GPS relative accuracy for two nearby units

[chrisN]

2 questions about GPS, when read by two different gps units:

First, as posed by Peter H on the AF447 thread: [What is] the difference in readings between two bog-standard GPS receivers at the wing-tips. Bog-standard, without superfast processor and GPS chips.

The issue being, whether they would have the same error as each other in positional accuracy and hence would be very accurate in relative position to each other (e.g. to measure bank angle?)

The second issue, which I am interested in for reasons totally unconnected with AF447 – in fact, when two gliders using Flarm are in close proximity: As well as whether two GPS units have the same GPS position error, when using the same satellites because they are close to each other: if two adjacent gliders are differently banked, or say on opposite sides of a steeply turning circle, could they be calculating positions by reference to different, and continuously changing, satellites; and if so, could this lead to quite different errors and so not have accurate relative positions and heights?

(Flarm works by comparing the position and projected track broadcast by one unit with the position and projected track known to the other, and hence projected whether potentially colliding or not). In the case of two different gliders each with its own Flarm device, they may be made by different manufacturers – albeit using the same software and/or algorithms – but if they might use different makes of GPS engine, I wonder if they would anyway be susceptible to picking up different satellites and getting different local errors in position.

Chris N.


[edit: posted before I was able to see EmBee’s helpful answer to the first question above on the other thread.

And, by the way, I think the second issue also has implications for ADS-B as a proximity warning, particularly for light aircraft, if that ever comes about, as ADS-B seems to be a sort of Flarm-like concept AIUI.]