No. 88 - Trajectory Control Manoeuvre
29 October 2007 16:26
Report for period 13 - 26 October 2007
The reporting period covers a period of 2 weeks of Active Cruise Mode characterised by activities linked to the preparation of the Earth Swing-by. Frequent tracking passes are taken in order to assess the spacecraft trajectory. On 18 October a Trajectory Control Manoeuvre has been performed to set the spacecraft onto the right track for the Swing-by.
At the end of the reporting period (DOY 299) Rosetta was at 15.8 million km from Earth (0.1 AU; one-way signal travel time was 53 seconds). The distance to the Sun was 164 million km (1.09 AU).
The RCS fuel consumption 38 Grams.
On the plot below, the red cross and surrounding 3-sigma error ellipse were the estimates for the location in the Rosetta B-plane, corresponding to the 2nd Earth swing-by, that were made on the basis of an orbit determination using data from the time of NSHM exit up to the end of the pass on 04 October.
The orange cross and surrounding 3-sigma error ellipse were the estimates made on 15 October that were used to plan the trajectory control manoeuvre (TCM) that was executed on 18 October. Using data up to the end of the NNO pass on 21 October, the blue cross and surrounding 3-sigma error ellipse shows the current estimate in the B-plane. The location is 3 km from the target. The target location corresponds to a perigee altitude of 5299 km (relative to the Earth's equatorial radius). The estimate corresponds to an altitude of 5296.8 km with a 3- sigma uncertainty of 17.0 km.
The estimated time of closest approach (perigee passage) is 20:57:22.9 UTC on 13 November.
The 3-sigma uncertainty is now 2.6 seconds.
From the B-plane plot it is obvious that the TCM performance was near nominal. However, it was very small and the information content within the tracking data is not sufficient to give a precise calibration. If the manoeuvre direction was perfect then from the change in the Doppler it would be known that the manoeuvre overperformed by 2.6%. But the calibration within the orbit determination gives a small direction error so that the overperformance estimate is reduced to +0.6% (+0.2 mm/s). The 1-sigma uncertainty on this value is 2.9% (1.0 mm/s).
The preparation of the Earth Swing-by phase is proceeding according to the plan. Latest orbit determination indicates that a trajectory correction manoeuvre at ESB2-2w is very likely not needed. A final decision will be taken on 30 October with the most up-to-date navigation data. The closest approach during the swing-by will take place on the 13th of November at 20:57 UTC.