Fix AFSPC gsto: use IAU/gstime polynomial to match Vallado C++ reference#22
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neilberkman wants to merge 1 commit into
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Fix AFSPC gsto: use IAU/gstime polynomial to match Vallado C++ reference#22neilberkman wants to merge 1 commit into
neilberkman wants to merge 1 commit into
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Vallado's initl() in SGP4.cpp computes gsto using two formulas: 1. Lines 1261-1270: the AFSPC d₁₉₇₀ polynomial (stored in gsto1) 2. Line 1273: gstime_SGP4(epoch + 2433281.5) — the IAU polynomial The second assignment unconditionally overwrites the first, making the AFSPC sidereal time formula dead code. The reference C++ always uses gstime_SGP4 for gsto regardless of opsmode. This crate's from_elements_afspc_compatibility_mode was passing afspc_epoch_to_sidereal_time which implements the dead-code formula. Changed to iau_epoch_to_sidereal_time which matches gstime_SGP4. The difference is ~1e-10 rad in gsto, which accumulates during long propagations. For a GEO satellite propagated ~1 year (500,000 min), this reduces position error from 3.7 mm to < 1 mm vs the C++ reference. Includes regression test that fails on master and passes with this fix. Updated doc comment to reflect the change.
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Summary
from_elements_afspc_compatibility_modeusesafspc_epoch_to_sidereal_timefor gsto (Greenwich Sidereal Time at epoch). However, the Vallado C++ reference implementation never actually uses that formula.In
SGP4.cppinitl(), gsto is computed twice:gsto1)gsto = gstime_SGP4(epoch + 2433281.5);The second assignment unconditionally overwrites the first. The AFSPC sidereal time result is dead code —
gstime_SGP4(the IAU polynomial) is always used, regardless of opsmode.This PR changes the sidereal time function passed by
from_elements_afspc_compatibility_modefromafspc_epoch_to_sidereal_timetoiau_epoch_to_sidereal_time, matching the C++ behavior.Impact
The gsto difference is ~1e-10 rad — negligible for short propagations but accumulates over time. For a GEO-resonant satellite propagated ~1 year (500,000 min), the position error drops from ~3.7 mm to < 0.13 mm vs the C++ reference.
Test plan