Re: satellite altitude ?

From: duh (heyyouthere12000@aim.com)
Date: Mon Dec 09 2013 - 22:03:05 UTC

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    On 12/09/2013 01:36 AM, Gavin Eadie wrote:
    > I’ve written a few a satellite tracking programs and, comparing with other programs, I notice we agree very precisely on location but differ in altitude.  With ISS, for example, our altitudes agree to within 100 meters on equator crossing, but are much further off (~20 Kms) at high latitudes.  I presume this is because I am taking the oblate Earth into account, so I was wondering if there’s a tradition or standard for satellite altitude value when the “altitude of ground level” varies.  It’s not a huge deal, but the question has been nagging me, and this seems The Place to ask!  Thanks .. Gavin
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    The same question has been in my mind for years. It does seem that a low 
    perigee has a much different
    level of significance  for a polar orbit, if perigee is  over one of the 
    poles vs being at the equator. Satellite
    life expectancy would be significantly different at least in most cases. 
    Supposedly such situations had to
    be taken into account if the space shuttle was landing on a descending 
    node vs an ascending node, back
    if the ancient days of early manned space travel. (A sad attempt at 
    humor!!!!)
    
    FWIW, I eventually convinced myself that people were using the radius of 
    the earth at the equator as the reference
    point for the perigee. However, relatively recently I remember seeing 
    something that indicated that it was the
    average of the earth's radius at the poles and the equator.
    
    The first assumption is just that -- an assumption by me and hence no 
    reference is available. The second is from the
    seasat list someplace *if* (and that is a big if) I am remembering 
    correctly, and therefore I will not be able to give
    you a specific detailed reference.
    
    As a further distraction, there was a discussion recently in one of the 
    nasaspaceflight.com forums regarding the
    supersynchronous geo transfer orbit for SES-8, and what the significance 
    of the TLE elements were. By Google-ing
    "tle elements", I found some interesting stuff that merely confused the 
    picture even more. Effectively, how the tle's
    are derived masks what the perigee and apogee are referenced to unless 
    you want to go through all the source
    code. Yes, the source code is available out on the web. I did not save 
    the references.
    
    Hopefully, someone can cut through all the murky space (pun in there 
    somewhere?) and provide some insight.
    
    Enough nonsense from me. However, you at least know that one person has 
    read your post and is similarly puzzled.
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