Perhaps the most important role of the OBDH is to ensure that data generated by the payload are transferred to the communications subsystem ready for downlinking to the ground. In some cases, these payload data need to be processed by the OBDH subsystem computer(s), and this processing involves things like storage, error detection and correction and compression (see below). For some spacecraft, such as communication satellites or those with imaging payloads, the volume of data produced by the payload can be large. For LEO spacecraft, sometimes a ground station is not in view as the payload data are being generated, and so the data must be stored on board while waiting for an opportunity to downlink to a ground station. These storage devices are part of the OBDH subsystem, and in the past tape recorders were commonly used. More recently, solid-state memories, like those in a desktop computer, have been used instead. They provide huge amounts of data storage, up to hundreds of Gbits (where the "G" stands for "Giga," meaning 1000 million), but they are prone to radiation-induced errors, as mentioned in Chapter 6. To minimize the effects of such data errors, the memory is continuously scanned by error detection and correction computer programs, as part of the OBDH subsystem function. In some cases, the data volume generated by the payload is too large for the downlink communications to handle, and so the data need to be compressed using OBDH software. The raw payload data can be compressed (or reduced in volume) in such a way as not to compromise the value of its content too much. This is done by eliminating redundant or duplicated content, removing unwanted information, or reducing the resolution of digital imagery.
The other main job of the OBDH subsystem is to generate telemetry, which we discussed briefly in Chapter 7. Various sensors are placed around the spacecraft, to monitor the health and operating status of the onboard equipment. They check the temperatures of electronic equipment, the pressures in fuel tanks, voltages and currents in power supplies, and so on. The operational status of equipment is also monitored in terms of whether items are switched on or off. All these accumulated data are converted into a digital bit stream, which is then downlinked with a data rate of a few kbps (kilobits per second) to the ground for display on computer monitors in the operations room. In this way, any problems that occur on the spacecraft can be quickly spotted and corrected by the operations team.
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