Journal notes, FOM vehicle filtering, clicking Pol. PS
GELLER Labs "Backyard Science"
Thoughts on a proton precession magnetometer design - a Proton Magnetometer Project. Build an Earth's field magnetometer.
The FDM MAGNETOMETER1 project is a low cost high performance proton magnetometer (a digital magnetometer) kit under development for universities and amateur scientists to be able to accurately measure and monitor changes in the Earth's total magnetic F field and to observe geomagnetic storms. Magnetic storms can cause large excursions in the field and are of concern to interests ranging from electrical power grids, radio communications, and satellite operations, to aurora watchers and amateur radio operators.
1 Filter Diagonalization Method "FDM" (harmonic inversion), see Jan 21 and Jan 23 entries, based on: Vladimir A. Mandelshtam, Howard S. Taylor, Harmonic inversion of time signals and its applications, Journal of Chemical Physics (1997), Volume 107, Issue 17, 1997, Pages 6756-6769
(be sure to hit refresh to pick up our latest changes and entries)
Project Documentation (very early stages)
Journal Notes:
Monday, September 13, 2010
The usual departing and arriving morning garbage trucks, school buses and cars show in the magnetogram. The FOM filter discussed in months past rejects moving vehicles, however vehicles that stop within several hundred feet (larger ferrous masses cause detectable offsets at greater distances) unfortunately do perturb the DC field at that counter-wound coil sensor and register as an offset field value (the actual field at the sensor during the 1.1 second digitization window of the measurment cycle).
It should be possible to add software filters to remove one or two point offsets, such as caused by momentarily stopped garbage trucks, or a long term offset (a baseline or offset restoration) caused by one or more newly parked or departed vehicles. On the other hand, with too many filter algorithms, there is a risk that a field change attributable to an actual impulse or other large change caused by an actual magnetic storm could be inadvertantly removed.
Another possibility would be to add fluxgate sensors at some distance (perhaps tens to hundreds of feet) from the FDM proton magentometer counter-wound sensor coil pair. The signals from the fluxgate sensors could be used to positively identify changes caused by vehicles, by comparing changes from two or three fluxgate sensors. There would be large differences between the measurements caused by a local ferrous mass typically closer to one of the sensors than the other one or two sensors. I suppose with three or more fluxgate sensors, the direction of the offending ferrous mass could be determined (analogous to radio direction finding), although that information would seem to be less useful for correcting the scalar field value from the main FDM proton magnetometer field value (to remove a vehicle caused offset). Note that removal of such an offset does not directly use the field value or change in field value at the fluxgate sensor. The fluxgate sensors are simply used to identify that a particular field change (typically one or two distinct measurement points or a step change from one measurement to a following measurement) is artificial and should be removed (automatically or manually) from the record.
Tuesday, September 14, 2010
Overnight: PDF, TXT, NASA Costello Geomagnetic Activity Index, PDF. The geomagnetic field here in upstate NY was slight disturbed overnight, however remained at sub-storm levels. The ~25 nT swing with a minimum around 4 am contrasts with the relatively constant fields after 12 am of the last week or so (See, for example, the 9/12 overnight PDF). There were none of the relatively high dB/dt points (relatively fast transitions of the field) seen in the 5/30/10 G1 PDF and 8/4/10 G2 PDF minor magnetic storms.
Wednesday, September 15, 2010 FDM - Proton Precession Magnetometer Block Diagram
Overnight: PDF, TXT, NRCan OTT PDF, NASA Costello Geomagnetic Activity Index PDF. Sample spectra PDF, log spectra PDF.
Again minor substorm activity overnight, a ~15 nT negative going pulse or impulse around 10 pm, a smaller pulse after 3 am, an interesting shaped triangular wave just before 6 am.
I started to work on the next article about the overall design of the FDM Proton Precession Magnetometer. Here is a block diagram of the overall system showing how the previously discussed Analog and Digital boards fit in PDF.
Thursday, September 16, 2010
Overnight: PDF, TXT. Very quiet steady geomagnetic field overnight. Compare the average field, now under 58,760 nT overnight, to a magnetogram from earlier this year (5/7/10 to 5/11/10) PDF. Our local F scalar value changes by about -144 nT / year. Check your local field values at the NOAA National Geophysical Data Center web resource page.
Friday, September 17, 2010 - First Draft - Part III project article posted
Overnight: PDF, TXT. There were a couple of minor swings in the geomagnetic field overnight, however it is still relatively quiet overnight here in upstate, NY.
I posted a first draft of our Part III project article (Part III gives an overview of the FDM magnetometer) in the project articles section. Comments, corrections, notice of typos are all most welcomed.
Saturday, September 18, 2010
Overnight: PDF, TXT, couple of day view PDF, quiet.
The relatively inexpensive HY1803D supply has one annoying characteristic. In the crossover mode (voltage to current regulation) at about 9.4 V, there is a relatively loud internal relay clunk. As our outside temperatures fall in the evening, the cable/sensor copper has cooled to where under 9.4 V is needed to provide the 1.5 A polarization current. An easy fix is to slightly increase the polarization current or to add a small series resistor to keep the voltage above 9.4 V during the 2 second polarization interval. Since the clunking noise was getting annoying, I went back to the Agilent E3615A overnight. As discussed earlier, a power supply with auto-crossover is thought to give a more constant and repeatable field, however, an inexpensive fixed voltage supply with no crossover mode can work okay too (might need some manual tuning of the voltage for seasonal changes to maintain a desired nominal polarization current). Note, however, that we have been running problem free on the inexpensive HY1803D for many months.
In the couple of day view PDF, you can see slight less scatter in the data, however, yesterday was also very clearly more active in a geomagnetic sense, so no inference should be made on that basis alone regarding the polarization power supply. It might be worth a quick look at the average figure of merit (FOM) values from the text file when time allows. I suppose another comparison would be to look for any changes in the number of rejected measurments by comparing the raw file TXT to the selected (see the FOM filter auto-retry block diagram PDF) and plotted data file TXT.
Project Documentation (very early stages)
QUESTIONS/COMMENTS/notice of typos, etc. send email to joegeller at gellerlabs dot com
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