The use of three-axes attitude control for all mission phases simplifies slewing for TCM burns and outer cruise communications. During cruise, the ACS propagates attitude estimates using star camera and IMU data. During outer cruise transmit periods, the RCS control law keeps the Earth within 18 beamwidth of the Medium-Gain Antenna. Roll axis control is maintained within a 60 deadband. IMU accelerometer data is used to cut-off the burn for accurate V. Sun vector information is obtained from the digital Sun sensors for Sun Acquisition and Safe modes.
Entry begins after a final star camera attitude update and cruise stage separation.
Navigational and attitude information during EDL is maintained by propagating IMU measurements. After release of the aeroshell, the LR is turned-on to obtain altitude and velocity data. At an altitude of 1.43 km, the descent engines are started and the parachute is released. The LR measures both altitude and velocity relative to the surface. Horizontal velocity information is derived from sequential range rate measurements of four beams.