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Comment by matt_wulfeck | Hacker News Reader

Testing against GPS is an interesting challenge. It's technically a critical public service, so any disruption of it should also be broadcast. Any weapon therefore must be tested out in the open.

The development of these weapons probably has some influence on the Navy's decision to bring back celestial navigation[1].

Come to think of it, I don't think I even own a compass.

[1] http://www.npr.org/2016/02/22/467210492/u-s-navy-brings-back...

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Seems like the risk/disruption involved in bringing down a critical public service would require them to do it somewhere else. The second largest city in the US is a poor choice.

Aren't there relatively isolated places where this could be tested with much less risk? The middle of the South Pacific? The Australian Outback? Or even Northern Canada?

Actually that is the best place to test it. LA might be right there, but almost all of Nevada is government owned. Imagine doing this test on the east coast and knocking out NYC, Philly, Chicago, DC, etc.

"not the worst" doesn't really make it the best.

There are still vastly less-densely populated areas of even the continental US.

> Come to think of it, I don't think I even own a compass.

If you have an iPhone or (nearly any) Android phone, you have a compass.

A noisy battery driven compass though. If you are planning for the end of GPS, it doesn't seem like a great idea to lean on your cell phone.

Easier, I think, to just make oneself slightly more autonomous with a solar-panel USB charger, than to acquire the sheer number of items that the smartphone phone has replaced. Just the number of physical maps you'd have to carry in place of one offline navigation app would make one regret the project.

Not to sound too crazy prepper, but getting a compass and a couple waterproof paper maps of the region around me wasn't a huge lift.

I tried to use my Android phone as a compass for aiming my roof antenna... it was not very accurate.

It's too sensitive. the ones attached to a physical pointer have inertia, and therefore resist oscillating or brief interference. Whereas the ones in your phone can actually give you an instantaneous reading, which is way more vulnerable to the odd magnetic fluctuation in the air.

If your android compass took a hundred readings a second and gave you a rolling average of the last two seconds, that would probably be better...

(note, just a theory, but one that seems consistent with my observations)

The earth's isn't the only magnetic field to which the phone is exposed, but unless one installs an antenna in a problematic location it should certainly be the dominant field. I have a hard time believing that the behavior described is "too good". It seems more likely that the phone is interfering with itself, and the circuitry required to avoid that simply isn't justified from a BOM perspective.

Also, any metallic object in the vicinity. Took me a while to realize why the compass goes haywire whenever I hold the phone with my left hand: a ferromagnetic-metal ring right beneath the device.

you're joking! That's awful.

I kinda like it. The ring, I mean. That specific phone is crappy for a host of reasons; this is just icing on the cake, so to speak.

If that was the problem then it would be easy enough to implement a damping filter in software.

does the hardware sensor report the strength of the magnetic field?

Can't average out your readings unless they have a magnitude...

AIUI the hardware reports X, Y and Z components of the magnetic field strength.

Okay, I looked one up, the HMC5883L-BB, and it seems you're right.

In that case somebody in the phone's hardware design department must just be dumb.

Apropos celestial navigation: its usage isn't unknown in aircraft, surprisingly:

https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#Astro...

Fancy automatic systems like that one are rare, but celestial navigation done by hand used to be quite common on board commercial aircraft. Making a long flight over the ocean in decades past, there really was no other way to keep track of where you were with any sort of accuracy. This lasted well into the jet age. Early 747s had a sextant port so that the navigator could use it to get a position fix.

There are rumors that the 747's sextant port fit a hose pretty well, and could be used as a nice vacuum cleaner while at altitude.

> The mounting location for the sextant remains in current 747 designs since it was originally designed and certified with an installed sextant. The sextant (and all associated hardware) has long since been removed with the advent of multiple, highly reliable long-range navigation systems. When installed, there was no requirement to depressurize the aircraft.

http://www.airliners.net/aviation-forums/tech_ops/read.main/...

No requirement to depressurize, but it had the potential.

The "vacuum hose on the sextant port" thing sure sounds like an urban legend. On the other hand, it also sounds like the sort of thing some of my old pilot friends would try.

EEVBlog did a teardown part of one of these systems from a B-52 (https://www.youtube.com/watch?v=hzGaO4Kdz2Y). IIRC, it used a completely mechanical analog computer.

If you have a very good faraday cage / RF test chamber (such as a third party certification lab that does paperwork for FCC compliance has) you can test GPS jammers and their resultant effects on your gear...

ISTM a "good Faraday cage" would disable GPS with or without the presence of a GPS jammer.

yes, what I meant is it's only legal to operate an actual GPS jammer (a transmitter that shits noise on the 1.4 to 1.6 GHz band spectrum) inside a faraday cage, and test how your equipment responds to that (perhaps while simultaneously operating a GPS simulator RF source in the same faraday cage), because you definitely do not want your jammer signal to escape from the cage and interfere with nearby unrelated third parties' equipment. The cage in this case being to keep your experiment contained inside, not to block outside signals from getting in.

I only took a year of physics, but in my experience a Faraday cage that blocks electromagnetic signals in one direction also blocks them in the other direction. GPS functions by receiving signals from several satellites at once.

Yes a Faraday cage blocks both directions. What walrus is saying I think is that if you are a good citizen, you should do any radiating GPS jammer testing inside a Faraday cage. Doing it outside a cage is probably illegal and could create a public safety hazard (911 on cell phones for example).

You could have a test setup bringing GPS inside the cage with coax+circulator or radiate a GPS constellation simulator inside to test the jammer.

You own a compass, most high end smartphones have one that can work completely offline to gps and airplane mode.

At least my $50 smartphone has one

Luckily, the physics on that one doesn't mandate overly technical fabrication.

http://m.wikihow.com/Make-a-Compass

There are GPS simulators for testing, things don't have to be tested only in the open.

Clearly there is some value in large-scale testing, thus the link and announcement.

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