12 
The magnetic induction vector is over-determined due to the measurement of three elements 
within the meridian plane. The difference C=F-(AF+Z 2 ) 1/2 represents the measurements’ inherent 
accuracy. The annual mean values of the error C amounted to: 
C in nT 
Number 
1996 
0.2 ±0.2 
52 
1997 
0.2 ±0.2 
52 
1998 
0.3 ±0.4 
51 
1999 
0.2 ±0.3 
52 
C is also shown in Tables 1 .x 
As a rule, the PPM of type V75 was used. This instrument shows a long-term drift of some 0.1 
nT depending on the components (Schulz and Carstens, 1979). Therefore, comparative 
measurements using the PPM of type V4931, which represents IMS (section 2.2), were carried out 
on a monthly basis. All base line values as well as the recorded minute spot values of F(section 4) 
are referred to this instrument. 
4 Digital recording system 
Minute mean values of the orthogonal components U, V, and Zas well as spot values of Fwere 
acquired by the primary digital system (V75 and FGE (No 125), section 2.1). The PPM is not only 
part of the recording system but also serves as an indicator of the PVM (section 3). 
Owing to over-determination, outliers, jumps and short-term base line instabilities between the 
base line measurements of all three components could be detected (section 4.1) and, under certain 
conditions, automatically eliminated. The following equation applies to Wingst: 
dF= 0.26 dU + 0.26 dV +0.93 dZ. 
Additionally, a fourth fluxgate was operated, which had been aligned in such a way that its W 
orientation satisfies the following equation: 
dW= 0.578(dL/+ d\/+ dZ). 
In this way, jumps and outliers of the secondary system could be monitored independently. 
A second suspended FM of type FGE (No 126), an FM of type EDA FM100B and a PPM of type 
PPM105(EDA) were operated as stand-by devices in case of failure of the primary system. 
4.1 Base line values 
Tables 1 .x show the base line values of the FGE125 referred to IMS. Figs. 1 .x show the results 
in graphical form. Absolute measurements of D and I (Dl-flux) are marked by circles, those of 
relative measurements as well as H and Z(PVM) by dots. / (derived from H and Z) is also displayed 
(dots). 
To obtain base line values, the dependence of the measured elements D, H, I, and Fon the 
recorded components U, V, and Z within the range of variations was developed up to terms of