Beam Divergence Measurement

[Matt Wilkinson]

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

[Matt Wilkinson]

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.

[Matt Wilkinson]

An alternative way to look at energy densities at satellite heights would be to make measurements at the satellite.  The Jason2 mission has recorded energies of SLR pulses since its launch in 2008 as part of the OCA/CNES time transfer by laser link (T2L2) payload.

The attached plot shows the average energies for individual stations recorded at Jason-2 for the year 2015.  The energies were normalised to the Jason2 height in the zenith (1336 km) and corrected for atmospheric attenuation

My thanks to P. Exertier and OCS and CNES colleagues for providing this data.

[Luis Manuel Cortina]

I'd like explain in the graphic  the strange value of divergence in the San Fernando Station.

It´s due to the pointing of our mounting. Our old mounting is, as built, driven by gearings ( added with springs and oil  shock absorbers ) , and always we have many problems to reach a exact positionating in the tracking. Besides, the static and dinamic equilibrium is dificult,too.

To remediate this,apart from working very much with software and  hardware,  empiric works on the trackings indicates que the best way it´s open the ray so much as possible, (the glonass is our reference).Up to now, we have had good results, much more of what system let !.

We are as the bad gunman,  shooted many bullets with the hope of hit the target.

Best regards.

[jose_sgf]

Hi, Luis

We liked your gunman analogy. It's as if you were shooting with a sawn-off shotgun, firing over a big area many pellets, each of which doing limited damage. If you closed the divergence you would be a blind sniper, firing powerful bullets with a rifle, but rather inaccurately.

Anyhow, I find it strange that the best you can do is to open the beam as much as possible. I don't know how wide you can make your divergence (or how bad is your pointing error), but shouldn't there be an optimal divergence value lower than "as much as possible"?

Cheers,

Jose

[Luis Manuel Cortina]

Hello.

Perhaps i don´t have explained all good. Really, we  measure the divergence using the old method of  project the ray on a wall at a distance known.
We varies the divergence of  the ray and computing its values.  It´s a easy calculus:  the base of the cone is the  diameter of the beam on the wall. Empirically , the best results to tracking it´s with 30-40" arc. After that, we do a finest adjust with  glonass.

Best regards.