Error Reading TLE

Orekit Python Wrapper usage
1

Hello dear Orekit team,

I recently faced a misunderstanding while reading a TLE. I wanted to get the sma using orekit but it gave me a wrong answer.

Here is the Orekit code :

tle_obj = TLE(tle[0], tle[1])

propagator = TLEPropagator.selectExtrapolator(tle_obj)

orbit = propagator.getInitialState().getOrbit()

sma = orbit.getA() / 1000

And here is the home made code delivering the right answer :

mu = 398600.4418  # km^3/s^2

line2 = tle[1].split()

n_rev_day = float(line2[7])

n = n_rev_day * 2*np.pi / 86400  # rad/s

sma = (mu / n**2)**(1/3)

Could you tell me why both don’t give the same results ?

(i’m in Orekit 13 by the way)

Thanks a lot

Augustin

2

This is because one version gives you the osculating value and the other gives you the mean value.
Converting TLE to any other orbit type should not be done on a single point.

1 Like
3

Hi there,

when you do this, you’re retrrieving the osculating state, not the mean one contained in a TLE.

For what you’re looking for, use computeSemiMajorAxis on the TLE Orekit object.

Cheers,
Romain.

2 Likes
4

Thank you very much :slight_smile: