EventDetector: ElevationDetector for Geostationary satellite

1

Hi forum
I would like to ask what is your approach to detect contact between satellites and ground points for geostationary orbits.

In this case, the ElevationDetector is always returning g function positive (or negative) and as it does not change the sign, the event is not detected.

Is there any solution to catch this use case?
Thanks!

Alberto

2

Hi @alberto-ferrero,

I don’t fully understand what you’re trying to achieve. Do you want to know when the satellite is above (i.e. at the zenith of) a ground point ?
An ElevationDetector with a very high elevation threshold could maybe work.
Or a GeographicZoneDetector with a very small ground zone around your ground point.

3

Hi @MaximeJ
Thanks for getting back to me quickly.

So my use case is:

  • ground point
  • satellite in geostationary orbit (let’s assume perfectly geostationary, no perturbations)

I add an ElevationDetector to capture if the ground point sees the satellite (with elevation margin 0 deg)

Now we have two conditions:

  • if ground point sees the satellite, g function will be > 0 for the entire propagation window
  • if ground point does not see the satellite, g function will be < 0 for the entire propagation window

Either ways, as it is a constant condition (perfect geostationary orbit), no event is detected as the g function does not change sign.
So checking the propagator EventLogger does not contain any detected ElevationDetector events. I would then assume that my point does not see the satellite, while actually it could.

So my curiosity is about, do we have any mean to capture visibility of a geostationary satellite? As apparently the ElevationDetector cannot catch it?
(or maybe I am doing something wrong?)

Thanks!
Alberto

4

Ok I see @alberto I was completely off-topic sorry :slight_smile:

You are right.

Maybe you could extend the elevation detector and work with its init function. Calling g here will allow you to log the “initial” event depending on the sign of g ?
I haven’t tried it though.

Hope this helps,
Maxime

5

Great intuition @MaximeJ , yes capturing the init state will do the job.
I will get inspired by

Thanks :slight_smile:
(as always)
Alberto

1 Like
6

You’re very welcome Alberto.

Indeed the post you linked seems to be exactly what you need, nice catch.
I didn’t think of the AdapterDetector and the problem with the withXXX methods !

7

Hi @MaximeJ
Still not sure if my implementation is correct as adding the ElevationDetector as follows, it still does log 0 events:

      AdapterDetector elevationDetector = new AdapterDetector(new ElevationDetector(
          60.0,
          1e-4,
          topocentricFrame)
          .withConstantElevation(0.0)
          .withHandler(new ContinueOnEvent<>())) {
        @Override
        public void init(final SpacecraftState s0,
                         final AbsoluteDate t) {
          getDetector().init(s0, t);
          eventOccurred(s0, g(s0) > 0);
        }
      };
     propagator.addEventDetector(logger.monitorDetector(elevationDetector.getDetector()));

I notice that actually, using the debugger, the code never run the lines with the @Override of the init.

It does pass through the @Override of the init adding to the propagator directly the AdapterDetector, so:
propagator.addEventDetector(logger.monitorDetector(elevationDetector)));

But in this case, I have regression issues as other ElevationDetector tests with LEO orbits are failing as no events are captured, it seems that

Am I doing something wrong?
Thanks!

Alberto

8

It is normal you don’t get an event. Events are triggered when the condition changes.
A geostationary satellite does not move with respect to Earth, hence you can check if it is visible or not at init time, but you will never get an event.

9

Thanks
So there is no actual mean to force the EventDetector to log an event, but it is only based on g function sign change.
The workaround I can think is to check after the propagation if the g function is indeed positive at the initial state s0 and at final state sf.
Something like:

if (elevationDetector.g(s0) > 0) {
    //Do something
}
if (elevationDetector.g(sf) > 0) {
    //Do something else
}

This workaround works for my application.
Thanks
Alberto

10

Sorry @alberto-ferrero I thought calling the eventOccured method would log the event.
Guess I was wrong. Thank you for the workaround !

11

Hi again @alberto-ferrero,

I made it work, it is the monitored event that needs to be adapted, not the “raw” event.
Here’s a piece of code based on your example above.

// "Raw" detector
ElevationDetector elevationDetector = new ElevationDetector(
          60.0,
          1e-4,
          topocentricFrame)
          .withConstantElevation(0.0)
          .withHandler(new ContinueOnEvent<>()));

// Monitor this detector, this uses the "LocalHandler" from "EventsLogger"
EventDetector monitoredElevationDetector = logger.monitorDetector(elevationDetector);

// Adapt this monitored event detector
AdapterDetector adaptedMonitoredElevationDetector = new 
AdapterDetector(monitoredElevationDetector) {
            @Override
            public void init(final SpacecraftState s0,
                             final AbsoluteDate t) {
                getDetector().init(s0, t);
                // This below will call EventsLogger.LocalHandler.eventOccured
                // Which will in turn call EventsLogger.LoggingWrapper.logEvent
                eventOccurred(s0, g(s0) > 0);
            }
        };

// Add this last event detector to the propagator
propagator.addEventDetector(adaptedMonitoredElevationDetector);

Hope this helps,
Maxime

12

@MaximeJ I confirm it works!
so no wonky workaround with initial and final SpacecraftState but I can capture the event at the beginning of the propagation.

Thanks a lot!

Alberto