//! SGP4 physical constants from Vallado's "Revisiting Spacetrack Report #3" (2006).
//!
//! WGS-72 constants used by the SGP4 propagator for compatibility with
//! NORAD TLE element sets. WGS-84 constants used for geodetic transforms.

use std::f64::consts::PI;

// ── WGS-72 constants (SGP4 propagator) ──────────────────────

/// Gravitational parameter of Earth (km^3/s^2), WGS-72 value.
pub const MU: f64 = 398600.8;

/// Earth equatorial radius (km), WGS-72.
pub const EARTH_RADIUS_KM: f64 = 6378.135;

/// XKE = 60.0 / sqrt(a_E^3 / mu) where a_E in km, mu in km^3/s^2.
/// This is sqrt(GM) in units of earth-radii^1.5 / min.
pub const XKE: f64 = 0.07436685316871385;

/// KE = sqrt(GM_Earth) in earth-radii^1.5 / min (same as XKE for WGS-72).
pub const KE: f64 = 0.0743669161;

/// Velocity conversion: km/s per (earth-radii/min).
pub const VKMPERSEC: f64 = EARTH_RADIUS_KM * XKE / 60.0;

/// J2 zonal harmonic, WGS-72.
pub const J2: f64 = 1.08262998905e-3;

/// J3 zonal harmonic, WGS-72.
pub const J3: f64 = -2.53215306e-6;

/// J4 zonal harmonic, WGS-72.
pub const J4: f64 = -1.61098761e-6;

/// J3 / J2 ratio (used frequently in SGP4).
pub const J3OJ2: f64 = J3 / J2;

/// CK2 = 0.5 * J2 * a_E^2 (in SGP4 normalized units).
pub const CK2: f64 = 0.5 * J2;

/// CK4 = -3/8 * J4 * a_E^4 (in SGP4 normalized units).
pub const CK4: f64 = -0.375 * J4;

// ── General constants ────────────────────────────────────────

/// Two pi.
pub const TWOPI: f64 = 2.0 * PI;

/// Minutes per day.
pub const MINUTES_PER_DAY: f64 = 1440.0;

/// Seconds per day.
pub const SECONDS_PER_DAY: f64 = 86400.0;

/// Degrees to radians conversion factor.
pub const DEG_TO_RAD: f64 = PI / 180.0;

/// Radians to degrees conversion factor.
pub const RAD_TO_DEG: f64 = 180.0 / PI;

// ── WGS-84 constants (geodetic transforms) ──────────────────

/// Earth equatorial radius (km), WGS-84.
pub const EARTH_RADIUS_WGS84: f64 = 6378.137;

/// Earth polar radius (km), WGS-84.
pub const EARTH_POLAR_RADIUS: f64 = 6356.752314245;

/// WGS-84 flattening.
pub const FLATTENING: f64 = 1.0 / 298.257223563;

/// WGS-84 first eccentricity squared: 2f - f^2.
pub const EARTH_ECCENTRICITY_SQ: f64 = 2.0 * FLATTENING - FLATTENING * FLATTENING;

/// WGS-84 second eccentricity squared: (a^2 - b^2) / b^2.
pub const EARTH_ECCENTRICITY_SQ2: f64 =
    (EARTH_RADIUS_WGS84 * EARTH_RADIUS_WGS84 - EARTH_POLAR_RADIUS * EARTH_POLAR_RADIUS)
        / (EARTH_POLAR_RADIUS * EARTH_POLAR_RADIUS);

/// Earth rotation rate (rad/s), WGS-84.
pub const OMEGA_E: f64 = 7.292115e-5;

/// Threshold for near-earth vs deep-space branching (minutes).
/// Satellites with period >= 225 minutes use SDP4.
pub const DEEP_SPACE_THRESHOLD_MIN: f64 = 225.0;

/// Small value for floating-point comparisons.
pub const SMALL: f64 = 1.0e-12;

/// Maximum iterations for Kepler's equation solver.
pub const MAX_KEPLER_ITERATIONS: u32 = 10;