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Re: AO-40



Matt Donohoe <therelic@dingoblue.net.au> wrote:

>Just curious as to why there is now a need to call on
>'emergency' procedures to get the controllers back into
>the satellite....

Systems like these often have two independent reset
systems, both intended to keep the system from going
berserk.

The short-term safeguard is the watchdog timer. Should
the timer not be reset in a specified interval (seconds,
typically), the system is assumed to have locked up
and is reset. This timer is hardware, and is entirely
independent of the software.

The long-term safeguard is having the system place
itself in a safe mode if it doesn't receive any commands
for some interval (days, typically). Suspend activities,
place command receivers in some least common denominator
configuration, and wait.

A good references, as usual, is _Space Mission Analysis
and Design_. There are almost certainly others - anybody?

Laura Halliday VE7LDH     "Que les nuages soient notre
Grid: CN89mg               pied a terre..."
                                   - Hospital/Shafte
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