Thomas Fly wrote: > As with life in general, luck plays a big part in the pursuit of transits ;-) So it seems, but I admire your efforts to limit its role to that of the weather. I have noticed an interesting inconsistency between NASA/JSC/MCC's predicted elements, and USSTRATCOM (via NASA/GSFC/OIG) historical (i.e. derived from tracking) elements. While trying to recreate the transit of Jupiter by ISS videotaped by Gary Trapuzzano, I noticed that the near contemporaneous MCC elset was much more accurate than the OIG equivalent. For fun, I tried various OIG elsets at having epochs up to several days past the event, and found that they became progressively more accurate. In an attempt to gain insight as to the cause, I propagated the OIG elsets, using SGP4, to the same ascending node as the MCC elset. Two things jumped out at me: - the OIG elset closest in epoch to that of the MCC had a slightly lower RAAN. - the RAAN of subsequent OIG elsets moved toward that of the MCC elset See for yourself: MCC 04134.54700828 predicted: 1 25544U 98067A 04134.54700539 .00020000 00000-0 20000-3 0 05 2 25544 51.6255 142.2773 0010945 129.4068 230.7900 15.69378694 32881 OIG 04134.87933009 propagated; 1 25544U 98067A 04134.54700728 .00010115 00000-0 89161-4 0 05 2 25544 51.6266 142.2681 0011104 129.7984 230.3992 15.69369079 02 OIG 04135.00732516 propagated; 1 25544U 98067A 04134.54700632 .00010472 00000-0 92098-4 0 08 2 25544 51.6265 142.2685 0010957 129.3258 230.8712 15.69371328 05 OIG 04136.15504506 propagated; 1 25544U 98067A 04134.54700628 .00011312 00000-0 98954-4 0 04 2 25544 51.6264 142.2713 0011056 129.3047 230.8931 15.69374760 09 OIG 04137.17601787 propagated; 1 25544U 98067A 04134.54700685 .00011602 00000-0 10125-3 0 02 2 25544 51.6264 142.2740 0011178 129.1983 231.0008 15.69375923 01 OIG 04138.12902256 propagated; 1 25544U 98067A 04134.54700729 .00011559 00000-0 10080-3 0 02 2 25544 51.6263 142.2768 0011256 128.8841 231.3162 15.69377325 08 OIG 04139.70387731 propagated; 1 25544U 98067A 04134.54700740 .00011825 00000-0 10284-3 0 07 2 25544 51.6259 142.2807 0011326 128.8360 231.3649 15.69378142 09 A similar phenomenon is evident around the time of Venus' transit of the sun. All of the following elsets have been propagated using SGP4 to ascending node near 2004 Jun 08 at 09:51:59 UTC: MCC 04159.51929464 propagated: 1 25544U 98067A 04160.41108680 .00020000 00000-0 20000-3 0 09 2 25544 51.6330 10.4829 0005386 210.6051 149.4632 15.68831850 05 OIG 04159.93711806 propagated: 1 25544U 98067A 04160.41109702 .00014804 00000-0 13139-3 0 05 2 25544 51.6330 10.4767 0005447 207.5034 152.5675 15.68813933 04 OIG 04160.11829168 propagated: 1 25544U 98067A 04160.41109648 .00014969 00000-0 13275-3 0 07 2 25544 51.6331 10.4766 0005497 206.0234 154.0487 15.68815010 01 OIG 04160.18136574 propagated: 1 25544U 98067A 04160.41109673 .00014863 00000-0 13184-3 0 07 2 25544 51.6330 10.4769 0005479 206.4696 153.6022 15.68814610 04 OIG 04160.34703704 propagated: 1 25544U 98067A 04160.41109690 .00014830 00000-0 13154-3 0 07 2 25544 51.6332 10.4772 0005492 206.6148 153.4568 15.68814629 02 OIG 04160.42390752 propagated: 1 25544U 98067A 04160.41109664 .00014992 00000-0 13290-3 0 08 2 25544 51.6329 10.4770 0005397 206.6555 153.4165 15.68815449 06 OIG 04160.55179646 propagated: 1 25544U 98067A 04160.41109655 .00014891 00000-0 13202-3 0 09 2 25544 51.6331 10.4772 0005516 206.8128 153.2585 15.68815230 04 OIG 04160.67381583 propagated: 1 25544U 98067A 04160.41109672 .00014831 00000-0 13149-3 0 02 2 25544 51.6316 10.4764 0005530 206.9805 153.0905 15.68814915 00 OIG 04160.87472337 propagated: 1 25544U 98067A 04160.41109700 .00014823 00000-0 13139-3 0 04 2 25544 51.6325 10.4769 0005537 206.6555 153.4158 15.68815036 00 OIG 04160.94517177 propagated: 1 25544U 98067A 04160.41109720 .00014820 00000-0 13135-3 0 09 2 25544 51.6327 10.4782 0005586 207.1559 152.9146 15.68814978 06 OIG 04162.71808431 propagated: 1 25544U 98067A 04160.41109751 .00014629 00000-0 12945-3 0 08 2 25544 51.6329 10.4826 0005769 207.0419 153.0278 15.68816731 01 OIG 04165.52719547 propagated: 1 25544U 98067A 04160.41109971 .00013370 00000-0 11839-3 0 05 2 25544 51.6325 10.4888 0006111 209.0389 151.0268 15.68824061 07 Notice that OIG's 04162.71808431 elset propagates to nearly the same RAAN as the MCC's 04159.51929464. Here is the OIG elset in question: 1 25544U 98067A 04162.71808431 .00014629 00000-0 12945-3 0 9665 2 25544 51.6329 358.7195 0005482 215.8978 222.4628 15.68884229317308 Tom, I would be interested to know how well this one does at predicting the recent ISS transit of Venus during the latter's transit of the sun. I suspect it will agree rather well with the MCC's elset. I am not certain as to the exact of the discrepancy. If it could be due to one or both of the following: - a systematic incompatibility between MCC and OIG orbital models; if so, we may be able to correct for it. - since OIG's elsets are derived from observations, i.e. are historical, they may require several days to catch up to MCC's predicted elements; if so, there may not be much that we could do about it I suspect a combination of the above. A linear regression of the propagated RAAN vs the original day of year of the OIG elsets reveals a very strong correlation. For the first data set above, I get: propagated RAAN = 141.91421 + 0.002623628 * DOY; r^2 = 1.0000000 I get a similar slope, 0.00216 deg/day, from the second set, using only three elsets (4159.937, 4162.718, 4165.527). What, if anything, this means, remains to be learned. Ted Molczan ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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