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The ADS-B system is only as accurate as the location data source it uses. In the US the 2020 ADS-B mandate requires that horizontal location data be from a GPS source with minimum performance standards set out in 14 CFR § 91.227.1

The aircraft in the Lamia crash in Colombia had ADS-B out that was based on an inertial system resulting in their ADS-B track being off from their actual position. It was off by 2 km when the flight began and drifted further during the flight, possibly being off by as much as 4 km by the time of the accident.2

Although ATC radar will still get an accurate location from the transponder, other aircraft might be using the data for traffic advisories through a system like ATAS. Using a system that could be sending inaccurate data has the potential to be dangerous.3 No info would be preferable to incorrect info. Is there any reason to install an ADS-B out system that does not use a GPS data source?


1ADS-B Frequently Asked Questions (FAQs)

2Satcom Guru: LMI2933 LAMIA AVRO RJ85 Medellín Deadstick

3At this point there is no reason to believe inaccuracy of location data had any bearing on the LMI2933 accident.

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    $\begingroup$ BTW, source 2 in the question is a fascinating read. He did a pretty thorough analysis of the ads-b data in the accident. $\endgroup$ – TomMcW Jan 4 '17 at 1:45
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    $\begingroup$ No, there is no reason to do this, as the FAA will not currently provide type certificates to ADS-B equipment that does not meet or exceed 91.227, (TSO-C166b and TSO-C154c). There is already one case where the FAA has recinded the type certificate for ADS-B equipment for uncomplying hardware (Navworks has words too). This results in a huge waste of money on the part of the owner, as the aircraft is not compliant for airspace requiring ADS-B. $\endgroup$ – Ron Beyer Jan 4 '17 at 2:05
  • $\begingroup$ @RonBeyer Wow, That's quite a snafu! $\endgroup$ – TomMcW Jan 4 '17 at 3:02
  • $\begingroup$ Not only that, but the FAA is recommending that owners not only deactivate the equipment, but have it removed. They effectively grounded all the aircraft with this equipment installed because now you have non-TSO avionics installed in a certified aircraft. This is the reason that my aircraft is getting a Garmin GTX345 from a major manufacturer rather than a start-up bargain ADS-B. $\endgroup$ – Ron Beyer Jan 4 '17 at 4:01
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    $\begingroup$ @RonBeyer I guess you get what you pay for. Everybody's gotta start somewhere, though. I used to deliver to Garmin when they were working out of a small warehouse in Lenexa KS $\endgroup$ – TomMcW Jan 4 '17 at 4:06
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There are still quite a lot of aircraft that do have ADS-B based on inertial navigation position.

The reason for this is quite simple; they don't have GPS or the GPS is not connected to the transponder. Somewhere during the lifetime of the aircraft the transponder has received an update or has been replaced and is now transmitting ADS-B messages, based on the only position source available to the transponder; the Inertial Navigation System. That is fully legal, but it does not meet the ADS-B requirements of the various ADS-B mandates around the world.


Is this a danger to the safety of flight when ADS-B IN based systems like ATAS are used?

No, not at all. ADS-B IN systems do not use the data from these aircraft as the ADS-B messages contain several quality indicators that must meet certain conditions before a track is being displayed in the cockpit. Transponders transmitting INS positions indicate that the position integrity and accuracy is very low. In addition the source integrity level (SIL) will also be set to 0. Each of these indicators will result in the track not being displayed.

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