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

The Physics of GPS

perthirtysix.com

Post reply on HN
Print view View original

21–30 of 40 posts

Re: The Physics of GPS

#21

Post by saltcured »

seanalltogether wrote: post #15

Do the satellites broadcast their own position, or is that all held in a database on your phone? Also why is it so draining on your battery to get GPS location, if it's just solving a simple calculation.

The information necessary for a fix is broadcast. The locally stored database helps improve the time to get a fix. A GPS receiver, going from a cold start, needs to listen for many minutes to acquire and decode enough signals to have the required satellite position and timing information to do the calculation.

Most of the power consumption is for the radio reception that has to detect and decode signals from multiple constantly shifting sources, dealing with their very low signal-to-noise ratios and other challenges like multipath distortion due to atmosphere and surface reflections.

It's pretty remarkable how much miniaturization has improved the efficiency of these radios. E.g. going from the early "portable" GPS units that essentially had a lead-acid car or motorcycle battery to today's wearables that run on a tiny power budget while supporting a wider range of satellite constellations and radio bands.

Re: The Physics of GPS

#22

Post by oofbey »

petee wrote: post #19

Maybe i missed it, but the first step kinda skips over how the inital time is calculated - the cell can't know when the signal was transmitted without some prior time or location knowledge?

Good catch. The trick is you don’t need a good clock on the phone. Really all you’re measuring is the difference in time signals between the satellites. The clocks on the satellites are (effectively) perfectly synced with each other. So what you measure is that one satellite is ### meters further away from another. Not absolute distance to each satellite.

It means you need to connect to one more satellite to remove that extra degree of freedom. If your phone had an atomic clock you could get your absolute position in 3D only listening to three GPS satellites, but because of local clock skew you need a signal from a fourth satellite.

Re: The Physics of GPS

#24

Post by empiricus »

I found much more interesting the way the gps electronics work. What do you mean you need to know the exact moment you receive a message from a satellite with nanosecond precision? when the message itself is several seconds long.

Re: The Physics of GPS

#25

Post by [deleted] »

petee wrote: post #19

Maybe i missed it, but the first step kinda skips over how the inital time is calculated - the cell can't know when the signal was transmitted without some prior time or location knowledge?

[deleted]

Re: The Physics of GPS

#26

Post by UltraSane »

Waterluvian wrote: post #7
ikidd wrote: post #5

Also, RTK is an interesting way to correct the signal to get sub-centimeter accuracy. Using the timing differences between satellites with a stationary unit and then sending the that to the rover is a cool workaround and can be used without expensive equipment now.

VRS RTK can even get 1cm RMS without needing a stationary unit. Just need atmospheric correction data for your approx location. Which has been amazing for outdoor mobile robotic applications.

They sell robotic lawn mowers with LIDAR and RTK GPS. LIDAR combined with RTK GPS is very neat because you can place every point in the point cloud on earth's surface.

Re: The Physics of GPS

#28

Post by dan-robertson »

throw0101a wrote: post #2

Reminder of Bartosz Ciechanowski "GPS" article: * https://ciechanow.ski/gps/ * 2022: https://news.ycombinator.com/item?id=29981188 * 2023: https://news.ycombinator.com/item?id=36180316 * Others: https://news.ycombinator.com/from?site=ciechanow.ski Standford's "An Introduction to Satellite Navigation" course is also instructive (recorded 2014): * https://www.youtube.com/playlist?list=PLGvhNIiu1ubyEOJga50LJ...

I like http://lea.hamradio.si/~s53mv/navsats/theory.html which has a different focus from the Ciechanowski article.

Re: The Physics of GPS

#29

Post by userbinator »

dan-robertson wrote: post #28
throw0101a wrote: post #2

Reminder of Bartosz Ciechanowski "GPS" article: * https://ciechanow.ski/gps/ * 2022: https://news.ycombinator.com/item?id=29981188 * 2023: https://news.ycombinator.com/item?id=36180316 * Others: https://news.ycombinator.com/from?site=ciechanow.ski Standford's "An Introduction to Satellite Navigation" course is also instructive (recorded 2014): * https://www.youtube.com/playlist?list=PLGvhNIiu1ubyEOJga50LJ...

I like http://lea.hamradio.si/~s53mv/navsats/theory.html which has a different focus from the Ciechanowski article.

That's a 404 for me, but I think the site moved; this works: http://s53mv.s5tech.net/navsats/theory.html

Re: The Physics of GPS

#30

Post by throw0101a »

Lukas_Skywalker wrote: post #14
throw0101a wrote: post #11

Earlier quoted context omitted.

> With one satellite, you won't get a circle on the earths surface, but a sphere. With one satellite you get a sphere in 3D space , but if you are on a surface (like that of the Earth), that gets translated into circle. If you are in a plane in the sky (3D space), then you get a spherical 'location fix'.

True. But the GPS receiver doesn't know whether you are on the surface or not (and at what elevation), so it must always assume 3d space, hence a sphere.

Not wrong, but I think most folks are interested in lat-long.

If you have lat-long and place it on a pre-canned map, then it's unlikely that most people will be either underground or in the air, and so whatever the surface elevation of the lat-long on the map is, that's most likely (>99% of the time?) where the person is.

At least from a UX/responsiveness perspective, this is probably a good way to do the math. (In the background get a more accurate 3D fix.)

Post reply on HN