At the risk of being accused of "propaganda:" Poul-Henning Kamp <[EMAIL PROTECTED]> writes:
> They last longer. and Tom Clark, K3IO (ex W3IWI) <[EMAIL PROTECTED]> 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 (978) 232-1461 Voice [EMAIL PROTECTED] (Business) (978) 927-4099 FAX [EMAIL PROTECTED] (Personal) (339) 927-7896 Mobile _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
