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
A watchdog timer (WDT, or simply a ''watchdog''), sometimes called a ''computer operating properly timer'' (''COP timer''), is an electronic or software timer that is used to detect and recover from computer malfunctions. Watchdog timers are wide ...
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.
Other uses
Command Loss Timers are not limited to spacecraft, as any scientific instrument that is in a situation where remote command could be lost could use this safety measure.
This feature is common with devices connected over the Internet, but not all Internet connected devices use it.
See also
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Safe mode (spacecraft)
Safe mode is an operating mode of a modern uncrewed spacecraft during which all non-essential systems are shut down and only essential functions such as thermal management, radio reception and attitude control are active.
Safe mode is entered aut ...
References
{{CCSDS
Consultative Committee for Space Data Systems