28 
To support very high accuracy requirements, it will be 
necessary to use local correction sources (like DARC i.e.). 
It is interesting to see, from the Figures 3 and 4, how 
accuracy is higher in the direction perpendicular to the signal 
diffusion one. It is particularly clear for ALF signal. 
For each component (N, E in UTM - ED50 system, H on 
WGS84 ellipsoid) of each recorded file, we drew up a graph 
reporting satellites’ number and relative PDOP value too. 
Some examples are shown in Figures 8, 9, 10. 
It is very interesting to notice (Tables 5, 6) the high 
repeatability obtained in the mean values of H coordinates for 
all the adopted systems. 
Something not yet detected is the great variation in all the 
components for some points of the ALF Nov 24 lh file, even if 
it can be a possible consequence of a bad satellites’ geometric 
configuration, as there is also a corresponding lost of a 
satellite and a higher PDOP value. 
DARC NOV 24th 
5158082.000 n 
E 416537.000 
Fig. 8: DARC diffused from Villach RTCM corrections (Nov 24 lh 1999 session): 
N, E coordinates in UTM - ED50 system, H on WGS84 (m), PDOP values, satellites’ number 
ALF NOV 24th 
Fig. 9: LW from Frankfurt diffused RTCM corrections (Nov 24 lh 1999 session): 
N, E coordinates in UTM - ED50 system, H on WGS84 (m), PDOP values, satellites’ number