Magnetometer for UAV?

[dakrause]

Greetings,
I am part of a group that is building a UAV to help find a Cessna-182 that was lost in the Venezuelan jungle two years ago:
http://www.findingbobnorton.org/

Noting that the Boeing 737 that was lost in the Brazilian jungle in 2006(Flight Gol-1907) was only found when a Cesium-magnetometer equipped C-130 was brought into the search effort, I would like to fit a magnetometer to our UAV.

Perhaps a magnetic tensor gradiometer based on Honeywell MR sensors could be a viable option for a low-cost UAV. Does anybody have information(or know of information sources) that could be of help?

David A. Krause
Washington, DC
[email protected]

[dakrause]

From: Ross

Thanks for your email. I will take the opportunity to jump in here and tell you what I know about using magnetometers on UAV’s. I probably know as much as anyone.

There are 2 or 3 issues that will complicate matters for the search. First, there is typically not much ferrous material in an aircraft. Perhaps the hardened parts of the engines, landing gear other structural components. This means that the percentage of material will be small and therefore the anomaly will be small. Second, the anomaly falls off as the inverse cube with distance. This means that if say there was 1000 lbs of ferrous material, then at 100 feet altitude (if you flew right over the target) you would only get about 0.5nT of “signal”. At 200 ft it would be 1/8th or 0.06nT! That is essentially undetectable by magnetometry when considering earth noise and platform movement noise.

The third issue has to do with UAV self-signatures In other words there are four types of magnetic effects we will see “generated”: induced, permanent (or remanent), eddy current and electrical current. You may want to watch some movies on our Video Library called Introduction to Marine Magnetometry which discuss the first three of these.

So the issue is that on a small aircraft like a UAV, you cannot get very far away from these sources of magnetic contamination. Remember the inverse cube rule. So being close to something magnetic has a big effect on the magnetometer. Most UAV’s are not capable of towing a suspended “bird” and thus the magnetometer will have to be hard mounted as you will see in the attached pictures. But you will notice that in standard helicopter and fixed wing installations the magnetometer is in a boom that removes the magnetometer from the immediate vicinity of the magnetic bits of the platform. This is not feasible in a UAV. Therefore we must do magnetic compensation to remove the effects of the local platform and its motion.

We are currently under contract to the US ESTCP Technology Development board to develop commercially available electrical current compensation for an AUV (autonomous underwater vehicle) to go with our standard induced, permanent and eddy current compensation software. We expect that a UAV will have on-board electronics which will generate magnetic transients which will have to be removed in order to see “outside” the platform. This is an experimental project and although we have excellent first order results we don’t expect the project to be completed until next year.

I wish I had better information about using a UAV magnetometer for your search effort but I believe that it will not be feasible to get the kind of data you need to unequivocally (e.g., to separate the anomaly from naturally occurring geologic signal) determine that an anomaly of interest has been detected. A better method may be EM but again, this is not well suited for UAV operation due to the amount of power required for transient EM operation.

Best Regards,
Ross

[Fred]

Towing another bird should not be that difficult, so mounting the mag on a small sailplane for example, with carefully shielded (minimal) electronic equipment, should be possible.

[msolc]

Some 25 years ago I was part of the team measuring magnetic deviations and it was done with 2 motor plane towing a NMR magnetometer pickup on 100m long cable behind it, the magnetometer bird was having small wings with ailerons for steering and stabilisation and an explosive cable cutter in case that bird gets stuck in trees. Scanned grid was cca 100m dense, and the project lasted 4 years (covered whole countries)

[machineman]

heart beat

hi would like to find out if this project is still moving or packed away, aw I had similar thoughts using a second towed "glider(stabilised). I would be keen to colaborate if it is still moving. msolc if possible could I find out more about your project please is the a website relating to the history etc
regards

[dakrause]

Yes, the project is still moving. The sensors piece has stalled due to 2 limitations:
-Available magnetometers-based systems are not sensitive enough to find a Cessna 182
-Available radar-based systems are too heavy to fit on a UAV

More information at:
http://findingbobnorton.org/

[msolc]

It was a NATO project in Germany in beginning of `80 years, mapping magnetic anomalies for long range artillery fire corrections. It was done with a bird with ppm towed 100m behind 2-mot airplane...therefore there is no web about it...final results are not known to me.

[mrkhrdr]

Yes, the project is still moving. The sensors piece has stalled due to 2 limitations:
-Available magnetometers-based systems are not sensitive enough to find a Cessna 182
-Available radar-based systems are too heavy to fit on a UAV

More information at:
http://findingbobnorton.org/

Because passive magnetometry doesn't seem to be sensitive enough for use with a UAV, have you considered an active system? For example, one vehicle to tow a coil transmitting an EM pulse, and another following with a light weight magnetometer. Perhaps one of the Honeywell (or 3, they sell x,y,z modules) magnetoresistive units. These latter are small and lightweight. As for the UAV, these can be made in all sorts of sizes, and I believe there are commercial versions. Perhaps one of these can be hired for your project? Hope this has been helpful. Keep us in touch,
Mark

[mrkhrdr]

...because any metal will hold eddy currents in response to a varying mag field. Thus, aircraft aluminum would generate a signal in response to a sufficiently powerful alternate current field or pulse train. The second aerial vehicle could, in principle, detect this time-varying echo. Portable devices like this have been manufactured and used in the field, albeit on the ground, to detect deep conductive targets in geological formations. They can be carried by hand.
Also, circuit boards carrying a variety of sensors, like accelerometers, gyroscopes, & magnetic compasses coordinated by on-board microcontrollers enable drones to fly autonomously.
Perhaps you are already aware of these things, but I thought I would add them nonetheless.

[mrkhrdr]

Speaking of the ultimate UAV - satellites - space physicists are searching for smaller, lighter instruments to include in microsatellites. I saw this article in which the authors experimented with the Honewell HMC1001 magnetoresistive sensor. I haven't read it through yet, but I thought I'd share anyway. The authors were searching for something lighter than fluxgates, which have been the standard sat instrument so far, apparently. It seems they have come up with circuitry that allows the HMC1001 to measure smaller fields with less noise and more resolution than the usual. If anyone can make sense of this and wants to comment, I'd appreciate it. The Honewell sensors are inexpensive, on the same order as the Speake fluxgates.

http://iopscience.iop.org/0957-0233/...3_2_025902.pdf