Shuttle Pre-Planned Elements vs. Actual Elements

From: Gerhard HOLTKAMP (grd.holtkamp@t-online.de)
Date: Thu Jul 06 2006 - 14:35:30 EDT

  • Next message: Bram Dorreman: "PPAS obs 2006 Jun 28 & 29 BD"

    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
    
    
    
    
    
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