Editor's note: since this question has been asked, the website in question has gone defunct and is now squatted . There are other websites available that perform this work now, but this one should no longer be visited

In a comment to this question, @PcMan suggested my stuffin.space for satellite data. Now I am not quite sure of how to interpret the data, take for example the following screenshot:

enter image description here

  1. Why is the altitude larger than the apogee value in this case? Isn't the apogee value the maximum altitude? Is it because the apogee (and perigee) values reference to some kind of mean surface level (i.e. earth idealized as sphere) and the altitude gives the value above the ground directly under the satellite (i.e. including mountains etc.)?
  2. How to get from this data the correct value of the semimajor axis : The apogee and perigee values seem to be taken from ground level, so the major axis should be the sum of the apogee value, the perigee value and the earth diameter (and the semimajor axis then the half of this value). But what would be the correct value for the earth radius to use? Is it the nominal radius (6,378.1 km), the arithmetic mean radius (6,371.0072 km), volumetric radius or some other conventional radius?
  • 1
    $\begingroup$ Not a duplicate because your question is much more detailed and application-specific, but answers to Satellites below perigee on Stuff in Space website may be somewhat helpful here. Also see How is the altitude of a satellite defined, given that the Earth is not spherical? $\endgroup$
    – uhoh
    Commented Oct 21, 2021 at 20:08
  • $\begingroup$ @Tristan - the stuffin.space website has apparently moved to sky.rogue.space, a site owned by three-year old startup Rogue Space Systems. On the new site the browser thumbnail still lists it as Stuff in Space. They apparently let the stuffin.space domain name license lapse so the redirect broke. You may want to simply edit the URL in the question to the new URL. The old site can be seen on the Wayback Machine $\endgroup$ Commented Oct 23, 2023 at 12:45
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    $\begingroup$ @StevePemberton They may have cloned the frontend of the site, but the information presented there is not correct. In particular, the TLEs being used to propagate orbits are utterly ancient, so you end up with nonsensical stuff like starlink satellites halfway out to the moon. The original website had a backend job that would pull current TLEs in for the frontend to process. They've also removed the github link and they appear to have no connection to the person who actually made it, so I'm not inclined to promote them as the replacement $\endgroup$
    – Tristan
    Commented Oct 23, 2023 at 14:51
  • $\begingroup$ @StevePemberton Looks like a better successor would be keeptrack.space $\endgroup$
    – Tristan
    Commented Oct 23, 2023 at 15:02
  • $\begingroup$ @Tristan - How odd. I used the work "apparently" in my comment about who produced Stuff in Space because looking at Stuff in Space on the Wayback Machine (my previous link didn't work), it didn't seem to have any identifiers of who produced it. Then a nearly identical looking page appeared as sky.rogue.space, which vaguely seems to bill itself as Stuff in Space, and contains an unexplained link to Rogue Space Systems. Which is a company that in my opinion seems a little shaky, even though they recently received some AF money. $\endgroup$ Commented Oct 23, 2023 at 18:39

1 Answer 1


Answers to this can be found by examining the website's code on GitHub.

Apogee and Perigee are numbers that are valid at the epoch of the active TLE for the satellite in question. They are derived from the mean motion and eccentricity values and reference plain old 6371.0 as earth radius.

The altitude comes from the propagator and is in reference to the geodetic ellipsoid.

So, to answer directly:

  1. The altitude can be larger than the apogee from two sources: propagation drift as time passes from the TLE epoch (though this is probably unlikely), and the difference in reference values -- notably, apogee and perigee being in reference to a sphere with radius 6371 km, while the altitude is based on height above the geodetic ellipsoid, which varies between 6378 km and 6356 km, depending on latitude.

  2. To get semimajor axis, use the apogee and perigee values with an earth radius of 6371 km.

  • 2
    $\begingroup$ Another the way to reconcile these data ("apogee, perigee, altitude"): take a perfect circular orbit. Put the orbit on the Equator: Altitude=Apogee=Perigee. Swing the orbit by 90° (polar orbit), the Altitude varies with time, maximum over the poles, minimum over the Equator. The listing pointed to by @Tristan uses 6378.1 km for the Equatorial radius (var a), and 6356.7 Km for the Polar radius (var b). $\endgroup$
    – Ng Ph
    Commented Oct 21, 2021 at 21:40

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