GRACE Orbital Configuration (plots updated daily)
The twin GRACE satellites are labeled GRACE-1 and GRACE-2 by the Operations Team. The Science Data System uses the labels GRACE-A and GRACE-B. Occasionally, the press has called them Tom & Jerry.
(Some other suggestions for names that got nowhere include: Drucken/Ziehen, Tweedledum/Tweedledee)
The "front" of each satellite is the side that contains the K-Band transmitter/receiver horn, and the "top" is the narrow roof containing the GPS receiver antenna. The schematic layout of the satellites can be seen here.
From mission start to December 10, 2005, GRACE-1 has flown "backwards", with its K-Band horn pointed towards the corresponding horn on GRACE-2. On December 10 2005, the satellites swapped positions, after which GRACE-2 is flying backwards while leading.
Evolution of GRACE Mean Orbits
GRACE satellites were launched on March 17, 2002, on-board Rockot, from Plesetsk Cosmodrome in Siberia.
The satellites were injected into a 500 km altitude, near circular polar orbit.
Since then, the satellite orbit and its ground-track have been allowed to drift naturally. The mean semi-major axis for GRACE-B is shown below.
The plot of mean eccentricity shows the characteristic 94-day perigee period. The periodicity in the mean inclination shows diverse effects. The near 160 day (S2 alias) period is the influence of the Sun on the orbit plane; and the slower half-cycle seen over 3 years (K1 alias) could be related to luni-solar effects and the orbital precession.
The next plot shows the evolution of the mean perigee and node. The node precesses barely at all, due to the polar inclination of the satellite. The perigee completes one cycle every 94 days.
The next plot shows the angle between the Earth-Sun line and the orbit plane (or the beta_prime angle). This angle is defined such that it is zero when the Sun is within the orbit plane.
Relative Orbit Evolution (plots updated daily)
The orbit of GRACE-A relative to GRACE-B is shown in this section. Discounting some early orbit adjustments, the mean semi-major axis difference (shown in the first plot) between the two satellites averages around 0 meters. Step changes in the semi-major axis difference appear when orbit maneuvers are executed in order to keep the separation between 170 and 220 km. In the early days of the mission, some changes were also caused by an attitude mode loss on board one of the spacecrafts, which leads to increased drag.
The semi-major axis difference and drag acceleration differences are the largest contributor to the macro-scale evolution of the inter-satellite range. The following plot shows the range between GRACE-A and GRACE-B at midnight each day.
The inclinations of the two satellites are slightly different, as shown below.
Over a smaller time scale, the intersatellite range has a largely 1-cycle per revolution variation of approximate 2-3 km amplitudeā a sample for an arbitrarily chosen day is shown below. The drift from start to end of day is part of the large trend seen before.
The range-rate is of the order of 2 m/s amplitude, as shown below.
Embedded within these large signals, are variations at the level of few tens of microns, or few tenths of micron/seconds, which are caused by mass re-distribution processes within the Earth system. It is these small, hidden signatures that the GRACE Science Data System attempts to extract as models of the Earth gravity field.