That's not true. Military receivers can use the Y Code which is encrypted for military use only. There are some really fancy civilian receivers out there that can leverage Y code to a limited extent without knowing the encryption key but full accuracy is only possible on a military receiver that can decrypt and use the Y code.
Also, I've been wondering, how effective the old scheme with a secret military and a public code was - certainly an old GPS reciever shows up sometimes at a military surplus store. I wouldn't be surprized if enemy nations bought one of those, and extracted the secret parameters from the now quite old chips using modern technology.
Semi-codeless receivers can use it anyways. The reason is that the W code has a much lower data-rate (500KHz I think).
Take the incoming signal, correlate with the aligned P code. The resulting signal will be a MHZ wide signal, bandpass to it, and square the result. The squaring wipes off the residual W code. Because you bandpass filtered before the squaring you don't get so much squaring loss. Lock the resulting carrier, and use RTK signals and long observations to resolve integer ambiguities.
For a dual frequency rx you can exploit that P(Y) is the same on both frequencies and correlate them against each other to find the ionospheric delays.
E.G.
P code 1001011010010110101010100010101
W code 00001111000000001111111100001111
And since it's modulated with the W code this would leak information. Not really sure how off base that part is, that's just a rough explanation I was told many years back.
https://en.wikipedia.org/wiki/GPS_signals#Military_.28M-code...
Though I'm honestly surprised I've "known" of GPS for this long and never realized there were a variety of codes. Legacy--some still operating, modern, military, and more. I'm going to enjoy binge reading this wiki/math stuff tomorrow.
Obviously the self-driving cars of the future will also need other sensors as GPS doesn't work under tunnels and bridges.
[1] https://agriculture.trimble.com/precision-ag/products/steeri...
Most if not all airliners equipped with autoland systems use ILS rather than GPS for position as well.
Knowing the lane of the highway you are may be useful as well.
Yesterday I read an idea from a guy who wanted to create a mariokart-style "ghost" via AR to pace you while you are on a run. That's one example.
Would you have links to any examples?
Some will even switch of the radio once one of these limits are reached and not enable it until it's been reset so make them a pain to use for even if you just wanted the location data after your payload comes back down within the limits.
EDIT: But yeah the so-called COCOM limits still apply to civilian equipment (esp to a company such as Broadcom). Unsure on the regulations for modifying the firmware on a device that doesn't put these lockouts at the silicon layer. For example: https://github.com/swift-nav/libswiftnav/blob/master/src/pvt...
The original limits appear to be for units available for export, but I would guess manufacturers didn't want to deal with the headache of producing different models for import/export and just slapped the restriction down on everything. The poster does claim the original restriction was indeed an OR[1].
And current restrictions may only be 600 m/s? [2]pg 58
[0]https://space.stackexchange.com/questions/14687/current-situ...
[1]https://www.ngs.noaa.gov/corbin/class_description/5700-5800V...
"Immediate access to satellite measurements and navigation results is disabled when the receiver’s velocity is computed to be greater than 1000 knots, or its altitude is computed to be above 18,000 meters. The receiver continuously resets until the COCOM situation is cleared."
[2]http://mtcr.info/wordpress/wp-content/uploads/2017/07/MTCR-T...
That kind of limit is implemented in firmware (all the math is done in software), and they aren't generally protected beyond being an obscure architecture. When you've decided whatever vliw dsp architecture they use, finding a simple 'if' condition near the output logic is quite easy.
> In May 2000, at the direction of President Bill Clinton, the U.S. government ended its use of Selective Availability in order to make GPS more responsive to civil and commercial users worldwide.
> The United States has no intent to ever use Selective Availability again.
The military has better precision, only because they buy better hardware. Similar hardware is used in commercial drones to improve GPS accuracy.
> Using two GPS frequencies improves accuracy by correcting signal distortions caused by Earth's atmosphere. Dual-frequency GPS equipment is commercially available for civilian use, but its cost and size has limited it to professional applications.
http://www.gps.gov/systems/gps/performance/accuracy/