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

Show HN: Building a GPS receiver

axleos.com

Post reply on HN
Print view View original

21–30 of 135 posts

Re: Show HN: Building a GPS receiver

#21

Post by darkhorn »

Rebelgecko wrote: post #15
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?

Regular jamming just degrades the signal quality. You can blast out fake data, but depending on what you mean by "large area" and a "point", I don't think what youre suggesting is possible. To trick GPS receivers you end up broadcasting fake signals from multiple GPS satellites, so receivers in different areas will be processing it differently and come up with different coordinates.

So does this means that in recent days there wasn't GPS jamming but GPS satalites were broadcasting fake signals? In south Turkey people were appearing to be in Beirut airport.

Also see this one: https://www.lloydslist.com/LL1148748/War-zone-GPS-jamming-se...

Re: Show HN: Building a GPS receiver

#22

Post by pictureofabear »

codyd51 wrote: post #14
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?

GPS jamming or spoofing on a wide scale is definitely a viable attack! As another commenter noted well, making everyone appear at exactly the same location would be nigh-impossible, though. GPS receivers will look for the 'strongest' PRN signal in the noise, so broadcasting louder than the (incredibly weak!) C/A signal is a valid way to jam or spoof GPS. It is, however, generally illegal for civilians. GPS receivers…

Just to be a little clearer, jamming and spoofing are the not the same thing. Jamming obfuscates a signal, usually through noise. Spoofing impersonates the signal.

Re: Show HN: Building a GPS receiver

#23

Post by tialaramex »

codyd51 wrote: post #11
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…

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 a single channel receiver and minute compute capability) but they more importantly include the ephemerides, precise data about exactly where the birds are and the atmospheric conditions, replaced hourly by a ground station. You can't "brute force" these - they're parameters measured by someone with objective truth like "I, a massive NASA satellite ground terminal in Florida, am definitely not moving, therefore this GPS bird #14 is 0.08 metres away from where it should be, I will adjust the data for the next hour accordingly".

Re: Show HN: Building a GPS receiver

#24

Post by tylerchr »

Super impressive. Can’t agree more with the author that GPS is a stunningly clever engineering achievement.

For those interested in the story of the development of GPS, I found “GPS Declassified” by Richard Easton to be an engaging retelling.

Re: Show HN: Building a GPS receiver

#25

Post by pictureofabear »

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?

You are talking about spoofing, not jamming.[1]

This may be theoretically possible but is, in reality, practically impossible.

Embedded within the GPS signal is the ephemeris data which, among other things, includes each satellite's location in space.

Receivers calculate position by calculating the difference between the time a signal was received and the time stamp encoded in the signal itself.

By analyzing the signals from a minimum of four satellites (one for each dimension in time and space), a receiver calculates where it is.

To spoof all phones on Earth, you would need to trick each receiver individually. Since receivers are passive, they don't identify themselves, and there would be no way to target each individual receiver, making them think they're somewhere they're not.

1. Jamming is obfuscating a signal, usually by creating a lot of noise that makes the real signal hard to find. Spoofing is impersonating a signal.

Re: Show HN: Building a GPS receiver

#27

Post by aaronax »

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…

Related: Satellite-based augmentation system (SBAS), of which Wide Area Augmentation System (WAAS) is the USA implementation.

https://en.wikipedia.org/wiki/GNSS_augmentation

Re: Show HN: Building a GPS receiver

#28

Post by michaelt »

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 might track 12.

So this software-defined receiver actually implements quite a bit of what would otherwise be hardware. And of course it can track every satellite in view.

The software-defined approach has some powerful benefits - for example, initial satellite acquisition involves calculating cross-correlation between the received signal and various gold codes. Being able to do this in the fourier domain lets you acquire signals pretty fast!

If you want a hardcore DIY GPS receiver, going right down to the transistor level, you'd probably enjoy reading https://lea.hamradio.si/~s53mv/navsats/theory.html - an 1990s era DIY GPS receiver, complete with hand-drawn schematics, hand-drawn PCBs, even a hand-made antenna.

Re: Show HN: Building a GPS receiver

#29

Post by shrx »

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…

“If you wish to make an apple pie from scratch, you must first invent the universe.”

― Carl Sagan, Cosmos

Re: Show HN: Building a GPS receiver

#30

Post by progbits »

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'm actually glad it uses RTL-SDR, I wasn't aware it was good enough to get interesting results.

Ever since I've seen the project by Andrew Holme (mentioned in sibling comments) years ago it has been on my wish list to replicate, but analog/RF signals are dark magic to me.

Now I feel like I can skip the hard RF frontend bit and play with the software by using the SDR I already have.

Post reply on HN