Combination of multi-satellite techniques at the observation level

Satellite techniques are very important for the IERS products EOP (incl. nutation) and terrestrial reference frame (station coordinates and their velocities). Currently the results from various satellite techniques are combined together at the normal equation level. The only link between them are the local ties of the collocation sites. For some years now, however, quite a number of satellites have been launched, which are equipped with instruments of more than one technique. In this paper we call it ‘collocation (of multi-techniques) on board satellites‘. Examples are: GPS35 and GPS36 (GPS sender + SLR); ENVISAT (SLR + DORIS); Jason and T/P (GPS receiver + SLR + DORIS); CHAMP and GRACE (GPS receiver + SLR). In order to fully exploit on-board collocation, combination at the observation level is necessary.