additional constraint for the ambiguity 
resolution. Due to these constraints the number 
of potential solutions which have to be tested is 
reduced by a significant amount and hence the 
on-board processing requirements is minimal. 
Further, the resolution of the ambiguities is 
much more reliable as for the positioning case. 
However, for the airborne, kinematic attitude 
determination the baseline can not enter the 
attitude computations as error-free. Due to 
structural deformations of the aircraft body and 
wings during the flight, caused by the 
aerodynamic behaviour, the baseline length can 
vary significantly (see also COHEN/PARKINSON 
[1992]), 
It has been mentioned earlier, that the highest 
measurement accuracies are required for GPS 
attitude determination. Therefore, error effects 
like multipath, antenna phase center variations, 
wing flex and receiver noise have to be kept as 
small as possible. A more detailed review about 
the error effects and the data-processing 
techniques for real-time, kinematic attitude 
determination with GPS can be found e.g. in 
HARTL/WEHR [1986]. Due to its simple handling 
its robustness and the minimal data processing 
requirements GPS attitude determination is 
highly suitable for real-time mapping 
applications. The potential of this sensor has 
been analyzed under conventional 
photogrammetric conditions, yielding attitude 
accuracies in the range of 0.1 - 0.2 degrees. 
Figure 3 shows a comparison of GPS attitudes 
and the attitudes computed from a conventional 
photogrammetric block adjustment. A more 
detailed description of this testflight can be 
found in SCHADE ET AL. [1993]. 
  
1 
[gon] A heading 
067 € pliich 
  
9 rol 
  
02+ & 
02+ 
  
  
0.8 + 
  
  
1 : | | : | 
375 395 415 435 455 475 
  
[Image nr.] 
[Figure 3 Differences between Photogrammetric 
and GPS Attitudes 
  
  
  
4. SUMMARY AND CONCLUSIONS 
The usage of GPS for the real-time 
determination of exterior orientation parameters 
in the context of airborne mapping applications 
has been discussed. From experimental, airborne 
kinematic GPS data an accuracy potential for 
real-time positioning of 0.1 - 5 m has been 
achieved, depending on wether the correct 
carrier phase cycle ambiguities can be 
determined or not. Attitude parameters with 
multi-antennae GPS receivers can be 
determined in real-time with an accuracy of 0.1- 
0.3 degrees. Although, the attitude accuracies 
are not sufficient for the direct stereo-evaluation 
of overlapping images, they can be used as 
approximate values for image matching 
techniques. On the contrary, the accuracy of the 
attitude parameters may be sufficient for the 
direct exterior orientation of non-imaging 
sensors, monoplotting purposes or the direct 
exterior orientation of airborne-remote sensing 
sensors. In conclusion, it has become obvious 
that with the advent of the Global Positioning 
System the real-time exterior orientation of 
airborne sensor systems can be achieved with 
sufficient accuracy for most mapping 
applications. 
5. BIBLIOGRAPHY 
BLACKWELL E.G. [1986]: Overview of 
Differential GPS Methods, Global Positioning 
System, Papers published in Navigation, Vol. 
III, The Institute of Navigation, Washington, 
1986 
COHEN C.E, PARKINSON B.W. [1992]: Aircraft 
Applications of GPS Based Attitude 
Determination, Proceedings of the ION GPS-92, 
Fifth International Technical Meeting of the 
Satellite Division of the Institute of Navigation, 
pp. 775-782, Albuquerque, USA 1992 
FREI E., BEUTLER G. [1990]: Rapid Static 
Positioning Based on the Fast Ambiguity 
Resolution Approach "FARA": Theory and 
Results, manuscripta geodaetica, Vol. 15, 
Springer Verlag, pp. 325-356, 1990 
188 
  
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