Some Background
But before we delve into the data handling, it would be a good idea to briefly introduce the main actors in the drama: the imagers (also referred to as the cameras) and the spacecraft's computer.
The primary computer on Phoenix was built around a RAD6000 CPU running at a maximum clock rate of 20 MHz, although it could also be operated at slower clock rates to conserve battery power. No cutting-edge technology here; this was basically a radiation-hardened, first-generation PowerPC with a mix of RAM and flash memory all crammed onto a set of VME circuit boards. After dealing with the landing chores, its primary functions involved handling communications with Earth (uplink and downlink in jargon-speak; see the sidebar UPLINK AND DOWNLINK), monitoring the spacecraft's health, and coordinating the activities of the various science instruments via commands sent up from the ground. It used WindRiver's VxWorks real-time operating system (RTOS) with numerous extensions provided by the spacecraft contractor, Lockheed Martin. All of the flight software was written in C in accordance with a set of specific coding rules.
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