On 05/05/2013 01:31 AM, Mark Spencer wrote:
The article available for download via this URL contains some history about
development issues with Rb and Cs Clocks for GPS. It seems at one point after
the GPS system was placed into service a development program for new Cs GPS
clocks failed and by necessity there was a shift towards Rb (at least for a
period of time.)
http://www.insidegnss.com/node/281
I'm also speculating that the end of the cold war may have led to less emphasis
being placed on the GPS system being able to operate for long periods of time
without ground based intervention which would have further reduced the need to
develop new and improved Cs clocks for the new GPS satellites. (In the cold
war era I recall seeing estimates of how long the GPS system could operate
without ground based attention.)
I seem to recall that they even extended the capability with AUTONAV
functionality, which would significantly prolong the time without ground
control to 180 days, but beyond the cold war ending, the actual
performance of the system and also that of the infrastructure has
allowed a more relaxed situation. Just the long-livety of the birds
themselves is a factor, and then the precision you achieve by correction
of time through the regular updates is not too bad.
Also, as many Cs/Rb sats moved to Rb only operation, it has not meant
any large threat to the system, so launching birds without Cs has been
less of an issue.
Cheers,
Magnus
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