and ground station segments of the operation. Leica equipment 
has been chosen for two purposes: first of all because the 
photogrammetric equipment to which the navigation system 
had to be linked is Leica and specifically Wild RC 30, finally 
CGR already had two GPS Systems 200 and an experienced 
staff able to handle GPS data and Leica Ski software. 
ia ER Ru RET uU 
E. à 
  
  
Figure 3 - Assembled mosaic of the aerial photo coverage of 
Ireland 
It is important to state that the Ascot system has performed 
navigation functions (flight project, airplane navigation control 
and analysis of the flight path) fairly well, but some 
deficiencies have been pointed out as regard to the handling of 
the phase measures to achieve the antenna phase center 
positions. The Ascot system (Leica ASCOT, 1995) consists of 
a hardware element and a software element. The hardware 
element consists of a single frequency GPS receiver with 
kinematic antenna, a control unit computer with video graphic 
display for navigation, and a video graphic display for the 
pilot. The software element includes programs for the flight 
project planning, control of the GPS during the flight linked to 
the computer, the storage of the GPS data and the flight 
analysis once finished. The GPS receivers linked to the 
control computer can be different: Leica uses a Magnavox 
series MX9212 single frequency receiver with 12 channels; 
the choice was made with the intention of using cheap but 
reliable equipment for kinematic purposes. The standard data 
recording frequency is 1 Hertz, as given in literature as the 
frequency necessary to adequately sample the flight path 
without excessive use of receiver memory. The receiver has 
156 
an RTCM interface for a radio link to a remote station: which 
makes differential corrections in real time (DGPS) possible. 
The antenna must be kinematic at single or double frequence 
according to the receiver. Leica sells a Sensor System antenna 
at double frequence L1/L2 of quite compact sizes (® = 89 mm 
and h=15 mm) as well. 
3.1 The Topographic Control Framework. 
In order to achieve the relative precision of 1 m RMSE 
required for the GPS controlled photo centers, differential 
GPS corrections need to be determined by utilising base 
stations with permanently recording receivers. 
Following previous experience on a test field in the North of 
Italy the location of the base station should be no more than 
200 km from the unknown position to be determined (i.e. the 
aircraft), with the particular receivers' system to be used 
(Leica MX9212 system which uses single frequency and C/A 
code measurements in Kinematic mode). Thus, three base 
stations were required to cover Ireland. These were located in 
the Dublin, Sligo and Ennis Ordnance Survey Offices (figure 
4). 
  
Northern Ireland 
  
  
  
  
Figure 4 - Ground stations coverage 
OS Offices were chosen as they provided secure, permanently 
manned facilities, with ample provision for power and 
communications. The GPS antenna's were placed on new 
stations built on the roofs of the offices. 
Each base station was marked, then co-ordinated by static 
differential GPS observations relative to survey stations with 
known co-ordinates in the Irish National Grid, using Trimble 
SSE geodetic receivers, geodetic antenna and Trimble post 
processing software. Having solved the GPS solution, GPS 
vectors were then used to obtain the positions of the base 
stations in terms of the Irish National Grid. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996 
  
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