Laser used in Lidar aerial mapping

There are basically two kinds of lasers used in LIDAR mapping. These are the pulse lasers and the continuous wave (cw) lasers. Floodrefers to these as small footprint, time-of-flight laser altimetry and large footprint waveform digitizing.

The pulse laser emits a narrow laser pulse in the near infrared region of the electromagnetic spectrum. Each discrete pulse is then reflected off a surface on the earth and returned to the receiver.

This signal yields a small footprint on the surface of the earth. One of the problems with this method of LIDAR mapping is that acceptable results may be somewhat difficult to achieve in dense and complex canopies [Flood, 2001]. While the signal may penetrate to the ground through holes in the canopy, many returns have to be filtered for correct classification of the ground surface. The cw laser emits a continuous signal stream where the receiver captures the full return wave. Distances are determined from phase measurements. The return signal covers a wider footprint and contains the entire structure of the return signal.

There are two distinct types of LIDAR systems based on the environment in which they are being used. A topographic lidar mapping system, which is the topic of this paper, is used over land and operates in the infrared portion of the electromagnetic spectrum. Over water, the infrared signal is partially absorbed by the water resulting in almost no return signal. A bathymetric system is used over water and it utilizes the blue-green portion of the electromagnetic spectrum, thereby allowing penetration and a return signal though the water.

While the speed of light is well known in a vacuum, one would expect that it would vary in the actual atmosphere. Thus, the raw distance, or sometimes called the range, is influenced by the variation in the actual speed of light. This variation can be modeled and corrected for in the processing of the raw laser signal.

The laser scanner is mounted in an aircraft just like an aerial camera. It can emit upwards to 50,000 pulses per second1. The laser mapping scan data is collected using a scanning mirror that rotates transverse to the direction of flight. The scan angle is generally less than 20 degree in both directions from the nadir line, although some system may scan up to 30 degree. The laser scan signal forms a footprint on the ground, which is referred to as the instantaneous field of view (IFOV). If the aircraft is completely level and if the laser scan is in the vertical position, then the IFOV will be a circle. As the laser scan signal moves off the vertical, the IFOV will become elongated, forming an ellipse, along the scan direction thereby enlarging the footprint .

source: http: lidarnews