II) Description of the on-board equipment 
  
GPS receiver 
For each operation IGN used a GPS TR5S-B.receiver, built by 
SERCEL company ; this material has been already described in a 
previous communication, during ISPRS - Commission I symposium 
held.in Stuttgart in 1986. Let us call.again that,this 
equipment comprises an antenna with its preamplifier, a recei- 
ving - processing unit, a colour CRT screen unit and a control 
keyboard (Fig 1). The receiver can simultaneously process all 
the signals coming from 5 satellites, on Ll frequency (1,57542 
GHz) and C/A code. It is organized to realize an accurate mea- 
surement of pseudo-distance and phase, in each of the 5 channels, 
at a rate of 0,6 second. 
The TR5S-B receiver is equipped of an internal precessor and a 
fast coprocessor for computing in real time, every 0,6 second, 
a complete 3D + T position solution using pseudoranges smoothed 
by accumulated phase. 
The quality of antenna and its setting are dealt with care, in 
order to obtain a delay and a phase center position stable and 
independent of the direction of the arrival and to restrain 
multipath effects residuals. 
From the static version of the receiver, developed for geode- 
tic purposes, SERCEL company has extended applications to dy- 
namic positionnement, first on sea, then on airplane. Develop- 
ment concerned antenna design and data processing by integrated 
phase techniques, in order to take into account speeds up to 
250 knots and 1 g acceleration. 
The first flightshave been realized on low-speed twin engine 
Aerocommander aircraft, using antenna and receiver developed 
for ships. The first results showed the interest to extend the 
dynamic capabilities of TR5S-B. For flight tests on twin jet 
Mystere 20, an omnidirectional DORNE-MARGOLIN antenna, with a 
half of sphere shape, was used at speeds up to 350 knots. 
. Camera 
Aerial survey, for each mission, was performed with a WILD RC10 
camera equipped with a UAg lens cone of focal length 152 mm. 
Exposure pulse is picked up by a relay on a reed-switch which 
commands exposure mechanism of the lens cone, with a delay 
which is a function of exposure time. This delay can be deter- 
mined experimentaly in laboratory : it is a constant for a gi- 
ven lens cone and a given exposure time. Its scale of size is 
some milliseconds for the usual exposure times. The exposure 
pulse is sent directly on an external port of the TR5S-B recei- 
ver. 
. Synchronization and data recording 
  
The receiver can deliver simultaneously raw data and calculated 
positions via two RS 232 C ports. The raw data message presents 
a fixed configuration : it is composed of ephemerides of each 
satellite (up to 5), GPS time, pseudoranges, phases, signal-to- 
noise ratio, etc, at a rate of 0,6 second. Recording is done on 
a Data Track 3 M cassette recorder. 
The calculated data message is configurated in external mode,