[time-nuts] time-nuts Digest, Vol 21, Issue 22

[Robert Lutwak]

At the risk of being accused of "propaganda:"

Poul-Henning Kamp <phk at phk.freebsd.dk> writes:

>    They last longer.

and Tom Clark, K3IO (ex W3IWI) <K3IO at verizon.net> writes:

>    The Cesium clocks in GPS have been less reliable (probably because
>       they are more complicated) than the Rb clocks in early GPS 
> satellites;


You are correct that cesium clocks are much more complicated than rubidium 
oscillators, but there is no evidence that the GPS cesiums have been any 
less reliable than the rubidiums.  On the contrary, the cesiums on the block 
IIA satellites have performed long beyond their design life, which has led 
to delays in the expected demand for the IIR (and ultimately IIF) launches.

Regardless of any perceived reliability (or performance) differences between 
the clocks, the JPO has always required two separate clock technologies, 
from two different manufacturers, on each satellite, for redundancy and 
protection from latent design flaws.  The only reason that this policy was 
violated on IIR was because the low-bid contractor for the cesium clocks 
failed to deliver.  For IIF,  they have returned to the previous cesium 
vendor and the usual mix of two each of Rb and Cs.

With that said, the Perkin-Elmer rubidium on the IIR satellites is an 
outstanding clock, unquestionably the best rubidium ever manufactured for 
any application, with short-term stability an order-of-magnitude better than 
the IIF cesiums.  The drift is also remarkably low for a rubidium 
oscillator, compared to commercial rubidium, and, with daily updates, they 
will outperform the cesiums, though the cesiums will beat them outside of a 
week or so.

The IIF satellites, which are expected to begin launching in 2008, will 
carry two Perkin-Elmer rubidiums and two Symmetricom 4410 cesiums.  In 
addition to providing the drift-free long-term stability which typifies 
cesium, the 4410 has other system-level advantages.  It'll be the first 
microprocessor-controlled clock of any sort to be deployed on GPS, with all 
of the self-diagnostics and telemetry that we've come to expect from modern 
cesium clocks (a la 5071A or CsIII).  With increasing reliance on GPS for 
civilian safety-of-life applications, the enhanced telemetry and integrity 
monitoring of the 4410 will lead to a more robust system and more rapid 
identification of problems and clock changeover, when necessary.

Again, I don't want to be accused of "propaganda," but all of the clocks on 
GPS, both cesium and rubidium, from a number of manufacturers, have been 
extraordinarily reliable.  I believe that every GPS satellite has outlived 
its design lifetime and many have been eventually decommissioned for reasons 
unrelated to clock failure.

...and speaking of propaganda, what on earth is the agenda of that Space 
Review article?

-RL

------------------------------------------------------------
Robert Lutwak, Senior Scientist
Symmetricom - Technology Realization Center
34 Tozer Rd.
Beverly, MA 01915
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(978) 927-4099   FAX             Lutwak at Alum.MIT.edu  (Personal)
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