Alpha-5 format export not behaving as I would naively expect ;)

Short version, if Satellite.satnum is greater than 99999, TLEs with no object number are produced. Here’s a demonstration script; I was expecting that if satnum is an integer greater than 99999, the exporter would correctly encode it as alpha-5.

#!/usr/bin/env python3

from scipy.stats.contingency import expected_freq
from sgp4.io import twoline2rv
from sgp4 import exporter
from sgp4.earth_gravity import wgs72

# from https://celestrak.com/NORAD/elements/stations.txt
original_tle = (
    "1 25544U 98067A   21245.66660925  .00003909  00000-0  80185-4 0  9991",
    "2 25544  51.6444 319.5951 0003037 346.9445 145.3381 15.48588378300608",
)

# can I round-trip the TLE successfully?
iss_one = twoline2rv(original_tle[0], original_tle[1], wgs72)
iss_one_tle = exporter.export_tle(iss_one)
assert iss_one_tle == original_tle

# Unnecessary paranoia
iss_two = twoline2rv(iss_one_tle[0], iss_one_tle[1], wgs72)
assert iss_one.intldesg == iss_two.intldesg
assert iss_one.ecco == iss_two.ecco
assert iss_one.a == iss_two.a

# Can I increment the satnum?
bump_satnum = 12345
iss_two.satnum += bump_satnum
modified_tle = exporter.export_tle(iss_two)
modified_obj = twoline2rv(modified_tle[0], modified_tle[1], wgs72)
assert modified_obj.satnum - iss_one.satnum == bump_satnum

# Can I export this new alpha-5 format?
a5 = "A1234"
iss_two.satnum = a5
modified_tle = exporter.export_tle(iss_two)
assert a5 in modified_tle[0] and a5 in modified_tle[1]

# what happens if I try to load it
modified_obj = twoline2rv(modified_tle[0], modified_tle[1], wgs72)
final_tle = exporter.export_tle(modified_obj)
if iss_two.satnum != modified_obj.satnum:
    print(f"round trip fail: {modified_obj.satnum} != {iss_two.satnum}")
if modified_tle != final_tle:
    print(f"A5 test fail\n\texpected:{modified_tle}\n\tcomputed:{final_tle}")

$ python iss.py 
101234
round trip fail: 101234 != A1234
A5 test fail
	expected:('1 A1234U 98067A   21245.66660925  .00003909  00000-0  80185-4 0  9991', '2 A1234  51.6444 319.5951 0003037 346.9445 145.3381 15.48588378300608')
	computed:('1 U 98067A   21245.66660925  .00003909  00000-0  80185-4 0  9991', '2  51.6444 319.5951 0003037 346.9445 145.3381 15.48588378300608')

I’m using python-sgp4 2,20 on xubuntu 21.04 x86_64

[brandon-rhodes]

Not yet — given my schedule, it might not be til late October that I’m able to sit down for a code review? We’ll see!

[ckuethe]

If you’re amenable to having a setter, then I’m amenable to doing up a PR.

I have use cases where I might want to change the satellite number, and use cases where I do have to construct a whole satellite record from orbital elements. In this case I was exploring Alpha-5 and adding 100000 to an otherwise valid TLE seemed like the fastest way to see what would happen.