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1、Laser rangefinder A long range laser rangefinder is capable of measuring distance up to 20 km; mounted on a tripod with an angular mount. The resulting system also provides azimuth and elevation measurements. A laser rangefinder is a device which uses a laser beam to determine the distance to an obj
2、ect. The most common form of laser rangefinder operates on the time of flight principle by sending a laser pulse in a narrow beam towards the object and measuring the time taken by the pulse to be reflected off the target and returned to the sender. Due to the high speed of light, this technique is
3、not appropriate for high precision sub-millimeter measurements, where triangulation and other techniques are often used.Pulse The pulse may be coded to reduce the chance that the rangefinder can be jammed. It is possible to use Doppler effect techniques to judge whether the object is moving towards
4、or away from the rangefinder, and if so how fast.Precision The precision of the instrument is determined by the rise or fall time of the laser pulse and the speed of the receiver. One that uses very sharp laser pulses and has a very fast detector can range an object to within a few millimeters.Range
5、 Despite the beam being narrow, it will eventually spread over long distances due to the divergence of the laser beam, as well as due to scintillation and beam wander effects, caused by the presence of air bubbles in the air acting as lenses ranging in size from microscopic to roughly half the heigh
6、t of the laser beams path above the earth.These atmospheric distortions coupled with the divergence of the laser itself and with transverse winds that serve to push the atmospheric heat bubbles laterally may combine to make it difficult to get an accurate reading of the distance of an object, say, b
7、eneath some trees or behind bushes, or even over long distances of more than 1 km in open and unobscured desert terrain. Some of the laser light might reflect off leaves or branches which are closer than the object, giving an early return and a reading which is too low. Alternatively, over distances
8、 longer than 1200 ft (365 m), the target, if in proximity to the earth, may simply vanish into a mirage, caused by temperature gradients in the air in proximity to the heated surface bending the laser light. All these effects have to be taken into account.Calculation The distance between point A and
9、 B is given byD=ct/2where c is the speed of light in the atmosphere and t is the amount of time for the round-trip between A and B.where is the delay which made by the light traveling and is the angular frequency of optical modulation.Then substitute the values in the equation D=ct/2,D=1/2 ct=1/2 c/
10、=c/(4f) (N+)=c/4f (N+N)=U(N+)in this equation, U stands for the unit length. stands for the delay part which does not fulfill .N stands the decimal value.Discrimination Some instruments are able to determine multiple returns, as above. These instruments use waveform-resolving detectors, which means
11、they detect the amount of light returned over a certain time, usually very short. The waveform from a laser pulse that hits a tree and then the ground would have two peaks. The first peak would be the distance to the tree, and the second would be the distance to the ground.Using wavefront sensing, i
12、t is possible to determine both the closest and the farthest object at a given point. This makes it possible for aircraft-mounted instruments to see through dense canopiesclarification needed Please explain how lasers see through canopies and other semi-reflective surface such as the ocean, leading
13、to many applications for airborne instruments such as: 1. Creating bare earth topographic maps - removing all trees 2. Creating vegetation thickness maps 3. Bathymetry(measuring topography under the ocean) 4. Forest firehazard Technologies Time of flight - this measures the time taken for a light pu
14、lse to travel to the target and back. With the speed of light known, and an accurate measurement of the time taken, the distance can be calculated. Many pulses are fired sequentially and the average response is most commonly used. This technique requires very accurate sub-nanosecond timing circuitry
15、. Multiple frequency phase-shift - this measures the phase shift of multiple frequencies on reflection then solves some simultaneous equations to give a final measure.Interferometry - the most accurate and most useful technique for measuring changes in distance rather than absolute distances.Applica
16、tionsMilitaryAn American soldier with a GVS-5 laser rangefinder. A Dutch ISAF sniper team displaying their Accuracy International AWSM .338 Lapua Magnum rifle and Leica/Vectronix VECTOR IV laser rangefinder binoculars.Rangefinders provide an exact distance to targets located beyond the distance of point-blank shooting to snipers and artillery. They can also be used for milita
