Groot, Jos 
  
2.2 Orbit and beam direction control. 
Orbit control of PHARUS is normally done with a GPS (Global Positioning System) and (pressure) height 
sensors. In combination with the auto-pilot of the Citation this leads to orbit repeatability accuracies of up to 
hundreds of meters. This is not sufficient for interferometry. The 10 m condition was planned to be met by using 
the NLR developed PRS (Position Reference System, which includes an intertial reference system), of which the 
information was fed into the auto-pilot. The necesarry input orbit accuracy for the PRS was estimated to be 1 m. 
To be able to meet the 10 m condition, the PRS had to be set to the code-tracking differential mode. This mode 
requires a ground-based reference station close to the operating area of the aircraft. The reference GPS data is 
sent to the aircraft by means of a telemetry uplink, where in real time an orbit is calculated with =1 m accuracy. 
In order to fly the required flight track under all wind conditions, the attitude of the aircraft changes in response 
to the auto-pilot generated steering command. Therefore, to approach the 0.3° requirement antenna beam 
steering had to be used. The beam direction of the phased array antenna of PHARUS can be steered 
electronically in the slant range plane, in 0.5° steps. By using (in-flight available) orbit and attitude data it is 
possible to point the beam in the desired direction. For this experiment we implemented the so-called 
"geographical beam steering" mode: the beam is pointed (every second) in a specific horizontal direction 
(perpendicular to the ideal flight track), fixed with respect to the geographical north. 
To aid off line processing (i.e., by providing a precise orbit to the SAR processor) a second GPS receiver was 
mounted in the aircraft. It provided a high precision («10 cm) flight track, sampled at 1 Hz. 
3 THE EXPERIMENTS 
A river dike near Gorinchem (The Netherlands) was selected as the main target of the measurements. (Fig.1). 
  
Figure 1. River dike near Gorinchem. Bumpy road caused by deformations. 
Eight corner reflectors (RCS 26.8 and 33.1 dBm?) were deployed to act as stable phase references. They were 
deployed at places on and close to the dike which were expected to deform, and some more stable places further 
away. The reflector positions and heights were levelled several times between the end of 1998 and the beginning 
of 2000. The levellings revealed a deformation rate of 1.2 cm per month at the end of 1998. 
  
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International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part B5. Amsterdam 2000.