What detector for satellite inside geostationary satellite fov?

Orekit usage
1

Hi,

What detector is best to use for detecting when LEO,MEO or HEO satellite passes within fov from a geostationary satellite defined by longitude and half cone angle?

I have looked at the EclipseDetector and the InterSatDirectViewDetector, but I cannot find a perfect fit for the task.

If I was only looking at LEO satellites I could probably make an approximate ground area covered by the cone from the geostationary satellite, but since I have to take HEO satellites into account I guess it can cross the cone wall (fov) at any height.

I would really appreciate any pointers in the right direction :slight_smile:

Thanks
Tommy

2

I would suggest registering a FieldOfViewDetector on a simple geostationary satellite propagator, but that would imply having an anaytical representation of the target satellite ephemeris (because using a NumericalPropagator as the target PVCoordinatesProvider would be too costly).
If you need to have the event detection registered on the target satellite itself, perhaps creating you own event detector would be simpler.

3

I decided to make my own detector since the events are connected to the orbiting satellite.

Thanks for the guidance.

I attach the code here. I got within 2-3 seconds of STK for a polar orbiting satellite at approx 630km.
It works, but don’t take as super scientific answer.

-Tommy

class InsideConeDetector(PythonAbstractDetector):
    def __init__(self, time_step: float, half_cone_angle: float,
                 frame: TopocentricFrame, handler: EventHandler = None):

        self._frame = frame
        self._half_cone_angle = half_cone_angle

        if handler is None:
            handler = StopOnEvent()
        super().__init__(float(time_step),
                         PythonAbstractDetector.DEFAULT_THRESHOLD,
                         PythonAbstractDetector.DEFAULT_MAX_ITER,
                         handler)

    def init(self, state: SpacecraftState, date: AbsoluteDate):
        pass

    def g(self, state: SpacecraftState):

        # https://en.wikipedia.org/wiki/Geostationary_orbit
        geo_altitude_above_equator_m = 35786000 # meters

        sat_elevation = self._frame.getElevation(state.getPosition(), state.getFrame(),
                                    state.getDate())
        sat_altitude = EARTH.transform(state.getPosition(), state.getFrame(), state.getDate()).getAltitude()
        sat_latitude = EARTH.transform(state.getPosition(), state.getFrame(), state.getDate()).getLatitude()

        #https://gis.stackexchange.com/questions/20200/how-do-you-compute-the-earths-radius-at-a-given-geodetic-latitude
        earth_a = 6378137
        earth_b = 6356752.3142
        earth_radius_at_latitude_m = sqrt((pow(earth_a*earth_a*cos(abs(sat_latitude)),2) + 
                                           pow(earth_b*earth_b*sin(abs(sat_latitude)),2))/
                                           (pow(earth_a*cos(abs(sat_latitude)),2) + 
                                           pow(earth_b*sin(abs(sat_latitude)),2)))
        
        # Half cone angle at 300km and 0km altitude
        half_cone_angle_at_300km = asin((earth_radius_at_latitude_m + 300e3)/(geo_altitude_above_equator_m+earth_radius_at_latitude_m))
        half_cone_angle_at_0km = asin((earth_radius_at_latitude_m)/(geo_altitude_above_equator_m+earth_radius_at_latitude_m))

        max_elevation = atan(earth_radius_at_latitude_m/(earth_radius_at_latitude_m+sat_altitude))

        hypotenus_sat = self._frame.getRange(state.getPosition(), state.getFrame(),state.getDate()) # hypotenus, slantrange

        # Distance from line, between geo relay and nadir point, to satellite
        x_sat = hypotenus_sat * cos(abs(sat_elevation))
        
        # Height of satellite, above or below nadir point of geo relay
        y_sat = hypotenus_sat * sin(abs(sat_elevation))

        # Calculate radius of cone at satellite altitudes
        if (sat_elevation > 0):
            cone_radius_at_sat_altitude = tan(radians(self._half_cone_angle))*(geo_altitude_above_equator_m-y_sat)
            cone_radius_at_300km = tan(half_cone_angle_at_300km)*(geo_altitude_above_equator_m-y_sat)
            cone_radius_at_0_altitude = tan(half_cone_angle_at_0km)*(geo_altitude_above_equator_m-y_sat)
        else:
            cone_radius_at_sat_altitude = tan(radians(self._half_cone_angle))*(geo_altitude_above_equator_m+y_sat)
            cone_radius_at_300km = tan(half_cone_angle_at_300km)*(geo_altitude_above_equator_m+y_sat)
            cone_radius_at_0_altitude = tan(half_cone_angle_at_0km)*(geo_altitude_above_equator_m+y_sat)

        # Check that satellite is not in earth shadow marked by cone_radius at 0km altitude
        # and that satellite is not inside 300 km grazing altitude 
        if ((sat_elevation < -max_elevation and x_sat < cone_radius_at_0_altitude) or 
            (sat_elevation < -max_elevation and x_sat < cone_radius_at_300km)):
            _log.debug("Dato: %s, elevation: %.2f (max: %.2f), y_sat: %.3f km, x_sat %.3f =? cone_radius_at_0km: %.3f km, cone_radius_at_300km: %.3f km" %
                        (state.getDate().toString(), degrees(sat_elevation), degrees(max_elevation), y_sat/1e3, x_sat/1e3, 
                         cone_radius_at_0_altitude/1e3, cone_radius_at_300km/1e3))
            return -1.0
       
        res = cone_radius_at_sat_altitude - x_sat

        #if (res > 0):
        #    _log.debug("Dato: %s, elevation: %.2f, y_sat: %.3f km, x_sat %.3f =? cone_radius: %.3f km, res = %.3f" %
        #                (state.getDate().toString(), degrees(sat_elevation), y_sat/1e3, x_sat/1e3, cone_radius_at_sat_altitude/1e3, res))
        #else:
        #    _log.debug("Elevation: %.2f, y_sat: %.3f km, x_sat %.3f =? cone_radius: %.3f km, res = %.3f" %
        #                (degrees(sat_elevation), y_sat/1e3, x_sat/1e3, cone_radius_at_sat_altitude/1e3, res))

        return res