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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?

lemonincider
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    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 http://www.garmin.com/us/intheair/ads-b/squit/ – JScarry Jun 25 '17 at 13:47
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    @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 – lemonincider Jun 25 '17 at 15:16

1 Answers1

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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.

DeltaLima
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    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? – lemonincider Jun 25 '17 at 15:11
  • You could have an ADS-B transmitter on 1090MHz that is not a Mode S transponder, but that is unusual. – DeltaLima Jun 25 '17 at 15:20
  • Okay I'll take it as yes. Thank you for your detailed explanation. – lemonincider Jun 25 '17 at 15:30
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    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. – Jan Hudec Jun 25 '17 at 17:22
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    @JanHudec you are right of course, but effectively only GPS is used at the moment. – DeltaLima Jun 25 '17 at 17:23
  • @DeltaLima, also, even if there are already dual-system chips in use (they are in use in handheld devices for quite some time, but certification for aviation takes time), I doubt any chip would only use some other system and pass on GPS. – Jan Hudec Jun 25 '17 at 17:29
  • The other thing is, however, that it's still quite common to see aircraft report just INS position, which shows on FR24 as aircraft landing say a mile off the runway. – Jan Hudec Jun 25 '17 at 17:31
  • I believe that the position can be from any approved position source, which just happens to be GPS. In theory one could derived the position from a non-GPS source such as Galileo, GLONASS or INS/IRS if it meet the position source requirements. See https://www.ecfr.gov/cgi-bin/text-idx?node=14:2.0.1.3.10#se14.2.91_1227 – Steve Kuo Jun 26 '17 at 15:52
  • FYI, some older plane's have a waiver to transmit a position derived from non-GPS (usually DME/DME/IRU) sources, because there is no certified GPS available for those types. – StephenS Mar 10 '21 at 19:11