# Can I use FieldOfViewDetector to compute ground station attenna access of satellite?

Hi! I’ve been recently studying the Orekit. While I was reading the help doc,I got a question about the FieldOfViewDetector.The help doc says the FieldOfViewDetector is a finder for target entry/exit events with respect to a satellite sensor `Field Of View`.But what if the sensor is on a ground station.Can I use the propagation&event method to compute the ground sensor access to the satellite?

Hi @AstroL, welcome.

Yes you can use this feature on ground, there is a dedicated `GroundFieldOfViewDetector` for that.

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Thanks a lot!That works!
I have test both circumstances and check them with the STK results.They are all fine to me.Although the logic here is a little confused to me.Because the setup code of access computation for sat2station and satation2sat are almost the same.

``````	    		FieldOfView fov = new CircularFieldOfView(Vector3D.PLUS_K, FastMath.toRadians(60.), FastMath.toRadians(0));

final FieldOfViewDetector fd = new FieldOfViewDetector(stationFrame, fov);
final ElevationDetector ed = new ElevationDetector(gpsFrame).withConstantElevation(FastMath.toRadians(5.0));
``````
``````FieldOfView fov = new CircularFieldOfView(Vector3D.PLUS_K, FastMath.toRadians(60.), FastMath.toRadians(0));

final GroundFieldOfViewDetector gfd = new GroundFieldOfViewDetector(stationFrame, fov);

final ElevationDetector ed = new ElevationDetector(gpsFrame).withConstantElevation(FastMath.toRadians(5.0));
``````

And I have some more questions.If I have a number of stations/satellites with a number of attennas and beams, and I have to calculate the access times and angle offsets.Is the a proper event handler to compute the uplink or downlink simutannously or do I have to make a list and deal them in a loop?

I would suggest in this case to wrap pairs of detectors with `BooleanDetector.andCombine` and set up the handler in the combined detector that will be triggered when both visibility conditions are met.