TABLE OF CONTENTS Direct georeferencing is the determination of time-variable position and orientation parameters for a remote sensing sensor. The most common technologies used for this purpose today are satellite positioning by GPS and inertial navigation using an inertial measuring unit (IMU). The fact that the method is in principle independent of available GCP has obvious economic advantages, especially in areas with poor or sparse control. The principle of direct georeferencing results in a number of systems that have been thoroughly tested and has resulted in georeferencing parameters of high accuracy through the use of integrated differential GPS and IMU systems in post-mission. Although each technology can in principle determine both position and orientation, they are usually integrated in such a way that the GPS receiver is the main position sensor, while the IMU is the main orientation sensor. The orientation accuracy of an IMU is largely determined by the gyro drift rates, typically described by a bias (constant drift rate), the short-term bias stability, and the angle random walk. Typically, four classes of gyros are distinguished according to their constant drift rate, namely (Schwarz and El-Sheimy, 2004):
Strategic gyros (0.0005 0.0010 deg/h) achieving an accuracy of degrees per month;
Navigation-grade gyros (0.002 0.01 deg/h) achieving an accuracy of degrees per week;
Tactical gyros (1 10 deg/h) achieving an accuracy of degrees per hour;
Low-accuracy gyros (100 10, 000 deg/h) achieving an accuracy of degrees per second.
Since strategic gyros are rare and costly, only the last three classes will be discussed...
