# Help understanding magnetic field orientation

#### jwsigler

Joined Feb 2, 2018
11
This is an electronics issue when you talk about the circuity which runs this technology.

BACKGROUND. Currently the US DOD is looking to develop air bursting (AB) projectiles (Mk310 projectile). The biggest problem with this technology is getting a projectile to air burst at the correct range (i.e. directly above the intended enemy personnel.) The muzzle velocity of standard projectiles will vary based upon a wide variety of environmental variables which would make using time as the AB trigger problematic. Northrop / ATK developed what they call a turns counting fuzing, which is based upon the assumption that a projectile will maintain the same revolutions per linear foot of flight as the twist rate found inside the gun barrel. If a gun barrel has a one turn per foot twist, then to get the projectile to explode at 1,000 feet, a turns counting fuze would need to detonate the fuze after counting 1,000 turns. I believe the way ATK counts turns in flight, is to use a hall effects type sensor to measure the earth's magnetic field. As a projectile spins in flight, the sensor will be rotating within the earth's magnetic field and it will produce a sine wave output with one cycle equal to one turn of the projectile.

ISSUE. In ATK's original patent (5,497,704 – MultiFunctional Magnetic Fuze), the inventor states “When the earth’s magnetic field is perpendicular to the spin axis (radial field), the tab-like portions of the core causes magnetic flux to alternate in direction through the coil thereby producing a sine wave voltage. As the alignment angle between the spin axis and the earth’s field vector direction changes, the sine wave voltage amplitude decreases with the cosine of the angle”. I need help in being able to explain this condition to non-technical people. It is doubtful that we are talking about whether we are shooting in different directions (i.e. north, south, east, or west) because a projectile that would only work based upon the direction you shoot would be worthless. My guess is where the inventor talked about the "sine wave voltage amplitude decreasing with the cosine of the angle between the projectile's spin axis and the earth's field vector direction"; this means that if a projectile is fired directly upward, then the angle between the projectile's spin axis and the earth's field vector would be 90 degrees and the sine wave amplitude would be zero; thus the fuze would not function. If this is correct, then an enemy could create a localized magnetic field which was 90 degrees to the earth's magnetic field and this would prevent a turns counting fuze from counting and essentially neutralize the fuze as long as it is within the localized artificial field.

THE QUESTION. I believe that DOD personnel are either complete idiots or have been bribed to continue funding any turns counting fuze technology. Any technology where the enemy could disable the fuze function, even for a short time, is a waste of our tax dollars and will kill our soldiers. If the field generator is next to the enemy, the fuze could not count or denotate near the enemy. If the field generator is between the enemy and the US forces, when the fuze passes through the field it would momentarily stop counting and when it restarts, it would end up detonating well past the enemy. To explain these issue to non-tech people, I need a graphic which shows the earth's magnetic field but I am not sure exactly how that would look like. If my graphics are off, people supporting this technology purely for money can get decision makers to ignore the danger here even if the basic premise is correct and this technology can be easily countered in order to kill Americans. I should explain that air bursting projectiles are desired as ammunition for US fighting vehicles to engage and kill enemy Anti-Tank teams whose sole purpose is to destroy US troop carrying vehicles; and most defense contractors are only interested in money and not how many US soldiers will died when their technology is easily defeated.

Finally, does anyone know where I can get a sensor which could measure the earth's magnetic field. I have tried calling around to find a hall effects sensor that could, but the reasonably priced ones I have found are supposedly not sensitive enough to measure the earth's field. I also do not want to have to pay hundreds or thousands of dollars for a demo sensor to do the work that our incompetent DOD research and development agencies should be doing. I have a machine shop and could easily make a demo which would show a spinning projectile generating a sine wave from the Hall Effects Sensor and then rotate the projectile to point up where the sine wave output would disappear. I could also include a magnetic field generator which when turned on would result in the sine wave also going to zero. If a newbie electronics person like me could figure out how to defeat this weapon system, I am sure the Chinese, Russia, Noth Koreans, and most Arab countries can do it too.

#### Delta Prime

Joined Nov 15, 2019
1,326
Hello there

If a newbie electronics person like me could figure out how to defeat this weapon system, I am sure the Chinese, Russia, Noth Kore
Don't worry we won't tell those guys either I don't believe we can pose an answer to your question it violates the spirit of AAC and a whole bunch of legal stuff

#### Ya’akov

Joined Jan 27, 2019
5,524
The orientation to the earth's magnetic filed only has to change, and if the projectile is rotating, it will. It doesn't have to change any particular way, just cyclically.

https://patents.google.com/patent/US5241270A/en shows Hall effect sensors for compasses, which would be detecting the earth's field. You can make one yourself, plenty of DIY projects out there.

You say you can figure out how to defeat the system but all you said was "generate a local magnetic field" how would you do that? Saying it isn't doing it. What is your proposed method?

#### ZCochran98

Joined Jul 24, 2018
205
From reading the patent, it appears that it has multiple timing modes (turns-based, timer-based, proximity-to-ferromagnetic-material-based, impact-based) to compensate for each other in case one system fails. Furthermore, it appears that it only relies on the turns-based means to estimate velocity and range for the first 1000 meters (which it would exceed in a little more than 1 second), after which time it relies on the timer (knowing, at that point, the velocity).

In any case, by generating a large magnetic field at an arbitrary point, all you do is increase or decrease the amplitude of the sinusoid that the rotational part of the fuse generates. The turns counter relies on the frequency of rotation, rather than the magnitude. There is a problem with the turns-counter, yes, if you generate it so that the magnetic field nearly-perfectly counterbalances the earth's magnetic field at that point, but to make it remotely effective, you'd need to cover a LARGE area (both in surface area and altitude!) near the point where it was launched with such a finely-tuned-and-oriented magnetic field, which is both impractical and would potentially cause other issues with equipment (for both sides involved).

It's an important question, to be sure, but that's why there are safeguards built into the fuse itself. It's also possible that the fuse design has since been improved, considering the patent was filed in 1993 and expired in 2013.

#### jwsigler

Joined Feb 2, 2018
11
From what I understand from the original patent, the inventor stated " As the alignment angle between the spin axis and the earth’s field vector direction changes, the sine wave voltage amplitude decreases with the cosine of the angle". At 90 degrees, the cosine is zero which would lead me to believe that if I can locally create a magnetic field at 90 degrees to the earth's field with a strength greater than the local earth's magnetic field, the turns counting fuze would stop counting turns as long as it is within the field. As far as creating a local magnetic field is concerned, I have tested creating a local magnetic field by using a OTS walkie talkie and a normal magnetic compass. As I key the walkie talkie and move it around the compass, I can create enough of a disturbance in the earth's magnetic field to move the needle of the compass. If I can use a radio transmitter to create a local magnetic field at 90 degrees to the earth's, then any turns counting fuze entering that local field would stop counting.

One more point, as far as someone saying discussion of a countermeasure for a US weapon system is in violation of some sort of laws or rules, the greater crime is sending US forces into the field with a system that can be spoofed by opposing forces. Currently it appears that the Government and the contractors are willing to turn a blind eye to the fact that a turns counting fuze can be spoofed because they do not want to spend the time and effort to develop a proper system. This is no different than if someone knew a defense contractor was providing our servicemen with substandard ballistic vests. Someone would need to speak up and stop those vest from going out. All I want to do is stop millions of tax dollars being wasted on developing a system that will fail or can be spoofed to the point that the US soldier could not engage and kill an enemy ATGM team. Unfortunately I have seen this behavior before with other defense contractors. I have seen a defense contractor know absolutely that a system has a glaring deficiency and the contractor, and the Government people, turned a blind eye to the facts. I have worked for these types of contractors and seen them use my ideas in a configuration that will fail under combat conditions and they went ahead and sold the system to the Government. IMO these contractors mainly want the contract dollars and are not willing to have a honest discussion about the real possibility that their system can be spoofed and leave a US vehicle crew without a useable weapon system.

#### jwsigler

Joined Feb 2, 2018
11
Here is a pretty good graphic. Note that the Earth's axis of rotation and the magnetic pole do not point in the same direction.

https://www.livescience.com/21668-why-earth-magnetic-field-wonky.html

I looked at your graphic and it is great, but I am having difficulty understanding the original patent's comment when the inventor stated " As the alignment angle between the spin axis and the earth’s field vector direction changes, the sine wave voltage amplitude decreases with the cosine of the angle". Of course the spin axis is the spin axis of the projectile which will be roughly parallel to the earth's surface. Whether you are shooting a projectile in a north/south direction, or in an east/west direction, I have to believe that the turns counting fuze will work for these two directions. If it did not, that would be an even bigger problem. So if the angle between the spin axis and the earth's field vector is not approaching 90 degrees for east/west, or north/south directions, then I would think that the only direction where the spin axis would be at 90 degrees to the earth's field vector would be if the projectile is fired straight up; but I am not absolutely sure of this. If there is no direction where the spin axis is 90 degrees from the field vector, then there would be no reason to have even stated this in the original patent.

If I am chasing a ghost and there is no condition where one could generate a local magnetic field which would stop a turns counting fuze from counting, that is fine; but my gut is telling me that it is possible to have a local field generator that would create a local field that would overpower the earth's magnetic field and stop this fuze from being able to count and locally detonate. I know this sounds kind of SyFy with talk of a field generator, but if the possibility exists, then it will happen and would be a rude awakening for a US gunner trying to air burst a round over a ATGM team and have a zero chance of getting a round to detonate close enough to kill the ATGM team. A lot of time companies, and the Government, get so invested in a technology that they are unwilling to examine the tech with open eyes and investigate possibilities where the tech will not work.

#### Ya’akov

Joined Jan 27, 2019
5,524
From what I understand from the original patent, the inventor stated " As the alignment angle between the spin axis and the earth’s field vector direction changes, the sine wave voltage amplitude decreases with the cosine of the angle". At 90 degrees, the cosine is zero which would lead me to believe that if I can locally create a magnetic field at 90 degrees to the earth's field with a strength greater than the local earth's magnetic field, the turns counting fuze would stop counting turns as long as it is within the field. As far as creating a local magnetic field is concerned, I have tested creating a local magnetic field by using a OTS walkie talkie and a normal magnetic compass. As I key the walkie talkie and move it around the compass, I can create enough of a disturbance in the earth's magnetic field to move the needle of the compass. If I can use a radio transmitter to create a local magnetic field at 90 degrees to the earth's, then any turns counting fuze entering that local field would stop counting.
Your understanding is wrong. You cannot use a radio transmitter to “cancel” the earth’s magnetic field. And, the counter just needs a field to act against, not any particular field.

Your naive approach to defeating the system is just that, naive. It isn’t simple as you would like it to be. Your experiment isn’t relevant and your idea of how the system would operate is just wrong.

#### Papabravo

Joined Feb 24, 2006
18,108
You should be able to find an estimate of the strength of the Earth's magnetic field to compare with any source you might have. One of the implications of Maxwell's equations is:
A changing magnetic field will produce an electric field, and a changing electric field will produce a magnetic field. Since these are both vector fields, you can change the magnitude or the direction, or both. It's challenging to keep track of everything that is changing.​

#### GeorgeBC

Joined Mar 22, 2019
13
I have an idea that wouldn't rely on the Earth's magnetic field. Within the projectile, there would be a sensor that does not rotate with the body of the projectile. Then you can accurately count rotations by referencing between the "stationary" sensor and the rest of the projectile. How do you keep a portion of the projectile from not turning you ask? A frictionless bearing mounted along the axis of the projectile. The bearing (call it sensor if you like) would be held within a vacuum and magnetically suspended (like the leviton trains). No rotational energy would be imparted to the sensor as the projectile is fired, nor as the projectile continues to spin. I'm not a mechanical engineer, but can someone blow up (no pun intended) this idea?

#### shortbus

Joined Sep 30, 2009
9,327
I have an idea that wouldn't rely on the Earth's magnetic field. Within the projectile, there would be a sensor that does not rotate with the body of the projectile. Then you can accurately count rotations by referencing between the "stationary" sensor and the rest of the projectile.
That was done back in the VietNam War, with something called the M79 grenade launcher or Blooper. Only trouble is how do you program the number of rotations to match the distance? With the Blooper it had a preset distance of, IIRC, 30yards before it was armed.

#### GeorgeBC

Joined Mar 22, 2019
13
That was done back in the VietNam War, with something called the M79 grenade launcher or Blooper. Only trouble is how do you program the number of rotations to match the distance? With the Blooper it had a preset distance of, IIRC, 30yards before it was armed.
The sensor mechanism I describe would only interface to an adjacent hall or optical sensor pick-up. The ignition could be set remotely before launching based on the detected quantity of rotations. Thanks for your input!

#### Ya’akov

Joined Jan 27, 2019
5,524
I have an idea that wouldn't rely on the Earth's magnetic field. Within the projectile, there would be a sensor that does not rotate with the body of the projectile. Then you can accurately count rotations by referencing between the "stationary" sensor and the rest of the projectile. How do you keep a portion of the projectile from not turning you ask? A frictionless bearing mounted along the axis of the projectile. The bearing (call it sensor if you like) would be held within a vacuum and magnetically suspended (like the leviton trains). No rotational energy would be imparted to the sensor as the projectile is fired, nor as the projectile continues to spin. I'm not a mechanical engineer, but can someone blow up (no pun intended) this idea?
There are no frictionless bearings and the G forces the shell experiences would make any sort of delicate arrangement impossible at reasonable cost. The reason for the sold state device is the ruggedness required.

#### GreatKing

Joined May 5, 2013
2
It seems to me that questions about the earth's magnetic field lines are better posed to geologists. I am no scientist, but if you want to see pictures of the earth's magnetic fields you can find some with explanations at http://www.geomag.bgs.ac.uk/education/earthmag.html. That site also mentions daily fluctuations as well as the influence of the 11-year sunspot cycle on the earth's magnetic fields. So you cannot depend on the earth's magnetic field being fixed.

There are munitions that use GPS to locate their target. (Picture from the US Army via Wikipedia) seems to me that these things are more accurate than using a compass.

#### shortbus

Joined Sep 30, 2009
9,327
.
Based on information from that website the gov't paid $349M for 6,676 TOW missiles. That works out to over$50,000 each. The cost of war!
What do you think a GPS guided ordnance costs? We can spend as much as we want on war but yet our(US) bridges and such are failing, and people are starving. Eisenhower warned us and we didn't listen.