What's Mode-S/ADS-B

[Hamish McTorsk]

Mode-S is a Secondary Surveillance Radar (SSR) technique with a selective interrogation of aircraft with its unique 24-bit worldwide address. This enables a unique identification of aircraft removing the risk of confusion due to overlapping signals.
Mode-S operates in the same Radio Frequency (1030-1090 MHz) band as conventional SSR systems.
Mode-S employs airborne transponders to provide altitude and identification data, with ADS-B adding global navigation data typically obtained from a GPS receiver
The position and identification data supplied by Mode-S/ADS-B broadcasts are available to pilots and air traffic controllers.
Mode-S/ADS-B data updates rapidly, is very accurate and provides pilots and air traffic controllers with common air situational awareness for enhanced safety, capacity and efficiency. Further, It can provide a cost-effective solution for surveillance coverage in non-radar airspace.

[DaveReid]

In Europe, Mode S also carries a lot of other download parameters via EHS (Enhanced Surveillance), which is mandatory for larger/faster aircraft.  These include vertical intention (i.e. selected altitude), TAS, IAS, heading, Mach no, roll angle, track angle rate, etc.

Decoding those without access to the uplink is fairly challenging (though not impossible), which is presumably why SBS and RadarBox don't.

[Fenris]

In Europe, Mode S also carries a lot of other download parameters via EHS (Enhanced Surveillance), which is mandatory for larger/faster aircraft.  These include vertical intention (i.e. selected altitude), TAS, IAS, heading, Mach no, roll angle, track angle rate, etc.

Decoding those without access to the uplink is fairly challenging (though not impossible), which is presumably why SBS and RadarBox don't.

If they're broadcast in the same way as other ADS-B information Dave, why is decoding them more challenging? And why do you mention access to the uplink specifically? I thought the point of these extended squitters is that they provide self-contained data.

[DaveReid]

If they're broadcast in the same way as other ADS-B information Dave, why is decoding them more challenging? And why do you mention access to the uplink specifically? I thought the point of these extended squitters is that they provide self-contained data.

If they were broadcast in the same way as ADS-B information, I agree there woudn't be any problem decoding them.

But they aren't.  In fact they aren't squittered at all.  Instead, a ground radar interrogates an ELS/EHS-equipped aircraft by requesting the contents of any of the 255 transponder data registers.  Normally the request is confined to one of three registers, for ELS, and an additional three for EHS.  In the case of the EHS registers, the response packet doesn't identify which register is being sent - that's not a problem for the ground radar, since obviously it knows which register it has asked for, but of course we don't have access to that data since we don't receive the 1030MHz uplink packet.

You're probably sorry you asked now  :-)

[Fenris]

If they're broadcast in the same way as other ADS-B information Dave, why is decoding them more challenging? And why do you mention access to the uplink specifically? I thought the point of these extended squitters is that they provide self-contained data.

If they were broadcast in the same way as ADS-B information, I agree there woudn't be any problem decoding them.

But they aren't.  In fact they aren't squittered at all.  Instead, a ground radar interrogates an ELS/EHS-equipped aircraft by requesting the contents of any of the 255 transponder data registers.  Normally the request is confined to one of three registers, for ELS, and an additional three for EHS.  In the case of the EHS registers, the response packet doesn't identify which register is being sent - that's not a problem for the ground radar, since obviously it knows which register it has asked for, but of course we don't have access to that data since we don't receive the 1030MHz uplink packet.

You're probably sorry you asked now  :-)

Not at all, I now understand how it works.

What I don't understand is why this information is not sent as extended squitters when other similar data is, unless it's an attempt to restrict the amount of data sent to maximise the capacity of the 1090MHz frequency.

[DaveReid]

What I don't understand is why this information is not sent as extended squitters when other similar data is, unless it's an attempt to restrict the amount of data sent to maximise the capacity of the 1090MHz frequency.

That could well be the case.  Bear in mind also that ELS/EHS is used solely within the Eurocontrol area, not in the rest of the world.

See www.eurocontrol.int/msa/public/standard_page/implementation_els_ehs.html