Why each GPS satellite holds four atomic clocks?

[SciencePF]

Each of the 24 GPS satellites holds four atomic clocks! Why four and not one or two? They are not cheap!
Thanks

 

[russ_watters]

Redundancy.

And compared to the cost of launching them into space, they probably are pretty cheap.

 

[SciencePF]

Ok, redundancy, but ... why 4 and not 5 or 3 or ... XY?

Thanks.

 

[Danger]

Perhaps for the same reason as the redundancy of space shuttle computers; they essentially 'vote' upon decisions. Three might not produce enough of a majority to be fail-safe, and five is overkill.

 

[mgb_phys]

The block 1 satelites used rubidium clocks and had three of them as the normal backup - 3 can vote and still have a backup if one fails.
Prototype ceasium clocks where developed and one clock was added alongside the existing 3 ( so the rubidium ones could be used if it didn't work).
In later model satelites 2 rubidium and 2 ceasium were used, so each had a backup.
Then when ceasium proved best they were switched to all ceasium, but since the design hardware and software was for four clocks it was as easy and safer to fit four of them.

(Not sure of exactly what changes where when and on what block numbers)

 

[Danger]

Thanks for a great explanation, Mgb.

 

[Vanadium 50]

It's also worth pointing out that "not cheap" is relative statement. Nothing in orbit is ever cheap. So the differential cost isn't always driven by the cost of the instrument.