From: Jim Ray (USNO 202-762-1444) Message-Id: <200007311353.JAA03806@maia.usno.navy.mil> Subject: [IGSMAIL-2962]: USNO Ultra-rapid clock predictions To: igsmail@igscb.jpl.nasa.gov Date: Mon, 31 Jul 2000 9:53:24 EDT Sender: owner-igsmail Precedence: bulk ****************************************************************************** IGS Electronic Mail 31 Jul 06:53:35 PDT 2000 Message Number 2962 ****************************************************************************** Author: Jim Ray The USNO analysis group has begun to report predicted clock estimates in our sp3 submissions for the IGS Ultra-rapid (IGU) combinations. These began with the predicted products for 26 July 2000. The IGS Ultra-rapid products are under development as described in the position paper by G. Gendt, P. Fang, and J. Zumberge presented at the Analysis Workshop at SIO in June 1999; they are not yet "official" but will become so in the near future. The procedure used for our clock predictions is to extrapolate the estimated satellite clock values from the observational data preceding the prediction period. The extrapolations use fits for the following models: linear + sinusoid for satellites with Cs clocks quadratic + sinusoid for satellites with Rb clocks where the period of the sinuoid equals the orbital period. Some satellite clocks show very pronounced sinusoidal variations (most prominently PRN06) while most show little. Continuity between the observed clocks and the predictions is enforced. If the RMS of the fit to the observed clock of any satellite exceeds 5 ns, then no clock predictions are reported for that satellite. I would like to acknowledge the helpful advice of Arthur Dorsey (Lockheed Martin Corp.) in designing this strategy, as well as Steven Hutsell (USNO/Colorado Springs) in pointing out the significant sinusoidal variations. The USNO clocks are estimated relative to the clock state of a chosen tracking receiver (equipped with a H-maser external standard), which is not adjusted as the reference. Following standard practice for other IGS clock products, all submissions in the IGU combination are aligned to GPS broadcast time by removing an overall linear trend based on the observed period. Note that these referencing procedures can cause occasional problems for both our observed and predicted clocks when the chosen reference clock suffers a reset or when a satellite clock is reset. Users are cautioned that these clock products should be regarded as experimental. Further evaluations and refinements will be made. It is important to note that until other analysis centers contribute to the IGU clock combination these products will not be as reliable or as robust as other IGS products. Given the rapid progress already demonstrated for the IGU orbits, the clock development is also likely to be rapid. With the ending of Selective Availability on 02 May, it should be possible to produce clock predicions at the few-nanosecond level, although it remains to realize and demonstrate this potential.