SGP4 TLE propagation from TLE seems inaccurate

Orekit Python Wrapper usage
#1

Hi Orekit Team,
I am trying to convert TLEs collected over the past 6 months into x,y,z positions in order to observe “ground truth” x,y,z positions. I try to extract the ground truths via the following code logic

mytle= TLE(tle_line1_str, tle_line2_str)
propagator = TLEPropagator.selectExtrapolator(mytle)
pv_truth = propagator.propagate(mytle.getDate().shiftedBy(0.0)).getPVCoordinates(frame=FramesFactory.getEME2000())

and from pv_truth.getPosition, I can extract x,y,z

PROBLEM: however, the extracted “ground truth x,y,z” seems to be very wrong when plotted. Because for satellite such as IRIDIUM id#=42807, we can see there is inclination of 86.4 °, but the extracted “ground truth x,y,z” shows ~0 degree inclination (shown as blue in attached ipynb picture)

Please see python notebook here:
convert-tle-to-ephemeris-truth-vs-propagates-FORUM-SUBMISSION.ipynb (67.1 KB)

Please see past 6 months TLEs here:
past6months_tles.txt (621.2 KB)

Please let me know if I am using TLEPropagator incorrectly? and how am I using it incorrectly?

#2

Hi,

Without going into your example (some external dependencies that are not readily available), I could recommend you tried a smaller test with just that Iridium TLE and the TLE propagation notebook example at:

Note also that you are only getting the coordinates from the time of the epoch of the TLE (maybe there is a mistake in the .shiftBy(0.0)?)

Regards

#3

Hi,

I made a small Java code to calculate the keplerian elements based on the first TLE of IRIDIUM.

public static void main(String[] args)  {

    // Initialize Orekit
    final File home       = new File(System.getProperty("user.home"));
    final File orekitData = new File(home, "orekit-data");
    final DataProvidersManager manager = DataContext.getDefault().getDataProvidersManager();
    manager.addProvider(new DirectoryCrawler(orekitData));

    // TLE
    final String line1 = "1 42807U 17039E   21032.21352857  .00000081  00000-0  21984-4 0  9999";
    final String line2 = "2 42807  86.3951  93.5661 0002498  92.7212 267.4270 14.34217473188743";
    final TLE tle = new TLE(line1, line2);

    // Propagator
    final TLEPropagator propagator = TLEPropagator.selectExtrapolator(tle);

    // Extract the orbit
    final SpacecraftState finalState = propagator.propagate(tle.getDate());
    final PVCoordinates pvECI = finalState.getPVCoordinates(FramesFactory.getEME2000());
    final KeplerianOrbit kepOrbit = (KeplerianOrbit) OrbitType.KEPLERIAN.convertType(finalState.getOrbit());

    // Print
    System.out.println(finalState);
    System.out.println("PV EME2000:" + pvECI);
    System.out.println(kepOrbit);

}

The output is:

SpacecraftState{orbit=Cartesian parameters: {P(-445286.17044032563, 7145074.863214257, 3.8191719204909695), V(-467.4569435600412, -38.807271277088404, 7448.645419187451)}, attitude=org.orekit.attitudes.Attitude@439f5b3d, mass=1000.0, additional={}}
PV EME2000: {2021-02-01T05:07:28.868, P(-411587.9082939392, 7147095.167906658, 793.2008453674401), V(-452.59107933642565, -36.55895397728144, 7449.574823129494), A(0.4471519983929933, -7.764654985239574, -8.6174071867045E-4)}
Keplerian parameters: {a: 7161968.392685218; e: 0.0013607999899432305; i: 86.3974212550252; pa: 71.95036237682125; raan: 93.56610000248601; v: -71.95033174996804;}

As you can see, the inclination is about 86.4 degrees, as expected.
Maybe there is a error in your example?

Bryan

#4

Hi Bryan,
Thank you for your Java code. I agree with you that the Loaded TLE does yield appropriate inclination calculation value 86.4. I also checked that this holds true in my python code (and it does).

However, There is a discrepancy between inclination and x,y,z positions extracted/propagated by delta T=0 over many TLEs (span of past 6 months). In other words, if there were 1TLE collected per month, then we would have 6 TLEs collected over the past 6 months. from those 6TLEs we could convert them to 6 <x,y,z> and plot those 6 positions on a 3D plot. And hopefully we would get an ellipse that shows inclination of 86.4?

SGP4problem
SGP4problem1242×612 74.2 KB

these extracted x,y,z positions plotted (in blue) on a 3D graph show inclinations of ~ 0degrees even though the SpacecraftState indicates inclination:86.4 (as you have shown in your java code)

#5

Hi Petrus,

I am using .shiftBy(0.0) because I am trying to extract the x,y,z position from each historic TLE. Is there a better alternative way to extracting historic x,y,z positions?

#6

You’re seeing aliasing. Change the “delta T” to other values and watch the blue circle move up and down.

#7

Upon further inspection, the TLEs for IRIDIUM id#=42807 are sampled at intervals when the satellite repeatedly returns to roughly the same x,y,z positions,

thus not showing the obvious inclinations of an elliptical orbit

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