2.1. Clock differences as a by-product of geodetic VLBI analysis
No technique is capable of providing absolute clock parameters at each site. Therefore it is necessary that one clock at one selected station in a VLBI network is kept fixed and all other clocks of the other stations in the network will be related to this reference clock. By doing so, clock differences can be obtained from geodetic post-processing, i.e. the adjustment of the unknown parameters. Since the clock difference parameters need to be estimated together with the other unknowns, one needs to pay attention that neither unmodeled effects bias the clock estimates, nor correlations among the unknowns absorb clock variations in other parameters. Moreover, the parametrization of the clock unknowns has to reflect the physical behaviour of the clock, i.e. providing a temporal sampling which can follow a random walk (in phase) noise process.