Hi,
This is a problem in certain companies which have the kit approved to do this ...but do not have the training...access to NOTAMs which tells you where / when it is likely to be unreliable. The sun is a major contributor to GPS unreliability...over the next few years the sun is entering a major period of instability and we will see much more of this. In my experience when it goes wrong (three times for me) it goes wrong spectacularly. One old /wise (but slightly bitter) bod said to me treat it like a woman.... love it to bits...but don't trust it.
