[HN Gopher] Calculating position from raw GPS data (2017)
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Calculating position from raw GPS data (2017)
Author : fanf2
Score : 234 points
Date : 2024-07-14 17:42 UTC (1 days ago)
(HTM) web link (www.telesens.co)
(TXT) w3m dump (www.telesens.co)
| cmsj wrote:
| 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
| Jerrrrrrry wrote:
| Just remember to intentionally handicap yourself as to not
| infringe upon ITAR restrictions.
| AlexanderTheGr8 wrote:
| Can you explain what you mean by that? The only thing I can
| think of is that GPS has 2 modes : one civilian and other
| military use. But military use-case uses encryption so
| civilians can't use that mode anyway.
| cmsj wrote:
| 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 aircraft, those able to decode the still-military-
| only encrypted signals that piggyback GPS and provide
| greater positioning precision, or those designed for use in
| rockets/UAVs.
|
| So, it's reasonable to assume that if you were to build
| your own, and do too good of a job, you would accidentally
| become subject to arms trade regulations, and that's
| probably not a place you want to unintentionally find
| yourself, particularly if you're publishing it as open
| source on the Internet :)
| AlexanderTheGr8 wrote:
| > 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?
| j16sdiz wrote:
| Technological restrictions and Legal restrictions not
| always aligns.
|
| That's where the "restrictions have largely been lifted"
| part comes in.
|
| You can decrypt most of them, but it is illegal to export
| without a license.
| dheera wrote:
| Actually you can use the phase information in the
| encrypted signals to improve RTK stability, without
| decrypting them.
| tverbeure wrote:
| 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...
| cmsj wrote:
| 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 ;)
| nobodyknowin wrote:
| Not GP but modern GPS / GNSS manufacturers intentionally do
| not allow their devices to function over some speed (1000
| mph or something like that).
|
| So they won't work on warheads as part of a guidance
| system.
| AlexanderTheGr8 wrote:
| 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?
| nobodyknowin wrote:
| Not if they buy an off-the-shelf solution.
| m463 wrote:
| Just don't let some future party use your code to do it.
| askvictor wrote:
| 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.
| cmsj wrote:
| 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.
| ssl-3 wrote:
| Any sufficiently smart person can accomplish the same
| original work as any other sufficiently smart person.
|
| But when the details of these things are published or
| otherwise made openly available, it doesn't take nearly
| as many smarts to duplicate these accomplishments.
|
| Quite often, that's good: It's easy for a dullard like me
| to build a circuit or to re-use some clever assembler
| code when someone else has published it for my own
| tinkering around the house. In this way, it's a pretty
| great world to live in; it is often very simple to stand
| on the shoulders of giants and get some things done that
| I could probably never do on my own.
|
| But sometimes, that's bad: We don't live in a perfect
| world. Enemies exist. Things like ITAR can't prevent a
| sufficiently smart person from doing anything, but they
| do make it a lot harder for them to get started.
| rich90usa wrote:
| I launched a high altitude balloon as part of a summer
| school program over a decade ago and we checked on edge
| cases. Off the shelf GPS are supposed to not work beyond a
| certain height 18,000 m and/or speed 515 m/s to be a
| barrier for use as a weapon. Some hardware treat that as
| AND; some treat that condition as OR. The term to look up
| is "CoCom Limits".
| Thaxll wrote:
| "The vast majority of civil and commercial GPS/GNSS equipment
| does not meet the Cat XII(d) criteria and is therefore not
| subject to ITAR"
|
| https://www.space.commerce.gov/itar-controls-on-gps-gnss-
| rec....
| gsich wrote:
| Any such restrictions are trivially removed in an OSS
| project.
| userbinator wrote:
| I find the project it links to (and admits to be inspired by)
| even more impressive:
| https://lea.hamradio.si/~s53mv/navsats/theory.html
| RF_Savage wrote:
| It's wild reading old issues of VHF Communications and
| finding advertizements for buying the PCB's for this project.
| So I,guess people really did try to reproduce it as well.
| eggsome wrote:
| Here is another good open source implementation:
| https://m.youtube.com/watch?v=dVD1Yws__v0
| Titan2189 wrote:
| Alternate and maybe more interactive explanation:
| https://ciechanow.ski/gps/
| namibj wrote:
| 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 with only minor
| degradation due to the changes in the GNSS pseudo-range
| measurements being predictable despite not holding still.
|
| Other interesting things enabled by it are e.g. auto-land of a
| model plane in a truck bed without needing wheels on the plane
| (and still preventing it from getting scratched up/depending on a
| grass landing strip).
|
| Even fairly good GNSS receivers aren't expensive to build, as
| long as power consumption isn't very critical, so why can't I
| just buy a pair for a hundred bucks?
| patze wrote:
| Sounds interesting! Does this approach work in indoor
| scenarios? Would you mind to provide some further links/papers
| to this topic?
| michaelt wrote:
| 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.
| johnwalkr wrote:
| GPS-denied navigation is the search term. It works well
| enough for driving through a tunnel, because eventually you
| get a known location to correct previous estimates. For a
| fully-indoor environment it's very difficult to get good
| results and the technique depends on the use-case.
|
| "Indoor GPS" using bluetooth beacons is technically possible
| but the space is filled with proprietary protocols/apps that
| try to make money by serving ads. It unfortunately doesn't
| seem to be evolving into a more useful extension of google
| maps.
| ragebol wrote:
| For indoor, there's UWB, ultra wide band. Can also use
| Bluetooth or even lights blinking at different (high, non-
| humanly-visible) frequencies to do indoor positioning.
| namibj wrote:
| For indoor you will prefer UWB[0] where you can set up base
| stations yourself that offer a service pretty much like GNSS
| signals (i.e., you can passively receive them from 4+ base
| stations and turn those pseudoranges into a position,
| provided you are told where they are and kept updated about
| their clock drift relative to each other (e.g. by them
| listening to their neighbors and piggybacking 2-way-ranging
| sessions on top of the beacons they already broadcast, each
| offering the clock offset to their neighbors in the data
| payload of their own beacon)), with the added benefit that by
| using a shared-key CSPRNG to generate the bitstream of the
| ranging code instead of a fixed known sequence, you can get
| authenticated ranging where MITM attacks are limited to
| artificially inflating the range (i.e., a trigger of "has to
| be close enough to the door handle that e.g. a pair of
| cameras looking at the areas right in front of either side of
| the door have it in view" can't be faked with a wireless
| MITM).
|
| There are some links/papers in the publications section[2] of
| [1].
|
| [0]: https://www.firaconsortium.org/ [1]:
| https://developer.android.com/develop/sensors-and-
| location/s... [2]:
| https://developer.android.com/develop/sensors-and-
| location/s...
| slow_typist wrote:
| Can't you?
| https://www.mouser.de/ProductDetail/u-blox/ZED-F9P-02B?qs=DP...
| namibj wrote:
| They are 114.62 USD each if you buy at least 100 of them. And
| you still need to add an active antenna and a good crystal.
| Considering those, this module is about 3x the price I asked.
| toast0 wrote:
| https://www.bestbuy.com/site/tracfone-blu-view-5-64gb-prepai...
|
| Here's a GNSS receiver for $30. It comes with some other junk
| you don't really need though.
| Terr_ wrote:
| Frequently posted in GPS comments--but for a good reason:
| https://ciechanow.ski/gps/
| Terr_ wrote:
| > The figure below shows how the user-source geometry affects the
| amount of uncertainty in the user position.
|
| Now I'm wishing there was a setting in mapping applications on my
| phone that changed that the shape used for position uncertainty,
| from a circle to these arc-intersection shapes.
| dingody wrote:
| 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?
| defrost wrote:
| > So, the generated data must ..
|
| What generated data are you talking about and who was it
| generated by?
|
| _If_ you mean the output of commercial GPS units, _then_ yes,
| all manner of error inducing effects have been compensated for
| in post aquisition processing that generates output.
|
| This article is about _raw_ GPS data .. which is a collection
| of raw data streamed from multiple satellites that then
| requires processing to generate an output, and quite often
| additional inputs from ground stations | naval corrections to
| improve accuracy.
|
| There are _many_ different GPS instrument providers who all do
| broadly similar things .. the devil is in the details.
|
| Several peer comments linked to https://ciechanow.ski/gps/ it's
| a good read.
| zokier wrote:
| from that link
|
| > Moreover, the clocks on satellites don't have to be
| explicitly slowed down to fix the cumulative relativistic
| speed-up of time. As part of their broadcasted message a
| satellite emits three coefficients that allow the receiver to
| correct for any offset or speed change of that satellite's
| clock.
|
| My understanding is that GPS satellites clocks are tuned to
| tick slow exactly to account for relativity.
|
| For example this (https://www.nist.gov/publications/global-
| positioning-system-...) paper explicitly states:
|
| > First, each GPS space vehicle (SV) clock is offset from its
| nominal rate by about -4.45x10^-10 (= -38 microseconds per
| day) to allow for the relativistic offsets between the
| differences between the SV and the ground. Of this -38
| microseconds per day, about -45 are due to the gravitational
| potential difference between the SV at its mean distance and
| the earth's surface, and +7 to the mean SV speed, which is
| about 3.87 km/sec. To this mean correction, each receiver
| must add a term due to the eccentricity of the GPS orbit.
| userbinator wrote:
| The "relativistic effects" _is_ part of how GPS works.
| karolist wrote:
| 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.
| salty_biscuits wrote:
| 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.
| drfuchs wrote:
| Exercise for flat-earthers: how does the GPS mapping on your
| phone work, without satellites orbiting a spherical Earth? Show
| your work.
| eru wrote:
| 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.
| mcmoor wrote:
| 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.
| cmsj wrote:
| In my experience, flat earthers are not capable of the kind of
| rigorous enquiry that would allow them to even attempt to
| answer this.
|
| Flat earth isn't a position arrived at by reason, it's one that
| is invariably reached by either confusion, or as a necessary
| consequence of some unshakable core belief (typically either an
| extremely literal reading of the bible, or a paranoid
| "everything official is a lie" delusion).
| yboris wrote:
| I came across a researcher working on collecting GPS data on
| aquatic creatures that only rarely (and for a short time) come to
| the surface. By recording raw data and post-processing, the power
| consumption and minimum duration of exposure to satellite signal
| are both drastically reduced (duration is under a second).
| zokier wrote:
| Next step PPP, and/or RTK. GNSS provides delightful rabbithole of
| possibilities.
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