With the speed of light known, and an accurate measurement of the time taken, the distance can be calculated. Time of flight - this measures the time taken for a light pulse to travel to the target and back. Technologies An OLS-27 IRST with laser rangefinder on the Sukhoi Su-27 In this equation, λ is the wavelength c / f Δφ is the part of the phase delay that does not fulfill π (that is, φ modulo π) N is the integer number of wave half-cycles of the round-trip and Δ N the remaining fractional part. ![]() The distance between point A and B is given byĭ = c t 2 All these effects must be considered.Ĭalculation Time-of-flight principles applied to laser range-finding. Alternatively, over distances longer than 360 m, if the target is in proximity to the earth, it may simply vanish into a mirage, caused by temperature gradients in the air in proximity to the heated surface bending the laser light. 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. 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. In good conditions, skilled operators using precision laser rangefinders can range a target to within a meter at distances on the order of three kilometers.ĭespite 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 water droplets 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. Smaller and less reflective targets return less information.Higher temperature and higher pressure (lower elevation) slightly decrease the speed of light through air.Humidity, snow, dust, or other airborne particulates will diffuse the signal.Environmental factors can significantly reduce range and accuracy: The precision of an instrument is correlated with the rise time, divergence, and power of its laser pulse, as well as the quality of its optics and onboard digital signal processing. 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. The pulse may be coded to reduce the chance that the rangefinder can be jammed. 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. 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. ![]() The resulting system also provides azimuth and elevation measurements.Ī laser rangefinder, also known as a laser telemeter, is a rangefinder that uses a laser beam to determine the distance to an object.
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