# Question:  What is the state of the art re LOS laser communications?



## Kirkhill (27 Aug 2005)

This question occurred to me as I was reading Lance Wiebe's comments on Robot Stryker's.  He noted that one of the limiting factors in remote controlled applications was bandwidth.

It got me to reconsidering something about communications.  

Much is made about bandwidth limitations, and also the vulnerability of broadcast technologies, which include GPS, to jamming.  Every now and then I am reminded of the old heliograph systems where morse code signals were sent by mirrors.

I know that some work has been done using lasers to communicate.  Are they in service? Reliable? Cheap? Useful? Suitable backup to broadcast? Jammable? Could they be used to update locations in place of GPS by lasing known points or by transmission to LOS relays like aircraft and aerostats?

Where are current developments in comms?


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## Radop (27 Oct 2005)

FOCA uses light beams (lasers) to send signals down a fiber cable.  That is the only comms application that I know of that uses anything with lasers.


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## willy (27 Oct 2005)

Directional LOS UHF+ comms are great for bandwidth, but are extremely limited by terrain and impossible to maintain on the move, as they have to be aligned precisely in order to work.   In the case of the robot stryker example, I can't think of a practical way for a LOS remote control to work reliably unless it were pretty much co-located with the robot it was controlling.   That would be an obvious limitation to the usefullness of the remote.


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## Radop (28 Oct 2005)

UHF is different than LOS as one is a point to point system (los) and the other is omnidirectional (aircraft).  A robot using UHF could go through a RRB or Repeater station that could be set up near the robots location.  It would be useless to use an LOS system for comms as it would be to limiting.  Aircraft use A-G-A to talk with pers on the ground and vise versa.  (and for those who worked MACS, yes they used HF as well)  

Often people misuse laser when they mean microwave comms.  Satellite comms rely on high end microwave transmitions to send signal.  Antennas for these systems, although technically LOS, have such high gain that the signal can be up to 30 deg off and still communicate with the satellite system.

As I stated above, the only use for lasers as I know is through fibre for comms.  They are using different colours to increase the number of "lines" a single piece of fibre can transmit.


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## Blue Max (28 Oct 2005)

There is a company called FSONA that has a line of sight laser SONET transmission system. It uses Laser technology (probably diodes) of 1550nm wavelength, which are supposed to be eye-safe. Transmission distance from Tx to Rx is stated as 4000m. The system is supposed to work through rain, sleet, snow & bird clusters.

FSONA's midrange product provides a bandwidth of OC3 SONET (that is 84xDS1 or 84x1.544Mbs). A very interesting product that I have been trying to get our engineering group to acquire for testing at work. 

With regard to military applications where Tx and Rx are moving, I do not believe that the technology is available at this time, but then again if you can get a Winnebago to keep a satelite TV signal strong while on the move, it is probably only a matter of time before someone designs mobile tracking/synchronization technology/software to make it work.

http://www.fsona.com/technology.php


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## willy (28 Oct 2005)

Radop said:
			
		

> UHF is different than LOS as one is a point to point system (los) and the other is omnidirectional (aircraft).
> \



My description of what I was talking about was poor in hindsight, what I should have said is that UHF+ comms are very good for getting high bandwidth, and are often LOS based.  It's true that when we talk about UHF in the military we're typically talking about an omnidirectional A/G/A system, but a UHF system is not neccessarily omnidirectional.


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## Radop (29 Oct 2005)

Correct as I was a Radio Relay guy for a few years but later was a Radio Node 2IC in TCCCS.  There we had 2 UHF systems.  One system was LOS (essentially the radio relay) and the other for AGA.  The bad part of the radio node was 21 emitters all in one location.   :threat:


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## Bintheredunthat (20 Nov 2005)

For all those interested, look for upcoming kit used primarily for LOS comms (UHF RR type stuff) to include the ability to use OTM (on the move)functions.  It's something that's being played with right now and could be making it's way to the "front lines" soon enough.  Uses an omni-d antenna to provide continued comms when someone has to move - but distances could be less than half that expected of the usual microwave shots.  And more than likely, bandwidth will also be lower.

Still, some is better than none.  Looking forward to more changes/upgrades down the road.

Bin


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## Kirkhill (20 Nov 2005)

How might these work with a Tethered Aerostat (Balloon tied to the ground like a WW2 Barrage Balloon) as a Radio Relay Station?  They "fly" at 10 to 15,000 feet and although they are tied to the ground they tend to "bob and weave" a bit in the wind.


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## Bert (20 Nov 2005)

Laser line of sight is different than RF line of sight.   The laser transmitter produces a relatively tight beam that must
strike the receiving detector with sufficient intensity for optimum reception in a straight or reflected line.   RF line of 
sight generally indicates the ability of the receiving antenna to be influenced by the transmitter's EMF from
a point to point perspective.

The act of suspending an RF repeater (receiver/transmitter) in the air simulates a super tall tower that improves the
line of sight communications and RF coverage area with remote transceivers.   

Mountains, trees, terrain, atmospherics, humidity, parasitic ground level comms, horizon,and obstructions may interfere with a 
remote transceiver's ability to contact another radio system especially in a ground horizontal orientation.   By suspending 
a radio   or repeater system in the air, the remote transceiver's line of sight with the other system is less obstructed 
improving the coverage area.   The repeater is able to retransmit the signal to another station with less obstruction.   
"Bob and weave" of the balloon is not an issue as RF EMF fields are broad enough within coverage areas but would
be extremely problematic if laser comms were used.

The drawbacks are suspended systems are more visible, could be an air hazard, are more difficult to repair in-service, 
take more maintenance due to the balloon, and vibration/moisture from higher elevation air movements may stress
the equipment.


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## Radop (21 Nov 2005)

Bert said:
			
		

> Mountains, trees, terrain, atmospherics, humidity, parasitic ground level comms, horizon,and obstructions may interfere with a remote transceiver's ability to contact another radio system especially in a ground horizontal orientation.   By suspending a radio   or repeater system in the air, the remote transceiver's line of sight with the other system is less obstructed improving the coverage area.   The repeater is able to retransmit the signal to another station with less obstruction.



These are the same problems that would be encountered with laser systems as well.  I would be interested in hearing more about these systems.  I am unaware of any systems other than laser rangefinders used with any equipment other than through foca.  I would think the system using lasers would have more problems because of rain, snow and dust than other LOS systems.


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## Bert (21 Nov 2005)

You are right Radop.  The paragraph quoted refers to RF radiation patterns and suspended radio
systems as Kirkhill speculated.  Obstacles and anything that would interfere with a laser's
beam path would reduce signal quality or stop it entirely.

The problem with communication lasers is how their light is generated and emitted.    Theres alot 
of info on the net but here is one example http://documents.exfo.com/appnotes/anote099-ang.pdf

The wavelengths generated by lasers are not distributed in wide patterns similar to RF communciation
devices.  Laser beams are relatively small in diameter, occupy a narrow band of spectrum, and
high in intensity. They are useful in low loss mediums like fibre optic cable or empty non-terrestrial
environments in point to point or point to multipoint applications.   Since laser light can be generated
at specific centre wavelengths, one can fit alot of lasers in a relatively narrow spectrum through
a medium creating high potential bandwidth for data.  RF carriers, how their generated and modulated 
by comms equipment, take up alot of spectrum per carrier and thus lower bandwidth. 

Another issue is laser detection/reception.  The beam must strike the laser detector with
sufficient intensity.  Any obsurity in the beam path, corruption of polarization, refraction,
or reflection reduces received signal strength.  The emitted laser light must strike the detector at all
times.  If the emitter or detector is moving in a quick unpredicatable manner, the beam is
less likely to strike the detector.  Systems that employ laser communications are usually fixed 
in position, have predicable movement, have measured movement with the use of radar, or 
employ quick "act of circumstance" data bursts.


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