3.4. Data Sampling and Filtering

INTERMAGNET requires that the one-second filtered data values be centered on the second. To minimize aliasing of higher frequency signals into the pass-band of the final second data series, anti-aliasing filters should be included in the analogue portions of magnetometers before analogue-to-digital conversion. The filter responses should be matched to the chosen primary digital sampling rate. Subsequent to the digital sampling, INTERMAGNET requires that a numerical filter be applied in order to obtain the final one-second data series. INTERMAGNET does not specify the use of a particular filter although the chosen filter must provide adequate attenuation to meet the required specifications in Section 3.3.

Examples of acceptable sets of filter coefficients, for use with various sampling rates of properly anti-aliased signals are presented in APPENDIX F2.

A scalar magnetometer must provide a sample centered on the same time as the output of the digital filter used with the vector magnetometer.