ERROR ANALYSIS OF TOTAL FIELD MODELS DERIVED FROM POGS DATA

Scalar magnetic data from the FOGS satellite are analysed for the 1991 -1993 period. Models of the main geomagnetic field are computed using an iterative least-squares method. Vector components of the field deri ved from these models display large errors known as the Backus effect. A method for estimating the noise level in global data in the form of an equivalent gaussian noise is introduced and FOGS data are shown to have a standard deviation of about sigma = 45 nT. Despite this high n oise level, it is shown that FOGS data can be used to improve models d erived from observatory vector data. It is also shown that the Backus effect can be significantly reduced by using some accurate vector grou nd-based measurements of the main field at observatories located near the equator. Monthly models are computed using both FOGS and observato ry data for which monthly mean values are available at FOGS period. Lo cal corrections applied to observatory measurements are computed by co mparing observatory monthly mean values with Magsat models. The result s are shown to be improved when these corrections are applied to the o bservatory data, confirming that they correspond essentially to statio nary short-wavelength crustal anomalies.