Helica curretly owns several LiDAR systems: among these there are the ALTM 3100 EA and the ALTM Gemini, produced by the Canadian company Optech. In order to correct the position data provided by the GPS system, it is necessary to use use one or more GPS ground stations, which are placed in points with known coordinates. The GPS data and the information recorded by the inertial measurement unit (IMU) are processed by the system as well.
All these stored data are then used in the post-processing stage, when the trajectory of the sensor is processed (DGPS - differential GPS) as well as in the following stage, when the laser points are spatially adjusted on the ground. Before every survey flight the on-board sensors are set and calibrated: this operation requires a number of flights over target areas, for which position and size were carefully measured before with traditional instruments.
This operation allows the determination of systematic errors in the instruments (e.g. errors in the pitch or roll constants). A flat surface measuring more than 650 m (of known coordinates and size) can be used as target in order to check the scaling. Helica's LiDAR systems are carried by both helicopters and airplane.