Launch & Early Operations Phase (LEOP)
The GRACE satellites will be launched together on a single vehicle. This phase, starting with lift-off, provides frequent opportunities for monitoring the satellite status to increase the opportunity for ground-based intervention.
In addition to the GSOC ground stations at Weilheim and Neustrelitz, NASA tracking stations at McMurdo, Spitzbergen, Poker Flatshall be available for both telemetry and command uplink during the LEOP phase.
The LEOP phase begins on lift-off and ends when the following conditions have been met:
- Both satellites are in safe, stable orbits with no danger of collision with each other, with the launch vehicle, or with co-passenger satellites.
- Both satellites have attained nominal attitude control including successful star-camera acquisition.
- Nominal uplink and downlink communications are achieved with GSOC stations
- No anomalies exist that pose a near-term threat to the mission.
- The nominal separation distance between the satellites (220 km ± 50 km) has been achieved and stabilized.
The sequence of events can be briefly described as follows. After the desired orbit has been achieved, the satellites are simultaneously released from the launch vehicle's upper stage with velocity relative to each other of approximately 0.5 m/s. The satellites are activated by separation from the launch vehicle, and autonomously begin to measure and remove angular rates imparted by the separation dynamics. In less than one minute the magnetometer boom is deployed and the low rate S-band transmitter activated, transmitting until turned off by ground command. During this initial acquisition period, or after experiencing an anomaly, the satellites must be capable of survival and attitude recovery with little or no real-time ground interaction.
Following the first pass over Germany, the GRACE satellites will reach the coverage area of Spitzbergen and subsequently that of Poker Flat. These two NASA polar network ground stations will be used for reception of telemetry and for relaying telecommands issued by GSOC. Subsequent orbits will be used to verify performance of both satellites and issue commands as needed to complete this Phase.
Following LEOP, the mission enters the Commissioning Phase, centered around the initial power-up and evaluation of the Science Instrument System. The following are some of the activities occurring during this phase:
- Turn-on and check-out science instruments, IPU and star camera
- Establish inter-satellite link and characterize its performance
- Initial calibration and trim of CM offsets in three axes
- Update power and thermal budgets based on measured variations
- Software patches and parameter updates
Following the successful completion of the Commissioning Phase, the mission enters a six-month Validation Phase focussed on providing an end-to-end characterization of the Science Instrument and Data Systems.
The following activities occur during this phase:
- Continuous records of science data are downlinked and data flow problems resolved
- KBR Boresight is calibrated and verified
- Precise orbit solutions are obtained and verified using terrestrial laser tracking data
- Initial solutions for gravity field, along with ACC calibration, are carried out
- Preliminary gravity field solutions are verified through a combination of internal consistency checks and comparisons with in-situ data.
Following the Validation Phase, the mission will enter the Observational Phase, in which science data is routinely gathered from the science payload. This phase will continue until the end of the mission, with the exception of brief periods for orbit maintenance and recalibrations.