Scott, thank you for sharing with us your orbital elements. As you noted in your follow up message, the USA 32 (19460 / 88078A) elset is unreliable. Quite a few others also contain unrealistic values. My guess is that you analyzed data over too short an arc. For example, here is your 90050C elset: 1 20691U 90050C 04141.10071233 .00000016 00000-0 24267-4 0 04 2 20691 72.4609 231.3801 0264349 308.1211 105.3500 13.38051517 08 For readers unfamiliar with the above numbers: http://www.satobs.org/element.html This object had not been observed for nearly 36 days, until David Brierley and Peter Wakelin observed it last night, producing single points on consecutive revolutions: 9005003267504052000352220 010 12174469 +13382 20 5 9005003201804052002251545 01 12130969 +37254 1 5 +7 S For readers unfamiliar with the above numbers: http://www.satobs.org/position/UKformat.html The object was only a fraction of a second late, and almost exactly on-track, relative these 36 day old elements: 1 20691U 90050C 04105.80204169 .00000020 00000-0 24267-4 0 02 2 20691 63.4170 307.5460 0372000 1.1505 358.8495 13.40452551 02 The inclination of your elset, 72.4609 deg, is way off that of the earlier elset, 63.4170 deg. The inclination of most orbits either is more or less constant over long periods of time, or slowly changing, so this large discrepancy is a clear sign of a problem in the orbit determination. When the difference between prediction and observation is as small as in the present example, there is no real need to update the elements, but if one chooses to do so, then only minor changes should be made to the earlier elset. In this case, I would propagate the elements to the ascending node prior to the latest observation, and allow the differential correction to adjust only the mean anomaly, so that the object is placed a bit more accurately within its orbit. Here is the result of propagating the epoch 04105.80204169 elset: 1 20691U 90050C 04141.08865757 .00000020 00000-0 24267-4 0 08 2 20691 63.4170 217.5180 0371993 1.1194 359.0579 13.40453962 08 (I know that you also used the epoch 04105.80204169 elset, because your result has the same B* term, 24267-4.) Here is the result after the differential correction of the mean anomaly: 1 20691U 90050C 04141.08865757 .00000020 00000-0 24267-4 0 08 2 20691 63.4170 217.5180 0371993 1.1194 359.0484 13.40453962 03 Once observers have generated a large number of points over a suitably long arc, then the differential correction can be trusted to produce meaningful results for all of the elements. Also, when analyzing NOSS orbits, such as 90050C, it is important to be aware that this class of orbit is perturbed by the odd zonal harmonics of Earth's gravitational field, such that eccentricity gradually increases (in some cases, decreases), and the inclination slowly decreases. Scroll through its orbital history, and you will see the dramatic change the orbit has undergone: http://www.planet4589.org/space/elements/20600/S20691 Orbital analysts need to be aware of such perturbations when assessing the reasonableness of their results. One final tip: always compare the residuals of your new orbit relative the observations from which it was derived. In the present example, I found large time residuals, of the order of 10 seconds - another indication of a problem. I am glad to see that you are interested in this aspect of the hobby, and I hope that you will keep at it. Ted Molczan ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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