EXPERIMENTAL TESTS ON FAST AMBIGUITY SOLUTIONS FOR AIRBORNE KINEMATIC 
GPS POSITIONING 
Prof. em. Friedrich Ackermann 
Institute of Photogrammetry 
University of Stuttgart 
Keplerstr. 11, 70174 Stuttgart 
Germany 
ISPRS Commission II 
KEY WORDS: Kinematic GPS, ambiguity solutions. 
ABSTRACT 
OEEPE has started experimental investigations into the accuracy and reliability of fast OTF ambiguity solutions, based on controlled test flights 
in Norway and Germany. The paper reports about first results of the testflight Vaihingen by the pilot centre Stuttgart. Ambiguity solutions were 
successfully obtained up to distances of 386 km to the GPS ground stations, although the restored trajectories after signal interruption show 
small discontinuities and biases. The comparison with check values at camera air stations (by aerial triangulation) showed no accuracy 
dependency on distance to ground station (with one exception in z), but confirmed sytematic errors. The results are preliminary, awaiting 
completion of the additional tests. 
1. INTRODUCTION 
1.1 Biased ambiguity solutions 
High precision airborne kinematic GPS positioning by differential 
phase observations has been applied in photogrammetry for several 
years. A particularly successful field of application has been GPS 
camera positioning for aerial triangulation, by which ground control 
points could be greatly reduced. 
The main problem has been, from the beginning, the question of 
obtaining continuous and absolute GPS flight trajectories. There has 
always been the risk of signal interruption during flying, especially 
during flight turns. Such interruptions had to be accepted as real, not 
being completely and safely avoidable. Software methods for re- 
establishing ambiguity solutions were not available, for a long time. 
The only accessible practical approach was, therefore, to re-solve for 
ambiguities, after interruption, on the basis of the less precise C/A 
code pseudo-range positioning. The resulting ambiguity solutions 
were biased, in this case, and there remained discontinuities in the 
GPS trajectories. And, the subsequent parts of a GPS trajectory 
showed systematic (drift) errors which, fortunately, remained 
approximately linear for some short time afterwards. Such potential 
(constant and linear) GPS drift errors could be described by linear 
correction terms which - in the case of aerial triangulation - were 
applied and solved for during the combined block adjustment. That 
approach represented a most successful engineering solution to the 
problem and has worked very well in practice. There was, however, 
one condition attached: In order to avoid singularities in the block 
solution some additional GPS controlled cross-strips had to be flown 
and used in the combined block adjustment. For general reasons the 
drift parameters were usually applied per strip. Also, it became 
customary, to rely on a few ground control points, for solving the 
datum problem. The method of linear GPS corrections implied that 
any additional constant or linear errors were compensated as well. 
That method has worked very well. It is accurate, reliable, safe. 
Many aerial triangulation projects have been treated in this way, 
without any problems. It is a special advantage, too, of the method 
1 
that the ground receiver station can be placed at great distance from 
the mission area, up to 500 km or more, which sometimes is highly 
essential. Also the conventional single frequency GPS receivers were 
applicable. Recent additional developments (2 GPS antennae on the 
aircraft, more than 1 ground receiver station) have made the system 
safer, but are not considered mandatory. 
It can be summarized that the approximate resp. biased ambiguity 
solutions of the method are unable to provide absolute GPS positions 
nor continuous trajectories. The post-solution via combined block 
adjustment is restricted to GPS application for aerial triangulation, i.e. 
in combination with aerial photo coverage. Other sensors (e.g. laser 
scanner) rely, however, on absolute GPS positioning. Hence, that 
method could and can only be considered an intermediate solution, 
awaiting more sophisticated techniques for fast ambiguity solutions, 
also known as OTF (on the fly) methods. 
1.2 Fast OTF ambiguity solutions 
The recent development of GPS hardware and software has changed 
the situation. Fast OTF solutions have been developed, based on dual 
frequency receivers. They are to provide continuous GPS trajectories 
by correctly restoring the ambiguity solutions after signal 
interruption. Successful applications have been reported. The method 
has started to be applied in practice. 
Only, the reliability of the method has remained unclear. Seemingly, 
the method does not always give successful solutions if the distance 
between roving and stationary GPS receivers is large. It is 
understood, more or less, to remain within a distances of 50 km, 
preferably 30 km, in order to be safe. Also, it is not really known on 
what effects the reliability might depend. Practical application can 
accomodate to such restrictions, in many cases. Nevertheless, larger 
ranges would be highly desirable. 
1.3 Experimental tests by OEEPE 
In that problem situation the European Organisation for experimental 
photogrammetric research (OEEPE) decided to take up experimental 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B6. Vienna 1996