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Super-Accurate GPS Chips Coming to Smartphones in 2018

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Re: Super-Accurate GPS Chips Coming to Smartphones in 2018

#81

Post by MertsA »

captainmuon wrote: post #76
MertsA wrote: post #45

Earlier quoted context omitted.

>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 b…

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.

Re: Super-Accurate GPS Chips Coming to Smartphones in 2018

#82

Post by msiebuhr »

gruez wrote: post #11
sgk284 wrote: post #8

Earlier quoted context omitted.

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.

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/

Re: Super-Accurate GPS Chips Coming to Smartphones in 2018

#83

Post by c517402 »

cnvogel wrote: post #75
throwaway613834 wrote: post #59

Earlier quoted context omitted.

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.

Re: Super-Accurate GPS Chips Coming to Smartphones in 2018

#84

Post by cantagi »

jtolmar wrote: post #69
colinwilson wrote: post #13

Earlier quoted context omitted.

You can see how off the trace can be here: https://i.imgur.com/eHJI8FH.jpg . You could probably do some smoothing to help fit some of this to a street but I've been on runs where I zig zag down streets just to add distance or to do hills.

I've been working on GPS filtering for three years and that's bad even by my standards. Chips with L5 should do a lot to clear that up though.

I have a question for you. If GPS locations in cities are affected by reflections off buildings, wouldn't the effect be deterministic and could you take advantage of that to reduce the noise?

Re: Super-Accurate GPS Chips Coming to Smartphones in 2018

#85

Post by dingaling »

throwaway613834 wrote: post #80
cnvogel wrote: post #75

Earlier quoted context omitted.

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

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.

Re: Super-Accurate GPS Chips Coming to Smartphones in 2018

#86

Post by j7ake »

gruez wrote: post #4

>will give the next generation of smartphones 30-centimeter accuracy instead of today’s 5-meters 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?

Why would consumers want this type of accuracy ?

Re: Super-Accurate GPS Chips Coming to Smartphones in 2018

#87

Post by michaelt »

chii wrote: post #55
MertsA wrote: post #45

Earlier quoted context omitted.

>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.

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...

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...

Re: Super-Accurate GPS Chips Coming to Smartphones in 2018

#88

Post by izacus »

haikuginger wrote: post #18
tomlock wrote: post #14

I may just be a super-cynical ad-tech exile, but I see this as a path to getting shown more ridiculously well-targeted ads based on the places and stores I've been in. Beacons haven't taken off and I'm glad they haven't. I'd really prefer a GPS in my phone with a similar granularity to the one I have now. Google's notifications that I should review places I've been are creepy enough.

It's almost as though using a phone whose operating system is created and maintained by an advertising company isn't the best idea if you want to keep things like your location private.

iBeacons, which push ads on you when you enter a store, were invented by Apple tho. Do we really need these OS tribalism posts in every topic?

Re: Super-Accurate GPS Chips Coming to Smartphones in 2018

#89

Post by michaelt »

c517402 wrote: post #83
cnvogel wrote: post #75

Earlier quoted context omitted.

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 :)

Re: Super-Accurate GPS Chips Coming to Smartphones in 2018

#90

Post by TheArcane »

michaelt wrote: post #87
chii wrote: post #55

Earlier quoted context omitted.

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...

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.
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