Allen Thomson wrote: > Ted said, > > > I have been unable to make sense of many of the time residuals in > > the Lacrosse 5 data set. In particular, many of my obs appear > > to be several 10ths of a second late relative the others - this is > > true of entire passes. > > > Are there sufficient post-launch observations for the other Lacrosse > missions to tell whether they also exhibited this behavior? > (The antecedent of "they" is meant to be ambiguous.) To my knowledge, only Lacrosse 4 has been tracked nearly as closely as Lacrosse 5, within the first week or so following orbit circularization. I have just completed an analysis of 40 positions obtained by Paul Gabriel and myself: Lacrosse 4 18.0 4.5 0.0 3.6 v 1 26473U 00047A 00245.37621282 .00000718 00000-0 14454-3 0 05 2 26473 67.9930 1.2775 0004612 270.4404 89.6192 14.61484425 08 Arc 2000 Aug 23.42 - Sep 01.39, WRMS residuals = 0.026 deg Only a few points had time residuals in excess of 0.15 s. I note that the typical frozen orbit of a 68 deg Lacrosse had already been achieved. This consists of a mean argument of perigee of 270 deg, and a mean eccentricity near 0.0004. Lacrosse 5 has yet to achieve the typical frozen orbit of a 57 deg Lacrosse, which consists of a mean argument of perigee of 90 deg, and a mean eccentricity near 0.0004. Our tracking reveals that the argument of perigee is between about 170 and 180 deg: Lacrosse 5 0.0 0.0 0.0 3.4 v 1 28646U 05016A 05131.96464533 .00000042 00000-0 10000-4 0 02 2 28646 57.0094 87.6328 0004183 172.2400 187.8679 14.53350437 00 Arc 2005 May 05.10 - 11.99, WRMS residuals = 0.047 deg It may be that an argument of perigee near 180 deg is required, or at least desirable, prior to making the manoeuvre to establish the desired frozen value, whether 90 deg or 270 deg. For example, Seasat (78064A / 10967), had argument of perigee = 174 deg, before manoeuvring to 278 deg: http://www.planet4589.org/space/elements/10900/S10967 1 10967U 78237.90713672 .00000790 00000-0 0 313 2 10967 108.0190 103.5586 0009648 174.5828 185.5369 14.31953582 8572 1 10967U 78238.95500845 .00000790 00000-0 0 326 2 10967 108.0219 105.6962 0001457 278.2590 77.3469 14.29719006 8727 (Though Seasat has long been dead, it has naturally maintained the frozen argument of perigee, as inspection of the above archive reveals.) EO-1's argument of perigee also was near 180 deg prior to manoeuvring to near 90 deg: http://www.planet4589.org/space/elements/26600/S26619 1 26619U 00075A 00345.36201492 -.00011868 00000-0 -27188-2 0 451 2 26619 98.2168 50.2940 0007144 176.1515 183.9663 14.55537197 2704 1 26619U 00075A 00346.73623477 .00003029 00000-0 68099-3 0 505 2 26619 98.2145 51.6492 0003821 95.7415 264.4020 14.57187415 2909 As was Icesat's: http://www.planet4589.org/space/elements/27600/S27642 1 27642U 03002A 03021.84791254 -.00002268 00000-0 -21401-3 0 141 2 27642 94.0085 185.5224 0010987 199.4818 160.6010 14.93547263 1314 1 27642U 03002A 03023.52287413 .00002625 00000-0 24746-3 0 156 2 27642 94.0069 186.3729 0010900 201.7235 158.4032 14.93818209 1567 1 27642U 03002A 03023.92676826 .00001787 00000-0 17689-3 0 178 2 27642 94.0101 186.5833 0010199 158.6939 201.4718 14.92154200 1623 1 27642U 03002A 03026.20827824 -.00003258 00000-0 -33027-3 0 180 2 27642 94.0117 187.7451 0010123 115.8321 244.3953 14.90786074 1968 1 27642U 03002A 03026.94662729 +.00000966 +00000-0 +99503-4 0 00208 2 27642 094.0114 188.1218 0010018 113.8092 246.4152 14.90793975002072 1 27642U 03002A 03033.52348385 .00003383 00000-0 34097-3 0 256 2 27642 94.0089 191.4772 0012245 96.2243 264.0378 14.91264490 3052 1 27642U 03002A 03034.53000706 .00003152 00000-0 31775-3 0 265 2 27642 94.0066 191.9882 0012277 92.1579 268.1053 14.91270475 3208 None of this explains the several second time residuals that have cropped up in tracking Lacrosse 5 at argument of perigee near 180 deg. Perhaps it is making small adjustments to argument of perigee and/or eccentricity, that result in the greater than normal time residuals. I expect that in the near future, Lacrosse 5 will rotate its argument of perigee to nearly the typical 57 deg Lacrosse mean value of 90 deg, at which point our residuals will settle down. Some suggested reading on frozen orbits: Earth Observing - 1 (EO-1) Ascent Strategy http://www.ai-solutions.com/Resources/Papers/EO1AscentStrategy.PDF Phase Plane Analysis and Observed Frozen Orbit for the Topex/Poseidon Mission http://jpltrs.jpl.nasa.gov/1995/95-1462.pdf Orbital Mechanics with MATLAB - Methods of Orbit Design http://www.cdeagle.com/ommatlab/odesign.pdf Ted Molczan ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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