A method based on multi-antennae linked to a common GPS receiver is pr
oposed. The goal of the technique is to improve height determination f
or baselines a few kilometres in length. The advantage of this techniq
ue resides in the elimination of relative clock parameters in the ''be
tween-antenna'' single difference observations. Because single differe
nce observations are free of clock errors more geometrical strength re
mains to determine the baseline components. This statement is valid as
long as intercable biases can be carefully calibrated. For millimetre
height determination, the intercable calibration must be done at the
same level of accuracy. Under this assumption it is shown that in gene
ral the height standard deviation improves by a factor of about three
compared to standard GPS data processing. With the proposed method, th
e effect of relative tropospheric zenith delay errors becomes a bit sm
aller (in absolute value), compared to standard data processing. To ab
sorb this error, a relative tropospheric zenith delay parameter may be
estimated. Even with this additional parameter in the solution the he
ight standard deviation remains two times smaller than the results of
standard processing techniques (without tropospheric zenith delay para
meter), and at least five times smaller than in the results obtained f
rom standard processing including one tropospheric zenith delay parame
ter.