The integration of GPS and INS was initially undertaken to support reliable commercial aircraft navigation, Unmanned Aerial Vehicle (UAV) guidance and control, and mobile mapping. This combination of technologies may also be referred to as a “GPS-Aided INS” or a “Position and Orientation System” (POS). Each of these technologies alone has limitations, but the integration of GPS and INS is a powerful solution for DG. In addition, a mathematical modeling procedure for removing noise (known as a Kalman Filter, implemented for lidar collection in the mid-1990s) provided a robust way to estimate and compensate for GPS/INS errors.
In a DG system, the GPS provides position and velocity, and the INS (shown in Figure 3.04) provides attitude, or orientation, of the sensor with respect to the ground. The laser ranging system measures the distances from the sensor to the mapped surface. The DG system and lidar sensor must be in a fixed position with respect to each other, rigidly fixed to the floor of the aircraft, and calibrated (boresighted) within the reference frame of the aircraft for meaningful results. During a flight mission, the DG system records position and orientation data, typically at a rate of ~200 Hz. It also records a corresponding time tag for each laser scan. The DG postprocessing software interpolates position and orientation of the laser reference point at each time tag. With this data and the range measured by the laser, the three-dimensional ground coordinates of every laser return may be computed. Figure 3.05 illustrates the relationship of the DG system, aircraft platform, sensor, and the ground GPS base station.