Evaluation of a DGPS/IMU Integrated Navigation System

The development of efficient systems for precise attitude and position determination of airborne sensor platforms is having a huge impact in different remote sensing applications, resulting in higher productivity levels and lower exploitation costs. This technology is also being successfully applied to different mobile mapping systems. Being aware of this fact, the authors have developed a navigation system that can be used either in real time or in post-processing mode, depending on the requirements of the user. The system is modular and can have different configurations. In the case reported here the airborne segment consists of a low cost IMU, two double frequency GPS receivers, an industrial PC and a radio communication link to allow real time DGPS. As a complement, a mini TV is used to display graphical information, such as the position of the aircraft over a map and a simulated Horizontal Situation Indicator (HSI) that the pilots use to follow pre-programmed flight profiles. The ground segment is based on a GPS reference station, a PC that compresses the GPS corrections (and also displays data that is being sent by the aircraft) and a set of radio transceivers that are spread over the surveying area. In July/August 1999 the system was subject to final testing on board a Portuguese Air Force aircraft. In order to evaluate the attainable accuracy, a photogrammetric camera was also installed and rigidly fixed to the aircraft's structure. More than 400 photos were taken and the triggering instants registered. Ground control was performed for image georeferencing, which allowed the determination of the position and attitude of the camera at the instant the shot was taken. Because the camera is coupled to the aircraft body, this information can be compared to the obtained with the navigation system and its accuracy assessed. In this paper, besides a general description of the system implementation and algorithmic structure, the real time and post-processing results obtained during these test flights are presented and compared to the photogrammetric results. To conclude, general remarks regarding the system performance and its sootiness to different remote sensing applications are discussed.