Greg Roberts wrote in: http://satobs.org/seesat/Dec-2003/0202.html > (1) Orbit plane search carried out for KH9-17 ( 82041C) and this > object sighted, placing it 382 seconds early on predictions. > Reasonably confident that it is the satellite in question. Nice work on the recovery, Greg! The object had not been seen in 85 days. 1 13172U 82041C 03263.90151978 .00000800 00000-0 10480-3 0 00 2 13172 95.9830 142.0298 0002000 341.9672 18.0326 14.80456533 09 Since the orbit is not published, the only remedy for aged elements is to conduct a planar search to find the object, so that regular tracking can resume. Two factors cause us to lose objects. First, the SGP4 orbit model (the basis of the 2-line elements) treats drag as a constant, even though it often varies a great deal. This makes it impossible to fit accurate orbits over long periods, and likewise makes it impossible to make accurate predictions very far into the future. The other factor in losing satellites is that we can only track them during periods when they are in sunlight and we are in darkness. We call these periods visibility windows. Certain objects spend months at a time outside the visibility of any of our observers. After several months of invisibility, objects in moderate to high drag orbits become lost to us - we no longer know their location within their orbit. Fortunately, predicting the location of an object's orbital plane is not nearly as sensitive to drag as predicting its location within the orbit. This is what makes possible the planar search technique. Think of an orbital plane as an imaginary ring around the Earth. A lost satellite is moving rapidly along the ring, at an unknown location. If we can stare at the ring for as long as it takes the satellite to circle the Earth, then we must eventually see it. Depending upon the height of the orbit, it may circle the ring in less than 90 minutes, or take many hours, so this is time consuming work, requiring considerable skill and patience. Since the Earth rotates under the orbit, the imaginary ring moves across the sky, so we cannot just aim in one place and expect to see the object - we must frequently adjust our aim to follow the ring. Most of us still use manually aimed binocular for searches, which is quite a chore. Greg Roberts has been pioneering the use of a computer-aimed video camera for both regular tracking and planar searching. Greg has put all that automation to good use, conducting a number of lengthy searches for some rather difficult objects. Perhaps the most challenging was Misty, aka AFP-731, aka USA 53, aka 90019B / 20516 - America's first LEO stealth satellite. Misty was last seen in 1997 by Russell Eberst as a faint unknown in this orbit: USA 53 (Misty) 18.0 4.0 0.0 1.5 v 1 20516U 90019B 97284.23458324 .00000027 00000-0 70436-5 0 01 2 20516 66.1631 65.2852 0005248 187.8717 231.2307 14.48751217 03 By the time I identified Russell's unknown as Misty, in late 2000, it had most likely been de-orbited. Nevertheless, Greg willingly conducted many hours of planar searches for it. One can never be certain about a stealth sat, but Misty's failure to show up for Greg, tends to support the theory that it is no longer in orbit. In any case, negative findings can be as important as positive ones. Regardless of the technology used, planar searching has much to offer those who enjoy the thrill of the hunt. Ted Molczan ----------------------------------------------------------------- To unsubscribe from SeeSat-L, send a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@satobs.org List archived at http://www.satobs.org/seesat/seesatindex.html
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