Indirect Georeferencing of Airborne Multi-line Array Sensors: a Simulated Case Study

Multi-line array sensors, carried on airborne or satellite, acquire images with along or across track stereo viewing and are used for photogrammetric mapping at different scales. The main characteristic of the imagery provided by this kind of sensors is that each image line is independently acquired with a different sensor external orientation (position, attitude). If positioning instruments (GPS/INS) carried on board provide the sensor external orientation of each line, the ground coordinates of the observed points can be estimated with direct georeferencing. Anyway the positional and angular displacements of the GPS/INS instruments with respect to the image frame with origin in the sensor perspective centre must be estimated, together with additional measurement errors contained in the observations. Therefore a triangulation integrated with the sensor external orientation modelling (indirect georeferencing) has been implemented. The algorithms have been tested on a simulated testfield, supposing an airborne three-line sensor with optical system consisting of one lens. After simulating the sensor trajectory and the coordinates of 40 object points, the image coordinates of each point in the three images were calculated with back projection. In order to test the indirect georeferencing model, some perturbations and constant offsets in the correct sensor external orientation were introduced and afterwards estimated with the proposed integrated triangulation. The RMS obtained on the checkpoints using different Ground Control Points (GCPs) and Tie Points (TPs) distributions are presented.