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Pirate Craft Detector

Method for Detecting Pirate Craft at a Distance under all Lighting Conditions
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To extend the range at which their approaches can be detected, especially at night, use a combination of thermal and optical cameras, plus a high-power laser. This combination is required to circumvent any conceivable tactic for evading detection such as: mirrors, coloration, thermal shielding or "invisibility cloaks".

The method:
Scan a circular path around the ship with both cameras, using the laser to illuminate any object present for the optical camera. This creates a "fence" which nothing can cross undetected, while reducing the physical area monitored to a minimum.
For each position along the path, check both optical and thermal images for any sign of an object. If nothing is detected, beam the high-power laser at that position with enough energy to both visibly illuminate it better and also to warm an area of it sufficiently to be detectable with the thermal camera.
The scan rate would need to be fast enough that no craft could completely cross the fence between scans.

The method relies on the fact that all solid materials on the sea can be distinguished from the water by one or more of several methods, even in total darkness:
* Optical reflection of natural light.
* Optical reflection of laser light.
* Optical emission.
* Natural thermal emission.
* Thermal emission after heating with a laser.

Anticipated Counter-Measures and Solutions:
* A mirror reflecting away from the ship would appear as an optical and thermal dark spot, just like the sea. To be warmed, the laser's wavelength should be chosen to match the higher absorbtion regions for quartz, glass and plastic. If their metal films might be placed on the outside surfaces, metal absorption spectra would also have to be considered:
----- * Quartz transmits UV light, but not infrared waves.
----- * Glass transmits infrared waves, but not UV light.
----- * Plastic ?
----- * Metals ?
* Any materials used for coloration would be warmed by the high-power laser.

It might be necessary to use several laser wavelengths to be sure all materials can be warmed sufficiently to be thermally detected.
Alvin, Nov 12 2011

congratulations. you've invented lidar. http://en.wikipedia.org/wiki/LIDAR
[Voice, Nov 14 2011]

[link]






       21 Quest,
This is for detecting pirates long before they get near enough to pose any danger. It's intended to be placed at a high position on a ship where it can scan a circle that's as close to the visible horizon as practical.
Alvin, Nov 12 2011
  

       It seems a lot of trouble to got to when what you really want to do is suck 'em in to small arms range before blasting them out of the water ...   

       All that is required is a couple of well trained chaps on watch with some night vision kit and a selection of appropriate weaponry. It has been conclusively shown that pirates who have had their boats and bodies ripped to bloody shreds by bursts of .50 cal gunfire and then left for the sharks are not likey to re-offend.
8th of 7, Nov 12 2011
  

       A stealthy radar-equipped drone which constantly circles the ship about 10 miles out and 5 miles up. That should give you enough time to wake up the chief cook.
FlyingToaster, Nov 12 2011
  

       // the chief cook //   

       Stephen Seagal, presumably ...
8th of 7, Nov 12 2011
  

       Ok, so what do you do after it triggers on the first five hundred legitimate fishing boats?
MechE, Nov 12 2011
  

       ....maybe just some pattern recognition that can pick up on eye-patches at a nautical mile?
not_morrison_rm, Nov 13 2011
  

       21 Quest,
Sorry 21 Quest, I misunderstood what you were saying. The only addition this device needs to let it detect vessels through a mist, is a maser strong enough to "burn a hole" through it, such as is done for punching holes in clouds to make a path for a laser beam.
Alvin, Nov 13 2011
  

       // such as is done for punching holes in clouds to make a path for a laser beam. //   

       Done by whom, exactly?
Alterother, Nov 13 2011
  

       //detect vessels through a mist is a maser// hmm... doesn't water absorb microwaves ?
FlyingToaster, Nov 13 2011
  

       Mist doesn't absorb microwaves (much). Only biggish bits of water do.
spidermother, Nov 13 2011
  

       I forgot exactly who was said to use masers to punch holes in clouds, but I believe it was some astronomers. The holes would disappear very quickly of course, but allow for some very fast optical operations to be performed through them. The way the masers work though, is the water droplets absorb them (FlyingToaster) until they heat up enough to vaporize and form a transparent path for infrared and optical wavelengths. I don't know why there would be much difference between clouds and mist for such an application. Both are composed of water droplets and both are somewhat turbulent.
Alvin, Nov 14 2011
  

       Voice,
Unless I've missed something, the description of LIDAR in that article doesn't mention heating solid, transparent or reflective materials until they emit detectable infrared radiation. I'll study it more carefully just in case.
Alvin, Nov 14 2011
  

       Actually, to spot a Pirate ship yopu need LIDAAAAAAARRRRR!
gnomethang, Nov 14 2011
  

       21 Quest,
Although the maser-punched holes disappear very quickly, there's enough time for light to travel through them before they do.

The scanning process consists of repeatedly checking a series of positions along the fence for the presence of any craft at all. The positions checked would be spaced at a distance less than the smallest dimension of any conceivable craft, to be sure it doesn't miss one.
Also, this system doesn't distinguish between friends and foes, it just makes sure that they are all detected as early as possible.

Another thing is that heating up a spot on a craft (if present) to make it emit infrared waves is done using a higher power laser beam, not by keeping a low-power beam aimed at it for a longer period.
Alvin, Nov 14 2011
  

       There may be a problem with going around at sea directing high-powered laser beams at anything that moves...   

       However, for many materials, including wood, plastics, fibreglass-resin, etc., the thermal infrared response to a high-power laser would be practically instantaneous.*   

       Thick metal hulls would be more difficult to detect, due to their thermal conductance, and aluminised polyester would defeat this method entirely, as its emissivity is extremely low both for all laser wavelengths and for thermal IR.   

       * I ducked outside and did a quick test of this. My cheap infrared thermometer shows a clear response to the IR emissions from a piece of wood placed in sunlight in œ second, which is the thermometer's temporal resolution.
spidermother, Nov 14 2011
  

       spidermother,
The high-power laser would only be activated when nothing was visible by optical or infrared sensors at the position being checked. That would indicate that if there was something at that spot, then it's someone who's trying to conceal their presence and that possibility would warrant use of the high power laser to make sure there's no one there.

I'm not sure how to tackle the aluminised polyester at the moment. Certainly there's a way to detect it though.
Alvin, Nov 14 2011
  

       Just for completeness, I ran the same test on a piece of emergency blanket (aluminised polyester). As expected, the readings were all over the place, and not noticeably influenced by the presence or absence of direct sunlight. IR readings from such a surface reflect (literally) the thermal properties of the surroundings, rather than the surface itself.   

       However, a craft simply wrapped in aluminised polyester would stand out like a beacon to radar; it would need to have a faceted stealth design to be reliably mistaken for sea or sky.
spidermother, Nov 14 2011
  

       Aluminised polyester shouldn't be too difficult to detect; for example, although it won't respond to your attempts to heat it with lasers, it will show up as a thermal anomaly in many situations; for example, as a 'cold' spot on the surface, due to reflection of zenith sky (which is 'colder' than horizon sky, at least in cloudless conditions). I'm fairly sure that stealth-by-shiny is only partly successful, otherwise the Black Hawk would be the Flying Discoball.
spidermother, Nov 15 2011
  
      
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