iPad Mini 4 - GPS altitude off by > 1000ft

Peter wrote:

What happens to the altitude is a good question….

Interestingly, mine shows the correct altitude when I’m sitting at home and am connected to WiFi. How that works, I have no idea…. And, as @Peter says, you may well get some altitude info acquired previously from somewhere else.

Lucius wrote:

The GPS altitude is not always off. There is no consistency. Sometimes it is accurate, sometimes not. I am wondering whether to return it.
Thank you.

In principle the ongoing GPS solution has to sacrifice one of the dimensions to obtain the most accuracy in the others. So any platform that is designed for earthbound movements will have it’s altitude the least accurate.

The GPS dimensions are: lat-lon-height-time.

Surely -

• the time must be known really accurately
• many years of experience has shown to me that the altitude is way more accurate than most people say

I suspect something is wrong with Lucius’ Ipad, to get such a massive error. Maybe the constellation has got corrupted and it cannot detect that. After all, these devices are – tongue in cheek – designed to locate the nearest Macdonalds And if there was such a bug I bet it would be very much under-reported. And even if Apple know about it, it’s hardly something for them to worry about. Probably 0.01% of their users ever check the altitude.

I have the same problem as Lucius with my old mini iPad. Usually fairly accurate (within 50 ft) but sometimes the error is closer to 1000 ft for some time. Definitely not a QNH issue at low altitude. I have no idea what it is about – there is no pattern except it only happens when flying.

I have never experienced a GPS that does not apply geoid correction to the altitude – android smartphone, iPad, sport tracker, handheld aviation unit. On some it is possible to turn it off, but in my experience it is always on by default.

Most non-augmented GPS’s seem to show within 30 ft of the correct geometric altitude on the ground. For some reason the non-aviation units I use generally read about 20 ft low. Smartphones, iPads and car GPS do that. I notice it when being on a shore – they usually tell me I am below sea level.

On the run-up pad, non-SBAS GPS altitude is usually just as accurate as – or a little better than – barometric altitude. With SBAS (“WAAS” – required for LPV approaches), it seems to be within 10 ft always.

huv wrote:

I have the same problem as Lucius with my old mini iPad.

Does it have a SIM card or is it WiFi only? Same Q applies to Lucius, who hasn’t been back to the discussion.

As suggested, the LPV / baro-NAV / temperature compensation discussion is here

Archie wrote:

In principle the ongoing GPS solution has to sacrifice one of the dimensions to obtain the most accuracy in the others. So any platform that is designed for earthbound movements will have it’s altitude the least accurate.

The GPS dimensions are: lat-lon-height-time.

I can’t believe it would be by 1800 feet.
The phase-lock loops wouldn’t be stable horizontally with that kind of dilution of precision of the fixed solution.
Perhaps a hundred feet (he says the horizontal is bang on) but the GPS constellation is truly 3D, which means the algo would have to be programmed to start rejecting satellites to retain the fixed solution.

Also, I don’t have an iPad, so I can’t say anything intelligent about the chips, but perhaps it is doing some sensor fusion?
i.e. using an in-built barometer or some other means to obtain altitude while using GNSS for position…

I’ve never seen more than 100m of sustained error with a proper fix/solution. I used global positioning professionally on a daily basis and saw all manner of craziness, but never 1800ft. Not even 1000ft (sustained).

I’m in agreement with everyone else about something else being the cause.

You might also check the number of satellites used for the solution. If it is 4 or less, you’re ‘fixed solution’ is probably not even…
See if you can cover up the GNSS receiver portion of the tablet to force a loss of lock/solution and then uncover it again. (I put mine under my leg sometimes)
Please wait about 2-3 minutes before bringing it out again, else it will still use the previous fix to recalculate your position…

If blocking the sky to force the re-initialization works, then it is either the OS’ problem, or it is the chipset. (likely the chipset) and you’d be best served either getting a new tablet, or wiping yours and hard-reseting to try again with a fresh OS. (I don’t use iOS, so I can’t say anything to the chipset interface)

This may be relevant. However, nobody seemed to know the exact answer as to how GSM is used to assist a GPS fix, and how much SIM functionality is required.