说明：  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 ?