Accurate orientation for airborne mapping systems

Stringent requirements on the accuracy of attitude determination are curren tly a major challenge for strapdown INS/GPS integration, which is at the co re of self-contained airborne remote sensing and mapping systems. This pape r reviews the error models for INS/GPS integration and focuses in detail on designing filtering meth ods for improving attitude accuracy in the bandwi dth in which an inertial system does not benefit from frequent GPS position /velocity updates. Several filtering methods are designed based on the spectral analysis of th e raw inertial signal in a dynamic environment. These include a spectral te chnique for dither spike removal and a class of low-pass finite-impulse-res ponse (FIR) filters operating in forward/backward manner for achieving zero phase distortion. The orientation performance of the whole system with dif ferent filters is evaluated by comparing it to the "true" attitude informat ion provided by a photogrammetric block adjustment. Results show clearly th at the choice of an appropriate filter is decisive for attitude accuracy. O verall, the INS/GPS integration combined with the most suitable pre-filteri ng method agrees with the external orientation reference to 0.005 degrees ( 19") RMS over the whole test period, while the flight-line consistency is t ypically 0.003 degrees (10") RMS. The best filter in the comparison has an RMS seven times smaller than the Butterworth filter wich is frequently appl ied in the industrial designs of INS.