Mars Global Surveyor Nadir Deck, 1996. Note locations of
Mars Orbiter Camera (MOC) and Mars Relay antenna.
Mars Global Surveyor MOC image of Nili Patera sand
dunes, 1999. Subframe of MOC image FHA-00451.
Because the Mars data relay process involves using some of the same computer memory in the MOC that would normally be used to store images, relaying data from the planet's surface means taking fewer or smaller pictures of the planet from orbit. One type of science (orbiter) is thus traded for another (lander).
Among the characteristics of the MOC that were unique for its late 1980s design were its ability to acquire data at rates ranging from a few hundred bits per second (health and welfare telemetry) up to 40 million bits per second (high resolution images), and to store up to 86 Mbits of data (although normally only about 78 Mbits are stored in order to provide margin on estimates of the efficiency of the image compression software).
These capabilities were required because, originally, MOC would only be able to transmit data to the spacecraft recorder at a few hundred bits per second. Even with vast improvement in the rate at which the spacecraft can accept data from MOC (record rates are 1105, 2856, and 9240 bps and realtime rates are 29260 and 63580 bps), the MOC buffer remains an enabling technology for high resolution imaging. Owing to these capabilities, it was logical to use the MOC as the interface between the MR radio (MR) and the Mars Observer (MO) and MGS spacecraft.
The MOC consists of three physical cameras--the high resolution "narrow angle" camera and two low resolution "wide angle" cameras. The MOC treats the MR as if it were a part of the imaging system. The MR is commanded as if it were a fourth physical camera. Camera commands are defined by their start time, duration, and data volume. MR "commands" that make the MOC receptive to data flowing through the MR are set similarly. As the MR data flow into the MOC, they are treated just like MOC image data (they are encoded with error correction coding while stored in the buffer, they are queued to be sent to the Earth either via the recorder or directly to the spacecraft radio system for realtime transmission, etc.).
Data from the MOC are broken into chunks of bits that are treated in a common way. All data within the MOC are stored in 240 KByte chunks, or "fragments," of the buffer memory, in a specific internal MOC data format along with ancillary, or label, information. This format is the MOC Science Data protocol (msdp). Engineering telementry is also accumulated and prepared for downlink in the MOC Engineering Data Protocol (medp). As the spacecraft's Payload Data System (PDS) polls the instruments for data, the MOC breaks up the msdp and medp files into packets of data ready for transmission, adds required header and other information to these packets, and places them into a small buffer where the PDS can grab them and move them to either the recorder or transmitter. Within the spacecraft telecommunications system, other communications protocols (labels, parity bits, etc.) are added, and then the data are transmitted to Earth.
As bits are received at the large antennas of the Deep Space Network, they are stored and processed to remove various spacecraft telecommunications protocols, transferred to computers at JPL for further protocol processing, and finally stored on a Project data server computer. At that point, MOC controllers at MSSS remotely query the server computer for MOC data, which are transferred to MSSS via a secure, high-speed data line. At MSSS, the original data as they existed in the MOC buffer are stripped from the remaining telecommunications protocol. Data that are images are reconstructed as images. MR data are placed into a file that reconstructs the exact pattern in which they were received from the MR (including the 16-second Basic Telemetry Time Sequence--BTTS--formatting). Special purpose MR software then separates the health and welfare data (called House Keeping TeleMetry, or HKTM) from the data received from the lander. The lander data are transferred back to JPL for distribution to the MER subsystem teams (Power, Thermal, Avionics, etc.). The HKTM is examined at MSSS to insure that the MR is working properly, to determine the location of the landers, and to assess the efficiency of the lander radio links. The HKTM data are also delivered to the operations teams at JPL.
The data flow is generally automated, with a latency at MSSS of only a few minutes at most. Manual procedures are also available to address problems that might arise.