Electronics

An overview of the VIS electronics is given in Figure 8. The instrument is interfaced with the spacecraft telemetry and command systems with two redundant Remote Interface Units (RIUs). The power interface is served by two independently switched and fused 28-volt spacecraft power buses. Each of these power buses feeds an isolated, current-limited DC/DC converter in the instrument. In addition to these main power lines there are two separately switched, pulse power lines. The pulse power is used to drive the motors and instrument heaters. A further non-redundant power line is used for instrument survival heaters in the event that the main power lines are inactive during a spacecraft load-shedding sequence. There are six 8085 microprocessors in the instrument data and control systems. Four of these microprocessors are devoted to image compression. The details for the data compression are described in the following section. The other two microprocessors, CPU A and CPU B, are redundant and either processor can be selected for control of the instrument by ground command. Each of these processors is equipped with 48 kbytes of memory and an additional 64 kbytes of extended memory. This control processor formats data, receives commands, and controls operation of the sensors, field stop and power subsystems. These activities include bi-axial mirror control for mosaicing and pointing, monitoring despun platform motion, generation of antenna blanking signals for the sensors, and processing of ancillary data. The CCD video signals from the sensors are digitized by one of the two 12-bit analog-to-digital converters (ADCs). These 12-bit words are quasi-logarithmically compressed to 8 bits. An 8 kbyte data window in the memory of the control processor is remapped into the memory space of one of the compression processors. This 8 kbyte block is configured as two-port RAM. The camera interface (I/F) electronics include the serial interfaces for sensor control. Each of the three cameras shown in Figure 8 is equipped with a complete set of electronics, i.e., amplifiers, gate array, low-voltage power supply, and intensifier high-voltage supply.

The electronics are configured to significantly decrease the probability of single-point failure. The cameras are operated independently of each other. The instrument is normally powered by one of the two main power buses and supplies. A power-switching network allows recovery of instrument regulated voltages in the case of failure of one or more of these power lines. The two control processors and their buses are redundant. The failure of a compression processor results in a decrease in frame rate.