NASA posts orbit elements for shuttle flights on http://spaceflight.nasa.gov/realdata/elements These data become available as pre-planned elements about two days before scheduled lift-off. After the launch these elements will be updated. How reliable are those pre-planned elements for our observation planning? I found the following spot check interesting: Trying to find out whether I could see STS-121 on the day after launch I came across a sun transit of the shuttle on 5-JUL-06, 15:28:35 UT some 20 km North of my place (21 hours after launch). For this calculation I used the pre-planned state vector labled Coasting Arc #4 (beginning with orbit 12) with an epoch of 5-JUL-06, 10:48:10 UT (the next available state vector - Arc #5 - had an epoch of 5-JUL-06, 21:45:15 UT so I had to use Arc #4). Later that day I downloaded updated (NASA) shuttle elements which must have been posted around the time of the predicted solar transit as the first arc there (beginning on orbit 15) has an epoch of 5-JUL-06, 15:49:11 UT. Using this updated state vector I now get a solar transit at 15:29:20 UT that is 45 seconds later than predicted by the pre-planned elements. Had I positioned myself at the best pre-planned site (I actually didn't attempt the observation) I would have missed the transit by about one solar diameter. Strictly speaking I should not have used the updated state vector which had an epoch some 20 minutes after my solar transit but there apparently were no maneuvers at the time and comparing the pre-planned with the updated shuttle position at the time of epoch (5-JUL-06, 15:49:11 UT) gives a distance of 291 km. A comparison at that same time with the Spacetrack TLEs of epoch 5-JUL-06, 17:03:34 UT and using the SPG4 propagation shows an exact match with the updated (NASA) state vector. (I think the Spacetrack TLEs are based on actual measurements.) With the multitude of maneuvers needed for the ISS rendezvous there seems to be some replanning due to optimizations etc. What it means for us is that although the pre-planned elements are good for some preliminary planning we should try to get updated elements before starting our observations if we have a chance. Gerhard HOLTKAMP Darmstadt, Germany ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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