Jeppesen defines the ADS-B as a surveillance system broadcasting GPS-derived positions and other information such as velocity over the data link. Yet, the Wiki article on ADS-B relates it to the Mode S transponder system. So the ADS-B operates either on a GPS or on Mode S. Is my understanding correct?
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asked Jun 25, 2017 at 13:28
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2$\begingroup$ Your ADS-B transmitter gets its position data from a GPS source. Then it transmits the data using either 1090ES or 978 Universal Access. Mode S transponders already transmit detailed information about the aircraft, called a 'squit'. Some can be upgraded to send 'Extended Squitter' info. The other alternative, in the US only, is to transmit the info on the 978 band. More info at garmin.com/us/intheair/ads-b/squit $\endgroup$– JScarryCommented Jun 25, 2017 at 13:47
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2$\begingroup$ @mins What I meant is closer to "What's required for the ADS-B to function? A GPS or Mode S Transponder?" I'll change the title $\endgroup$– lemoninciderCommented Jun 25, 2017 at 15:16
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ADS-B, Automatic Dependent Surveillance - Broadcast, is a broadcast of the aircraft's position and other information.
The position is determined by a GPS receiver. It is possible to use another qualified position source, but today it is de facto GPS. Other GNSS (Galileo, GLONASS, Bei-Dou) are other likely candidates in the future, but as far as I know, none have been certified for ADS-B yet.
The broadcast is usually by the Mode S transponder but can also be done by a dedicated transmitter / transceiver in the Mode S format and frequency (1090 MHz).
In the USA below 18 000 ft, in addition to the Mode S based ADS-B, a system based on Universal Access Transceiver (UAT) is used. This operates on 978 MHz
A third broadcast system was VDL mode 4, it was never rolled out on large scale.
So ADS-B uses GPS to determine the position and it operates either on 1090 MHz (a frequency also used by Mode S) or 978 MHz to broadcast information.
answered Jun 25, 2017 at 14:45
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1$\begingroup$ Then in order for the ADS-B to function, an aircraft should be equipped with both a GPS and a mode S transponder (or UAT). Without either of them, the ADS-B simply doesn't function. Correct? $\endgroup$ Commented Jun 25, 2017 at 15:11
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$\begingroup$ You could have an ADS-B transmitter on 1090MHz that is not a Mode S transponder, but that is unusual. $\endgroup$– DeltaLima ♦Commented Jun 25, 2017 at 15:20
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$\begingroup$ Okay I'll take it as yes. Thank you for your detailed explanation. $\endgroup$ Commented Jun 25, 2017 at 15:30
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2$\begingroup$ Nothing requires the position to be determined by GPS specifically. Requirements are just for precision and any GNSS can do that, so GPS, GLONASS, Gallileo (and BeiDou-2 is under construction and some more are planned). Since the principle and protocols are similar, modern receivers may use more than one. $\endgroup$ Commented Jun 25, 2017 at 17:22
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1$\begingroup$ @JanHudec you are right of course, but effectively only GPS is used at the moment. $\endgroup$– DeltaLima ♦Commented Jun 25, 2017 at 17:23