arika 0.2.0

Coordinate frames, reference transforms, epochs, and celestial body ephemerides for orbital mechanics.
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//! Earth: physical constants, reference ellipsoid, geodetic coordinates,
//! Earth rotation models, and Earth Orientation Parameters.
//!
//! # Submodules
//!
//! - [`ellipsoid`] — WGS-84 reference ellipsoid constants
//! - [`geodetic`] — WGS-84 Cartesian ↔ geodetic conversions, [`Geodetic`] type
//! - [`rotation`] — IAU 2009 WGCCRE rotation model (`EARTH` const)
//! - [`eop`] — Earth Orientation Parameters provider traits
//!   ([`Ut1Offset`](eop::Ut1Offset), [`PolarMotion`](eop::PolarMotion),
//!   [`NutationCorrections`](eop::NutationCorrections),
//!   [`LengthOfDay`](eop::LengthOfDay)) and [`NullEop`](eop::NullEop) placeholder
//! - [`iau2006`] — IAU 2006 / 2000A_R06 precession-nutation supporting math
//!   (angular units, fundamental arguments, precession polynomials; Phase 3A-1)
//!
//! Phase 3 will add `Rotation<Gcrs, Cirs>::iau2006` / `Rotation<Cirs, Tirs>::from_era`
//! / `Rotation<Tirs, Itrs>::polar_motion` constructors that consume these EOP
//! traits for the full IAU 2006 CIO-based Earth rotation chain.

pub mod ellipsoid;
pub mod eop;
pub mod geodetic;
pub mod iau2006;
pub mod rotation;

pub use ellipsoid::{WGS84_A, WGS84_B, WGS84_E2, WGS84_F};
pub use geodetic::{Geodetic, geodetic_altitude};

// ─── Physical constants ──────────────────────────────────────────

/// Earth gravitational parameter [km³/s²] (WGS-84).
pub const MU: f64 = 398600.4418;

/// Earth equatorial radius [km] (WGS-84).
pub const R: f64 = 6378.137;

/// Earth J2 zonal harmonic coefficient (WGS-84 / EGM96).
pub const J2: f64 = 1.08263e-3;

/// Earth J3 zonal harmonic coefficient (WGS-84 / EGM96).
pub const J3: f64 = -2.5356e-6;

/// Earth J4 zonal harmonic coefficient (WGS-84 / EGM96).
pub const J4: f64 = -1.6199e-6;

/// Earth rotation rate [rad/s] (IERS 2010).
pub const OMEGA: f64 = 7.2921159e-5;

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn mu_is_positive() {
        assert!(MU > 0.0);
    }

    #[test]
    fn r_is_positive() {
        assert!(R > 0.0);
    }

    #[test]
    fn surface_gravity_approximate() {
        // g ≈ μ/R² ≈ 9.798e-3 km/s² ≈ 9.798 m/s²
        let g = MU / (R * R);
        assert!((g - 9.798e-3).abs() < 0.01e-3);
    }

    #[test]
    fn j2_is_positive() {
        assert!(J2 > 0.0);
    }

    #[test]
    fn j3_is_negative() {
        assert!(J3 < 0.0);
    }

    #[test]
    fn j4_is_negative() {
        assert!(J4 < 0.0);
    }

    #[test]
    fn j2_dominates_higher_harmonics() {
        assert!(J2 > J3.abs());
        assert!(J3.abs() > J4.abs());
    }
}