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

We need to talk about polarisation!

At Herstmonceux, we are still emitting linear polarised laser light after an attempt to make it circular with a 1/4 wave plate failed. The circular polarisation was lost through the coudé mirrors before the beam left the emitter.

Would everyone please describe here the polarisation characteristics of their SLR systems.

• What is polarisation state of the laser light emitted by your SLR system?
• How are you achieving this?
• Have you tested this and know that you are sending what you think you are at all telescope azimuths and elevations?
• Can you control the emitted polarisation?
• What are the polarising properties of your receive path [At Hx, we found our receive path to be highly selective to polarisation and we had to replace our dichroic mirror]
  

In the future, we plan to look further at circular polarisation and would be grateful for any advice from those that are able to send circular polarised light.  We also hope to return to looking at controlling the emitted linear polarisation orientation using a 1/2 wave plate.

Thanks
Matt

The 4 S-NET (https://ilrs.cddis.eosdis.nasa.gov/missions/satellite_missions/current_missions/snt4_general.html) satellites were launched on February 01, 2018.  They have been difficult targets up to now. At Herstmonceux, we've had a number of unsuccessful attempts. And i think they have not been tracked by any station in the ILRS network.


However, next week (11th April) we expect to begin to see passes that are partially sunlit and so we will hopefully stand a better chance of getting some returns. This must also be true for other stations.

We will focus on S-NET4 as suggested in the recent ilrs-sta mail (https://lists.nasa.gov/pipermail/ilrs-sta/2018-April/005207.html).

Please use this space below to comment on your S-NET attempts, successes and failures over the coming weeks. Good luck!

To help contributors who want to include formulae in their posts I've installed a program called 'mimetex' which takes LaTeX formatted text and produces a gif image.  It works like this:

[ img]/cgi-bin/mimetex.cgi?f(x)=\int_{-\infty}^xe^{-t^2}dt [/ img]

which without the tag spaces gives:



Please test it out using the post preview.  See here http://www.forkosh.com/mimetex.html

Matt

Hi

I'd like to know more about FLARM and what it can offer SLR stations. https://flarm.com/products/powerflarm/

I see the Graz station is using it. Is anyone else?

Despite having a cautionary/avoidance zone for aircraft around the Herstmonceux SLR station, we do get some light aircraft, gliders etc flying over, especially on nice sunny days. This means the observer has to keep a look out.

If this system can raise a proximity alarm then i guess the observer would find it very useful.

Please post your experience. Cheers

We use an AEPX silicon photodiode from Centronic http://www.centronic.co.uk/products/3/high-speed which has a ~0.6ns rise time.

We align the diode, control the intensity with ND, observe the output signal and set the discriminator threshold to give the lowest possible calibration RMS.

Is there something out there that's better?

Hi all

I hope all those who attended had an enjoyable and productive Workshop in Riga.

I'm sorry i wasn't there. And I know I wasn't the only one who couldn't make it.

So, what did i miss?

• What did you take away from the Workshop?
• What were the things you learnt?
• What was of most interest to you?

Please share your thoughts below


Matt