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Show HN: Building a GPS receiver

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Re: Show HN: Building a GPS receiver

#91

Post by seba_dos1 »

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…

> GPS satellites don't send down a location, they only send down time

They do send a location - to be more exact, they send rough locations of all satellites in the constellation (almanac) and precise location of themselves (ephemeris). Devices like mobile phones, however, usually get that data from other sources because it's much faster than listening to data over GPS.

Re: Show HN: Building a GPS receiver

#92

Post by seba_dos1 »

nix0n wrote: post #51
jeffbee wrote: post #39

Earlier quoted context omitted.

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?

There's a difference between GPS vs A-GPS (Assisted GPS)[0][1]. Since A-GPS uses the cell tower to get the list of satellites in view, the GPS on some cellphones will keep working when cell service is lost but won't start working if cell service is unavailable. I think this means my Samsung doesn't actually have GPS, since fallback to unassisted GPS has never worked for me (yes, I've tried waiting far longer than 15…

> I think this means my Samsung doesn't actually have GPS

Usually this just means that it has a weak antenna or catches too much noise, which isn't as much of an issue when assisted. It will probably work just fine unassisted under perfect conditions, but struggles otherwise. Apparently it's pretty common for modern phones, but there are exceptions.

Re: Show HN: Building a GPS receiver

#93

Post by seba_dos1 »

kube-system wrote: post #40
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.

Also cellular towers.

Re: Show HN: Building a GPS receiver

#94

Post by AlexanderTheGr8 »

sizzzzlerz wrote: post #18

Brilliant! I have no idea what the technical background of the author is but for anyone to tease apart the vast, complex, details of the GPS universe is a massive feat. Coupled with his ability to craft software to both assist his analysis and to implement the final solution, he has created a magnificent project. I've been studying GPS and worked with it professionally for a number of years and I still don't know eve…

"Interestingly, the only thing stopping civilians from using the P code is the knowledge of the value of its chipping sequence. If the formula to generate the P code was publicly known, there’d be nothing stopping civilian GPS receivers from locking on to it, with the exact same techniques as are used for the C/A code."

I didn't finish reading the whole thing but was curious. Is there any way of brute forcing it or some other trick to get the chipping sequence to get the P code for more precise GPS?

Re: Show HN: Building a GPS receiver

#95

Post by AlexanderTheGr8 »

"Interestingly, the only thing stopping civilians from using the P code is the knowledge of the value of its chipping sequence. If the formula to generate the P code was publicly known, there’d be nothing stopping civilian GPS receivers from locking on to it, with the exact same techniques as are used for the C/A code."

I didn't finish reading the whole thing but was curious. Is there any way of brute forcing it or some other trick to get the chipping sequence to get the P code for more precise GPS?

Re: Show HN: Building a GPS receiver

#98

Post by RF_Savage »

nix0n wrote: post #51
jeffbee wrote: post #39

Earlier quoted context omitted.

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?

There's a difference between GPS vs A-GPS (Assisted GPS)[0][1]. Since A-GPS uses the cell tower to get the list of satellites in view, the GPS on some cellphones will keep working when cell service is lost but won't start working if cell service is unavailable. I think this means my Samsung doesn't actually have GPS, since fallback to unassisted GPS has never worked for me (yes, I've tried waiting far longer than 15…

One samsung phone I had problems per software in getting a gps lock in it's last years of use.

Fancier software would sit there forever trying to get lock, but more primitive one's could get a lock and after that the fancier ones could as well.

This meant I got to "boot" the lock by hamgps first and then I could switch to maps and other software.

Re: Show HN: Building a GPS receiver

#99

Post by OmarShehata »

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?

absolutely, and most people do! I encourage you to go around asking your friends and family this question (without judgement, and see if you can prod them along to the right answer on their own!) i think the world is better when the average person has a more accurate understanding of the tools they use

(and contrary to popular belief, I think the average person is interested in understanding this, they're more motivated to understand things that actually matter to them, that are in their hands every day. A lot just have an emotional aversion to math due to bad school experiences, but they are genuinely curious)

Re: Show HN: Building a GPS receiver

#100

Post by globalise83 »

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?

I can tell you with high confidence that a very large number of people have no mental model of what GPS is at all.
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