Ten days of GPS data from 1998 were processed to determine how the accuracy of a derived three-dimensional relative position vector between GPS antennas depends on the chord distance (denoted L) between these antennas and on the duration of the GPS observing session (denoted 7). It was found that the dependence of accuracy on L is negligibly small when (a) using the 'final' GPS satellite orbits disseminated by the International GPS Service. (b) fixing integer ambiguities. (c) estimating appropriate neutral-atmosphere-delay parameters, (d) 26 km less than or equal to L less than or equal to 300 km, and (e) 4 h less than or equal to T! 24 h. Under these same conditions, the standard error for the relative position in the north-south dimension (denoted S,, and expressed in mm) is adequately approximated b the equation S-n = k(n)/T-0.5 with k(n) = 9.5 +/- 2.1 mm . h(0.5) and T expressed in hours. Similarly, the standard errors for the relative position in the east-west and in the up-down dimensions are adequately approximated by the equations S-e = k(e)/T-0.5 and S-u = k(u)/T-0.5, respectively, with k(e) = 9.9 +/- 3.1 mm . h(0.5) and k(u) = 36.5 +/- 9.1 mm . h(0.5).
