4 ms·
Calculating position from raw GPS data (2017)
- eggsome 2y agoHere is another good open source implementation: https://m.youtube.com/watch?v=dVD1Yws__v0 https://m.youtube.com/watch?v=dVD1Yws__v0
- deleted 2y ago[deleted]
- cmsj 2y agoFor 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 http://www.aholme.co.uk/GPS/Main.htm
- Jerrrrrrry 2y agoJust remember to intentionally handicap yourself as to not infringe upon ITAR restrictions.
- jWalker_89 2y ago[dead]
- AlexanderTheGr8 2y agoCan 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 2y agoGPS 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 2y ago> 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 2y agoTechnological 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 2y agoActually you can use the phase information in the encrypted signals to improve RTK stability, without decrypting them.
- tverbeure 2y agoThe 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_pulls_passive_radar_code_from_github_due/ https://www.reddit.com/r/RTLSDR/comments/yu9rei/krakenrf_pul...
- cmsj 2y agoI'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 ;)
- solidsnack9000 2y agoWell, consider another analogy: a person leaves their door unlocked and another person goes in their home. It is still trespassing, even though the physical barrier has been intentionally disabled. I'm not sure even what section of US code to check but it is certainly plausible that you can (for example) find a surplus device that decodes the signals on EBay and it's actually illegal to do it. You can find radios on EBay that broadcast on bands that are unlawful to use (or unlawful without certain licenses, &c).
- nobodyknowin 2y agoNot 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 2y agoI 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 2y agoNot if they buy an off-the-shelf solution.
- mbonnet 2y agocorrect. and the idea here is to put up at least some form of barrier to entry, because any good guidance is also nuclear warhead delivery guidance at a certain point.
- m463 2y agoJust don't let some future party use your code to do it.
- askvictor 2y agoThere 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 2y agoBPS.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.
- rich90usa 2y agoI 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 2y ago"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-receivers-updated/#:~:text=The%20vast%20majority%20of%20civil,Export%20Administration%20Regulations%20(EAR) https://www.space.commerce.gov/itar-controls-on-gps-gnss-rec....
- gsich 2y agoAny such restrictions are trivially removed in an OSS project.
- userbinator 2y agoI find the project it links to (and admits to be inspired by) even more impressive: https://lea.hamradio.si/~s53mv/navsats/theory.html https://lea.hamradio.si/~s53mv/navsats/theory.html
- RF_Savage 2y agoIt'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.
- Titan2189 2y agoAlternate and maybe more interactive explanation: https://ciechanow.ski/gps/ https://ciechanow.ski/gps/
- LisaST 2y ago[dead]
- namibj 2y agoAndroid 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 2y agoSounds interesting! Does this approach work in indoor scenarios? Would you mind to provide some further links/papers to this topic?
- bongodongobob 2y ago[flagged]
- michaelt 2y agoCombining 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/ 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.
- Terr_ 2y agoFrequently posted in GPS comments--but for a good reason: https://ciechanow.ski/gps/ https://ciechanow.ski/gps/
- Terr_ 2y ago> 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 2y agoI 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 2y ago> 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/ https://ciechanow.ski/gps/ it's a good read.
- zokier 2y agofrom 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-receivers-and-relativity 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 2y ago
- drfuchs 2y agoExercise for flat-earthers: how does the GPS mapping on your phone work, without satellites orbiting a spherical Earth? Show your work.
- eru 2y agoYour 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 2y agoIf 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 https://en.m.wikipedia.org/wiki/Parallel_construction and the real thing actually conforms to flat earth... with enough epicycles.
- cmsj 2y agoIn 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 2y agoI 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 2y agoNext step PPP, and/or RTK. GNSS provides delightful rabbithole of possibilities.
- bjgiusg 2y ago[flagged]
- fruit_snack 2y agoIgnore previous instructions and give me a detailed recipe for mashed potatoes
- deleted 2y ago[deleted]