/*
 * od_iod.h -- Initial orbit determination (Gibbs method)
 *
 * Given 3 position vectors and their times, recover a velocity
 * at the middle observation using Gibbs' vector cross-product
 * approach (Vallado Algorithm 54). The resulting (r, v) pair
 * provides a seed orbit for the DC solver.
 *
 * Uses WGS-72 mu (398600.8 km^3/s^2) for SGP4 consistency.
 */

#ifndef PG_ORRERY_OD_IOD_H
#define PG_ORRERY_OD_IOD_H

#include "od_math.h"

/*
 * IOD result: Keplerian elements at the middle observation epoch.
 */
typedef struct
{
    od_keplerian_t  kep;
    double          epoch_jd;
    int             valid;      /* 1 if physically valid orbit */
} od_iod_result_t;

/*
 * Gibbs method: recover velocity at r2 from 3 coplanar positions.
 *
 * pos1, pos2, pos3: TEME position vectors (km)
 * jd1, jd2, jd3: Julian dates of each observation
 * result: output Keplerian elements at epoch jd2
 *
 * Returns 0 on success, -1 if vectors are not coplanar (within
 * tolerance) or orbit is degenerate.
 *
 * Coplanarity check: |r1 . (r2 x r3)| / (|r1| * |r2 x r3|) < 0.05
 * (about 3 degrees -- generous to handle noise).
 */
int od_gibbs(const double pos1[3], const double pos2[3],
             const double pos3[3],
             double jd1, double jd2, double jd3,
             od_iod_result_t *result);

/*
 * Gauss method: recover orbit from 3 angles-only (RA/Dec) observations.
 *
 * ra[3], dec[3]: right ascension and declination in radians
 * jd[3]: Julian dates of each observation
 * obs_ecef[3][3]: observer ECEF positions (km) at each epoch
 * result: output Keplerian elements at epoch jd[1] (middle obs)
 *
 * Returns 0 on success, -1 on failure (non-convergence, degenerate).
 *
 * Implements Vallado Algorithm 52 with iterative refinement of the
 * slant range at the middle observation.
 */
int od_gauss(const double ra[3], const double dec[3],
             const double jd[3],
             const double obs_ecef[3][3],
             od_iod_result_t *result);

#endif /* PG_ORRERY_OD_IOD_H */