ced by a cesium clock, more precise than the internal clock of 
the receiver. 
With the present GPS constellation of six satellites it is pos- 
sible on a given place to have two periods of six hours each 
day for navigation of surveys and remote sensing missions. 
VII) Conclusion 
IGN experimentations conducted with SERCEL company since 1986 
demonstrate all the interest of the GPS contribution for photo- 
grammetry. 
At medium scale (about 1/30000) Vichy and Lunel test flights 
show that all ground preparation can be eliminated except one 
control point at the center of the block for the connection to 
the geodetic network. 
Contribution of differential measurement is not obvious, on 
Vichy and Lunel. These measures allow to reduce systematisms 
for each flight strip but have little influence on final.accu- 
racy. Use of on-board pseudorange smoothed by phase receiver at 
high rate (0,6 second) seems therefore to be the best solution 
for photogrammetry. A second receiver on the ground would imply 
operational difficulties and cost increasing but would not 
really bring improvement of accuracy. 
Use of a simpler receiver based on only pseudoranges could be 
perhaps considered ; but a high rate remains absolutely neces- 
sary. It is notthe case for these materials. 
At great scale the Amiens study .cannot.allow to. conclude, due 
to .the.poor quality o£ control points and the lack.of transver- 
Sal flight strips for stiffness of the block. Another study is 
foreseen, in order to determine the greatest scale for which 
GPS data would be able to replace ground preparation. 
GPS measures are destined to generate in the near future very 
deep changes in photogrammetric methods, by elimination of the 
heavy and expensive ground preparation, at least for medium 
scales. 
Fig 1 — On-board GPS equipment for photogrammetric applications 
Principle diagram 
X 
DD Y Antenna 
zZ 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
WILD RC10 
Camera 
| | Preamplifier 
Raw data : 
CRT gl recorder | 
screen unit | ~~ TR5S-B LA | 
receiver HP 9825 A y ERE LE 4 : 
Control calculator A | 
keyboard x j | X 
a A Y Point of view 
x. EERILY Y 
Exposure pulse / 
| 
7 
| N 
10