SSR

Notes on SSR

Aircraft are assigned unique Mode 3/A codes by the airport departure controller.
General aviation aircraft flying under visual flight rules are not under constant positive control, and such aircraft use a common Mode 3/A code of 1200.
In either case, the assigned code number is manually entered into the transponder control unit by the pilot or a crew member.

Aircraft are assigned Mode3/A codes by airport departure controller. It's part of the Secondary Surveillance Radar (SSR) System. The aircrew enter this in their transponder before takeoff. During flight when an ATC radar interrogates that aircraft it will respond with its 4 digit code.

There aren't enough Mode 3/A codes for each aircraft to have one so they are assigned regionally and reused in different regions.
When the aircraft in this scenario is in Europe it has whatever Mode3/A code they were assigned by European ATC.
When they depart European airspace for the trans-Atlantic part of the route they change their transponder code to 2000, as do all civil aircraft crossing the Atlantic.
When that aircraft crosses into North American airspace the aircrew are supposed to change their Mode3A to whatever ATC tells them or to whatever they were preplanned to use - 3134 in this scenario.

Additional Considerations:
There are other predefined Mode 3A codes:

  • 1200 VFR
  • 2000 Oceanic
  • 7700 is emergency
  • 7600 is comms failure
  • 7500 is unlawful interference (hijacking)

You might see Mode3, ModeA or Mode 3A/C
Mode 3 is military name for Mode A. They are synonymous.
Some (most) Aircraft can include their pressure altitude as part of their Mode 3A response. This is called Mode C. So sometimes the whole thing is referred to as Mode3A/C or some variation.
ATC Radars do not measure elevation, just slant range and azimuth. This is why Mode C exists - to provide A/C altitude to ATCs.
Military Air Defence Radars like the TPS-77 at Cold Lake are 3D radars and do measure target altitude.

Fighters:
When tactical aircraft are flying in a formation, typically only one of them (lead) is operating their Mode3A/C transponder.
This means that an ATC radar will receive primary (skin) returns from all of the aircraft but only one will have an elevation and Mode 3A code.
Saw this with the snowbirds last year - Nine of them flew by my house but only one of them showed up on ADSB exchange.

Mode 3A/C has been around since the 60's
Mode S (military Mode 5) is new'ish (~20 years) and solves a lot of the shortcomings of Mode3A. Not all aircraft have this (especially small private aircraft) so Mode3A is still important.
ADS-B is another system - Mode 3 and Mode S are transponders - they respond to interrogations from the ground. ADS-B is broadcast and anyone can listen in on ADS-B with a $25 dollar USB SDR receiver and open source software.


Mode 5 is specified in NATO STANAG 4193 and DOD-AIMS 17-1000

Mode 5 defines four operational levels using two waveform classes:

  • L1, Q&A: a synchronous (triggered by interrogations) mode with Processing
    Gain

    • Level 1 is similar to Mode 4 Q&A but is enhanced with an Aircraft Unique PIN
    • L2, Position Reporting: an asynchronous (triggered and non-triggered) mode with Processing Gain & includes additional information such as Aircraft Position and Other Attributes
    • L3, Selective Interrogation: a synchronous or asynchronous mode (Future Use)
    • L4, Extended Data: an asynchronous mode with High Data rate (Future Use)
Level Format Data Requested
1 0 ID
1 1 Mode 1, 2 Codes
1 2 Mode 3/A, C Codes
1 3 Lethal (shooters)
1 4 PIN and NO
2 16 3D, PIN and NO
2 17 3D and Mode 1, 2 Codes
2 18 3D and Mode 3/A, C Codes
2 19 3D and Lethal (shooters)
2 20 3D, PIN and High-Res PIN

Mode 5 – provides a cryptographically secured version of Mode S and ADS-B GPS position.
Mode 5 is divided into two levels. Both are crypto-secure with Enhanced encryption, Spread Spectrum Modulation, and Time of Day Authentication.

  • Level 1 is similar to Mode 4 information but enhanced with an Aircraft Unique PIN.
  • Level 2 is the same as Mode 5 level one but includes additional information such as Aircraft Position and Other Attributes

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