GPS-supported Aerotriangulation with Ambiguity Resolution on the Fly

In the ordinary operation of GPS kinematic surveying on aerial triangulation,a long-time static initialization is necessary to solve a correct ambiguity before the start of the flight.Even as we know,the static initialization is a time-consuming,costly,and difficult procedure.Sometimes the flight mission has also to be interrupted and the static initialization must be executed again because the cycle slip in carrier phase measurements is created.In order to obtain continuous records of GPS signals,we stipulate some constraint norm for the aerial photographic flight of GPS blocks.In fact,this is harmful to the actual application of GPS-supported aerotriangulation.Therefore,it is important and urgent to investigate a kind of approach of camera air positions determining by differential kinematic GPS positioning with ambiguity resolution on-the-fly (OTF).This paper will describe such an approach adopted in our practice.Firstly,we use a recursive algorithm of phase smooth based on the carrier phase difference between adjacent epochs to get the smooth pseudo-range of the C/A code measurements at each epoch.Secondly,we solve a float estimation of ambiguity for the double difference carrier phase's observables at a strip using the above smooth pseudo-range.Finally,we determinate 3D coordinates of the exposure centers of the airborne camera by means of the obtained ambiguity solution.This approach is implemented strip by strip instead of whole block.It does not consider the cycle slips happening during the turn around from one flight line to the next,so that the aerial photography is able to perform in the conventional operation specification.For testing the accuracy of positioning projective centers of airborne camera by the above method and efficiency of GPS-supported bundle block adjustment,a set of actual aerial photos with GPS data taken from Tianjin project was selected and processed by software DDKIN (DGPS dual frequency kinematic surveying) and WuCAPS GPS (Wuhan GPS-supported bundle block adjustment).The achieved results have shown that our proposing scheme is feasible as well as the overall accuracy of GPS-supported bundle block adjustment is closer identical between ambiguity solutions initialized on the ground before taking off and resolved on the fly.The OTF algorithm,however,will greatly relax the operational constraint for the aerial photographic flight of GPS blocks and be of great importance to apply widely GPS-supported aerotriangulation into practical production.