Things have changed since the 90s.

> 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/

>The military has better precision, only because they buy better hardware.

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.

By the way, has anybody tried to crack P(Y) code using a kind of "rainbow table"? (I'm not advocating that ofc, just curious...) I mean, it gets it's security because the parameters to its generating function are not known. But the function repeats after about a week, and presumely can't be changed due to the large numbers of legacy devices in the field. So you could just record the whole signal for a week (I did a back of the envelope calculation some time ago and it would be about a TB IIRC). Then just build some kind of index, and after recording a few seconds, you can find your position in the code, and syncronize to it.

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.

P code itself has a period of a week but the W code that scrambles it doesn't have a known period AFAIK.

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.

What's really interesting is that there are some commercial receivers out there that will use P(Y) Code even though they can't decrypt it. IIRC the weakness was something along the lines of the lower rate W code made it so that while you couldn't decrypt it you could still use it as a reference to increase the accuracy of the C/A code.

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.

Don't you mean... M-code? (meaning "military" code)

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.

No, I'm talking about the P code or precise code. It's modulated with the encrypted W code to create the encrypted Y code.

m-code is the new hotness that is stuck in development hell currently.

Yeah. As I understand it, previously it wasn't possible to do dual-band GPS without using the encrypted military P(Y)-code because the unencrypted C/A code was only on the L1 frequency, and for best accuracy you need dual-band to measure and compensate for ionospheric delay.

is there any reason to be even more accurate than a 5 meter radius? I mean, if you are targeting a person in a building, i can't see why it has to be more accurate than that. Navigation of a drone that is less than 5 meters in size may require higher accuracy, i suppose...

The chip doesn't just improve maximum accuracy, it also vastly reduces the deviation when in poor conditions such as inside a building. The technique to do so just happens to also make peak operating conditions more accurate.

Self-driving vehicles. You can buy GPS-based automatic steering systems for tractors right now [1] (there's still an operator on board, so it's not truly autonomous)

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...

That's precisely the problem TomTom and Here HD Maps aim to solve.

Airplanes are already using the higher-accuracy GPS+GNSS to land safely on the runway in low-visibility conditions, guided in by the Flight Management Computer. It makes a big difference whether the plane is 5 meters above the runway or zero meters above the runway.

Commercial airliners use a radar altimeter, not GPS/GNSS to assess altitude.

Most if not all airliners equipped with autoland systems use ILS rather than GPS for position as well.

Agreed, but only the big iron. I spent 8 years working for Universal Avionics, who develops precision WAAS-SBAS GPS flight management systems for GA / BizJets: http://uasc.com/home/nextgentechnologies/precisionaviationwi...

A problem that I usually have with Google Maps is that, when I am driving in (or next to) an elevated highway, the GPS usually does not know if I am in the highway or in the road beneath it.

Knowing the lane of the highway you are may be useful as well.

Determining lanes for map navigation is a big problem. I had a buddy that was responsible for mass export of Verizon telemetry data from Verizon to a third company. Their "holy grail" was to be able to determine lanes through triangulation in a timely manner (2008-2009ish). Don't know if the problem ever got solved and it would have only been relevant in dense metropolitan areas.

Imagine pedestrian navigation. 5 meters (a rare, perfect accuracy) would place you in the center of a street instead of the sidewalk.

5 meters is the difference between one room and the next room. Huge potential for indoor navigation and contextual AR applications.

Mapping the boundaries of a property is usually desired to much higher precision than 5 meters!

Autonomous vehicles and drones (used for delivery) can benefit a lot from such accuracy.

Wow really? You would post that on a technology board?

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.

Guess I'm being downvoted for snark. But the definition of technology is to advance the state of the art. None of us would be here if we thought everything was already good enough.

> There are some really fancy civilian receivers out there that can leverage Y code to a limited extent without knowing the encryption key

Would you have links to any examples?

Is there not still a limit to GPS accuracy when civilian receivers go beyond a certain speed? e.g. to deter clandestine missile development?

iirc the requirements are an AND condition for speed and altitude. >1,000 knots AND >18,000m. But many civilian receivers implement it as an OR condition much to the annoyance of the near space balloon community.

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...

Leave it to stackexchange to shed some more light on the matter[0].

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...

Look in the right places and you'll find updated firmware images for various receivers which remove the limits.

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.

Selective Availability was turned off in 2000, prior to that error was about 50m. Differential GPS could work around it anyway, so there wasn't a lot of point to keeping it on.

Civilian GPS signals used to have intentional error added, but that was ended in 2000:

https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_...

There's other constellations now, so limiting the capabilities of civilian GPS hardware has reduced strategic value.

If I understand correctly, the best implementations treat the various satellites as one combined system, rather than trying to average the solutions.

There's usually restrictions that kick in at certain altitudes and/or hardware restrictions if you're going over a certain speed.

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.

This should be an "and", unfortunately many GPS receivers consider it an "or".

Makes them worthless for high altitude near space balloons :-(

But that's client side right? What's preventing someone from implementing a GPS receiver using a SDR that doesn't have those restrictions?

Copenhagen Suborbitals has a blog-post about those limits:

  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.

... and proceed to figure out a way around ...

https://copenhagensuborbitals.com/gps-without-limits/

Please don't use code blocks for quoting, makes it incredibly hard to read on mobile.

> 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.

More like parties that are not technologically advanced nation states. Russia has its own satellites, China will find 10 ways around the problem, but a few random dudes won't be able to DIY a cruise missile with cheap parts that would be precise enough to be dangerous.

Or at least that's how I always understood it.

Actually civilian GPS would work fine in low-cost cruise missiles because they fly relatively low and slow. The restrictions are to protect against ballistic missiles.

Nothing at all: http://www.aholme.co.uk/GPS/Main.htm

That's really cool! I'd like to build something like that One Of These Days, when I have more free time.

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.

Technically speaking, such a receiver would be classified as a munition and prohibited from export out of the US: https://en.wikipedia.org/wiki/GPS#Restrictions_on_civilian_u...

But practically speaking, almost all GPSes are made outside of the US and then imported...

How do these folks do the math? Do they understand general relativity? Or do they just pull the equations from somewhere?

There is no general relativity involved in calculating your position. Solving for your position is basic geometry, if you want to have better resolution, have good model for the effects of the atmosphere on the radio waves.

http://www.aholme.co.uk/GPS/user_position_solution.pdf

In "Exploring Black Holes" by Taylor & Wheeler, project A is titled 'Global Positioning System', where they say locating your position accurately depends crucially on General Relativity.

Before the satellites are launched, their atomic clocks have to be tuned to offset for time dilation.

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 :)

Huh, is the whole thing a myth then? I'd definitely read that GR is necessary, but now that I'm searching I see mentions of it being a myth.

I remember this question arising on HN about a year ago. GR is a necessary consideration in the design of the satellite system.

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.

GPS, Relativity, and pop-Science Mythology http://www.alternativephysics.org/book/GPSmythology.htm

Boy do I miss those types of sites. I need to find more of them.

Here's another, very detailed 4-part "build your own GPS" article from the early 90s:

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.

Note that while well explained, this is a very 90s way of implementing a GPS receiver. As I understand it modern receivers use FFT-based correlators that have much better cold-start performance than the old-fashioned design because they can test very large numbers of possible correlations at once (generally hundreds of thousands of them).

Anecdotically, some chinese GPS chips for cell phones don't have those restrictions, but they maybe have others. The restrictions have become dubious anyway, as most nations who have that kind of rocket have the facilities to build GPS receivers (OK, I don't know about NK...).

Good luck. You're right but implementing a GPS receiver using a SDR is quite a feat. It's sort of been done but not in a way that would make it practical to use for e.g. a hypothetical terrorist cruise missile.

The restrictions are just from the manufacturer and can be disabled on request. We have to do this for our high-altitude balloon experiment.

I don't see the point. You can just use a bigger bomb to overcome the inaccuracy.

I assume you're joking. Nevertheless, for missiles, it's more about detecting and correcting early when the error is still manageable. If you go at Mach something, a few meters off course can quickly become a few hundred.

Way to lose support from your own people.

I do see the point of the intention, but as far as I understand it, it never really worked for preventing that anyway.

Why would consumers want this type of accuracy ?

I prefer results to be as close to accurate as possible. I use a dedicated device, not a phone, when I put out game cameras or traps. The more accurate the results, the quicker they are to check.

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.

Lane detection, self driving cars, drone delivery and of course meeting your Tinder date in the middle of a rock concert.

Where signal strength is low (in the mountains or a building).

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.

For locating things accurately? Imagine I found a fossil in a rock field and wanted to share its location with you - you'd want more than ±5m accuracy to find it.

Accurate geotagging of photographs for starters. From my experience the results can be wildly off and at times my phone doesn't even get a GPS signal.

Better Pokemon Go.