TLE Propagator Parameters

[astroboy]

Hello,

I apologise if I am just missing something simple here. All I want to do is load a TLE and then check that the Keplerian orbital parameters match those in the TLE.

I have made a minimal working example below:

#!/usr/bin/env python3

import orekit
from org.orekit.orbits import OrbitType
from orekit.pyhelpers import setup_orekit_curdir
from org.orekit.propagation.analytical.tle import TLE, TLEPropagator

orekit.initVM()
setup_orekit_curdir()

tle_line_1 = '1 41999U 17008BD  21322.49427166  .00005929  00000-0  19476-3 0  9992'
tle_line_2 = '2 41999  97.3103  30.0492 0005406 261.3283  98.7346 15.31932939264278'

tle_object = TLE(tle_line_1, tle_line_2)    
tle_propagator = TLEPropagator.selectExtrapolator(tle_object)
tle_orbit_cart = tle_propagator.getInitialState().getOrbit()
tle_orbit_kep = OrbitType.KEPLERIAN.convertType(tle_orbit_cart)

print(tle_line_1)
print(tle_line_2)
print(tle_orbit_kep)

And this gives me the following output:

1 41999U 17008BD  21322.49427166  .00005929  00000-0  19476-3 0  9992
2 41999  97.3103  30.0492 0005406 261.3283  98.7346 15.31932939264278
Keplerian parameters: {a: 6854622.438115723; e: 6.816873331539523E-4; i: 97.30520232499347; pa: 53.73262282796879; raan: 30.04920000880782; v: -53.7326719339718;}

You can see slight differences in the eccentricity and the inclination with the argument of perigee being completely wrong (even taking into account some wrapping).

Am I making any oversights here with my code?

I tried converting from a Keplerian orbit to a cartesian orbit and back to a Keplerian orbit to see if there were any issues with the conversion (there weren’t) and I also tried propagating the TLE propagator from the epoch of the TLE instead which gave the same result (tle_orbit_cart = tle_propagator.propagate(tle_object.getDate()).getOrbit())

Any suggestions of where I could be going wrong?

Also thanks for your continued support on these forums you have answered many of my other questions through other posts, I appreciate your time.

Thanks

[bcazabonne]

Hi @astroboy

Welcome to the Orekit forum!

Your code is good.

As explained in the topic below, TLE parameters are significant only with respect to the TLE propagation model (i.e., SGP4 model represented by the TLEPropagator class of Orekit)

DSST Propagation results seems odd

Hi @JulienL for i in range(len(TLEs)): tle_i.append(TLEs[i].getI()) tle_omega.append(TLEs[i].getRaan()) tle_e.append(TLEs[i].getE()) tle_arg.append(TLEs[i].getPerigeeArgument()) tle_M.append(TLEs[i].getMeanAnomaly()) tle_date.append(TLEs[i].getDate()) tle_meanMotion.append(TLEs[i].getMeanMotion()) You can’t use directly the orbital parameters contained in the TLE. These parameters are only significant with respect to the propagation model (e.g. SGP4). Therefore, yo…

We can’t directly use the keplerian elements contained in the TLE. In other words, the keplerian elements printed by

print(tle_line_1)
print(tle_line_2)

are not the good ones. The correct ones are those calculated by

tle_propagator = TLEPropagator.selectExtrapolator(tle_object)
tle_orbit_cart = tle_propagator.getInitialState().getOrbit()
tle_orbit_kep = OrbitType.KEPLERIAN.convertType(tle_orbit_cart)

The JavaDoc of the TLE class explains this particularity.

Best regards,
Bryan

[lirw1984]

Hi there,

Beside the explaination of @bcazabonne, the eccentricity is very small so that the argument of perigee and mean anomaly may be not the same, but pa+v(or M) should be close.

Here, 261.3283 +98.7346=360.0629=0.629, pa+v= 53.7326+ -53.7326~0.

You can try others with lager eccentricity.

[astroboy]

Thank you both @bcazabonne and @lirw1984 for your answers. I appreciate your time.

Thanks