There are two payload elements on each GRAIL orbiter: the Lunar Gravity Ranging System (LGRS) which is the science instrument, and the MoonKAM lunar-imaging system which is used for Education and Public Outreach. The LGRS is based on the instrument used for the Gravity Recovery and Climate Experiment (GRACE) mission which has been mapping Earth's gravity since 2002.
The LGRS is responsible for sending and receiving the signals needed to accurately and precisely measure the changes in range between the two orbiters. The LGRS consists of an Ultra-Stable Oscillator (USO), Microwave Assembly (MWA), a Time-Transfer Assembly (TTA), and the Gravity Recovery Processor Assembly (GPA).
The USO provides a steady reference signal that is used by all of the instrument subsystems. Within the LGRS, the USO provides the reference frequency for the MWA and the TTA. The MWA converts the USO reference signal to the Ka-band frequency, which is transmitted to the other orbiter.
The function of the TTA is to provide a two-way time-transfer link between the spacecraft to both synchronize and measure the clock offset between the two LGRS clocks. The TTA generates an S-band signal from the USO reference frequency and sends a GPS-like ranging code to the other spacecraft. The GPA combines all the inputs received from the MWA and TTA to produce the radiometric data that is downlinked to the ground. In addition to acquiring the inter-spacecraft measurements, the LGRS also provides a one-way signal to the ground based on the USO, and is transmitted via the X-band Radio Science Beacon (RSB). The steady-state drift of the USO is measured via the one-way Doppler data provided by the RSB.
Each of the two GRAIL spacecraft, GRAIL-A and GRAIL-B, is about the size of a washing machine and has about 200 kg of mass. They are nearly identical, but the need to point antennas on each at one another requires differences in the MoonKAM mounting and in the angles of the star trackers used for attitude control and the antennas through which the orbiters measure the changing distances between them. These orientations also require that GRAIL-B precede GRAIL-A in lunar orbit.
The spacecraft design is based the Experimental Small Satellite-11 technology demonstration mission for the United States Air Force and the avionics are derived from NASA's Mars Reconnaissance Orbiter.
The Attitude Control subsystem, which provides three-axis stabilized control, consists of a sun sensor, star tracker, reaction wheels, and inertial measurement unit.
The electrical power subsystem includes two solar arrays and a lithium ion battery. Each solar array is capable of producing 700 watts at the end of the mission. They are deployed shortly after separation from the launch vehicle and remain fixed throughout the mission. Each battery has a capacity of 30 amp-hours, and is used to provide energy when the spacecraft orbits take them through the Moon's shadow.
The propulsion system includes a hydrazine catalytic thruster for lunar-orbit insertion and trajectory changes, and a warm-gas system with 8 thruster valves for attitude control and other small maneuvers.
The telecom subsystem includes the following:
- 2 S-band transponder antennas to communicate with Earth
- 2 X-band beacon antennas for Doppler ranging measurements from Earth of the Moon's near side
- S-band time-transfer system antenna, which sends a time-synchronization code back and forth between the spacecraft
- Ka-band ranging antenna for precision distance measurement between the spacecraft
Each of the first two pairs of antennas has one antenna mounted on the sunny side of the spacecraft and one on the dark side. The sunny-side antennas point to Earth during the full moon and the dark-side antennas point to Earth during new moons. This system avoids the need to mechanically rotate the antennas during the mission, which would alter the spacecraft's center of mass and disturb the science measurements.