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; a vehicle 
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ses can be 
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2. ACQUISITION OF GEO-REFERENCED 
IMAGE DATA 
The data acquisition platform is named VISAT (Video, Inertial 
system and SATellite), with its system architecture shown in 
Figure 1. The number of cameras can be as many as eight in 
the present configuration. High precision positioning is 
achieved by applying differential GPS, with an additional 
receiver located at a control point with known coordinates. 
The INS installed inside the van supplies the angular 
orientation parameters of the van in a three dimensional 
coordinate system. It also provides interpolated positions, 
corrects GPS cycle slips, and is used as a back-up during GPS 
outages. 
The integration of GPS and INS technologies is especially 
advantageous in situations where GPS signals cannot be 
received completely. For example, in downtown areas the 
GPS signals are obstructed by high buildings. In these 
situations, INS data can be used to compensate for the loss of 
GPS positional information. This ensures that the acquired 
images can be oriented geometrically at any time with a 
sufficient accuracy. 
Before each prolonged use of the mobile platform, a 
calibration procedure is necessary for determining the relative 
positions of each of the components, such as INS, GPS 
antenna and cameras. These parameters might change slightly 
over time because of the vibration caused by the movement of 
the platform. The on-board computer records the images and 
the associated navigational data. The data are downloaded 
onto the office computer server on which image data are 
preprocessed and managed. 
The data recorded in real time are not directly usable. First of 
all, the GPS data need to be processed because of the 
differential mode configuration. This procedure ensures 
navigational accuracy of the images. Secondly, some images 
may have poor quality. In addition, to make the objects in 
images clearly discernible against the background, some 
image enhancement work is needed. Filters are carefully 
chosen to avoid any pixel shift due to the processing. 
3. DATA PROCESSING ON VISAT-STATION 
The platform for geometric information extraction is named 
VISAT-Station. Basically, it consists of a SUN SPARC series 
workstation with two monitors, running MOTIF. One monitor 
is used for the so called MultiView subsystem, where stereo 
images and graphic user interface elements are displayed. 
Most of the interactive procedures are conducted on this 
screen (see Figure 2). The other monitor is used for the 
OverView application, with the base map and P-lines (i.e. 
route of the van) displayed. Whenever an object is measured, 
its projection on the base map is shown on the OverView 
screen. The main purpose of OverView is to give the user a 
reference for the measured data using a base map. For high- 
end applications, we can have more than one monitor for 
MultiView, or even several workstations networked together 
in case of time-critical surveying in which several operators 
can simultaneously carry out measurements of data from the 
same van run. For low end applications, OverView and 
Gee care Da 
  
  
  
Figure 2 : User interface of MultiView 
233 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996