The basic design of the MR communication protocol consists of a 16-second cycle, called the Basic Telemetry Time Sequence, or BTTS. When the MR is powered and in one of the communications modes, it transmits its beacon at one of the three specific beacon frequencies (RC1, RC2, RC3 in Table 3 above), and it listens at that mode's specified receiver frequency. As the MGS approaches a lander, the strength of the beacon signal the lander hears increases. While the signal is too weak for communication, the MR transmits the beacon for 14 seconds, followed by a continuous tone for 2 seconds. When the MR beacon signal is detected by the lander, it responds with a continuous radio tone of its own for a short period of time, followed by a "pseudorandom" number (PN) code that is used by the MR to synchronize to the expected telemetry stream from the lander. Once the MR is synchronized with the PN sequence, it changes the beacon modulation to the TC subcarrier. When the landed package recognizes the TC subcarrier it begins to broadcast its data to the MR. The MR receives the telemetry for approximately 14 seconds and then breaks the link by sending a continuous tone (that is, with no subcarrier). In the absence of the TC subcarrier (i.e., hearing a continuous tone rather than one with a subcarrier), the lander stops transmitting data. The MR then adds health and welfare data (voltages of various components and temperatures at various locations on the antenna and within the electronics), status of the hardware (what mode the MR is in, what the received signal strength is, whether it is receiving data, etc.), and other measurements to the data stream. Collectively, these data are called House Keeping TeleMetry (HKTM). The MR completes one BTTS while transmitting continuous tone. The next BTTS then begins by transmitting the beacon subcarrier, which the lander hears and responds to again with its continuous tone and PN code, to which the MR responds with the TC subcarrier, and the lander resumes transmitting data for another 14 seconds. When either the MR can no longer recognize the lander's continuous tone or synchronize with the PN code, or the lander can no longer recognize the beacon from the MR, the link is broken and the MR remains in the beacon mode until it again communicates with the lander or is commanded to another mode. Figure 2 illustrates the time history of the BTTS.

Figure 2: Basic Telemetry Time Sequence

Three times during each 16 second period (3, 8, and 13 seconds after the start of each TC period), the MR measures the frequency of the received signal from the lander. Frequency differences arise from the Doppler effect--the familiar phenomena wherein a tone (for example, a fire siren or train horn) becomes higher pitched as it approachs a listener and lower pitch as it moves away. As the MR aboard MGS approaches a lander, the differences show a specific trend; after it passes the lander, the values and signs reverse. Such doppler data can be used to determine the location of the lander by determing where the frequency measurements are made with respect to the Mars coordinate systems (this involves knowing the position of MGS at the time a specific measurement was made).

As the MR has no storage capability, it needs someplace to put both the lander data and the housekeeping data generated internally. Thus, the MR hardware is only half of the relay story.

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 (telemetry) up to 40 million bits per second (high resolution images), and to store up to 86 Mbits of data (normal MOC operations leaves about 10% margin). 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 and the MO and MGS spacecraft.

The MOC imaging system 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 this system. The MR is commanded as if it were a fourth physical camera. Commands to cameras are defined by their start time, duration, and data volume. MR "commands" that make the MOC receptive to data flowing through the MR are similar. 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 chucks, or "fragments," of the buffer memory, in a specific 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 format of 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 Malin Space Science Systems 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 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 lander teams (MPL or DS-2). 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.