Requirements for Obtaining High Accuracy with Proton Magnetometers Part 5

Signal-to-Noise Ratio

Limited S/N ratio will make measurement of the average period uncertain. Relative accuracy or repeatability of measurement, а prerequisite for absolute accuracy, depends to a large extent on S/N ratio of the obtained precession signal. Obviously, an effort must be made tо use all thе available information from the noisy precession signal, therefore usually all thе zero crossings are taken and used for calculation of the result.

Sensor Cleanliness

Obviously, any ferromagnetic inclusion in the sensor or its immediate vicinity may influence the field and cause erroneous result.

Presence of AC Magnetic Fields

AC magnetic fields caused by power networks or smilar may modulate measured magnetic field and cause errors. AC field collinear with the main magnetic field vector can be averaged out, but the field perpendicular to the main vector get rectified because of non-linear vectorial addition. The summ is a square root of the sum of dquared of the components. Fortunately this summation greatly suppressed smaller of the two components. For example, an AC component of 100nT at right angle to the main field will produce only 50pT offset in the field of 50,000nT (or 1ppm).

Conclusion

Proton magnetometers, although theoretically of higher absolute accuracy, require tender
care if the highest accuracy is ever to be achieved. Similar considerations apply for other
candidates for the highest absolute accuracy of measurement such as potassium.
(Potassium also exhibits nonlinearities in frequency-field relationship).

Fortunately for the magnetic observatories it is much more important to have repeatability of the results than the highest absolute accuracy. The reason for this is a
substantial spatial distribution of lосаl magnetic fields that masks the absolute accuracy and makes it virtually of academic importance.

The highest absolute accuracy of the magnetic field measurement has a metrological importance, but very little beyond this.