orbit-tools.c

/* -*- Mode: C; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
/*
    Gpredict: Real-time satellite tracking and orbit prediction program

    Copyright (C)  2001-2009  Alexandru Csete, OZ9AEC.
    Parts are Copyright John A. Magliacane, KD2BD 1991-2003 (indicated below)

    Authors: Alexandru Csete <oz9aec@gmail.com>
             John A. Magliacane, KD2BD.

    Comments, questions and bugreports should be submitted via
    http://sourceforge.net/projects/gpredict/
    More details can be found at the project home page:

            http://gpredict.oz9aec.net/

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, visit http://www.fsf.org/
*/

#include <glib.h>
#include "sgpsdp/sgp4sdp4.h"
#include "orbit-tools.h"


orbit_type_t
get_orbit_type (sat_t *sat)
{
     orbit_type_t orbit = ORBIT_TYPE_UNKNOWN;

     if (geostationary (sat)) {
          orbit = ORBIT_TYPE_GEO;
     }
     else if (decayed (sat)) {
          orbit = ORBIT_TYPE_DECAYED;
     }
     else {
          orbit = ORBIT_TYPE_UNKNOWN;
     }

     return orbit;
}


/** \brief Determinte whether satellite is in geostationary orbit.
 *  \author John A. Magliacane, KD2BD
 *  \param sat Pointer to satellite data.
 *  \return TRUE if the satellite appears to be in geostationary orbit,
 *          FALSE otherwise.
 *
 * A satellite is in geostationary orbit if
 *
 *     fabs (sat.meanmotion - 1.0027) < 0.0002
 *
 * Note: Appearantly, the mean motion can deviate much more from 1.0027 than 0.0002
 */
gboolean
geostationary  (sat_t *sat)
{
     if (fabs (sat->meanmo - 1.0027) < 0.0002)
          return TRUE;
     else
          return FALSE;
}


/** \brief Determine whether satellite has decayed.
 *  \author John A. Magliacane, KD2BD
 *  \author Alexandru Csete, OZ9AEC
 *  \param sat Pointer to satellite data.
 *  \return TRUE if the satellite appears to have decayed, FALSE otherwise.
 *  \bug Modified version of the predict code but it is not tested.
 *
 * A satellite is decayed if
 *
 *    satepoch + ((16.666666 - sat.meanmo) / (10.0*fabs(sat.drag))) < "now"
 *
 */
gboolean
decayed        (sat_t *sat)
{

     /* tle.xndt2o/(twopi/xmnpda/xmnpda) is the value before converted the
        value matches up with the value in predict 2.2.3 */
     /*** FIXME decayed is treated as a static quantity.
          It is time dependent. Also sat->jul_utc is often zero
          when this function is called
     ***/
     if (sat->jul_epoch + ((16.666666 - sat->meanmo) /
                           (10.0 * fabs (sat->tle.xndt2o/(twopi/xmnpda/xmnpda)))) < sat->jul_utc)
          return TRUE;
     else
          return FALSE;


}


/** \brief Determine whether satellite ever reaches AOS.
 *  \author John A. Magliacane, KD2BD
 *  \author Alexandru Csete, OZ9AEC
 *  \param sat Pointer to satellite data.
 *  \return TRUE if the satellite will reach AOS, FALSE otherwise.
 *
 */
gboolean
has_aos        (sat_t *sat, qth_t *qth)
{
     double lin, sma, apogee;
     gboolean retcode = FALSE;


     /* FIXME */
     if (sat->meanmo == 0.0) {
          retcode = FALSE;
     }
     else {

          /* xincl is already in RAD by select_ephemeris */
          lin = sat->tle.xincl;
          if (lin >= pio2)
               lin = pi - lin;

          sma = 331.25 * exp(log(1440.0/sat->meanmo) * (2.0/3.0));
          apogee = sma * (1.0 + sat->tle.eo) - xkmper;

          if ((acos(xkmper/(apogee+xkmper))+(lin)) > fabs(qth->lat*de2ra))
               retcode = TRUE;
          else
               retcode = FALSE;

     }

     return retcode;
}