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LEGEND: 
® GPS ad-hoc reference station (€) : GPS permanent reference station 
A : ground control ——»5 : flight path and sense (trajectory) 
——- 1 mapping area boundary/indicator shoreline 
A : airport mapping areas of interest, à = 1,2 
à : mission phase, = 1,...,7 
Figure 2: General aerial mission layout. 
kinematic applications when GPS SA (Selective Availability) is discontinued. See also (Witchayangkoon and Segantine, 
1999) for a discussion of absolute phase positioning with the PPP (Precise Point Positioning) service of NASA's JPL (Jet 
Propulsion Laboratory). All the mentioned achievements (Colombo et al., 1999, Ovstedal, 1999, Witchayangkoon and 
Segantine, 1999) are based on the raw data and derived products collected by the tracking reference stations of the IGS 
(International GPS Servive; see section 6.2). 
The bottom line for trajectory determination is that within the next decade, without SA, with a modernized GPS, with 
Galileo, and above all with IGS, precise absolute positioning will become a reality. 
5 TOTAL ORIENTATION PROCEDURES 
We start with an example and then generalize. The example scenario is schematically depicted in figure 2. Let us assume 
a standard aerial photography flight mission for mapping purposes. Let us assume, as well, that the airplane has its base 
at airport À and has to cover two mapping areas of interest, M; and M». M, is the main mapping area whereas M» is of 
secondary relevance but it has to be covered. To illustrate a not so uncommon situation we assume that M» is an island. It 
could be any piece of land away from and not connected with M;. 
We further assume that the company that does the survey has installed two reference, ad-hoc, GPS stations; one close 
to the airport, the other close to the main mapping area. Furthermore, there are a number of GPS permanent reference 
stations operated by some geodetic or mapping agencies. It would also be possible that the surveying company establishes 
a third GPS reference station at Ma. 
Now assume that the airplane is equipped with geodetic GPS receivers and has one or more IMU attached to its imaging 
sensors. Under normal flying conditions and air traffic regulations a typical mission process would be as follows. The 
airplane spends some time roaming at the airport platform before heading for the runway (1) and taking off (2). After 
  
194 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000.