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TLE & OMM

This tutorial covers Two-Line Element sets (TLE), Orbit Mean-Elements Message (OMM), and Orbit Parameter Message (OPM). You will learn to parse TLEs, propagate with SGP4, interchange between formats, and handle the critical distinction between mean and osculating elements.

Parsing a TLE

Parse a standard two-line element set from its three-line string form:

using IO.Astrodynamics;
using IO.Astrodynamics.Body;
using IO.Astrodynamics.TimeSystem;
using IO.Astrodynamics.SolarSystemObjects;

SpiceAPI.Instance.LoadKernels(new DirectoryInfo("Data/SolarSystem"));

var tle = new TLE("ISS (ZARYA)",
    "1 25544U 98067A   21020.53488036  .00016717  00000-0  10270-3 0  9054",
    "2 25544  51.6423 353.0312 0000493 320.8755  39.2360 15.49309423 25703");

Console.WriteLine($"Name:         {tle.Name}");
Console.WriteLine($"NORAD ID:     {tle.NoradId}");
Console.WriteLine($"Epoch:        {tle.Epoch}");
Console.WriteLine($"Inclination:  {tle.Inclination * Constants.Rad2Deg:F4} deg");

Mean vs Osculating Elements

Critical Rule

TLE elements are mean elements fitted to the SGP4/SDP4 theory. Calling ToStateVector() on mean Keplerian elements is not valid — it would mix mean elements with a two-body conversion and produce incorrect results.

To get a physical state vector, always use ToOsculating:

// Correct: osculating state vector at TLE epoch
var sv = tle.ToOsculating();

// Correct: mean Keplerian elements (for display or comparison only)
var meanKep = tle.ToMeanKeplerianElements();

// Correct: SGP4-propagated state vector at a specific time
var svAtTime = tle.ToStateVector(new Time(2021, 1, 21, 12, 0, 0));
  • ToOsculating() returns the osculating state at the TLE epoch.
  • ToStateVector(epoch) propagates with SGP4/SDP4 to the given epoch and returns the osculating state.
  • ToMeanKeplerianElements() returns the raw mean elements for inspection.

SGP4 Propagation

Propagate the TLE to a future epoch using the built-in SGP4/SDP4 propagator:

var futureEpoch = new Time(2021, 1, 25, 0, 0, 0);
var futureSv = tle.ToStateVector(futureEpoch);

Console.WriteLine($"Position at {futureEpoch}: {futureSv.Position}");
Console.WriteLine($"Velocity at {futureEpoch}: {futureSv.Velocity}");

SGP4 predictions degrade with time. For best accuracy, use the most recent TLE available.

Creating a TLE from a State Vector

Convert an osculating state vector back to a TLE:

var newTle = TLE.FromStateVector(sv, noradId: 25544, classification: 'U',
    internationalDesignator: "98067A", revolutionNumber: 25703);

The fitting process converts osculating elements to mean elements consistent with SGP4.

OMM: Orbit Mean-Elements Message

OMM is the CCSDS standard XML/KVN format for distributing mean elements. Load, validate, and convert:

var omm = Omm.LoadFromFile("satellite.omm",
    validateSchema: true, validateContent: true);

Console.WriteLine($"Object name: {omm.ObjectName}");
Console.WriteLine($"Object ID:   {omm.ObjectId}");
Console.WriteLine($"Epoch:       {omm.Epoch}");

OMM to TLE

Not all OMMs can be converted to TLEs (some lack the required fields). Check compatibility first:

if (omm.IsTleCompatible)
{
    var tleFromOmm = omm.ToTle();
    Console.WriteLine($"TLE line 1: {tleFromOmm.Line1}");
    Console.WriteLine($"TLE line 2: {tleFromOmm.Line2}");
}

TLE to OMM

Convert a TLE to OMM for CCSDS-compliant data exchange:

var newOmm = tle.ToOmm(originator: "My Organization");

COSPAR ID Format

TLE uses a compact international designator (e.g., 98067A) while OMM uses the full COSPAR format with a hyphen (e.g., 1998-067A). Conversions between the two formats are handled automatically.

OPM: Orbit Parameter Message

OPM carries a single osculating state vector in CCSDS format:

var earth = PlanetsAndMoons.EARTH_BODY;

var opm = Opm.LoadFromFile("spacecraft.opm",
    validateSchema: true, validateContent: true);
var stateVector = opm.ToStateVector(earth);

Console.WriteLine($"Position: {stateVector.Position}");
Console.WriteLine($"Velocity: {stateVector.Velocity}");
Console.WriteLine($"Epoch:    {stateVector.Epoch}");

Unlike TLE/OMM, OPM elements are osculating — they can be used directly for numerical propagation without conversion.

Summary

  • Parse TLEs from standard two-line format; use ToOsculating() or ToStateVector(epoch) for physical states.
  • Never call ToStateVector() on mean Keplerian elements — always go through the SGP4 propagator.
  • OMM and TLE are interchangeable when the OMM has TLE-compatible fields (IsTleCompatible).
  • OPM provides osculating state vectors in CCSDS format, ready for direct use.
  • COSPAR ID format differences between TLE and OMM are handled automatically.