Geopositioning

Calibrating LiDAR data begins with the proper installation/mounting of the LiDAR unit, GPS antenna, and IMU sensor on the aircraft, and the precise measurement of offsets in the x, y, and z directions between each of these sensors. The IMU usually serves as the point of reference and the precise distance between all units are measured with respect to the IMU. The precise location of the GPS base station, the antenna height, and the phase center information are required to process the differential GPS-IMU trajectory. The GPS-IMU trajectory is the precise aircraft trajectory that contains the 6 positioning and orientation parameters: x, y, z, pitch, roll, heading; along with a unique timestamp. The position information is derived from post-processing the aircraft GPS receiver data along with the GPS base station data using specialized differential GPS (DGPS) software. The LiDAR positions are calculated at 0.5 second steps. In a second step, an integrated position and orientation solution is calculated with the DGPS-position data and the IMU data by another software module, yielding position and orientation (roll, pitch, yaw) angles to better than one-hundredth of a degree. The IMU measurement rate is typically 200 Hz; the trajectory values are usually maintained at the same rate as the IMU, i.e. 200 records per second. Once the GPS and IMU data are processed and passes all QC checks, the data are combined with the laser range data. This processing step is performed in the LiDAR manufacturer’s developed software. Calibration is done at this stage of the processing. Although the methods of performing calibration are software-dependent (and hence manufacturer-dependent), the LiDAR vendor should test the calibrated data independently. This is usually done by interrogating data from four overlapping flight lines flown in opposite and perpendicular directions along building rooftops and flat surfaces such as airport runways. Any misalignment between the IMU and the LiDAR scanner can be determined using this approach. This information can be fed back into the calibration software to improve the overall calibration of the data. Calibration testing is recommended prior to each mission and is necessary when any of the LiDAR system components are remounted on the aircraft. Several different types of airborne LiDAR systems were developed in the research and scientific field since the late 1970s through the 1980s. These systems typically involved the use of laser profilers to generate a single line profile of the ground beneath an aircraft. The development of Global Positioning System (GPS) and Inertial Measurement Unit (IMU) technologies in the 1990s for civilian applications eventually led to the use of airborne LiDAR systems for accurate topographic mapping. The development of laser scanners (explained in Section 1-1.b above) during the same decade also enabled the use of these systems for wide-area topographic mapping. Airborne LiDAR systems can be broadly classified based on the following specifications: (1) Laser wavelength (2) Pulse energy, pulse width, and beam divergence; (3) Pulse Repetition Frequency (PRF); (4) Operating Altitude; and (5) Return type.