# References
## References
The equations and many of the unit tests underlying this work are drawn from the following sources:
- **"Fundamentals of Astrodynamics and Applications, Fourth Edition"**, D. Vallado, Microcosm Press and Springer, 2013.
<https://celestrak.org/software/vallado-sw.php>
- **"Satellite Orbits: Models, Methods, Applications"**, O. Montenbruck and E. Gill, Springer, 2000.
<https://doi.org/10.1007/978-3-642-58351-3>
## Models
This code makes reference to and relies on models generated by the following:
- **SGP4 Orbit Propagator** — <https://celestrak.org/software/tskelso-sw.php>
NORAD / SGP4 orbit propagator used to generate position and velocity states from orbital ephemerides described by Two-Line Element Sets (TLEs). This code base includes a pure-Rust translation of the SGP4 orbit propagator.
- **NRL MSISE-00 Density Model** — <https://ccmc.gsfc.nasa.gov/models/NRLMSIS~00/>
NRL model of air density, including density at high altitudes, used to compute satellite drag.
- **Gravity Models** — <http://icgem.gfz-potsdam.de/home>
International Center for Global Earth Models (ICGEM), collection and archive in a common format of all existing global gravity field models.
- **Space Weather** — <https://celestrak.org/SpaceData/>
Space weather used to modulate the air density used in drag calculations.
- **Earth Orientation Parameters** — <https://celestrak.org/SpaceData/>
Time-varying Earth orientation parameters used for time epoch conversions and high-precision rotations between the inertial and Earth-fixed coordinate frames.
- **IERS Conventions** — <https://www.iers.org/IERS/EN/Publications/TechnicalNotes/tn36.html>
International Earth Rotation and Reference Systems Service Technical Note 36 for rotation between inertial and Earth-fixed coordinate systems.
## Verification
The code includes Rust test modules and Python test modules for verification of calculations, including but not limited to:
- **JPL Ephemeris** — Via JPL-provided test vectors for Chebyshev polynomial calculation
- **SGP4** — Via SGP4 test vectors provided with original C++ distribution