Cannot Generate Valid TLE from sgp4init

I was trying to use sgp4 to construct tles from orbital parameters using latest version (2.12)

I am getting invalid tles back, however. This seems to be because export_tle expects epochyr and epochdays to be set, but they don’t seem to be set by sgp4init.

Maybe this is user error, but I couldn’t quite figure it out.

Here’s the test I constructed (using the sgp4init example from the documentation):

from sgp4.api import WGS72, Satrec
from sgp4.exporter import export_tle

satrec = Satrec()

satrec.sgp4init(
    WGS72,           # gravity model
    'i',             # 'a' = old AFSPC mode, 'i' = improved mode
    5,               # satnum: Satellite number
    18441.785,       # epoch: days since 1949 December 31 00:00 UT
    2.8098e-05,      # bstar: drag coefficient (/earth radii)
    6.969196665e-13, # ndot: ballistic coefficient (revs/day)
    0.0,             # nddot: second derivative of mean motion (revs/day^3)
    0.1859667,       # ecco: eccentricity
    5.7904160274885, # argpo: argument of perigee (radians)
    0.5980929187319, # inclo: inclination (radians)
    0.3373093125574, # mo: mean anomaly (radians)
    0.0472294454407, # no_kozai: mean motion (radians/minute)
    6.0863854713832, # nodeo: right ascension of ascending node (radians)
)

tle = export_tle(satrec)

assert tle[0][18:32] != '00000.00000000'

[chrisrosner]

I will attempt to validate correctness next. I have validated that there are realistic looking epoch dates present in the tle now.

I know it doesn’t make any sense, but for this application I don’t need precision.

I’ll comment today with a test for correctness.

[chrisrosner]

I am now getting epochs in the tles.

I cannot believe how responsive you were on this, thank you!