find_events on highly elliptical orbit satellite inconsistent

[ecdaero]

Hi! Thanks for the work you've done on Skyfield. I'm using it to find rise/set times of objects, and recently came across this corner case(?) where the find_events method is giving incomplete results.

The track of the object that I noticed this on:

I wrote a small program to demo this behavior:

#!/usr/bin/env python3

import datetime as dt
from datetime import datetime

from skyfield.api import EarthSatellite, Time, load, load_file, wgs84
from skyfield.toposlib import GeographicPosition
from skyfield.vectorlib import VectorSum


def main():
    """Attempt to find rise/set events for an example pass."""
    planets = load_file("de421.bsp")

    ts = load.timescale(builtin=True)

    site_latitude = 33
    site_longitude = -118
    site_altitude = 30
    min_elevation_angle = 10

    site = wgs84.latlon(site_latitude, site_longitude, elevation_m=site_altitude)

    # This object has a highly elliptical orbit:
    sat_target = EarthSatellite(
        "1 17328U 87008A   24305.23833080  .00000500  00000-0  22722-2 0  9997",
        "2 17328  63.0833   7.8955 7415190 253.9972  19.6892  2.00632911273252",
        "MOLNIYA 3-31",
        ts,
    )

    earth = planets["earth"]
    groundstation = earth + site
    satellite = earth + sat_target

    # This is an example pass where we noticed we weren't getting the right rise/sets
    obs_start_time = datetime(2024, 10, 31, 5, 28, 6, 0, tzinfo=dt.timezone.utc)
    obs_end_time = datetime(2024, 10, 31, 5, 40, 6, 0, tzinfo=dt.timezone.utc)
    t0 = ts.from_datetime(obs_start_time)
    t1 = ts.from_datetime(obs_end_time)

    event_times, events = find_events(site, groundstation, sat_target, satellite, t0, t1, min_elevation_angle)

    print("This should have found a 'rise':")
    print(events, event_times)
    print("...but it has not found a rise event.\n")

    # However, this *does* work:
    obs_start_time = datetime(2024, 10, 31, 2, 28, 6, 0, tzinfo=dt.timezone.utc)
    obs_end_time = datetime(2024, 10, 31, 7, 40, 6, 0, tzinfo=dt.timezone.utc)
    t0 = ts.from_datetime(obs_start_time)
    t1 = ts.from_datetime(obs_end_time)

    event_times, events = find_events(site, groundstation, sat_target, satellite, t0, t1, min_elevation_angle)

    print("Expand the boundary, and it finds events:")
    print(events, event_times)
    print("...including a rise and culmination.\n")

    obs_long_end_time = datetime(2024, 10, 31, 16, 40, 6, 0, tzinfo=dt.timezone.utc)
    t2 = ts.from_datetime(obs_long_end_time)

    event_times, events = find_events(site, groundstation, sat_target, satellite, t0, t2, min_elevation_angle)

    print("Expand it further, and it should have found a rise, culmination, and set:")
    print(events, event_times)
    print("...but it found a rise, two culminates, and no set.\n")

    # note: adjust the range fidelity to see more detail on the track
    track_time = ts.utc(2024, 10, 31, 5, range(20, 710, 4))
    print_track(groundstation, satellite, track_time)


def find_events(
    site: GeographicPosition,
    site_vector: VectorSum,
    satellite: EarthSatellite,
    sat_vector: VectorSum,
    t0: Time,
    t1: Time,
    min_elevation: float,
) -> tuple:
    alt_initial, az_initial, ssb_dist = site_vector.at(t0).observe(sat_vector).apparent().altaz()
    alt_final, az_final, ssb_dist = site_vector.at(t1).observe(sat_vector).apparent().altaz()
    print(f"Start/End (Az/El): {az_initial.degrees:.5f} / {alt_initial.degrees:.5f} ----> {az_final.degrees:.5f} / {alt_final.degrees:.5f}")

    # This fails to find the "rise" above 10 degrees elevation.
    event_times, events = satellite.find_events(site, t0, t1, altitude_degrees=min_elevation)

    return event_times, events


def print_track(groundstation: VectorSum, satellite: EarthSatellite, track_time: Time) -> None:
    """Print a minute-by-minute track of a satellite."""
    print("Propagated track:")
    for obs_time in track_time:
        ssb_alt, ssb_az, ssb_dist = groundstation.at(obs_time).observe(satellite).apparent().altaz()
        print(f"{obs_time.utc_strftime('%Y-%m-%d %H:%M')}: {ssb_az.degrees:.5f} / {ssb_alt.degrees:.5f}")


if __name__ == "__main__":
    main()

The output, trimmed a bit for brevity:

Start/End (Az/El): 187.17892 / 3.56302 ----> 181.04224 / 31.75064
This should have found a 'rise':
[] <Time tt=[]>
...but it has not found a rise event.

Start/End (Az/El): 8.53929 / -5.12598 ----> 342.28175 / 80.02753
Expand the boundary, and it finds events:
[0 1] <Time tt=[2460614.73033378 2460614.79156628]>
...including a rise and culmination.

Start/End (Az/El): 8.53929 / -5.12598 ----> 153.81261 / 25.31236
Expand it further, and it should have found a rise, culmination, and set:
[0 1 1] <Time tt=[2460614.73033391 2460614.79156647 2460615.17820007]>
...but it found a rise, two culminates, and no set.

Propagated track:
2024-10-31 05:20: 192.82956 / -19.59808
2024-10-31 05:24: 189.81945 / -7.95510
2024-10-31 05:28: 187.23907 / 3.29044
2024-10-31 05:32: 184.96974 / 13.76284
2024-10-31 05:36: 182.93587 / 23.21467
2024-10-31 05:40: 181.08645 / 31.55621
2024-10-31 05:44: 179.38535 / 38.82250
2024-10-31 05:48: 177.80579 / 45.11913
2024-10-31 05:52: 176.32707 / 50.57753
2024-10-31 05:56: 174.93248 / 55.32799
2024-10-31 06:00: 173.60790 / 59.48724
2024-10-31 06:04: 172.34077 / 63.15455
2024-10-31 06:08: 171.11921 / 66.41212
2024-10-31 06:12: 169.93118 / 69.32710
2024-10-31 06:16: 168.76358 / 71.95414
2024-10-31 06:20: 167.60085 / 74.33764
2024-10-31 06:24: 166.42309 / 76.51380
2024-10-31 06:28: 165.20258 / 78.51223
2024-10-31 06:32: 163.89761 / 80.35721
2024-10-31 06:36: 162.43976 / 82.06872
2024-10-31 06:40: 160.70536 / 83.66305
2024-10-31 06:44: 158.44144 / 85.15312
2024-10-31 06:48: 155.03239 / 86.54776
2024-10-31 06:52: 148.50069 / 87.84690
2024-10-31 06:56: 128.13765 / 89.00029
2024-10-31 07:00: 39.69036 / 89.30654
2024-10-31 07:04: 3.38661 / 88.37322
2024-10-31 07:08: 354.56943 / 87.32850
2024-10-31 07:12: 350.60919 / 86.30500
2024-10-31 07:16: 348.25847 / 85.31556
2024-10-31 07:20: 346.63795 / 84.36104
2024-10-31 07:24: 345.41435 / 83.43988
2024-10-31 07:28: 344.43383 / 82.55000
2024-10-31 07:32: 343.61550 / 81.68929
2024-10-31 07:36: 342.91262 / 80.85572
2024-10-31 07:40: 342.29623 / 80.04743
2024-10-31 07:44: 341.74737 / 79.26273
2024-10-31 07:48: 341.25300 / 78.50008
2024-10-31 07:52: 340.80385 / 77.75808
2024-10-31 07:56: 340.39309 / 77.03548
2024-10-31 08:00: 340.01557 / 76.33114
2024-10-31 08:04: 339.66727 / 75.64402
2024-10-31 08:08: 339.34503 / 74.97318
2024-10-31 08:12: 339.04628 / 74.31778
2024-10-31 08:16: 338.76892 / 73.67704
...
2024-10-31 12:32: 339.18931 / 51.25362
2024-10-31 12:36: 339.32071 / 51.16099
2024-10-31 12:40: 339.45316 / 51.07786
2024-10-31 12:44: 339.58656 / 51.00441
2024-10-31 12:48: 339.72082 / 50.94083
2024-10-31 12:52: 339.85584 / 50.88734
2024-10-31 12:56: 339.99152 / 50.84416
2024-10-31 13:00: 340.12775 / 50.81152
2024-10-31 13:04: 340.26446 / 50.78967
2024-10-31 13:08: 340.40153 / 50.77889
2024-10-31 13:12: 340.53889 / 50.77946
2024-10-31 13:16: 340.67642 / 50.79169
2024-10-31 13:20: 340.81404 / 50.81590
2024-10-31 13:24: 340.95166 / 50.85244
2024-10-31 13:28: 341.08918 / 50.90169
2024-10-31 13:32: 341.22653 / 50.96405
2024-10-31 13:36: 341.36361 / 51.03994
2024-10-31 13:40: 341.50034 / 51.12985
2024-10-31 13:44: 341.63665 / 51.23427
2024-10-31 13:48: 341.77246 / 51.35374
2024-10-31 13:52: 341.90769 / 51.48884
...
2024-10-31 15:48: 346.96691 / 69.50775
2024-10-31 15:52: 347.66689 / 71.36404
2024-10-31 15:56: 348.66738 / 73.45936
2024-10-31 16:00: 350.20602 / 75.84171
2024-10-31 16:04: 352.83769 / 78.56846
2024-10-31 16:08: 358.20033 / 81.69377
2024-10-31 16:12: 13.60105 / 85.15166
2024-10-31 16:16: 81.85831 / 87.11603
2024-10-31 16:20: 134.10787 / 83.04728
2024-10-31 16:24: 145.82440 / 76.28422
2024-10-31 16:28: 150.17777 / 67.48542
2024-10-31 16:32: 152.25411 / 56.25051
2024-10-31 16:36: 153.31592 / 42.23893
2024-10-31 16:40: 153.80583 / 25.74685
2024-10-31 16:44: 153.90416 / 8.11710
2024-10-31 16:48: 153.67806 / -8.82336

I would prefer to continue using Skyfield for finding rise and set events, and I don't think I'm doing anything wrong. Is this a bug/limitation in Skyfield? Are there intentional limitations here that I should work around?

Thanks again!

[nils10]

Hi,
I ran into a similar issue with pass prediction regularly failing for the recently launched Proba-3 satellite (that is also in a highly elliptical orbit). In case you want to have another example case to reproduce, some info below:

TLE:

PROBA3-estimation
1 99991U 24999A   24340.44722222  .00000010  00000-0  52554-3 0  0017
2 99991  59.0000 142.9829 8111000 188.0000 000.0000 01.22265304000001

Observer location: 53°N, 5°E
Min. elevation: 0.0 deg
Start time (t0): 2024-12-07 12:35 UTC
End time (t1): 2024-12-09 12:35 UTC

Calling the following code to find all passes/events between the start and end time (2 days) results in the following events being returned:
t, events = self.satellite.find_events(self.observer, t0, t1, altitude_degrees=0.0)

Passes/Events:

Pass 1 (correct):
2024-12-06 21:40:24.319836+00:00 Event: 0 | EL: 0.0 deg CORRECT
2024-12-07 00:31:40.045713+00:00 Event: 1 | EL: 21.1 deg CORRECT
2024-12-07 01:26:46.400018+00:00 Event: 2 | EL: -0.0 deg CORRECT
Pass 2 (correct):
2024-12-07 04:11:32.986707+00:00 Event: 0 | EL: 0.0 deg CORRECT
2024-12-07 06:22:31.528189+00:00 Event: 1 | EL: 10.1 deg CORRECT
2024-12-07 09:27:28.588537+00:00 Event: 2 | EL: -0.0 deg CORRECT
Pass 3 (NOT correct):
2024-12-08 00:26:41.619490+00:00 Event: 0 | EL: 0.0 deg CORRECT
2024-12-08 05:15:41.914492+00:00 Event: 1 | EL: 30.6 deg CORRECT
<MISSING: here the 3rd pass actually has LOS at ~12:10:55 UTC>
<MISSING: here a 4th pass actually has AOS at ~15:33:55 UTC>
2024-12-08 16:53:26.786992+00:00 Event: 1 | EL: 16.3 deg (TME and EL is correct, but this is of a 4th pass, not 3rd pass!)
<MISSING: here the 4th pass actually has LOS at ~17:05:39 UTC>
<MISSING: here a 5th pass actually has AOS at ~22:56:48 UTC>
2024-12-09 05:25:31.332216+00:00 Event: 1 | EL: 45.2 deg (TME and EL is correct, but this is of a 5th pass, not 3rd pass!)
2024-12-09 12:31:59.786383+00:00 Event: 1 | EL: 88.1 deg (TME and EL is correct, but this is of a 5th pass, not 3rd pass! Its correct that the 5th pass indeed has 2 TME's.

So in this example the prediction has missed that passes have ended/new passes have started. It would be great if this could be fixed as its hard to know in production when this happens and can have significant impact when missed!

I found a comment in the sgp4lib.py that might be a starting point to fixing this:
# Long-period satellites might rise each day not because of their own motion, but because the Earth rotates under them, so check position at least each quarter-day. We might need to tighten this even further if experience someday shows it missing a pass of a particular satellite.