激光测距仪外文翻译

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 object. 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 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 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

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 height of the laser beam's 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, beneath 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 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 B is given by

D=ct/2

where 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·φ/ω=c/(4πf) (Nπ+Δφ)=c/4f (N+ΔN)=U(N+)

in this equation, U stands for the unit length.

Δφ stands for the delay part which does not fulfill π.

ΔN stands the decimal value.

Discrimination

Some instruments are able to determine multiple returns, as above. These instruments use waveform-resolving detectors, which means 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, it 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 canopies[clarification needed Please explain how lasers see through canopies]and other semi-reflective surface such as the ocean, leading 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