The Physics of GPS

throw0101a

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...

TravisLS

I love these incredibly simple and elegant classic technologies. GPS is one of the best. It seems like it would be incredibly complicated and mysterious, but it's actually quite straightforward. I'm working on a presentation now to explain how GPS works to second graders. If they understand it, I'll take some photos and do a write-up.

dmk

The fact that they deliberately manufacture the satellite clocks to tick at the wrong frequency on the ground (10.22999999543 MHz instead of 10.23 MHz) so that relativity makes them tick correctly in orbit is one of my favorite engineering details in any system.

ikidd

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.

magneticnorth

A slightly related question, if anyone knows - has phone GPS gotten worse in recent generations? More reliance on local wifi networks or something like that? I ask because I do a lot of backcountry hiking, camping, and foraging and rely on true GPS-only navigation. My most recent two phones (iphone and pixel) have noticeably worse GPS performance than previous phones, and I even changed OS ecosystems mostly hoping for better GPS, but it didn't help. Maybe I've had bad luck, but two noticeably bad phones in a row seems like it may be a pattern. And is there any way to find phones with very good GPS performance?

ck2

Wish they could solve the GPS altitude weakness Watches that use GPS for altitude are terribly inaccurate It is interesting to run the opensource GPSTEST app on a smartphone and watch the MSL "settle" over time but each sat seems to disagree * https://github.com/barbeau/gpstest btw watches are now getting THREE multi-band L1+L5 GPS chipsets, should help things quad-band GNSS coming soon too! * https://the5krunner.com/2026/03/06/tri-band-gps-garmin/

Lukas_Skywalker

The explanation about the spheres is slightly inaccurate. With one satellite, you won't get a circle on the earths surface, but a sphere. GPS is not constrained to earths surface (or the oblate spheroid approximating it), luckily.

seanalltogether

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.

petee

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?

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.

linzhangrun

In principle it's actually easy to imagine: the speed of light is known, the orbital positions are known, the time differences give distances, and then it's just trilateration with three spheres. The idea isn't complicated. Engineering-level precision that’s hard to imagine: light can circle the Earth seven times in one second, so tiny deviations make a world of difference — the atomic clocks on GPS even have to account for relativistic effects. So cool!