Hi Toshi

These are really useful and i guess every station has something to think about. The SGF, Herstmonceux has been addressing its residual vs RMS plot by looking at our data reduction procedure.

It would be great if all stations could post here on the forum what they are doing to address the trends seen in these charts.

The pdf http://geo.science.hit-u.ac.jp/slr/bias/2017sp/SortDelay7.pdf has multiple entries for Potsdam, Greenbelt, Wettzell and others, is this correct?

Matt

Hi all

From about the beginning of 2017, the team at the SGF noticed some increased instability in our terrestrial range calibration values at the level of 2-3 mm.  It was particularly clear on the first calibration of the day being a low value and so we learnt to take at least two calibrations before starting an observing duty.

2 weeks ago we also saw an increase in the RMS of the calibrations and some periodic behaviour in the calibration range values.  We consequently took the kHz laser offline and began using our older 10Hz laser, which had very stable calibration values and RMS.

After some investigation and discussion with High-Q, the team found that the flow rate of the cooling water was much lower than it should have been.  The water used is a reservoir of distilled water, but after 10 years of use something had built up to block the waterways.

Temperatures in the laser bed were not being kept at a stabilised constant, causing energy fluctuations and range errors from the start diode. The temperature would change when the laser began firing after being off for a period (20+ minutes). It would stabilise after approximately 10 minutes, during which time calibration range differences were seen.

Running the flow backwards and using a descaler (Durgol was recommended) brought the flowrate up to specification levels.  Setting the best operating temperature removed the signals in the range values and now the calibration RMS and repeatability are now within acceptable levels.

Needless to say, we will be checking the flowrate more often from now on.

Matt

Hi Kalvis

That's quite a narrow FOV.  We are wondering here @Hx what would be the best approach, either to use a wider image to spot approaching planes and take action when necessary or to have a narrower field to immediately shutoff when any plane like object appears in the frame (perhaps using a comparison to an earlier frame).

We've not tested our camera at night yet.

Matt

The forum platform can run polls among its members.  Here's the first one - and it's an easy question!

I presented the results from the SLR beam divergence measurement campaign, carried out over the summer of 2016, at the NESC meeting during the 20th ILRS Laser Workshop held in Potsdam, Germany. Most stations were able to use the beam divergence procedure and get good results. These are presented in the first attachment.

Some stations however we not able to carry out the procedure for a number of reasons, including:

• Faulty energy sensor/meter
• High energy detection threshold
• Limited telescope pointing/tracking accuracy

Data was provided by some stations using alternative methods (shown with white hash lines), including:

• Satellite scanning
• Optical system ray tracing
• Measurements in the focal plane

Mount Stromlo, Australia carried out many measurements, see attachment #2.  There is some variability in the results and it seems that lower values were recorded for the GNSS satellites. The chart suggests that taking multiple measurements on different satellites on different nights will give a better average reading than a single measurement. Matera, Italy also took many measurements, see attachment #3. The results show the two different beam divergence settings used.

The procedure includes a secondary part where a operator can change the beam divergence setting and making a quick comparative measurement.  This was carried out at Herstmonceux, UK for a number of satellites at different times by 2 observers. The trend fit can now be used to relate the arbitrary beam expander setting value to a real divergence value.

Attachment #4 is the averaged values for each station. Included is a green line indicating the stated beam divergences in the ILRS Site Logs.
Attachment #5 shows the energy density from each station at a height of 1336km, calculated using these beam divergence values and the laser pulse energies from the ILRS Site Logs. The y-axis has been limited, the Wettzell result is 0.28 microJ/cm² and the Grasse result is 0.65 microJ/cm².

The beam divergence procedure is giving good consistent results for many stations with generally close agreement with the ILRS SIte Log entries.

Please add your comments below.

What experience has anyone had with laser beam spectral line width?

Is it correct that there is some broadening of the laser wavelength spectrum with shorter beam pulse lengths?

Has anyone measured this effect or know what their laser spectral pulse is?

At Herstmonceux we use a narrow band filter for daylight ranging, but we are concerned that we cannot select the narrowest filter possible because we must consider the spectral width of our kHz laser.

Hi

It is now essential that ILRS stations are able to estimate, or measure, the angular divergence of laser pulses emitted during satellite laser ranging. With this divergence value, along with the laser pulse energy, it is possible to calculate the energy density at satellite heights.  This is of high interest to satellite mission operators, particularly those with on-board equipment that could be sensitive to incident laser light.

A procedure to measure beam divergence was drafted by the NESC and is available to download http://ilrs.gsfc.nasa.gov/docs/2016/BeamDiv_procedure_201606.pdf

Stations are using this procedure and other methods to determine beam divergence with some success, particularly after some practice.

It would be helpful for forum members share their experience, advice and issues here so that all stations can get a reliable divergence measurement.

Matt

Hi Luis

Jose at the SGF, Herstmonceux has been in touch with your colleague Manuel on this subject and we will supply you with some of our FORTRAN code to help you to adjust the orbit to get flat residuals from which you can then make Normal Points using the method described here http://ilrs.gsfc.nasa.gov/data_and_products/data/npt/npt_algorithm.html.

I hope this will work for you

Matt