Accuracy of subpoint computation for satellites

Dear Brandon,

First of all thanks a lot for providing this amazing library, it is highly appreciated. I am currently working on a project which determines sensor accesses from a variety of platforms. To that end, I use skyfield to compute satellite accesses, making use of the Topos.at() and Topocentric.altaz() functionality.

As starting point of my custom sensor propagation routine, I first compute the time of zero elevation (using linear interpolation between SGP4 timesteps) and then require the sub-satellite point LLA.

satellite.at(TimeOfZeroElevation).subpoint()

To check the result, I compare the output with AGI’s STK software, which I am personally not a big fan of but trust very much in terms of results. Using the same TLE file as input (from celestrak), I found some major deviation, in particular in the latitudes and longitudes of the satellite subpoint at a given time:

• StdDev of start time (s): 1.16
• StdDevend time (s): 3.30
• StdDev lat start (deg): 0.070
• StdDev lat end (deg): 0.158
• StdDev lon start (deg): 0.041
• StdDev lon end (deg): 0.483
• StdDev alt start (m): 14.5
• StdDev alt end (m): 21.6 N = 178 accesses, used Worldview 1-4 for satellite accesses to Freiburg, Germany, in first week of February in 2020.

While the difference in altitude is marginal for my intended application, the Lat and Lon values correspond to a few 10s of km. And that causes the routine to miss a few sensor accesses every now and then, in particular, if you consider small field of views, e.g. for highly-resolved optical sensors.

I first believed that this had to do with inaccuracies stemming from the interpolation process, so I compared values at the defined time steps used in the SGP4 propagation. But this had no influence.

Next, I also used Orekit’s SGP4 propagator to check against STK and ended up with very similar values as STK. So my guess is that either I am doing something wrong or there might be a potential issue with the subpoint() computation.

I would be very happy if you could help me out.

[maxgulde]

Thanks a lot for your quick reply! I will check tomorrow (STK is way too expensive to have a personal license) about the intermediate values and come back with some more data asap.

[brandon-rhodes]

It’s beautiful to see so many leadings zeros. 😃 Thanks for sharing!