Recent Posts

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21
Timing / Re: Pulse collision avoidance
« Last post by Georg on June 13, 2018, 11:34:54 AM »
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
22
Open a Discussion / Existing software for Impulse Response Function determination
« Last post by ddequal on June 08, 2018, 08:38:17 AM »
Hi everybody from Matera,

I'm trying to make some analysis of the returns from Lageos ans Starlette satellites, and I'd like to perform a simulation of the retroreflected pulses from CCR array. So far I've used the John Degnan paper "Millimeter accuracy satellite laser ranging: a review". In particular, I used 6.1.4 and 6.1.8a for the cross section of each CCR and 6.5.1 for the time delay. The results don't match the data of figure 5.5-1 of "Prelaunch  Optical characterization of the Laser Geodynamic Satellite", where a different model is used (see appendix A). Unfortunately I couldn't find any other documentation of the RETRO program used for the prelaunch analysis.

I'd like to know if any of you have worked on IRF and has some advices on how to properly reproduce them, maybe including interference and far field diffraction patterns.

Cheers

Daniele
23
Lasers / Re: Polarisation
« Last post by jsteinborn on June 07, 2018, 02:07:34 PM »
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
24
Lasers / Re: Polarisation
« Last post by Matt Wilkinson on June 07, 2018, 01:56:09 PM »
Hi Jens

The full story of our battle with polarisation in our system is here https://cddis.nasa.gov/lw17/docs/papers/session10/07-Wilkinson_ILRS17_polarisation.pdf.

Bottom line is that we were losing laser energy through our coudé path depending on the mirror position and we were losing > 50% of our return signal through a polarisation selective dichroic mirror.

The first thing you should do is establish if it is caused on the transmit or receive side. My favourite test was to point the telescope at the zenith and rotate in azimuth and record the intensity of the laser light backscatter at night. The polarisation state is preserved in the reflection by the small spherical water droplets in the atmosphere. We saw clear variation in intensity on both the transmission and reflection side of the dichroic but in opposite magnitudes.

If you've got polarisation problems, switching to circular will not solve this, but would take out the variation. We've not been successful in transmitting circular so would be interested to know how you get on.

Matt
25
Lasers / Re: Polarisation
« Last post by jsteinborn on June 06, 2018, 02:33:47 PM »
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
26
Mission Tracking Feedback / Re: S-NET tracking
« Last post by Matt Wilkinson on June 04, 2018, 09:34:14 AM »
See the attached for a plot of kHz residuals from S-NET1 taken over the weekend at Herstmonceux SLR station
27
Mission Tracking Feedback / LARGE campaign 2018
« Last post by jose_sgf on May 30, 2018, 02:11:49 PM »
Here are the results of the 2018 LARGE campaign in terms of passes and NPs collected by the network. It appears that stations of the Russian network did not participate in the campaign (they only tracked GLONASS satellites), so they have not been included in the statistics.

We believe that the inferior tracking coverage of the Galileo constellation is to a great extent explained by the lower cross section of the retroreflector arrays relative to those mounted on the Glonass satellites. Our experience tracking these objects tells us that the LRAs mounted on the first Galileo spacecraft (Galileo 101-104) are obviously superior to the ones that suceeded them (Galileo 2XX). Among both the primary and secondary groups of Galileo targets there is one spacecraft carrying the first LRA versions (Galileo-102 and Galileo-103). Both of them have received significantly higher tracking coverage than the rest of spacecraft in their respective groups (~60% more NP data for Galileo-102, ~40% more NP data for Galileo-103). Meanwhile, the data yield for Galileo-103 and Galileo-104 is lower than for Glonass satellites by approximately 20% in terms of NP data. It must be noted that the LRAs onboard the Glonass satellites selected for the LARGE campaign have a higher cross section than the previously employed ones, and superior to those used in any of the Galileo constellation.

Except for Compass-M3, coverage for the Compass constellation has been low as a) the geosynchronous satellites are only partially visible from Europe (if at all); b) prediction quality for Compass-MS1 and MS2 is not adequate for intensive SLR tracking.

28
We are installing in Riga a network of SLR temperature sensors using RapsberryPI's and the software in Python.
One RapsberryPI with 2 sensors is monitoring the laser room (where the 25 meters calibration path single mode optical fiber is stored) and another sensor is on the PMT thermal box attached to the Telescope receiving Coudé path (on open air when tracking) but connected to the Laser Room sharing the heating/cooling.

The second RapsberryPI with 2 sensors is monitoring the Control/Electronics room and the CFD/Event Timer electronics

Here is a plot of the daily max/min temperatures at the PMT box
29
Station Equipment Questions / Meteorological station
« Last post by delpino@riga on May 30, 2018, 01:03:12 PM »
There is a point that was mentioned already by Kalvis Salmins in Annapolis:

the current log file do not have a dedicated line on the section 12 to report the difference height between the meteo sensors (in particular the barometric sensor) and the invariant point.
Anybody knows if and where this information is available for all the stations?

The pressure is reported on the CRD's in mbar in format 7.2, but according to the note on section 3.4.2. of the CRD format description, only the pressure changes by 0.1mB are significant enough to report.
what should be the recommended tolerance for measuring the height difference and/or maximum allowed height error if we want to set the sensor exactly at the invariant point level?

Another comment answering to pywang:
There is a work in progress from many years about having a traveling absolute barometer used to calibrate in-situ the SLR local barometers in Europe.

In this moment we have the GFZ Potsdam absolute barometer in Riga, and in July-August one intern from the bilateral Estonia-Latvia SpaceTEM Eslat project (https://estlat.eu/en/estlat-results/spacetem.html) will use it to calibrate our secondary Vaisala PTU300 sensor.
After that we will send it back to Potsdam during the autumn.

So if another station wants to do the same, contact Sven Bauer at GFZ about loaning the absolute barometer, and us for some practical hints on doing and processing the data.
30
Station Equipment Questions / Re: Meteorological station
« Last post by pywang on May 25, 2018, 06:46:42 AM »
In Graz, we are using  MET3A from Paroscientific.

And another pressure sensor--740-16B . Because pressure is the most critical met parameter, the 2 pressure sensor data are compared after each reading; a warning mesage is issued if the difference is > 0.3 hPa. Both are setup at different altitudes (not on the same as the reference point), and values are corrected to our invariant point.

All met devices are read by a dedicated PC in 1 Minute intervals. Any program in need for met data is accessing this data.

Unfortunately, since I am here(almost 4 years) never heard about somebody made any calibration.
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