Wednesday 12 February 2014

Performance comparison of power over ethernet (PoE) and battery magnetometers


I've been working to improve the performance and stability of the AuroraWatchNet magnetometers. It's apparent that both measurement noise and stability are considerably improved when the sensor is powered continually. Unfortunately with the sensor powered continually the batteries last for only a few weeks. I have therefore been testing a power over ethernet (PoE) version of the magnetometer. The hardware is essentially the same but the addition of the Arduino Ethernet shield requires the microcontroller to operate from 5V. Operating voltage is easily changed on my Calunium microcontroller board. With the power restrictions lifted the sampling interval can also be reduced. The test system has been operating reliably for almost two months, sampling every 5 seconds. With some minor configuration changes sampling every second is possible although I am not convinced the trade-off in measurement noise is worthwhile.

Performance comparison

Below is a plot comparing one hour of data from the new power over ethernet system with two of the existing battery-powered wireless models already in operation. The PoE system is on the same site as LAN1 but located nearer parked cars. I chose this period because it was free of man-made disturbances. I adjusted the baselines so that the plots overlap.Only the H component of the magnetic field is shown.

Power over ethernet compared with existing AWN magnetometers
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The graph shows that the power over ethernet version has much smaller measurement errors; it can probably operate with ~0.1nT accuracy compared to ~10nT for the battery-powered version. It's such an improvement that I found the measurement accuracy was being limited by the available resolution of the analogue-to-digital converter. The battery-powered versions derive the sample value from the median of 15 samples (taken as a burst over 4 seconds) to reduce noise. To improve the resolution for the PoE version it operates by taking the mean of 15 samples. Further improvement to the resolution may be possible by taking advantage of the programmable gain amplifier that is built into the analogue-to-digital converter.

Let's see how it compares against some observatory-grade measurements. The plot below shows the same interval but this time uses data from the British Geological Survey magnetometers at Eskdalemuir and Hartland. I obtained the data from the SAMNET data archive at Lancaster University, where the data has already been converted into HEZ magnetic coordinates. AuroraWatchNet also operates with HEZ magnetic coordinates, although usually the E and Z sensors are not present. As before, the baselines have been adjusted so that the plots overlap.

PoE magnetometer compared with BGS magnetometers
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The similarity between the different traces is striking. Some differences are to be expected since Eskdalemuir is approximately 140km north of Lancaster and Hartland is 350km SSW of Lancaster. This interval is interesting because it shows Pi2 pulsations, starting at about 01:15 and ending around 01:30. The period of the pulsations are about 90 to 120 seconds.


For a home-built citizen-science magnetometer which probably costs 25 times less than its observatory grade cousin I'm very happy with its performance. The detection of Pi2 pulsations means a low-cost magnetometer can now notify of substorm onset, not just the arrival of geomagnetic storms. A network of such devices has interesting possibilities for the study of magnetic field line resonances.

You might wonder why anyone would want to buy an observatory grade instrument, there are good reasons. At present I am relying on the sensor manufacturer's calibration, whilst an observatory grade instrument would be supplied with an official calibration certificate. The observatory instrument would also have better long-term baseline stability, lower temperature variation and higher cadence. Calibration is an issue I hope to tackle at a later date. For space-weather monitoring only short-term variations are of interest and
the better performance provided by an observatory system may not be needed.

Data credits

The Sub-Auroral Magnetometer Network data (SAMNET) is operated by the Space Plasma Environment and Radio Science (SPEARS) group, Department of Physics, Lancaster UniversityHartland and Eskdalemuir data is provided courtesy of the British Geological Survey.

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