LiDAR by Jethendra P N

LIDAR (which stands for light detection and ranging), like radar, is and active remote sensing technique. The use of lidar for accurate determination of terrain elevations began in the late of 1970s. Initial systems were profiling devices that obtained elevation data only directly under the path of the aircraft. This technology involves the use of pulses of laser light directed towards the ground and measuring the time of pulse return. The return time for each pulse back to the sensor is processed to calculate the variable distance between the sensor and the various surfaces present on (or above) the ground. Advantages of these LIDAR systems afford the opportunity to collect terrain data about steep slopes and shadowed areas and inaccessible areas ]Modern LIDAR acquisition begins with a “photogram metric aircraft “ equipped with air bone GPS (for x,y,z sensor location),an Inertial Measuring unit (for measuring the angular orientation of the sensor with respect to the ground ), a rapidly pulsing (15,000- pulses/sec)laser , a highly accurate clock , substantial onboard computer support, reliable electronics, and robust data storage . modern system are able to record up to five return per pulse. Which demonstration the ability of the lidar to discriminate not only such feature as a forest canopy and bare ground but also surface in between (such as the intermediate forest structure and under story ). Flight planning is crucial, so that data are collected for the entire project without missing areas, causing gaps between flight lines. The distance between sampling points is derived from the altitude and speed of the aircraft, the scanning angle, and rate. As with any airborne GPS actively, the LIDAR system require a surveyed ground base location to be established in the project are as differential post processing correction. In addition, a calibrated alignment process for the GPS position of the sensor and the orientation parameter is required to assure and verify the accuracy of the lidar data sets. These lidar applications are particularly well suited for generation of digital DEMs, topographic contouring, and automatic feature extraction. The whole lidar technology uses laser beam to scan the earth surface. Laser scanner mounted in aircraft emits laser beams with high frequency and receives the reflected beam. Time difference between emission of laser beam and reception of the reflected laser signal is recorded to get the distance (range). High frequency rotating mirror placed in front of the laser converts this laser into a laser scanner. GPS data recorded in air and on ground (DGPS base stations for every 25 kms) determine aircraft position. IMU record attitude variations (pitch, roll and yaw) of the platform for carrying out attitude corrections for the laser beam.

By Jethendra P N