1. what kind of noise/errors/uncertainties/unmodeled things affect

the GPS precision : AS, SA, orbits, clocks, ionosphere,

troposphere, antenna phase centers, centering (tribrachs), etc...

2. how do we evaluate GPS uncertainties ? (difference between

formal and a posteriori) can we trust them ?

3. difference between precision and accuracy (internal consistency

like repeatability is assertion of precision, comparison with other

method affected by different biases is accuracy)

4. reference frames : how do we map ? with what precision ? What

influence on results ?