Sorry for the late reply.

You are right, the intensities in the LAGEOS-2 report figure are funny. In any case, I wouldn't sweat it trying to match that data. As for the geometry, I get about 111 ps from the retro in the north pole and the third ring. It may be worth checking the expressions for the delay provided in the reports by D. Arnold, just in case what you are using is somehow different...

In what way your results don't match those of fig. 5.5.1? The models given in Degnan's paper are based on empirical approximations, if I remember correctly, so I wouldn't expect the intensities to be spot on. But the geometry (delay) should be fine. Did you get this bit right for the orientation shown in the figure, i.e. a laser incident on the north pole of the satellite?

Hello Daniel,

sorry for the late reply - I just saw your questions now ....

In Graz, we have tried to optimize this overlap avoidance procedures / minimize the rep rate losses, using the FPGA on our home-made PC card:

- This PC card creates the laser firing pulses, and triggers the HiQ laser
- The FPGA stores all future return event times of just fired laser pulses (sseveral 100 events for up to GEO satellites) in a FIFO
- On each laser start pulse event, the FPGA checks the time difference between this laser firing epoch, and the next return epoch time
- If the next return epoch time is closer than 100 µs (can be adjusted...), one additional 100 µs delay is inserted before the next laser fire command,
  followed then again with laser firing commands in the usual 500 µs intervals (then referred to this first delayed one)
- When the echo of this first delayed pulse arrives, and the overlap conditions is still alive, once again a 100 µs delay is inserted .... etc....
   as long as the overlap conditions is valid
- This procedure reduces the rep rate only during overlap, and only by a very small percentage (e.g. from 2 kHz to 1960 kHz or so; depends on satellite)

This overlap avoidance procedure can be enabled / disabled by setting a control bit via PC.

Works perfect for 2 kHz (and a bit higher); for > 10 kHz, the 50% operation might be necessary, but this is a rather high price ... it might be better to just neglect overlaps, which could be acceptable in view of much lower energy per pulse (i.e. less backscatter).

Georg

Thanks Matt,

I have the suspicion that we have the same problem. It seems that we are losing energy while pointing in specific directions.
We will definitely try the azimuth rotation test.

I will keep you informed.

Best
Jens

Hi Matt,

polarization effects were never thought as a problem for the original 10Hz design of our station and after
switching to kHz there was also no apparent reason to start an investigation.

Nevertheless we played around with a l/2 wave plate several month ago to check if the polarization plane of
out refurbished HighQ Laser has changed. We noticed variation in our calibration return rate if we rotate the
wave plate.

So the next step would be to try to make the polarization circular with a l/4 wave plate. We may run into the
same problem as you, but we should try at least.

How did you check that the circular polarization was lost?

Best
Jens