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Laser Troposcatter & Cloud Bounce Experiments!

By John, K3PGP 06/12/97 - Rev 1.03

As many of you are aware I have been experimenting with bouncing laser signals off the low cloud layers that we have been experiencing over the past few weeks. Using these signals I was able to optimize my system.

This past week I installed a new receive front end that I've been working on for the past couple of months. Using this new front end and the optimized system above I am now able to detect weak signals being scattered back from CLEAR SKY! That's right, NO CLOUDS! The characteristics of these scattered signals are completely different from those obtained using clouds as the reflecting medium.

Cloud Bounce

Using the cloud layer signals ALWAYS were the strongest when the system was pointed straight up. Signal levels would vary from +4 dB when aimed just above the horizon to greater than +27 dB above the noise floor when pointed directly overhead.

I'm sure a lot of the difference in signal level is due to the fact that the clouds that are directly overhead are quite a bit closer. However, the angle at which the laser beam hits the cloud layer may also enter into this. Although I can't prove it at this point, it's my feeling that when aiming at clouds near the horizon most of the energy is being reflected in the FORWARD direction and very little is being reflected or scattered back to the transmitting station. I won't be able to determine this until I get another station or beacon on that is over the horizon.

Troposcatter

The second mode of propagation was discovered over the weekend and has been observed every night so far. This mode involves crystal clear skies, something that we haven't seen in this area for months!

Unlike the cloud bounce experiments these signals are weakest when pointing straight overhead and peak just above the horizon. Signal levels are also much weaker (but a LOT more stable) than anything observed during the cloud bounce tests.

Typical signal levels run approx. -25 dB below the noise floor when pointed straight up. (DSP, FFT, and long term integration techniques are necessary to even detect these signals!) However, as I drop the elevation angle the signals gradually build in signal level peaking approx. +4 dB out of the noise when aimed just above the horizon. This is completely opposite of the cloud bounce tests! When one considers the fact that the atmosphere is thinnest looking straight up this makes sense. Less atmosphere = less scattering volume = less signal!

Exactly what is causing the backscatter during clear skies is not known at this time but I'm sure it's very similar to microwave troposcatter propagation with the added benefit that the much shorter wavelength (780 nm) can also be reflected and scattered by dust particles, water vapor, ice crystals, and pollutants in the atmosphere. When the sky is extremely clear, backscatter signals can be detected from as far away as 25 to 30 miles with the laser radar setup pointed straight up into the nighttime sky.

Observation

I find it interesting that signal levels detected just above the horizon were about the same (+4 dB) during both the cloud bounce and clear sky troposcatter tests. Perhaps what I detected on the horizon that cloudy night wasn't due to clouds! It's pretty difficult to determine what's on the horizon in the dark during cloudy weather! It's very possible that the skies were clear on the horizon in that direction and what I thought were signals bouncing back from clouds on the horizon may have been caused by backscatter from the atmosphere!

Early Experimenters

It is my feeling that over the horizon laser cloud bounce and troposcatter QSOs are indeed possible. After discovering the fact that I can detect these signals I made several trips to the library and discovered that test like these were conducted back in the late 60's by the Federal Telephone Company. They were using ruby rod transmitters and were able to communicate with other stations 150 miles away! I was amazed to learn that their original goal was to try to bounce laser signals (used for communication) off clouds. However, they also discovered that there was a residual signal present all the time. Exactly the same results that I am getting!

From "LASERS, Tools of Modern Technology, 1968 Page 82:

"Scientists at the International Telephone and Telegraph Federation laboratories in New Jersey have tried using clouds as the reflecting medium. They found that the typical cumulus clouds can scatter the beam to a receiver more than 150 km. from the transmitter."

From "Revolution in Light" by Irwin Stamber, 1972 Page 96:

"On the very first night the lidar was operated, experimenters gained new and sometimes unexpected information about the atmosphere. Myron Ligda, the head of the Stanford Research Institute Aerophysics Laboratory, found he could detect things in the atmosphere previously observable with difficulty or not at all. Among the new data were reflections from the upper atmosphere THIRTY MILES above the earth's surface. These indicated the presence of some light reflecting layers that meteorologists had not been aware of before."

Proof?

With the lens covered on my receiver or the laser beam blocked there is NO residual signal. Also the fact that the signal levels vary from -25 dB to +27 dB depending on the propagation mode and pointing angle seems to indicate that the signals are indeed coming from some distance away and not something generated in the equipment or light being scattered directly in front of the optics. This can be a severe problem during periods of high humidity!

The only station I have been able to use for verification of these results is approx. 5 miles from here and he is definitely NON line of sight as we are blocked with several very high hills between us. (I live on the west side of and in the foothills of the Allegheny mountains with the result that our elevation is all over the place from lows of 500 to 600 feet up to 2700 feet!) The fact that we can hear each other by each of us pointing up in the sky seems to indicate that the signals being detected are indeed the real thing. However, until I get someone on the air further away there's not a whole lot more I can do to determine what can be done using cloud bounce or troposcatter.

I have done ONE-WAY tests out to a distance of 26 miles and was still able to get 100% copy using BPSK and troposcatter. There is a digital recording of this signal available on my web site under Laser, Audio, Video, and Charts. However, as you can imagine it is quite weak and many will NOT be able to hear the signal despite the fact that it prints 100% solid using VE2IQ's COHERENT program!

How to Participate

I must add that in order to make use of troposcatter signals with the typical laser powers that most stations are running (<5 mw) it will be necessary to use computer processing to dig the signals out of the noise.

I do NOT suggest that all of us increase power levels because of the safety hazards that this would create plus the fact that we are pointed up in the sky creating an air traffic hazard! Although the air traffic hazard is minimal when aimed straight up and using cloud bounce, it appears that troposcatter is most easily worked when pointing near the horizon. If you try any of these experiments use your head and don't endanger yourself or anyone flying in the skies above! Be especially careful of horizon shots where an oncoming aircraft could be in your beam for many miles! Aim something (binoculars, telescope, or CCD camera with a telephoto lens) in the direction you're firing and watch for strobe lights!

If you want to jump on the computer processing bandwagon I have a couple of suggestions. First off make sure you're using the most sensitive receive setup that you can build. There is no substitute for a good front end! All the computer processing you can afford will never bring back a signal that isn't there in the first place and if you can't hear them you certainly aren't going to work them!

Weak Signal Software

I have discovered two weak signal computer programs that seem to work extremely well with weak laser signals. Keep in mind that these are intended for serious weak signal work only. If you primarily use your laser for line of sight communications you do NOT need either of these programs!

The first program is from VE2IQ. Actually there are two of them, FFT and COHERENT. FFT is useful for finding signals buried in the noise by using the integrate mode. COHERENT is a full blown communications program that will allow you not only to detect signals below the noise floor but be able to communicate with them! Signal levels of -25 to -30 dB are solid copy using this program! (All signal to noise measurements in this article are referenced to a 400 Hz receive band pass.) It's really weird to watch text coming across the computer screen when nothing can be heard coming from the speaker other than white noise!

Both of these programs are available for free! Log onto my web site at: http://www.qsl.net/k3pgp/index.html.

Follow the link: *Links to Other Sites *Weak Signal Software * VE2IQ BPSK & FFT Software. Keep in mind that BOTH programs require a special interface that plugs into the serial port of any IBM style PC and run from DOS. You can see details of the interface and FFT program in the January 1992 issue of QST.

The interface is available from VE2IQ and is in my mind a real bargain! Contact Bill, VE2IQ and tell him I sent you. Definitely check this one out !!!

VE2IQ BPSK & FFT Software / Email = bill@ietc.ca

The second program is available from AF9Y. It is also available via a link at my web site: http://www.qsl.net/k3pgp/index.html.

Follow the link: *Weak Signal Software * AF9Y FFTDSP Program.

AF9Y FFTDSP Program

This program is available in a demo form only. It will run for approx. 60 seconds or so then it will quit. It can be run as many times as you want but it will time out again. If you try this program and decide you like it, you must contact AF9Y for a key to unlock the program.

If your goal is unattended monitoring of a beacon then the AF9Y program is probably a good choice. You can let it run all night and check the results in the morning! However, it offers NO communication capability like the VE2IQ programs do. This program runs with any Sound Blaster compatible card but must be run from DOS.


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