Review and Teardown of a Cheap GPS Jammer

The actual enforcement action notice is at the link below. The FCC does not mess around. They started at $42k, but were 'nice' and offered him a 25% discount because he voluntarily relinquished the jammer.

https://apps.fcc.gov/edocs_public/attachmatch/FCC-13-106A1.p...

There's not much more going on with a physical layer jammer. That's all this jammer does. It jams the GPS carrier band so that a receiver can't "hear" the satellites.

You can put together such a jammer from a lab signal generator and a microwave power amplifier that can cover in the GPS band. You may have to tweak the signal generator to get a suitable wide signal. GPS is not a narrow, but a wide-band signal. This helps receivers to reject narrow interference.

+1 to this.

Men showed up at my doorstep... I removed the designs from github and they went away happy.

No fine or anything, but it was clear I didn't have a choice...

They were selling these online from shady resellers at least ten years ago. A friend picked one up and told me he could jam some phone calls on the train, depending on carrier and frequencies used I suppose. I think it broke a week or two later and he tossed it.

Second link from google:

http://www.jammerfromchina.com/categories/Mobile_Phone_Jamme...

Pretty much. Here's an expanded, but still very simple way to think of the way this noise jamming works:

- A GPS receiver needs a signal/noise ratio above X dB to get a lock

- Higher gain antennas, more sensitive receivers, and higher powered signals add to the numerator (good)

- Antennas with smaller sidelobes (often a consequence of a high gain antenna or nulling antennas), and better signal processing reduce the denominator (good)

- There is a pretty good S/N margin when using a modern receiver with a good antenna in open space

- Shadowing reduces the signal strength and thus reduces the numerator (bad)

- Multipath effects increase noise, and thus increases the denominator (bad)

- Jamming increases noise, and thus increases the denominator (bad)

- Jamming can be improved by cranking up tx power or using a higher gain antenna (or both)

- In open space, a jammer must overcome the entire S/N margin by itself. GPS systems are so highly optimized (very good algorithms) and have such a big S/N margin that you can often get a lock just by putting the GPS in a hole and that nulls enough of the jamming.

- In a city, shadowing and multipath eat into the S/N margin by quite a bit, so the jammer doesn't have to do as much. Sometimes the urban environment is so bad that GPS can't get a lock even without a jammer.

- Jamming strength decreases with the square of the distance (or 2x when talking in terms of dB, which are logarithmic)

- The margin reduction by the shadowing and multipath benefit the effective distance by the square of the reduction (or 2x when talking in terms of dB)

- So any natural jamming benefits the jammer by allowing it to operate much further away or operate at a lower power. There isn't much you can do to reduce natural jamming like multipath or shadowing other than moving to a different location.

edit: grr, I hit tab and enter before finishing.

You can get a good idea of the signal path to the antenna mounts with the photo provided, especially the photo visible via the underlying link to dx.com. Trace width would be set to match desired impedance (e.g. 50Ohm), generous spacing from adjacent ground plane for isolation, and big radii on corners to minimize self-capacitance. The 45deg component mounting is unusual, especially since the equivalent (?) PCB on dx.com doesn't have it. Perhaps this board was fabbed in low enough volume that optimizing pick & place to only do multiples of 90deg wasn't needed.