Command-loss timer
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Command Loss Timer Reset(s) are part of the CCSDS communications system to spacecraft either in Earth orbit or beyond Earth orbit.
The Command Loss Timer Reset, if it is not received in a timely manner generally forces the spacecraft to engage in safety and self protection procedures.
Command Loss Timer Reset systems involve both hardware and software, whereas watchdog timers are essentially hardware only affairs.
Technically the Command Loss Timer Reset is a glorified Watchdog timer, but with safety in mind not necessarily computer rebooting.
For those embedded systems that can't be constantly watched by a human, watchdog timers are often implemented. Most embedded systems need to be self-reliant, and it's not usually possible to wait for someone to reboot them if the software or hardware hangs.
Embedded designs, such as space probes, are simply not accessible to human operators. If their software or hardware ever hangs, such systems are permanently disabled. In cases similar to these, a watchdog timer can help in solving the problem.
In actual use
This is an extract from a Voyager Mission report
- Voyager Mission Operations Status Report #2009-12-25
- Command Transmission & Verification Operations
- Voyager 1 command operations consisted of the uplink of a Command Loss Timer Reset and HPOINT LINK with MRO on 12/21 [DOY 355/1350z]. The spacecraft received all commands sent and all commands were verified.
- Voyager 2 command operations consisted of the uplink of seven bracketed Command Loss Timer Resets sent on five-minute centers using 0.5 Hz steps on 12/23 [DOY 357/0212z]. The spacecraft received three of the seven commands sent.
Other uses
The Command Loss Timer Reset is not strictly 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.
References
See External links.
External links
Every report indicated how many CTLRs were sent and how many were received. So far this is the only NASA mission to indicate this aspect of spacecraft health.