Establishing ground control points for high-resolution satellite imagery using GPS precise point positioning

A GPS measurement and processing technique of precise point positioning (PPP) accurate to 2.5 m is developed for the purpose of establishing ground control points (GCPs) for rectifying medium and high resolution satellite imageries. In practice, the coordinates necessary in geometric rectification are usually derived from medium scale maps such as 1;50,000 or 1:25,000 scale topographic maps. However, the total positional error of any point on these maps may exceed 1.0 mm due to pointing uncertainty, position shift and exaggeration from cartographic generalization, paper shrinkage and etc. As spatial resolution of satellite imageries such as Landsat 7 panchromatic, ASTER VNIR, Spot 5 now in the range of 5-15 m, GCP coordinates derived from medium scale maps are no longer acceptable since the magnitude of positional error is up to several pixels. Larger scale maps may be used, but their coverages are limited to urban and high-density areas. This problem may be alleviated by adopting differential GPS (DGPS) technique which can provide coordinates of up to 1-m accuracy. However, the technique is effective only for short baselines because the un-modeled errors are directly proportional to the baseline length. The need of having to operate at least two GPS receivers simultaneously during data acquisition further makes the technique less desirable for most remote sensing applications. Developing a GPS technique that not only requires an operation of a single GPS receiver, but also provides accurate positioning results suitable for a GCP establishment of medium and high resolution satellite imageries is the crux of this research. Interest in single receiver positioning has been rekindled by the recent availability of post-mission satellite clock and ephemeris information generated by various data analysis centers, as byproducts of the data processing carried out under the auspices of the International GPS Service (IGS). All IGS analysis centers have established a free service to provide precise satellite clock corrections and precise orbit information via the Internet. This paper describes how the post-mission information and advanced data processing techniques are introduced into the PPP software to improve the positioning accuracy. The components of the PPP- are identified and simple post-observation processing software aimed for non-GPS specialists is developed. The results clearly demonstrate that an accuracy of better than 2.5 meters in horizontal components can be achieved, over a 15-minute observation period using a dual-frequency GPS receiver.