[Java] Constant event triggering with LongitudeCrossingDetector

Orekit usage
1

I’m trying to find the periods of a satellite when the satellite is above the coordinates. But when I propagate to a time further than the tle’s date LongitudeCrossingDetector is triggered excessively and single coordinate prediction does not complete

This problem doesn’t happen on python

Here is the relevant code:

    public static OneAxisEllipsoid EARTH;

    @BeforeAll
    static void beforeAll() {
        Utils.setDataRoot();    //load orekit data
        EARTH = new OneAxisEllipsoid(Constants.WGS84_EARTH_EQUATORIAL_RADIUS,
                Constants.WGS84_EARTH_FLATTENING,
                FramesFactory.getITRF(IERSConventions.IERS_2010, true));
    }

    @Test
    void test(){

        String[] tle = new String[]{
                "Test tle",
                "1 25544U 98067A   22257.49623145  .00008872  00000-0  16209-3 0  9998",
                "2 25544  51.6424 254.1285 0002269 232.3655 233.3422 15.50204391359035"
        };
        ZonedDateTime startDate = ZonedDateTime.parse("2022-09-14T17:51:34.39728Z");
        ZonedDateTime finalDate = ZonedDateTime.parse("2022-09-14T21:54:34.39728Z");

        //when
        getIntervals2(tle, startDate, finalDate, 0, 0, 10, 10);
    }

    private List<Pair<ZonedDateTime, ZonedDateTime>> getIntervals2(
            String[] tle,
            ZonedDateTime startDate, ZonedDateTime finalDate,
            int startLat, int startLon,
            int endLat, int endLon
    ) {
        var orekitTle = new org.orekit.propagation.analytical.tle.TLE(tle[1], tle[2]);
        var attitudeProvider = new NadirPointing(FramesFactory.getTEME(), EARTH);
        var sgp4 = TLEPropagator.selectExtrapolator(orekitTle, attitudeProvider, 466615.0);

        var lonEntryDetector = new LongitudeCrossingDetector(EARTH, FastMath.toRadians(startLon))
                .withHandler((s, detector, increasing) -> {
                    System.out.println(s + " " + increasing);
                    return Action.CONTINUE;
                });
        var lonExitDetector = new LongitudeCrossingDetector(EARTH, FastMath.toRadians(endLon))
                .withHandler((s, detector, increasing) -> {
                    System.out.println(s + " " + increasing);
                    return Action.CONTINUE;
                });
        sgp4.addEventDetector(lonEntryDetector);
        sgp4.addEventDetector(lonExitDetector);

        sgp4.propagate(toAbs(startDate), toAbs(finalDate));
        return null;
    }

    private AbsoluteDate toAbs(ZonedDateTime zdt){
        var utc = TimeScalesFactory.getUTC();
        long millis = zdt.toEpochSecond() * TimeUnit.SECONDS.toMillis(1);
        return new AbsoluteDate(new Date(millis), utc);
    }
2

Hi @Aytackydln

Welcome to the Orekit forum! :slight_smile:

I don’t know if it will fix your issue but I see one error. I think you initialize the
new LongitudeCrossingDetector(EARTH, endLon) with a longitude in radians. However, Orekit uses SI units. Could you convert the angles in radians? You can use FastMath.toRadians(...) for that.

Regards,
Bryan

3

Sorry about that, in my original code conversion is already applied. Just fixed and the issue still persists. Also edited my post

4

Did you try using directly the propagate method of sgp4 propagator?

Instead of

TemporalAmount timeStep = Duration.of(10, ChronoUnit.SECONDS);
        var itrf = FramesFactory.getITRF(IERSConventions.IERS_2010, true);
        var utc = TimeScalesFactory.getUTC();
        var date = startDate;
        while (date.isBefore(finalDate)) {
            long millis = date.toEpochSecond() * TimeUnit.SECONDS.toMillis(1);
            var absoluteDate = new AbsoluteDate(new Date(millis), utc);

            var sgp4Pv = sgp4.getPVCoordinates(absoluteDate, itrf);
            sgp4Pv.getPosition();

            date = date.plus(timeStep);
        }

Use

sgp4.propagate(finalDate);

Please note that you can also use EventsLogger class to save all the event detections during propagation and at the end of the propagation, access a lot of useful information for the detected events like the SpacecraftState at detection.

EventsLogger loggerStart = new EventsLogger();
sgp4.addEventDetector(logger.monitorDetector(lonEntryDetector));
... 
sgp4.propagate(finalDate);
List<EventsLogger.LoggedEvent> loggedEvents = logger.getLoggedEvents();
int numberDetection = loggedEvents.size();
PVCoordinates pvFirstDetection = loggedEvents.get(0).getState().getPVCoordinates();

Bryan

5

Behaviour is same. I’ve edited my code in the post to make it simpler

6

The point was for a single propagation, crossing event is triggered for each millisecond predicted. Since I can’t predict when it would happen this becomes unusable

7

The problem is not that the event is detected every millisecond, it is that when there are two longitude detectors, it keeps triggering pairs of events entry/exit at the first event date, and cannot proceed further.
The problem does not appear when only one detector is registered.

I have yet to find the reason of the problem.

Could you open an issue on the forge?

8