Lesson plan
Command-loss timer
In spacecraft, a Command-Loss Timer (CLT) is a software timer within the command and data system (CDS) which is restarted every time the spacecraft receives a command from Earth. If the CLT times out, it is assumed that the spacecraft's receiver has failed to reliably receive messages and, in response, the CDS will attempt to restore communication or perform safing procedures, or both. A Command Loss Timer is effectively a software watchdog timer which, upon timeout, initiates corrective actions that will in some cases swap to redundant communication hardware. Robotic spacecraft are accessible to human operators only by command telemetry. Consequently, a spacecraft need to be self-reliant, because its embedded systems cannot be physically accessed if a communication fault occurs. Without a CLT, such faults would result in the spacecraft becoming permanently disabled.
Article summary
In spacecraft, a Command-Loss Timer (CLT) is a software timer within the command and data system (CDS) which is restarted every time the spacecraft receives a command from Earth. If the CLT times out, it is assumed that the spacecraft's receiver has failed to reliably receive messages and, in response, the CDS will attempt to restore communication or perform safing procedures, or both. A Command Loss Timer is effectively a software watchdog timer which, upon timeout, initiates corrective actions that will in some cases swap to redundant communication hardware. Robotic spacecraft are accessible to human operators only by command telemetry. Consequently, a spacecraft need to be self-reliant, because its embedded systems cannot be physically accessed if a communication fault occurs. Without a CLT, such faults would result in the spacecraft becoming permanently disabled.
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Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
- Which claim in this article is most likely to be revised as the sources update? How would you check its current status?
- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
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