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Show HN: Building a GPS receiver

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Re: Show HN: Building a GPS receiver

#31

Post by codyd51 »

tialaramex wrote: post #23
codyd51 wrote: post #11

Earlier quoted context omitted.

Hi! Yes, "from scratch" is definitely always a bit of a funny term. I also implemented the receiver in Python, which is quite far from "scratch" =). What I mean by it in this context is that I'm taking a piece of hardware that knows nothing about GPS, and just has the ability to sample the EM field, and building up a receiver from there. Re. slow TTFF, or time-to-first-(position)-fix on older hardware, this essential…

The fast TTFF on a typical modern device e.g. your phone is because the device has the Internet, and so it can obtain all the information it needs from the Internet up front, it isn't magically brute forcing everything needed, that's not practical at all. The 12.5 minutes includes a rough multi-week almanac which you could perhaps brute force given available compute and receive capability (original GPS receivers have…

Yes, I forgot to mention downloading the orbital parameters over the network! Thanks for mentioning this as well.

In this case, I was meaning to refer to brute-forcing the Doppler-shifts and PRN phases of each satellite, not the orbital parameters themselves. The project in the OP is able to get a position fix in less than a minute because, if the subframe timings are convenient, you can retrieve the necessary ephemeris parameters from the subframes in that span (and down to as little as 18 seconds in ideal conditions, if my back-of-the-napkin is right).

Re: Show HN: Building a GPS receiver

#32

Post by [deleted] »

AdamJacobMuller wrote: post #5

Cool article. Whenever I see "from scratch," I'm always curious to see how from scratch the author actually means so I'll admit I was a bit disappointed to see that the hardware was just RTL-SDR. Still, the protocol decoding was very interesting and the result is great. > GPS was launched in 1978, which was 45 years ago at time of writing. Five billion people are currently under 40 years old, so well over half the wo…

[deleted]

Re: Show HN: Building a GPS receiver

#35

Post by amluto »

> Just one problem: you won’t find any SDR on the market that will claim to be able to sample a wave oscillating over a billion times a second.

This was true, but not any more. You can get truly impressive “direct RF sampling” or “direct RF conversion” receivers that are more than fast enough for GPS. For example:

Xilinx RFSoc: https://www.mouser.com/datasheet/2/903/ds889_zynq_usp_rfsoc_...

A nice National Instruments article: https://www.ni.com/en/solutions/aerospace-defense/radar-elec...

And their referenced off-the-shelf hardware: https://www.ni.com/en-us/shop/category/flexrio-custom-instru...

One might be forgiven for being a bit puzzled as to why NI thinks that direct RF conversion is cost-effective but nonetheless sells the device for $30k :) That being said, if I were prototyping a system that wanted phase-coherent wideband reception around 3 GHz and I had a proper lab and budget, I’d buy a few of these. If I were to go to production, I’d either wait for costs of a homemade board to come down a bit or see whether a traditional heterodyne receiver could do the trick.

Hmm. For military applications, if I were concerned about really advanced RF-seeking weapons pointed at me, a direct conversion receiver is probably great — there won’t be any leakage of the LO that an enemy device could try to detect.

Re: Show HN: Building a GPS receiver

#36

Post by OmarShehata »

Amazing! I also had exactly the same experience that led me to research this a few years ago, realizing that:

- GPS works even in airplane mode (while on a literal airplane) - It works without cell service, or wifi, or anything - The United States of America controls the GPS constellation, and they can (and have!) turned off GPS off certain regions at will when necessary (which has prompted other countries to launch their own GNSS constellations) - GPS satellites don't send down a location, they only send down time

I think it's a really fun exercise to do this with data you receive on your phone. Your phone has a direct link to satellite.

(side note: I recently learned the basic principles of star navigation, and while it is a completely different mechanism, it also relies very much on keeping accurate time, which I thought was a fun symmetry!)

Re: Show HN: Building a GPS receiver

#37

Post by magnat »

Note that GPS receiver capable (i.e. not artificially limited) of providing navigation data while moving 600 m/s or higher used to be considered munition by ITAR. The amount of legalese at updated ruling [1] is well beyond what I can make sense of, to the point I don't even know if it still applies.

While we're at SDRs, ITAR is also responsible for takedown of passive radar GNU Radio module made by Kraken RF team.

[1] https://www.space.commerce.gov/itar-controls-on-gps-gnss-rec...

Re: Show HN: Building a GPS receiver

#38

Post by makomk »

michaelt wrote: post #28
AdamJacobMuller wrote: post #5

Cool article. Whenever I see "from scratch," I'm always curious to see how from scratch the author actually means so I'll admit I was a bit disappointed to see that the hardware was just RTL-SDR. Still, the protocol decoding was very interesting and the result is great. > GPS was launched in 1978, which was 45 years ago at time of writing. Five billion people are currently under 40 years old, so well over half the wo…

> I'll admit I was a bit disappointed to see that the hardware was just RTL-SDR. Somewhat surprisingly, if you went back 15-20 years, a lot of what the author is doing in software here would have been done in hardware. GPS receivers used to market themselves by the number of tracking channels they had, as cheaper receivers might only have the hardware needed to track 6-8 satellites while a more expensive receiver mig…

I think even these days, a lot of what the author is doing here is still generally done in hardware for power efficiency reasons - it's just that nowadays, the hardware looks a lot like the software-defined architecture used here with a conversion to digital at the frontend followed by hardware accelerators for doing cross-correlation in the fourier domain. Actually, this kind of approach of implementing the architecture an SDR-based receiver would use as specialised hardware seems to be pretty common in general nowadays.

Re: Show HN: Building a GPS receiver

#39

Post by jeffbee »

OmarShehata wrote: post #36

Amazing! I also had exactly the same experience that led me to research this a few years ago, realizing that: - GPS works even in airplane mode (while on a literal airplane) - It works without cell service, or wifi, or anything - The United States of America controls the GPS constellation, and they can (and have!) turned off GPS off certain regions at will when necessary (which has prompted other countries to launch…

A person would have to have a quite flawed mental model of what GPS is to form the belief that it would stop working without data service, wouldn't they?

Re: Show HN: Building a GPS receiver

#40

Post by kube-system »

darkhorn wrote: post #9

Is it possible to trick phones with a jammer in a large area to make it look like all phones are in a specific point on Earth. Only with a jammer?

It’s not really that simple. Most phones these days do not rely solely on GPS for location data. Many use a combination of WiFi, Bluetooth, dead reckoning, GPS, Galileo, GLONASS, QZSS, and BeiDou.

Even if a couple of these signals are degraded, wrong, or missing, most phones will come up with a relatively accurate location using the remaining data.

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