How are the signals used by ATC

[Dean]

Hope someone can enlighten me.

I know what the ADS-B signals do etc but notice that a lot of commercial flights that come overhead etc etc don't transmit a signal.

Aside from making our hobby more interesting who uses the ADS-B signal and for what if not all flights have to transmit it or those that do have incorrect info being sent out?

Thanks.

Dean

[Fenris]

Essentially the ADS-B transmissions are used by the TCAS collision avoidance systems on the aircraft, and Mode S generally is used as an upgrade to Mode C altitude-encoding transponders.

The ATC radars obtain a skin paint by the normal process of reflecting pulses from an aircraft and detecting their return, but that is a very blunt instrument and provides no real information on the target thus seen. So Secondary Surveillance Radar is used, which is a bit of a misnomer, because it isn't radar it is a transponder system. The interrogator operates at 1030MHz and the aircraft then transmits back at 1090MHz, the S in Mode S means 'select' which is a selective interrogation method whereas with Mode C all aircraft in range would respond to incoming 1030MHz signals and in dense environments that leads to many signal collisions.

I'm sure some Googling will lead to more information on the subject.

[DaveReid]

Essentially the ADS-B transmissions are used by the TCAS collision avoidance systems on the aircraft.

While that's true, TCAS works perfectly well in the absence of ADS-B data and the latter is only used to enhance perfomance of the basic TCAS system, which is based on 1030/1090Mhz standard interrogation/responses and, in suitably-equipped aircraft, Mode S acquisition squitters.  TCAS isn't normally identified as one of the drivers for the introduction of ADS-B.

The FAA has a very good Introduction to TCAS at www.faa.gov/about/office_org/headquarters_offices/ato/service_units/nextgen/research_tech_dev/eng_dev_serv/tcas/media/pdf/tcas.pdf (makes no mention at all of ADS-B).

[Fenris]

Fair enough Dave, I was trying to write something that was short and fairly simple. You are correct that TCAS predates ADS-B and works in its absence.

[viking9]

Surely the simple answer to the OP is - nobody, except for development purposes as it has not yet been introduced into any ATC service that I know of.

Tom

[tarbat]

Surely the simple answer to the OP is - nobody, except for development purposes as it has not yet been introduced into any ATC service that I know of.

Aberdeen ATC use ADS/B (and multilateration) for north sea offshore helicopter operations beyond their normal radar range.  Last I heard, it will be fully operational by June 2010.

[DaveReid]

Aberdeen ATC use ADS/B (and multilateration) for north sea offshore helicopter operations beyond their normal radar range.  Last I heard, it will be fully operational by June 2010.

And ADS-B is also used operationally by Nav Canada to provide separation for aircraft over Hudson Bay.

See www.navcanada.ca/ContentDefinitionFiles/Services/ANSPrograms/ADS-B/ADS_B_Brochure_EN.pdf

[viking9]

Must admit I forgot about the Canadian one, but as far as I know the Aberdeen one is still in development with only ten helicopters fitted.