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In certain locations GPS units can recieve an SBAS correction signal (WAAS, EGNOS etc) but are outside the coverage area of a base station. For example, EGNOS signals can theoretically be recieved in Africa but lack local base stations that can provide correction data.

  • In such areas should SBAS be turned off?
  • In the event that SBAS is turned on what kind of error is likely to be introduced as a result of incorrect correction?

I am specifically interested in consumer GPS units (such as Garmin).

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If I'm understanding correctly, there should be no correction applied since there's no base station to calculate it from. Since the GPS knows the approximate location, it will know whether any correction signal it receives is applicable or not.

...so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit.

From http://www8.garmin.com/aboutGPS/waas.html

As for whether or not it should be turned off, I found as much discussion out there on that as to what the actual benefits even are. In terms of location error, whether it's on or off should make no difference if no correction is available in the first place. So then you're looking at other factors/reasons. There seems to be a lot of anecdotal evidence both for and against disabling to conserve battery power.

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  • As a developer of marine AIS systems using GPS we have found that in the Southern Pacific the enabling of SBAS can cause the fix to be off significantly by several ten's or even hundreds's of meters - much worse then with SBAS disabled. My assumption was that the Japanese MSAS system was being applied inappropriately; but this would suggest that corrections are being applied outside of the coverage area. Given your answer, do you have an explanation for this issue? We recommend our Australian and New Zealand customers disable SBAS, but a few round-the-world trippers have been caught out.
    – Clifford
    Jan 8, 2016 at 12:02

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