How do the radio modem telecom protocols work?
The rover and lander UHF radios communicate using a relatively simple protocol. The rover is the master of the link and controls the initiation of all communication sessions. The session types are: • Heartbeat • Time Request • Command Request • Telemetry The function of the lander radio is to upload fairly small command sequences to the rover and receive data and telemetry downloads from it. Data transfer is performed in sessions whereby individual frames are exchanged in a well controlled manner. A frame is a group of byte-aligned data that is transmitted during keying of the RF modem. The maximum length of a data frame is 256 bytes and many, like a 250 byte telemetry frame, are shorter. A complete frame consists of: • A 6 byte acknowledge (ACK) frame, which contains a Sync code, a frame ID (FID), a frame number (FNUM) and a CRC value. • Followed by the 256 bytes of data, for a total of 262 bytes. Commands, data (like images and APXS spectra) and telemetry are organized into packets.
