Re: X-37B: Altitude history of OTV 1-1 ad 2-1

From: Robert Clark (bobbygc2001@yahoo.com)
Date: Fri Dec 02 2011 - 20:06:49 UTC

  • Next message: Ted Molczan: "RE: X-37B: Altitude history of OTV 1-1 ad 2-1"

     Hmm. Sometimes the data shows up for the satellite orbital elements on that page, sometimes not. You can find the orbital elements for a particular satellite by entering it in the search bar on the page, http://www.n2yo.com/ . For the X-37b enter in the search box:  OTV 2.
    
    
       Bob Clark
    
    
    
    ----- Original Message -----
    From: Ted Molczan <ssl3molcz@rogers.com>
    To: seesat-l@satobs.org
    Cc: 
    Sent: Friday, December 2, 2011 2:54 PM
    Subject: RE: X-37B: Altitude history of OTV 1-1 ad 2-1
    
    Robert Clark asked:
    
    >  On this page X-37b the altitude frequently dips down to around 296 km:
    > 
    > http://www.n2yo.com/?s=37375
    > 
    >  Why does the perigee listed in your file only go down to about 320 km?
    
    I followed the link you provided, but the data that you referred to was not there, and it was not immediately obvious to
    me how to display it; however, you raise an important point. There are many ways to compute altitude relative Earth, and
    to my knowledge there is no standard method. Therefore, it is helpful to define the method used, which I failed to do.
    Here is a revised version, with a box added at lower left, which defines what I mean by mean altitude:
    
    http://satobs.org/seesat_ref/misc/X-37B_OTV_1-1_and_2-1_altitude_history.pdf
    
    The method I used is fairly common, though there probably are variations in how semi-major axis is calculated; and
    sometimes altitude is expressed relative Earth's mean radius instead of the equatorial radius.
    
    Another common method is to compute the actual altitude above the geoid. That certainly has its uses, but the method I
    used is arguably best for showing the effects of manoeuvres, because it omits the variations in altitude due to Earth's
    oblateness (the long-term oscillation of eccentricity with argument of perigee, as well as the variation in radius of
    the Earth).
    
    I will leave it to you to ask Chip how he obtained the values you cited.
    
    Ted Molczan
    
    
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