revolutions, with a typical overlap of 2 days between successive arcs. Nowadays, orbit determination is made weekly using an arc of about 10 days duration and therefore with similar overlaps. The differences between the values of common range residuals computed from successive orbit determinations provide an indication of the effect of orbit determination errors on the accuracy of the residuals. Outside of periods of superior solar conjunction, these differences are almost always below 3 m. The range residuals that are retained are taken from the earlier of each overlapping solution.

[Figure: Fig. 2. Impact of the asteroid ring on the position of the SSB over 100-years integration.]

4. Presentation of MEX and VEX observations The tracking of Mars Express (MEX) and Venus Express (VEX), while orbiting their planetary namesakes, comprises two-way coherent Doppler and range measurements. The Doppler data are converted to range-rate: the component of the spacecraft’s velocity relative to the ground station along the direction from the station to the spacecraft. The determination of the spacecraft orbits uses only the range-rate data since, under normal circumstances, the additional inclusion of the range data leads to only insignificant improvement in the accuracy of the orbit solutions. Within the orbit determination, the computed values of the observables rely on high fidelity modeling of the dynamics and the signal path (Budnik et al. 2004; Moyer 2000). As part of this modeling, the orbital states of Mars and Venus are taken from the JPL DE405 planetary ephemerides (Standish 1998b). Over time the range residuals exhibit a signature whose predominant contribution is the error in the distance from the Earth to the planet as given by the planetary ephemerides. The range residuals are therefore derived data that are very useful for improving the accuracy of planetary ephemerides. The accuracy of the range residuals depends on a number of factors, which are described in the following subsections.
4.1. Accuracy of the MEX orbit determinations
4.2. Accuracy of the VEX orbit determinations
MEX, the first European mission to Mars, was launched on 2nd June 2003 and inserted into Mars orbit on 25 December 2003 and is presently extended until May 2009. MEX is tracked with both ESA 35 m deep space antennas and those of the NASA Deep Space Network (DSN) (mainly at the Goldstone complex), in almost equal proportions in terms of tracking duration. Most of the ESA tracking is from New Norcia (NNO), in Western Australia, the rest from Cebreros, near Madrid in Spain.
VEX, the first European mission to Venus, was launched on 9th November 2005 and inserted into Venus orbit on 11th April 2006. Its mission is presently extended until May 2009. The primary ground station supporting VEX is Cebreros, which tracks the spacecraft almost every day. Between 27th April 2006 and 13th March 2008, there were 663 Cebreros passes whose accumulated duration accounts for 97.7 % of the total tracking duration.

New Norcia has provided two-way coherent radiometric data during 12 passes and NASA DSN stations during 26 passes, of which 16 were made with DSS 43 at Canberra, mainly during the superior solar conjunction in autumn 2006.

The sampling rates of the range data and the random noise on the measurements are the same as for MEX.

The operational orbit is polar and highly elliptic.

There is hardly any precession of the line of apsides, so the periapsis, whose argument is currently at 95◦, remains close to the planet north pole.

The apoapsis altitude is about 66500 km and the periapsis altitude has been controlled to stay within the range of 185 km to 390 km.

The orbital period is nominally 24 hours and the maximum excursion has never been more than 6 minutes.

Every orbital revolution, in the vicinity of apoapsis but not within ground station visibility, the momentum of the reaction wheels is off-loaded by thrusting.

The perturbing ∆V into the orbit is typically in the range 15-25 mm/s which is substantially higher than for the MEX wheel off-loadings.

The spacecraft attitude and the direction of the thrust are chosen so that the manoeuvres help to control the orbit phasing.

The control is such that the signal elevation from the daily Cebreros passes rises to 10 ◦(when telecommanding may start) always close to 2 hours after periapsis passage.

The pass ends either 10 hours later, close to apoapsis, or at 10◦descending elevation, whichever is earlier.

Tracking data obtained in X-band up and downlink are thus almost never acquired during the descending leg of the orbit, nor around periapsis.