Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Calculating position from raw GPS data (2017)

telesens.co

Post reply on HN
Print view View original

31–40 of 58 posts

Re: Calculating position from raw GPS data (2017)

#31

Post by tverbeure »

AlexanderTheGr8 wrote: post #16
cmsj wrote: post #12

Earlier quoted context omitted.

GPS was built primarily for the military and the signals available for civilian use were crippled to reduce accuracy. GPS receivers that provided high accuracy fell under the ITAR rules (ie you needed proper licensing to trade them) Over time such restrictions have largely been lifted, but there are still US export controls on GPS receivers with particular features, such as those designed for use in high speed aircra…

> those able to decode the still-military-only encrypted signals How? I was under the impression that military-only signal was encrypted. And if someone breaks that encrpytion, blame should go to the poor handling of encryption rather than the person breaking it? Analogy : if you leave a classified document on the train and a passenger reads it, whose fault is it?

The Kraken RF project, which has multiple SDR radios next to each other, had demo code of a passive radar: the slight time delay by which signals arrived at the antennas could be used to detect large metal bodies (IOW: planes) in the sky.

They took down the demo code they were informed that they were violating ITAR regulations.

https://www.reddit.com/r/RTLSDR/comments/yu9rei/krakenrf_pul...

Re: Calculating position from raw GPS data (2017)

#32

Post by eru »

drfuchs wrote: post #27

Exercise for flat-earthers: how does the GPS mapping on your phone work, without satellites orbiting a spherical Earth? Show your work.

Your phone could do arbitrary 'magic' (ie technology) to get good positioning.

You would first need to make some kind of pretty complicated argument why your phone couldn't do this without satellites.

Somewhat easier for you, and harder for flat-earthers to push aside: the ISS is just about visible to the naked eye and definitely to someone with a backyard telescope. Similar for Starlink satellites.

Re: Calculating position from raw GPS data (2017)

#33

Post by karolist »

dingody wrote: post #23

I heard that GPS is one of the few applications in daily life that needs to consider relativistic effects. So, the generated data must have already excluded these relativistic effects, right?

Time ticks at different rates depending on the strength of the gravity "force", or rather the rate at which objects fall into the space-time distortion caused by mass of the earth. Time also ticks slower for objects moving at higher speeds than the observer (satellites move pretty fast). Since GPS depends on time being in sync between the observer and satellites, the time tick is offset to account for the effects of special and general relativity.

Re: Calculating position from raw GPS data (2017)

#34

Post by slow_typist »

namibj wrote: post #8

Android provides carrier phase access for a while now, and with that relative positioning between such devices in about the same neighborhood can be done with enough precision that you have to care about where in the device the GNSS antenna lurks. That alone isn't too fancy; it gets good once you throw in the accelerometer and gyroscope in each device. Because with that you get this not only in realtime, but also wit…

Can‘t you? https://www.mouser.de/ProductDetail/u-blox/ZED-F9P-02B?qs=DP...

Re: Calculating position from raw GPS data (2017)

#35

Post by gsich »

Jerrrrrrry wrote: post #6
cmsj wrote: post #3

For anyone wanting to build their own GPS receiver, here's a fully open source project that explains a great deal of the theory too: http://www.aholme.co.uk/GPS/Main.htm

Just remember to intentionally handicap yourself as to not infringe upon ITAR restrictions.

Any such restrictions are trivially removed in an OSS project.

Re: Calculating position from raw GPS data (2017)

#36

Post by mcmoor »

drfuchs wrote: post #27

Exercise for flat-earthers: how does the GPS mapping on your phone work, without satellites orbiting a spherical Earth? Show your work.

If you believe hard enough that the government is trying to screw you, you could think that our taught satellite physics is a parallel construction https://en.m.wikipedia.org/wiki/Parallel_construction and the real thing actually conforms to flat earth... with enough epicycles.

Re: Calculating position from raw GPS data (2017)

#37

Post by michaelt »

patze wrote: post #11
namibj wrote: post #8

Android provides carrier phase access for a while now, and with that relative positioning between such devices in about the same neighborhood can be done with enough precision that you have to care about where in the device the GNSS antenna lurks. That alone isn't too fancy; it gets good once you throw in the accelerometer and gyroscope in each device. Because with that you get this not only in realtime, but also wit…

Sounds interesting! Does this approach work in indoor scenarios? Would you mind to provide some further links/papers to this topic?

Combining GPS with accelerometer and gyroscope is sometimes called "sensor fusion" - one common way of doing it is with a "Kalman filter" and if you google that you'll find a wealth of information.

If you've got plenty of money, companies like https://www.oxts.com/ have working systems available off the shelf. They're used for things like measuring the dynamic performance of cars.

Such systems will sorta work indoors but not very well. The gyroscope/accelerometer will gradually drift; when you're outdoors with a clear view of the sky, the GPS can compensate for that but when you're indoors that isn't possible. How fast the drift happens depends on how much money you spend to get the highest performance gyroscopes and accelerometers.

It's also possible to integrate other sensors, but that's application-dependent; for a car going through a tunnel, an odometer is a great addition, for a smartphone not so much.

Re: Calculating position from raw GPS data (2017)

#38

Post by cmsj »

AlexanderTheGr8 wrote: post #16
cmsj wrote: post #12

Earlier quoted context omitted.

GPS was built primarily for the military and the signals available for civilian use were crippled to reduce accuracy. GPS receivers that provided high accuracy fell under the ITAR rules (ie you needed proper licensing to trade them) Over time such restrictions have largely been lifted, but there are still US export controls on GPS receivers with particular features, such as those designed for use in high speed aircra…

> those able to decode the still-military-only encrypted signals How? I was under the impression that military-only signal was encrypted. And if someone breaks that encrpytion, blame should go to the poor handling of encryption rather than the person breaking it? Analogy : if you leave a classified document on the train and a passenger reads it, whose fault is it?

I'm relatively certain that the US government will be unmoved by your argument that it would be their fault if you built a GPS receiver that could decrypt the precise positioning signals ;)

Re: Calculating position from raw GPS data (2017)

#39

Post by cmsj »

askvictor wrote: post #28
AlexanderTheGr8 wrote: post #15

Earlier quoted context omitted.

I never really thought of that. That's a pretty interesting restriction. Although any party with access to warheads that can fly 1000+ mph probably can bypass the GPS restriction, no?

There was a guy in NZ (Bruce Simpson) who detailed on his blog how to make a DIY cruise missile. I think he got politely asked to stop doing so at some point.

BPS.Space on youtube is working on a DIY space-capable rocket and in a recent video he mentioned that he is not doing this as a tutorial and that his guidance system likely already wanders into ITAR territory, and thus he's self-censoring which parts he shares and which parts happen off-camera.

Ultimately these kinds of regulations are fairly silly because a sufficiently determined smart person can recreate the covered technologies from scratch, but here we are.

Re: Calculating position from raw GPS data (2017)

#40

Post by salty_biscuits »

dingody wrote: post #23

I heard that GPS is one of the few applications in daily life that needs to consider relativistic effects. So, the generated data must have already excluded these relativistic effects, right?

The error budget in the pseudorange to the satellites has various factors due to relativity, but they are just lumped into the errors in the least squares problem that you solve to get the position estimate (as per the article). So relativity is important, but you don't need to know much about it to solve for your position. Various flavours of long baseline/network RTK will need more sophisticated modelling though.
Post reply on HN