2.4. Data Sampling and Filtering
In its Resolution 12 from the 1979 Assembly in Canberra, IAGA noted the desirability of digital magnetic observatories using a sampling rate no slower than once every 10 seconds. In that resolution, IAGA also stated that the one-minute means must be centered on the minute (mm:00).
To minimize aliasing of higher frequency signals into the pass-band of the final minute 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 minute data series.
One digital filter that is widely used by INTERMAGNET can be achieved by applying the following coefficients (for a Gaussian filter) to a series of 19 samples of 5-second data :
C0 |
0.00229315 |
C1 |
0.00531440 |
C2 |
0.01115655 |
C3 |
0.02121585 |
C4 |
0.03654680 |
C5 |
0.05702885 |
C6 |
0.08061140 |
C7 |
0.10321785 |
C8 |
0.11972085 |
C9 |
0.12578865 |
C10 |
0.11972085 |
C11 |
0.10321785 |
C12 |
0.08061140 |
C13 |
0.05702885 |
C14 |
0.03654680 |
C15 |
0.02121585 |
C16 |
0.01115655 |
C17 |
0.00531440 |
C18 |
0.00229315 |
For a filter output value to be centered on the minute, coefficient C0 is applied 45 seconds before this minute and coefficient C18 is applied 45 seconds after the minute.
In addition to the attenuation provided by the numerical filter, a “natural filter” applies, estimated at -9 to -18 dB/Octave typically, caused by the decrease in energy of the natural field with increasing frequency.
Examples of other acceptable sets of filter coefficients, for use with various sampling rates of properly anti-aliased signals are presented in APPENDIX F1.
A scalar magnetometer must provide a sample centered on the same time as the output of the digital filter used with the vector magnetometer. INTERMAGNET recommends that the scalar magnetometer is sampled at the highest possible rate and that the data are filtered to one-minute values using the filter specified in this section.