[riverrock]

Your cellular iPad can do an initial triangulation using mobile phone network and known WiFi locations, then download the almanac via the phone network rather than from satellites. It can then connect to GLONAS and Galileo as well as GPS and can show you a rough / guessed position rather than one it's sure off.
So yes - will be much quicker!

[Paul_Sengupta]

Where is Paul Sengupta when you need him?

At an 80th birthday party (not mine!) and then a BBQ!

Stevelup is your man for all things iPad, where is he these days, he seems to have gone quiet? I'd only be guessing. But my guess would be the same as others on here, that Apple uses "location services" or such like to provide a position. This doesn't use the GPS directly, so you never actually know if you're getting a position from the GPS or another method. I think. Or so I've read on here. So if the GPS hasn't locked on yet, you may see an erratic location.

This could be due to various factors, airframe shielding, jamming, ground shielding, not having had enough time with enough visible satellites, etc. As you climb, you'll both get away from jamming, and provide a clearer view for seeing satellites.

I'm not sure how you can tell you have a GPS lock on an iPad, are there GPS apps which will tell you which satellites you're receiving and which are being used for your position fix?

[Mr Dinos]

Thanks all for your input. I guess there’s a few things for me to try out. I think the theory that it’s not getting a good GPS lock initially due to some factor either on the ground or within the aircraft sounds plausible.

[dublinpilot]

A thought. You mention that it happens when close to the ground.

But could it be that it happens when you're landing and taking off from your home airport only?

If so, maybe it's something to do with the aircraft radio? After departure, you change frequency and the problem disappears? When returning you change back to the local frequency and make your arrival call, and suddenly the problem reoccurs?

Maybe your radio is transmitting something on that frequency that is causing interference?

Does it happening when taking off and departing from other airfields where you are using a different frequency?

[Mr Dinos]

@dublinpilot That’s a good point and something I had thought about. It doesn’t seem to happen at other airfields. It happens only on takeoff at Elstree.

[Sooty25]

keep that 1:500,000 and stopwatch handy!

[GrahamB]

keep that 1:500,000 and stopwatch handy!

Why?

It’s still a half-mill (or one of a number of other scales) without the GPS working, and one that’s likely to be more up to date.

[patowalker]

Simplest test would be Aeroplane mode On.