Thursday, July 20, 2017

A 173 mile (278km) all-electronics, FSO (Free Space Optical) contact: Part 1 - Scouting it out

Nearly 10 years ago - in October, 2007, to be precise - we (exactly "who" to be mentioned later) successfully managed a 173 mile, Earth-based all-electronic two-way contact between two remote mountain ranges in western Utah.

For many years before this I'd been mulling over in the back of my mind various ways that optical ("lightbeam") communications could be accomplished over long distances.  Years ago, I'd observed that even a modest, 2 AA-cell focused-beam flashlight could be easily seen over a distance of more than 30 miles (50km) and that sighting even the lowest-power Laser over similar distances was fairly trivial - even if holding a steady beam was not.  Other than keeping such ideas in the back of my head, I never really did more that this - at least until the summer of 2006, when I ran across a web site that intrigued me, the "Modulated Light DX page" written by Chris Long (now amateur radio operator VK3AML) and Dr. Mike Groth (VK7MJ).  While I'd been following the history and progress of such things all along, this and similar pages rekindled the intrigue, causing me to do additional research - and I began to build things.

Working up to the distance...

Over the winter of 2006-2007 I spent some time building, refining, and rebuilding various circuits having to do with optical communications.  Of particular interest to me were circuits used for detecting weak optical signals and it was those that I wanted to see if I could improve.  After considerable experimentation, head-scratching, cogitation, and testing, I was finally able to come up with a fairly simple optical receiver circuit that was at least 10dB more sensitive than other voice-bandwidth circuits that were out there.  Other experimentation was done on modulating light sources and the first serious attempt at this was building a PIC-based PWM (Pulse-Width Modulation) circuit followed, somewhat later, by a simpler current-linear modulator - both being approaches that seemed to work extremely well.

After this came the hard part:  Actually assembling the mechanical parts that made up the optical transceivers.  I decided to follow the field-proven Australian approach of using large, plastic, molded Fresnel lenses in conjunction with high-power LEDs for the source of light emissions with a second parallel lens and a photodiode for reception and the stated reasons for taking this approach seemed to me to be quite well thought-out and sound - both technically and practically.  This led to the eventual construction of an optical transceiver that consisted of a pair of identical Fresnel lenses, each being 318 x 250mm (12.5" x 9.8") mounted side-by-side in a rigid, wooden enclosure comprising an optical transceiver with parallel transmit and receive "beams."  In taking this approach, proper aiming of either the transmitter or receiver would guarantee that the other was already aimed - or very close to being properly aimed - requiring only a single piece of gear to be deployed with precision.

After completing this first transceiver I hastily built a second transceiver to be used at the "other" end of test path.  Constructed of foam-core posterboard, picture frames and inexpensive, flexible vinyl "full-page" magnifier Fresnel lenses, this transceiver used, for the optical emitter and transmitter assemblies, my original, roughly-repackaged prototype circuits.  While it was neither pretty or capable of particularly high performance, it filled the need of being the "other" unit with which communications could be carried out for testing:  After all, what good would a receiver be if there were no transmitters?

On March 31, 2007 we completed our first 2-way optical QSO with a path that crossed the Salt Lake Valley, a distance of about 24 km (15 miles.)  We were pleased to note that our signals were extremely strong and, despite the fact that our optical path crossed directly over downtown Salt Lake City, they seemed to have 30-40dB signal-noise ratio - if you ignored some 120 Hz hum and the occasional "buzz" from an unseen, failing streetlight.  We also noted a fair amount of amplitude scintillation, but this wasn't too surprising considering that the streetlights visible from our locations also seemed to shimmer being subject to the turbulence caused by the ever-present temperature inversion layer in the valley.

Bolstered by this success we conducted several other experiments over the next several months, continuing to improve and build more gear, gain experience, and refine our techniques.  Finally, for August 18, 2007, we decided on a more ambitious goal:  The spanning of a 107-mile optical path.  By this time, I'd completed a third optical transceiver using a pair of larger (430mm x 404mm, or 16.9" x 15.9") Fresnel lenses, and it significantly out-performed the "posterboard" version that had been used earlier.  On this occasion we were dismayed by the amount of haze in the air - the remnants of smoke that had blown into the area just that day from California wildfires.  Ron, K7RJ and company (his wife Elaine, N7BDZ and Gordon, K7HFV) who went to the northern end of the path (near Willard Peak, north of Ogden, Utah) experienced even more trials, having had to retreat on three occasions from their chosen vantage point due to brief, but intense thunderstorms.  Finally, just before midnight, a voice exchange was completed with some difficulty - despite the fact that they never could see the distant transmitter with the naked eye due to the combination of haze and light pollution - over this path, with the southern end (with Clint, KA7OEI and Tom, W7ETR) located near Mount Nebo, southeast of Payson, Utah.

Figure 1:
The predicted path projected onto a combination
map and satellite image.  At the south end
(bottom) is Swasey Peak while George Peak is
indicated at the north.
Click on the image for a larger version.
Finding a longer path:

Following the successful 107-mile exchange we decided that it was time to try an even-greater distance.  After staring at maps and poring over topographical data we found what we believed to be a 173-mile line-of-sight shot that seemed to provide reasonable accessibility at both ends - see figure 1.  This path spanned the Great Salt Lake Desert - some of the flattest, desolate, and most remote land in the continental U.S.  At the south end of this path was Swasey Peak, the tallest point in the House range, a series of mountains about 70 miles west of Delta, in west-central Utah.  Because Gordon had hiked this peak on more than one occasion we were confident that this goal was quite attainable.

At the north end of the path was George Peak in the Raft River range, an obscure line of mountains that run east and west in the extreme northwest corner of Utah, just south of the Idaho boarder.  None of us had ever been there before, but our research indicated that it should be possible to drive there using a high-clearance 4-wheel drive vehicle so, on August 25, 2007, Ron and Gordon piled into my Jeep (along with a 2nd spare tire swiped from Ron's Jeep as recommended by more than one account) and we headed north to investigate.

Getting there:

Following the Interstate highway nearly to the Idaho border, we turned west onto a state highway, following it as the road swung north into Idaho, passing the Raft River range, and we then turned off onto a gravel road to Standrod, Utah.  In this small town (a spread-out collection of houses, really) we turned onto a county road that began to take us up canyons on the northern slope of the range.  As we continued to climb, the road became rougher and we resorted to peering at maps and using our intuition to guide us onto the one road that would take us to the top of the mountain range.

Luckily, our guesses were correct and we soon found ourselves at the top of the ridge.  Traveling for a short distance, we ran into a problem:  The road stopped at a fence gate that was plastered with "No Trespassing" signs.  At this point, we simply began to follow what looked like road that paralleled the fence only to discover, after traveling several hundred feet - and past a point at which we could safely turn around - that this "road" had degenerated into a rather precarious dirt path traversing a steep slope.  After driving several hundred more feet, fighting all the while to keep the Jeep on the road and moving in a generally forward direction, the path leveled out once again and rejoined what appeared to be the main road.  After a combination of both swearing at and praising deities we vowed that we would never travel on that "road" again and simply stay on what had appeared to have been the main road, regardless of what the signs on the gates said!

Looking for Swasey Peak:

Having passed these trials, we drove along the range's ridge top, looking to the south.  On this day, the air was quite hazy - probably due to wildfires that were burning in California, and in the distance we could vaguely spot, with our naked eyes, the outline of a mountain range that we thought to be the House range:  In comparing its outline and position with a computer-simulated view, it "looked" to be a fairly close match as best as we could guess.

Upon seeing this distant mountain we stopped to get a better look, but when we looked through binoculars or a telescope the distant outline seemed to disappear - only to reappear once again when viewed with the naked eye.  We finally realized what was happening:  Our eyes and brain are "wired" to look at objects, in part, by detecting their outlines, but in this case the haze reduced the contrast considerably.  With the naked eye, the distant mountain was quite small but with the enlarged image in the binoculars and telescope the apparent contrast gradient around the object's outline was greatly diminished.  The trick to being able to visualize the distant mountain turned out be keeping the binoculars moving as our eyes and brain are much more sensitive to slight changes in brightness of moving objects than stationary ones.  After discovering this fact, we noticed with some amusement that the distant mountain seemed to vanish from sight once we stopped wiggling the binoculars only to magically reappear when we moved them again.  For later analysis we also took pictures at this same location and noted the GPS coordinates.

Continuing onwards, we drove along the ridge toward George Peak.  When we got near the GPS coordinates that I had marked for the peak we were somewhat disappointed - but not surprised:  The highest spot in the neighborhood, the peak, was one of several gentle, nondescript hills that rose above the road only by a few 10's of feet.  Stopping, we ate lunch, looked through binoculars and telescopes, took pictures, recorded GPS coordinates, and thought apprehensively about the return trip along the road.
Figure 2:
The predicted line-of-sight view (top) based on 1 arc-second SRTM terrain data between the Raft River range
and Swasey peak as seen from the north (Raft River) side.
On the bottom is an actual photograph of the same scene at the location used in the simulated view.  As can be seen,
more of the distant mountain can be seen than the prediction would indicate, this being due to the refraction of
the atmosphere slightly extending the visible horizon.  Under typical conditions, this "extension" amounts to
an increase of approximately 10/9th of the distance than geometry would predict.  This lower picture was produced
by "stacking" multiple images using software designed for astronomy.
Click on the image for a larger version.

Returning home:

Retracing our path - but not taking the "road" that had paralleled the fence line - we soon came to the gate that marked the boundary of the private land.  While many of the markings were the same at this gate, we noticed another sign - one that had been missing from the other end of the road - indicating that this was, in fact, a public right-of-way plus the admonition that those traveling through must stay on the road.  This sign seemed to register with what we thought we'd remembered about Utah laws governing the use of such roads and our initial interpretation of the county parcel maps:  Always leave a gate the way you found it, and don't go off the road!  With relief, we crossed this parcel with no difficulty and soon found ourselves at the other gate and in familiar territory.

Retracing our steps down the mountain we found ourselves hurtling along the state highway a bit more than an hour later - until I heard the unwelcome sound of a noisy tire.  Quickly pulling over I discovered that a large rock that had embedded itself in the middle of the tread of a rear tire.  After 45 minutes of changing the tire and bringing the spare up to full pressure, we were again underway - but with only one spare remaining...

Analyzing the path:

Upon returning home I was able to analyze the photographs that I had taken.  Fortunately, my digital SLR camera takes pictures in "Raw" image mode, preserving the digital picture without loss caused by converting it to a lossy format like JPEG.  Through considerable contrast enhancement, the "stacking" of several similar images using an astronomical photo processing program and making a comparison against computer-generated view I discovered that the faint outline that we'd seen was not Swasey Peak but was, in fact, a range that was about 25 miles (40km) closer - the Fish Springs mountains - a mere 150 or so miles (240km) away.  Unnoticed (or invisible) at the time of our mountaintop visit was another small bump in the distance that was, in fact, Swasey Peak.

Interestingly, the first set of pictures were taken at a location that, according to the computer analysis, was barely line-of-sight with Swasey Peak.  At the time of the site visit we had assumed that the just-visible mountain that we'd seen in the distance was Swasey Peak and that there was some sort of parallax error in the computer simulation, but analysis revealed that not only was the computer simulation correct in its positioning of the distant features, but also that the apparent height of Swasey Peak above the horizon was being enhanced by atmospheric refraction - a property that the program did not take into account:  Figure 2 shows a comparison between the computer simulation and an actual photograph taken from this same location.

Building confidence - A retry of the 107-mile path:

Having verified to our satisfaction that we could not only get to the top of the Raft River mountains, but also that we also had a line-of-sight path to Swasey Peak, we began to plan for our next adventure.  Over the next several weeks we watched the weather and the air - but before we did this, we wanted to try our 107-mile path again in clearer weather to make sure that our gear was working, to gain more experience with its setup and operation, and to see how well it would work over a long optical path given reasonably good seeing conditions:  If we had good success over a 107-mile path we felt confident that we should be able to manage a 173-mile path.

A few weeks later, on September 3, we got our chance:  Taking advantage of clear weather just after a storm front had moved through the area we went back to our respective locations - Ron, Gordon and Elaine at Inspiration Point while I went (with Dale, WB7FID) back to the location near Mt. Nebo.  This time, signal-to-noise ratios were 26dB better than before and voice was "armchair" copy.  Over the several hours of experimentation we were able to transmit not only voice, but SSTV (Slow-Scan Television) images over the LED link - even switching over to using a "raw" Laser Pointer for one experiment and a Laser module collimated by an 8" reflector telescope in another.

With our success on the clear-weather 107-mile path we waited for our window to attempt the 173-mile path between Swasey and George Peak but in the following weeks we were dismayed by the appearance of bad weather and/or frequent haze - some of the latter resulting from the still-burning wildfires around the western U.S.

To be continued!


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