Mission Scenario

Both GRACE satellites will be launched  simultaneously with a ROCKOT launch vehicle from Plesetsk/Russia (62.7° N, 40.3° E) presently scheduled for November, 2001.

ROCKOT launch vehicle services are provided by  EUROCKOT Launch Services GmbH, a joint venture company of DaimlerChrysler Aerospace and Khrunichev State Research and Production Space Center. ROCKOT is a three-stage launch vehicle, comprising an adaption of the SS-19 for the first and second stages, and the maneuverable and re-ignitable BREEZE third stage. With a launch mass of 107 tonnes, ROCKOT can place up to 1900 kg payload into a low Earth orbit.

The  characteristic orbit will be almost polar (i=89.0°) and near circular (e<0.005) starting with an initial altitude between 485 and 500 km. The anticipated lifetime for scientific operations is 5 years.

The near-polar orbit was chosen to get a homogeneous and  global coverage of the Earth's sphere which is necessary for a precise estimate of the gravitational geopotential.  The advantage of the  89.5° orbit vs. a dawn-dusk sun-synchroneous orbit is the local time variation of the satellite's ground tracks which is essential  for the separation of constituents of periodic phenomena like tides or day-night variations of the atmosphere.

An  initial altitude of about 485 - 500 km was chosen (a) in order to guarantee a multi-year mission duration even under severe solar activity conditions and (b) to get a good compromise between  gravity field solutions, which desire an even lower altitude, and atmospheric/ionospheric applications, which  in contrast would benefit from an higher altitude.

Due to atmospheric drag the altitude of the GRACE satellites will  decrease over the 5 years mission duration. As GRACE will be launched around the next solar activity maximum, the predicted natural decay depends on the magnitude of the actual solar activity cycle and may amount to more than 200 km (high activity) or only 50 km (low activity). In order to ensure the mission lifetime of 5 years, the altitudes of both satellites may be re-boosted once, if deemed to be necessary.

GRACE decay scenario

During science data collection the satellites will be nominally Earth oriented. For precise pointing of the two K-band ranging system antenae both satellites have to be pitched by about -1°.  This causes drag force differences which  result in along-track separation changes. Therfore station-keeping manouvres will be carried out every 30 - 60 days to keep the two satellites at their nominal  separation of  220 ± 50 km. To ensure the uniform exposure and aging of the K-band antennae the leading and the trailing satellite will be exchanged once during the mission.

The payload and sub-system data collected onboard GRACE amount to about 100 MB per day (tbc) which will be downloaded during  4-5 passes per day to DLR's ground receiving stations in Neustrelitz and Weilheim. Mission operations and satellite control will be performed by the German Space Operation Center (GSOC) in Oberpfaffenhofen. For further information please refer to the descripion of the Mission Operation System (MOS).

The GRACE mission shall be divided into five different phases:

The Pre-Launch Phase lasts up to the launch of  the satellites. During this phase the satellites, the instruments and the ground systems will be designed, integrated and tested.

The Launch and Early Operations Phase (LEOP) starts at time of launch and ends when the following conditions are fulfilled:

1. both satellites are in safe, stable orbits, approximately 220 km apart, with no danger of collision with each other, with the launch vehicle, or with co-passenger satellites,
2. both satellites have attained nominal attitude control ,
3. nominal uplink and downlink communications are achieved with GSOC stations and
4. no anomalies exist that pose a near-term threat to the mission.

In the absence of major unexpected events, the LEOP should be completed within the first 5 - 10 days after launch

Following LEOP a Commssioning Phase starts in which initial power-up and evaluation of the science instruments is performed. Additionally the SST link will be established , the center-of-mass offsets in all three axis will be calibrated and trimed, and - if necessary - software patches and parameter updates commanded.

Then the mission will enter a 6 months Validation Phase focussed on providing an end-to-end characterization of the science instrument and data systems. The following activities occur during this phase:

1. continuous record and downlink of science data, data flow problems resolved,
2. K-band ranging boresight is calibrated and verified,
3. precise orbit solutions are obtained and verified using SLR data,
4. initial gravity field solutions and accelerometer calibration are derived and
5. these gravity field solutions will be verified by internal consistency checks and compared with in-situ data.

Afterwards the science data will be routinely gathered in the Observational Phase which lasts until the end of the mission, with the exception of brief periods for orbit maintanance and instrument re-calibrations.