SGP4/SDP4 validation against the community reference

Kshana's orbit propagator is validated against the canonical community reference for SGP4/SDP4 (SGP4: Simplified General Perturbations 4; SDP4: Simplified Deep-space Perturbations 4): the verification vectors published with Vallado et al., "Revisiting Spacetrack Report #3" (AIAA (American Institute of Aeronautics and Astronautics) 2006-6753), distributed as SGP4-VER.TLE (the test TLEs (two-line element sets)) and tcppver.out (the expected TEME (true equator, mean equinox) state at each time). This is the same reference the public C++/Python/MATLAB implementations validate against, so agreement here is agreement with the de-facto standard.

Result#

The deep-space and resonant cases matter specifically for this project: GNSS (global navigation satellite system) satellites sit in ~12 h orbits that are deep-space and resonant, which a two-body + J2-secular model cannot reproduce — SGP4/SDP4 can.

Method#

• The implementation (src/sgp4.rs) is a dependency-free Rust port of the public-domain Vallado algorithm; epoch handling is days-since-1950, the improved (not afspc) mode is used, and WGS-72 (WGS: World Geodetic System) gravity constants are applied as the reference specifies.
• The test (tests/sgp4_verification.rs) parses the vendored SGP4-VER.TLE and tcppver.out fixtures (tests/fixtures/sgp4/), propagates each test satellite to every reference time, and asserts the TEME position and velocity match within the tolerances above. The compared-row count is pinned at exactly 666 so a fixture or skip-behaviour regression cannot quietly reduce coverage.

Reproduce it#

cargo test --test sgp4_verification -- --nocapture

The test prints the number of rows compared and the worst-case position and velocity errors. The fixtures are committed, so the check is fully offline and deterministic.

Independent cross-check against the sgp4 crate#

Matching the published reference proves correctness against a table. To also prove correctness against an independent implementation, tests/sgp4_crate_comparison.rs runs a widely used independent Rust SGP4 library, the sgp4 crate (neuromorphicsystems/sgp4, version 2.4), over the same 666 AIAA vectors and compares the two codebases head-to-head. Both are driven with the WGS72 (World Geodetic System 1972) gravity model the vectors are defined in (the crate's default from_elements uses WGS84 (World Geodetic System 1984), which differs from the WGS72 reference by ~km — a modelling choice, surfaced honestly, not an error).

Two independent implementations agree to sub-micron on near-earth and resonant orbits and to 4.12 mm worst-case across all regimes — the same 4.12 mm that is Kshana's own deviation from the reference, i.e. the disagreement is that one deep-space case, with the crate sitting between Kshana and the table. The full table (with per-regime row counts and the four pathological cases the crate rejects at construction) is committed at tests/fixtures/sgp4_comparison.md and regenerated via KSHANA_REGEN_FIXTURES=1 cargo test --test sgp4_crate_comparison.

Status of "publishing" this result#

The cross-validation itself is complete and in-tree. Submitting the result to external venues (a community catalogue, a short note, a DOI-archived (DOI: digital object identifier) record) is a maintainer action tracked separately on the roadmap; this document is the in-repository record those submissions would point to.