One or two GPS fixed ground stations are used to correct the positional data supplied by the onboard GPS. The data recorded by the stations is used in the post-processing of the sensors trajectory (DGPS Differential GPS) and subsequently in the spatial determination of the laser points on the ground.
Before each collection project, Helica carries out calibration and adjustment of the onboard sensors.
Before each project, Helica calibrates and adjusts the onboard sensors. This operation involves flying repeatedly over “target” areas and their position and dimensions are very carefully surveyed using traditional methods. In this way, system errors in the apparatus can be identified, for example, the pitch and roll bias. A flat surface of over 650 metres (with known co-ordinates and dimensions) is used as a “target” to calibrate the scale and delay.
The table below shows an example of the calibration parameters used on a remote sensing job with pitch, roll and scale corrections:
| Altm Serial No | 2D133 |
| ImuType | LN200A1 |
| ImuRate | 200 |
| ScannerScale | 0.9983 | ScannerOffset | 0.05005 |
| FirstPulseRange | -0.09 |
| LastPulseRange | -0.22 |
| IMURoll | 0.054 |
| IMUPitch | -0.054 |
| IMUHeading | 0.000 |
| UserTolmuEx | 0.010 |
| UserTolmuEy | -0.020 |
| UserTolmuEz | 0.350 |
| UserTolmuDx | -0.101 |
| UserTolmuDy | -0.005 |
| UserTolmuDz | -0.111 |
| UserToRefDx | -0.051 |
| UserToRefDy | -0.027 |
| UserToRefDz | -0.500 |