required to transmit all the phase observations within the 
measurement update rate. 
For the tests which have been carried out in the scope of 
this paper no radio link was used. The coordinates which 
have been derived from the real-time code solutions of 
the airborne receiver were compared with the results 
from the aerial triangulation. The differences between 
these solutions are shown in figures 3a and b for the two 
receiver types. 
30 
  
20 4-4 
  
   
10. 
Difference [m] 
  
  
Image Nr. 
[-] 
Figure 3a Differences (Xy,z) between Aerial 
Triangulation and Real-Time Computed Positions (Leica 
9212 Aero) 
30. 
  
20. 
  
  
10. of 
0 à 
Difference [m] 
  
  
Image Nr. 
[-] 
Figure 3b Differences (x,y,z) between Aerial 
Triangulation and Real-Time Computed Positions (Leica 
SR 399) 
For the Leica 9212-Aero a root mean square value of 
16.24 m has been achieved over all 3 coordinate 
components (x,y,z). As expected the performance of the 
SR 399 is slightly better (r.m.s. = 14.25), because the 
receiver is capable of reducing the effect of the 
ionosphere using the observations on both frequencies. 
Also, it is expected, that the signal to noise ratio of the 
SR 399 is superior to the one of the 9212-Aero receiver, 
as special techniques allow P-code aided tracking on 
both frequencies. Similar accuracies could have been 
achieved in further tests under varying conditions. Apart 
from the above mentioned features the dual frequency 
receiver does not have apparent advantages over the 
  
  
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cheaper single frequency navigation receiver. When 
comparing the achieved results with the demands for 
photoflight navigation and automatic camera release, it 
can be seen, that using the tested hardware the given 
accuracy requirements can be met even for large scale 
applications. 
6. ACCURACY OF POST-PROCESSED DGPS 
CAMERA COORDINATES FOR REDUCTION OF 
GROUND CONTROL 
Besides navigation and camera control, GPS is used 
more frequently for the determination of the camera 
perspective centers. The economic benefits, due to a 
significant reduction of ground control points have 
convinced photogrammetrists to use post-processed 
GPS observations in the block adjustment. Although, the 
majority of the GPS error sources can be eliminated by 
differential positioning, the code observations of modern 
receivers still can not provide the required centimeter 
accuracy for large scale applications. To achieve this 
accuracy it is necessary to use the GPS carrier phase 
observations. However, the problems with the phase 
observations is, that it is necessary to determine the 
correct set of cycle ambiguities in order to exploit the 
inherent accuracy of a few millimeters. Several methods 
have been proposed to fix the cycle ambiguities in an 
airborne environment. These methods will be briefly 
reviewed here: 
1. As long as no loss of lock or cycle slips occur, the 
ambiguities remain constant integer values. In 
principle, they can be estimated and fixed in a static 
initialization at the beginning of a continuous 
trajectory, but due to banking angles in flight turns 
and the highly kinematic environment losses of 
phase lock and cycle slips are frequent in 
photogrammetric applications. 
2. Ambiguity resolution on the fly (AROF), tries to 
resolve the carrier phase ambiguities from the GPS 
data alone. Sophisticated statistical tests are used 
to distinguish between the correct and incorrect 
ambiguity sets. 
3. The third method is the combined adjustment of 
GPS and photogrammetric image data (CBA). The 
ambiguity resolution is done in a two step 
procedure. In a first step the ambiguities are fixed 
only roughly in a GPS post-processing step and the 
final determination of the camera perspective center 
coordinates is done in the block adjustment using 
the image coordinate measurements and the post- 
processed GPS positions (FrieR [1990], 
Ackermann/Schade [1993]). 
It is obvious, that the first method has lost importance, 
due to the doubtful reliability and the unfavorable 
economic aspects as flat turns increase the flying time 
considerably. 
Today, photogrammetrists are usually choosing between 
the methods 2 and 3. Under geodetic conditions 
ambiguity resolution on the fly has already reached an 
operational status (see e.g. Frei/Beutler [1990], Hatch 
[1990]). The statistical tests which are used in AROF 
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