Message-Id: Date: Thu, 6 Dec 2001 08:58:22 -0800 To: igsleo@igscb.jpl.nasa.gov From: Thomas P Yunck Subject: [IGSLEO-32] SLR Comparisons Sender: owner-igsleo Precedence: bulk ****************************************************************************** IGS LEO Mail 06 Dec 09:23:09 PST 2001 Message Number 32 ****************************************************************************** Author: T. P. Yunck To: IGS LEO List Here are some comments I sent yesterday to Henno regarding the new SLR comparisons with GPS CHAMP orbits (before I had seen Da Kuang's comments). These deal principally with the anomalously low JPL-NCL ratio discussed at the end of Henno's web page. --Tom --------------- Dear Henno, I've been examining your lovely new website with the SLR comparisons and I must say it's remarkable. You've done a tremendous amount in a very short time, and done it very rigorously. Your ratios of direct agreement to combined SLR agreement are especially interesting. I suspect there's a fairly simple reason for the anomalously low JPL-NCL value -- the same reason, in fact, for the anomalously close JPL-NCL agreement in Fig 2 of your original comparison. No, no-one is using SLR data in their orbit solutions; but not only do JPL and NCL use the same software, I suspect they both (I don't actually know what NCL does) adopt a more highly kinematic strategy than the others. That means their solutions at each step depend more strongly on the momentary observing geometry and measurement error, which are common to all solutions, and are less sensitive to arc lengths, epoch times, initial conditions, etc, that can strongly affect the details (though not necessarily the overall performance) of dynamic solutions. In the kinematic limit, the JPL and NCL solutions would become virtually identical, even while their absolute error (departure from SLR) would increase, and your ratio would approach zero (just as the diagonal elements of your ratio matrix must be zero). That's just a longwinded way of saying that kinematic solutions from the same set of data will tend to be more highly correlated than dynamic solutions, while not necessarily being more accurate, and thus will yield lower ratios. And that of course is why the SLR comparisons are so valuable as an independent, absolute check. Nothing more is really needed. Also, I notice that SLR site 7 is rather problematic -- everybody disagrees with it, all by about the same rather large amount -- so it should probably be omitted. Eyeballing your charts, I estimate that without site 7 the revised 1-way RMS values would be roughly 10.4, 15.2, 17.5, 18.8, and 32.3 cm. Even allotting 3 or 4 cm to SLR error, the GPS-only error would not come down much (e.g., 9.6 cm for JPL assuming a 4 cm independent SLR error). To estimate the 3D error, I might divide your 2.33 multiplier by root-2 (i.e., remove the denominator 2 from your combining equation), giving about 1.65 (which is not too far off a plausible a priori guess of root-3 in going from 1D to 3D). That would then agree well with our own estimates of the JPL 3D CHAMP orbit error, derived by other means -- mainly overlap comparisons. -- ----------------------------------------------------------- T. P. Yunck Ph: 818-354-3369 JPL, M/S 238-540 Fax: 818-393-6686 4800 Oak Grove Drive e-mail: tom.yunck@jpl.nasa.gov Pasadena, CA 91109 -----------------------------------------------------------