5
$\begingroup$

I'm relatively new to orbital mechanics and I know there are a ton of resources and similar questions on the web, but I can't seem to get a straight answer/guidance from any of them.

I have a set of data regarding any particular satellite which consists of the following:

  1. Mean motion
  2. Mean anomaly
  3. Argument of perigee
  4. Eccentricity
  5. Inclination
  6. Right Ascension of Ascending Node (RAAN) AKA Longitude of Ascending Node

This data is extracted using PyEphem library. What I would like to calculate is the longitude, latitude and altitude of the satellite at any given time. I know what each parameter means but I don't know how they relate to my unknowns.

I tried reading papers, tried several simulations and checked out lots of articles about Keplerian elements conversion but I still can't figure out how to connect the dots. The main resources I have been studying are:

http://fgg-web.fgg.uni-lj.si/~/mkuhar/Pouk/SG/Seminar/Vrste_tirnic_um_Zemljinih_sat/Keplerian%20Elements%20Tutorial.html#epoch

https://au.mathworks.com/matlabcentral/fileexchange/54875-geostationary-satellites-tracking

https://au.mathworks.com/matlabcentral/fileexchange/982-gui-based-satellite-tracking-system

and I also read most of the related questions on this website. Would it be easier if I used real-time tracking data instead of using TLE to predict my unknown?

Thanks in advance!

$\endgroup$
  • $\begingroup$ You can also try reading related questions and answers within this site. It's usually necessary here to put a little effort first (and do describe the effort here). It would not make sense to ask for a new answer to something that's been answered here already. In this case what you are asking for is quite a big calculation, getting a spacecraft's position and then calculating the latitude and longitude of the ground track on the rotating Earth. Add some links showing the resources you have found and read so far, and explain just what it is that you are having difficulty with. $\endgroup$ – uhoh Dec 17 '17 at 8:22
  • $\begingroup$ Here is a link from a search for the term RAAN within this site, just for example; space.stackexchange.com/search?q=+RAAN $\endgroup$ – uhoh Dec 17 '17 at 8:27
  • 1
    $\begingroup$ Do you think it's easier if I get real-time data? Cause from your description I understand the calculation process and it's not easy as you mentioned. I'm currently using PyEphem to get my satellite parameters. This link for keplerian elements; fgg-web.fgg.uni-lj.si/~/mkuhar/Pouk/SG/Seminar/… This one for RAAN to longitude conversion: sciencing.com/calculate-longitude-right-ascension-6742230.html And almost all the related question on stackexchange $\endgroup$ – Will Dec 17 '17 at 8:34
  • $\begingroup$ Oh in that case you should really go back and edit your question and explain all of that in the question! Right now readers have no idea where you are in the process. If you can calculate position in Earth-centered coordinates, all you need is how to get the ground track. It turns out I've asked a similar question for the Syfield Python package; Better way to get approximate ground track for a satellite using Skyfield? $\endgroup$ – uhoh Dec 17 '17 at 9:25
  • 1
    $\begingroup$ @uhoh Thanks so much for your tips. The Skyfield thread was really helpful! $\endgroup$ – Will Dec 17 '17 at 22:34
2
$\begingroup$
  1. Calculate eccentric anomaly. It can be done by solving Kepler's equation. Example Python code for Newton's method:

    M   = ...           # your mean anomaly
    ecc = ...           # your eccentricity
    maxIter  = 15       # maximum number of iterations
    maxError = 1e-11    # maximum error
    
    i = 0
    E = M if (ecc < 0.8) else math.pi
    F = E - ecc * math.sin(M) - M
    while ((abs(F) > maxError) and (i < maxIter)):
        E = E - F / (1 - ecc * math.cos(E))
        F = E - ecc * math.sin(E) - M
        i += 1
    
    return E
    
  2. Use this paper to calculate the cartesian position from eccentric anomaly and other elements. You'll need to rotate some vectors, you can do that with rotation matrices described here.

  3. Now you need to convert your cartesian position to spherical coordinates. The Pyhton expressions are as follows:

    r   = math.sqrt(x**2 + y**2 + z**2)
    lat = math.asin(z / r)
    lon = math.atan2(y, x)
    if lon < 0:
        lon += 2 * math.pi
    

Note that r here is the distance from the Earth barycenter. You should subtract Earth radius from it to calculate altitude. You can just take 6371 km as a mean value or use a more precise one from some ellipsoid models described here and here.

Also note that lon is relative to the equinox vector, not Earth's prime meridian, so you'll need to account for Earth rotation (get current Earth rotation angle about its axis relative to the equinox vector and subtract it from your derived longitude).

$\endgroup$
1
$\begingroup$

if you're using pyephem, you can get the latitude and longitude directly from the body object.

body = ephem.EarthSatellite() 
body._raan = ....  
body._ap = ....  

then get the lat, lon at datetime dt

body.compute(dt)  
body.sublat  
body.sublon 
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.