil ATC, modes A,C and S are important.

The uplink signal (interrogation) is a simple pulse encoding (Mode A/C) or a Differential Phase Shift Key modulation (Mode S / TCAS) on 1030 MHz.

The downlink signal (replies, unsolicited squiters) is a simple pulse encoding (Mode A/C) or Pulse Position Modulation (Mode S / TCAS / ADS-B)

Originally only Mode A & C were available. Mode A and Mode C are unaddressed; every transponder receiving a Mode A or Mode C interrogation will reply*. This causes two problems, garbling and FRUIT.

Garbling is caused by the overlap of replies from multiple transponders. A transponder replies on 1090 MHz in the from of a number of pulses. For Mode A & C, there are at most 12 pulses between a set of framing pulses. When multiple aircraft reply to the same interrogation, these pulses mix up and it becomes difficult to find out which transponder sent which pulse. This leads to false altitudes or false squawk codes.

FRUIT (False Replies Unsynchronised to Interrogator Transmission) is caused by replies that are triggered by other radars. When multiple radars operate in an area, transponders can be quite busy and the radar is easily mislead by a reply to another radar. In addition to the timing (range) being wrong, the reply can be Mode A (squawk code) while the radar interrogated Mode C (altitude). There is no way to see what kind (A or C) reply was sent.

To overcome these problems of Mode A/C, Mode S was introduced. Mode S is quite different from Mode A/C and requires a more sophisticated radar and transponder.

  • Most Mode S interrogations are addressed; the interrogation contains the unique 24 bit address of the aircraft it is interrogating. This reduces the probability of garbling since other aircraft will not reply

  • Unaddressed Mode S interrogations will solicit replies that contain the address of interrogator (radar). This prevents FRUIT since the radar can verify that the reply is correctly addressed.

  • Unlike Mode A/C which does not have any error checking, a Mode S reply contains a 24 bit CRC check code.

  • Mode S Altitude and identity (squawk) replies have their own identifier so they can be distinguished.

  • Mode S radars can be operated in clusters and coordinate between them who is interrogating which aircraft at which time. This further reduces the message load and garbling / fruit probability.

Despite all the enhancements, Mode S radars are still receiving garbled messages and FRUIT, but are much better equipped to cope with it than a Mode A/C radar due to the addressing and CRC checks.

Since TCAS works on the same frequency as secondary radar, radars will occasionally receive TCAS messages as well. But since these have another downlink format (DF 0, DF 16) they are ignored. The same holds for ADS-B (DF 17 / 18)

Garbling remains a concern. Modern Mode S radars are relatively resilient against garbling due to their narrow beamwidth and advanced degarbling processors. But other systems like TCAS, ADS-B in and Wide Area Multilateration systems with their omnidirectional antennas receive overlapping messages quite often. But when the signal strength of the messages differs the messages can be often untangled with modern digital signal processors.

In the Mode S (& TCAS) a reply can be either short (56 bit) or long (112) bit. ADS-B messages are 112 bit long. Every bits takes 1μs, and there is a preamble that takes 8μs. So a message takes either 64μs or 120μs.

Every second, a Mode S transponder sends 1 unsolicited short squitter, and at most 6.2 long squitters when ADS-B equipped.


*To reduce number of Mode A/C replies, Mode S radars will include an extra pulse when interrogating in Mode A or C, so that Mode S capable transponders will not reply to these A/C interrogations.

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  • $\begingroup$ I agree with you on the Mode S. The 24 bit ICAO address is used to selectively target aircrafts. Lets say a radar sends out a Mode S interrogation to an aircraft. But simultaneously, a radar in an airport will be continuously receiving squitters from aircrafts upto 100 miles away right? So won't these squitters get mixed up with the Mode S reply? Also what about the case of ADS-B 1090 ES where signals will be sent from and between all aircrafts on the same 1090 MHz every second? Won't this definitely cause a mix up? How can there ever be a proper reception then? $\endgroup$ – Clive Sep 13 '14 at 2:43
  • $\begingroup$ What about ADS-B IN and ADS-B OUT for each aircraft? Again, both are using the same 1090 MHz which only increases the chance of message collisions further. $\endgroup$ – Clive Sep 13 '14 at 2:48
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    $\begingroup$ @Clive The beamwidth of a mode S radar is quite narrow. It does not receive squitters from aircraft all the time. It will only receive squitters from those aircraft that are in the beam, and they are in the radar beam only for a brief time. The probability of overlapping messages within the beam is not that high. Secondly if the signal strength of the messages differs by about 3dB, then it is usually no problem to degarble the overlapping replies. $\endgroup$ – DeltaLima Sep 13 '14 at 8:35
  • $\begingroup$ @Clive Initially ADS-B OUT increases the probability of message collision, it adds at most 6.2 120μs messages per second to the frequency occupation for each aircraft. ADS-B IN will not add messages, it is only listening. On the longer term ADS-B OUT/IN will reduce the need for TCAS interrogations as hybrid ACAS solutions will partially use of ADS-B for conflict detection. Currently most of the frequency occupation is caused by TCAS so the nett effect will be a reduced probability of message overlap. $\endgroup$ – DeltaLima Sep 13 '14 at 8:42
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    $\begingroup$ ADS-B receivers have in general antennae that cover 360 degrees. They have a much higher change of receiving overlapping messages. But as long as the signal strength difference between the overlapping messages is more than 3dB, they can usually be untangled. If not, the message will just get lost which is not a problem, there will be another one soon enough. A terminal area radar will rotate once every 5 seconds. ADS-B transmits position 10 times in that period. To have a radar like update rate from ADS-B, you can loose about 90% of the messages. $\endgroup$ – DeltaLima Sep 13 '14 at 8:59

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