Accuracy of GPS coordinates

[vjongene]

I am going on a liveaboard in French Polynesia in November, and I am taking a Nautilus Marine Rescue GPS unit with me. When I contacted the company running the boat about this, they said "Please note that we no longer use GPS rescue devices ourselves. Our experience is that the devices often give the wrong coordinates, and therefore give a false feeling of safety."

Could anyone comment on this? It is obviously worrisome.

Thanks already.

 

[dlofting]

Don't know about the Nautilus specifically but I do know that some PLB's were tested in Utah slot canyons and the coordinates sent were close enough to be useful to Search and Rescue. I can't see what would inhibit the signal in the open ocean.

 

[Storker]

Reads like a CMA memo to me.

Yes, all GPS readings have some error. Yes, that error may - in some rare cases - be significant. No, you shouldn't trust that your GPS device is able to accurately pinpoint your location. OTOH, nine times out of ten (insert disclaimer here), you'll be close enough to the reported coordinates that it'll make it a lot easier for the SAR service folks to find you.

But for a company selling those services and facing the possibility of lawsuits, I can sort of understand that they need the CMA clause. OTOH, what's the better alternative?

 

[Charles2]

During the 1990s, GPS employed a feature called Selective Availability that intentionally degraded civilian accuracy on a global basis. Perhaps this is where/when the folks in French Polynesia got their experience. In May 2000, at the direction of President Bill Clinton, the U.S. government ended its use of Selective Availability in order to make GPS more responsive to civil and commercial users worldwide.

Today, the government commits to broadcasting the GPS signal in space with a global average user range error (URE) of ≤7.8 m (25.6 ft.), with 95% probability. Actual performance exceeds the specification. On May 11, 2016, the global average URE was ≤0.715 m (2.3 ft.), 95% of the time.

To be clear, URE is not user accuracy. User accuracy depends on a combination of satellite geometry, URE, and local factors such as signal blockage, atmospheric conditions, and receiver design features/quality.

I copied this from the government website: GPS.gov: GPS Accuracy which contains a lot of interesting tidbits that should give you more confidence in your Nautilus purchase.

By the way, there are other countries that have their own satellites with GPS systems. I can't speak to the relative accuracy of those devices.

 

[Johnoly]

..Could anyone comment on this? It is obviously worrisome.

Different GPS models "may" use different algorithm datums. Datum's are how the GPS interprets and tells you where you are at based on a known reference point. There are different datum reference points all over the world, so a US datum is different from an asian datum. An example is Google maps and Garmin marine don't line up exactly on the water. The error distance(?) can be as much at 3/4's of a mile. Which in open water is very easy for a good captain to spot an SMB. But diving around an island with a loop current may position you behind a beach that the captain does not have direct line of sight to you and adds to the search confusion. But it gets the SAR's close enough to cut down the time needed to find you. I've said before that I like VHF marine radios over PLB's because you'll know immediately you are talking to the captain.

 

[vjongene]

Thanks for the informative answers. It does indeed seem like a CMA, probably after they once failed to rapidly locate a diver carrying a GPS device. This will not stop me for taking one, and the boat IS equipped to receive its signals.

 

[Stoo]

Personally, I think even a crappy GPS location is better than none at all in an emergency. Even if it's off by a kilometre (which it absolutely won't be) it narrows down a search area from "he's right there, ya know, in the ocean somewhere".

 

[DrMack]

I think there are enough success stories with the Nautilus Marine Rescue GPS from around the world to negate the statement, "the devices often give the wrong coordinates". It is more likely that the people receiving the coordinates are malfunctioning.

 

[lowflyer]

A testament to the accuracy of GPS is that commercial airliners full worldwide use GPS approaches to airports (to land in instrument conditions).