Didn't the US scramble the civilian signals specifically so it wouldn't be too accurate? And they're just okay with civilians having access to ultra accurate GPS receivers?
> 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.
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.
https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_...
If I understand correctly, the best implementations treat the various satellites as one combined system, rather than trying to average the solutions.
Depending on the source of signals used, the military can also just change the precision for civilian bands. But in times of peace / low threat levels, there's not a particular need to do so.
Makes them worthless for high altitude near space balloons :-(
By default all GPS units have some built-in limits (so-called COCOM
limits) that prevents them from providing any data if the velocity
exceeds 515 m/s at altitudes above 18 km. These artificial limits are
built into GPS receivers to prevent bad guys from using them in
missiles and other nasty stuff.
I was confused. So here's a translation: Bad guys = US enemies or non-allies. Not bad guys in any moral sense.
Or at least that's how I always understood it.
Also gives you the point - if one guy can build a receiver on his own... I mean, I don't want to make it easy for rogue nations and terrorist groups to get ICBM-mounted nuclear weapons, but with the difficulty of building and testing the missiles and bombs, it seems a little silly to expect restricting GPS receivers to accomplish anything.
But practically speaking, almost all GPSes are made outside of the US and then imported...
GPS satellites are all tuned this way, and it has the desired effect.
However, for people implementing GPS receivers, the signal arrives already corrected for time dilation. So the people making that part of the system don't have to know about general relativity at all :)
See page 5 of DTIC document: Relativistic Effects in the Global Positioning System
http://www.dtic.mil/dtic/tr/fulltext/u2/a158720.pdf
Without correction the drift due to combined coarse relativistic effects would be on the order of 100,000 nanoseconds per DAY.
In fact the first NTS-2 satellite was launched with the clock unadjusted for GR, due to doubters, but after three weeks the syntheziser had to be activated to compensate. The calculated uncorrected drift on that clock alone was 38,000 nanoseconds per day.
As the document notes, to further refine Navstar to greater levels of precision would require tackling yet another set of tougher relativistic effects, plus more mundane atmospheric influences.
http://lea.hamradio.si/~s53mv/navsats/theory.html
No FPGAs, RPis, or SDRs --- it's mostly done in discrete logic with a 68000 doing the back-end processing. As a bonus, he also implemented GLONASS. Furthermore, the software takes up less than 32KB of ROM.
I do see the point of the intention, but as far as I understand it, it never really worked for preventing that anyway.
I guess I'd reverse the question. Why wouldn't you want the greatest level of accuracy you can get? Maybe you want to track your property lines? Maybe you want to accurately mark where your access point is for your septic tank? Maybe you just want to hide Easter eggs or buried treasure?
I'm not sure why anyone would want a tool that is less accurate than it can be, assuming it is also reasonably affordable.
When moving slowly (walking).
When precise coordinates are needed (self driving vehicles).
When precise altitude is needed (airplanes and drones).
When tracking smallish things on a person (head tracking for augmented reality).
When performing certain scientific measurements (continental drift).
Finally, so that when I'm sitting at a stop light my navigation doesn't freak out and suddenly think I'm moving in the opposite direction.
I'm sure there are tons more.