Light Convergence Failure

Heyo @brandon-rhodes thanks for skyfield!

We do a lot of satellite math using your package, and I bumped into this gem while working with it. Here is a reproduction case:

def test_convergence_demo():
    tle_0 = """STARLINK-1885"""
    tle_1 = """1 47168U 20088AY  22221.92842503  .01483002  11676-2  34717-2 0  9991"""
    tle_2 = """2 47168  53.0385 305.4734 0002390 239.0067 121.2198 15.96617970 96526"""

    load
    satellite = EarthSatellite(tle_1, tle_2, tle_0)
    capp = wgs84.latlon(37.76115,-122.418056,12)
    ts = load.timescale()
    time = ts.tt_jd(2459831.9170483043)

    _planets = load('de421.bsp')

    # ssb = Solar System Barycentric
    location_ssb = _planets["earth"] + capp
    target_ssb = _planets["earth"] + satellite

    # NOTE(meawoppl) - altaz accepts temp/humidity correction...
    location_ssb.at(time).observe(target_ssb).apparent().altaz()

This code takes a road less traveled and reports the following:

observer = <Barycentric BCRS position and velocity at date t center=0 target=WGS84 latitude +37.7612 N longitude -122.4181 E elevation 12.0 m>
target = <VectorSum of 3 vectors:
 'de421.bsp' segment 0 SOLAR SYSTEM BARYCENTER -> 3 EARTH BARYCENTER
 'de421.bsp' segment 3 EARTH BARYCENTER -> 399 EARTH
 EarthSatellite 399 EARTH -> STARLINK-1885 catalog #47168 epoch 2022-08-09 22:16:56 UTC>

    def _correct_for_light_travel_time(observer, target):
        """Return a light-time corrected astrometric position and velocity.
    
        Given an `observer` that is a `Barycentric` position somewhere in
        the solar system, compute where in the sky they will see the body
        `target`, by computing the light-time between them and figuring out
        where `target` was back when the light was leaving it that is now
        reaching the eyes or instruments of the `observer`.
    
        """
        t = observer.t
        ts = t.ts
        whole = t.whole
        tdb_fraction = t.tdb_fraction
    
        cposition = observer.position.au
        cvelocity = observer.velocity.au_per_d
    
        tposition, tvelocity, gcrs_position, message = target._at(t)
    
        distance = length_of(tposition - cposition)
        light_time0 = 0.0
        for i in range(10):
            light_time = distance / C_AUDAY
            delta = light_time - light_time0
            if max(abs(delta)) < 1e-12:
                break
    
            # We assume a light travel time of at most a couple of days.  A
            # longer light travel time would best be split into a whole and
            # fraction, for adding to the whole and fraction of TDB.
            t2 = ts.tdb_jd(whole, tdb_fraction - light_time)
    
            tposition, tvelocity, gcrs_position, message = target._at(t2)
            distance = length_of(tposition - cposition)
            light_time0 = light_time
        else:
>           raise ValueError('light-travel time failed to converge')
E           ValueError: light-travel time failed to converge

This case was hard to find… I suspect we have called an equivalent of this subroutine maybe… a few million times without experiencing it…

Happy to dig in more if there isn’t something obvious that springs to mind.

[rmathar]

So the problem appears to be that the program is trying to observe a satellite on 2022-09-09 which decayed in 2022-08-14 and was for that end-of-life starting an extreme (non-physical) elliptic orbit approximately 2 days earlier while actually crashing into the atmosphere: https://isstracker.pl/en/satelity/47168 . The extreme LTE parameter set to 0.0148 (on 2022-08-10) for the change in the n-parameter is in some way indicating that sort of problem.

[meawoppl]

Also, if you ever find yourself at or near that lat/long, I owe you at least a couple drinks 🍻