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Show HN: Building a GPS receiver

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Re: Show HN: Building a GPS receiver

#61

Post by AnarchismIsCool »

jeffbee wrote: post #39
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?

Mostly in the same way the average person would assume "Hacker News" is anything but a place for SV investment bros to hang out and become rabid Libertarians.

Functionally on most devices losing network coverage renders GPS useless. I keep telling people to download OsmAnd if they want to be able to view maps on a plane or get home from their hike outside cell range. Google maps will try to cache maps to some degree nowadays but it tends to be very flakey and it seems to be very easy to accidentally get it to drop its cache when you're outside cell coverage.

Re: Show HN: Building a GPS receiver

#62

Post by ruuda »

> Have you ever noticed that your Maps app still works during a flight?

I noticed the opposite, it always fails to locate any satellites, even when GPS is still turned on in aeroplane mode. I'm not sure why.

Re: Show HN: Building a GPS receiver

#63

Post by opello »

nomel wrote: post #55
magnat wrote: post #37

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…

> takedown of passive radar GNU Radio module made by Kraken RF team. https://hackaday.com/2022/11/19/open-source-passive-radar-ta...

I did some brief searching of their Twitter feed and in general but couldn't find any update on this. Does anyone know the current state of passive radar and KrakenSDR?

[1] https://www.rtl-sdr.com/sdrdue-updated-passive-radar-softwar...

Comment thread from 2023-02-10:

> We are attempting to clarify if it is legal for us (KrakenRF, a US company that provides a physical SDR product) to also provide our own open source software that is made by us. As that could be seen as providing a full PR system.

Is the latest I found from them.

Re: Show HN: Building a GPS receiver

#64

Post by Rebelgecko »

darkhorn wrote: post #21
Rebelgecko wrote: post #15

Earlier quoted context omitted.

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...

I'm having a hard time telling from the article - are the ships' GPSes actually reporting that they're at the airport? Or is the disconnect happening somewhere between GPS and AIS (eg they're intentionally transmitting bad location data- it's been common-ish in the past for ships to do this when doing something naughty (blockade running or illegal fishing) and given what's going on in the region I can see the value of keeping your location obscure for safety

Re: Show HN: Building a GPS receiver

#65

Post by error503 »

ruuda wrote: post #62

> Have you ever noticed that your Maps app still works during a flight? I noticed the opposite, it always fails to locate any satellites, even when GPS is still turned on in aeroplane mode. I'm not sure why.

You're more or less inside a Faraday cage. I find you'll generally get a strong lock just holding your phone near the window, though TTFF can be relatively slow compared to normal due to the lack of AGPS.

Re: Show HN: Building a GPS receiver

#66

Post by londons_explore »

amluto wrote: post #47
elevation wrote: post #44

Earlier quoted context omitted.

> there won’t be any leakage of the LO that an enemy device could try to detect Why would an LO be more of an issue than your sample clock? edit: missing word

I don’t know all the details of this kind of technology, but I would imagine that one direct RF receiver’s sample clock looks effectively identical to any other similar receiver’s sample clock. So if these devices become popular, then a military sample clock is indistinguishable from a civilian sample clock. In contrast, an LO is rather application-specific.

For military use, where you are trying to blend in with civilian equipment, either the direct sampling clock, or the local oscillator frequency, could be randomly chosen in quite a wide range at bootup and still have the device work.

In todays world with everything software reconfigurable, changing the sampling rate or local oscillator frequency is very do-able.

Re: Show HN: Building a GPS receiver

#67

Post by londons_explore »

opello wrote: post #63
nomel wrote: post #55

Earlier quoted context omitted.

> takedown of passive radar GNU Radio module made by Kraken RF team. https://hackaday.com/2022/11/19/open-source-passive-radar-ta...

I did some brief searching of their Twitter feed and in general but couldn't find any update on this. Does anyone know the current state of passive radar and KrakenSDR? [1] https://www.rtl-sdr.com/sdrdue-updated-passive-radar-softwar... Comment thread from 2023-02-10: > We are attempting to clarify if it is legal for us (KrakenRF, a US company that provides a physical SDR product) to also provide our own open source…

The lawyer likely advised not talking about it more publicly.

Almost any lawyer won't present the world as black and white, but rather in quantities of risk - and even saying "we've taken the project down and it wont be coming back" is a risk if that attracts attention to your past distribution of the software and causes others to mirror it from archives.

Re: Show HN: Building a GPS receiver

#68

Post by error503 »

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…

> making everyone appear at exactly the same location would be nigh-impossible, though.

I don't think this is actually the case. In a spoofing scenario, all of the rogue signals would typically be generated by a single terrestrial station. The time of flight of all of the generated signals will be the same, so all that matters is the position solution reflected in the transmitted signals, as the fundamental principle of GPS based on TOF is no longer in play. So I'd think that in a typical spoofing scenario, all receivers thinking they're in more or less an identical location is what you'd expect.

It might be possible in a borderline case for the receiver to receive some spoofed signals and some real signals simultaneously, in which case you'd expect weird results, but I think you'd definitely see a correlation around the position being broadcast by the spoofer.

Re: Show HN: Building a GPS receiver

#69

Post by Scoundreller »

londons_explore wrote: post #67
opello wrote: post #63

Earlier quoted context omitted.

I did some brief searching of their Twitter feed and in general but couldn't find any update on this. Does anyone know the current state of passive radar and KrakenSDR? [1] https://www.rtl-sdr.com/sdrdue-updated-passive-radar-softwar... Comment thread from 2023-02-10: > We are attempting to clarify if it is legal for us (KrakenRF, a US company that provides a physical SDR product) to also provide our own open source…

The lawyer likely advised not talking about it more publicly. Almost any lawyer won't present the world as black and white, but rather in quantities of risk - and even saying "we've taken the project down and it wont be coming back" is a risk if that attracts attention to your past distribution of the software and causes others to mirror it from archives.

> quantities of risk

Even that’s asking for a lot.

Qualities of risk is more likely.

Re: Show HN: Building a GPS receiver

#70

Post by tialaramex »

michaelt wrote: post #52
tialaramex wrote: post #23

Earlier quoted context omitted.

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…

You don't need to brute force the almanac - why would you? But it's very much feasible to 'brute force' your initial signal lock by searching for all gold codes at a range of frequency offsets. And it doesn't take 12.5 minutes to get the ephemerides - the almanac is sent in paginated form which is why it takes so long, the ephemerides are sent more often - they repeat every 30 seconds, and they're enough for a naviga…

> You don't need to brute force the almanac - why would you?

I have no idea, but the claim was that you get faster fix with brute force when I know that's not why it's fast in practice.

> But it's very much feasible to 'brute force' your initial signal lock by searching for all gold codes at a range of frequency offsets.

I hadn't even imagined this constituting "brute force". Is my phone using "brute force" to find the WiFi router? At some point it's not really "Brute force" it's "There are a handful of options, try all of them" and GPS seems past that point especially on modern hardware.

This actually reminded me of a (possibly no longer extant) design choice in Encrypted Client Hello - we don't necessarily know if the encryption was done with key F we gave out yesterday afternoon or key G which we just began using an hour ago, do we need a way to signal that in the connection? No, just try all valid keys. If you can't afford to try more than two keys, make sure you only roll them slowly so you won't need to.

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