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The Physics of GPS

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Re: The Physics of GPS

#11

Post by throw0101a »

Lukas_Skywalker wrote: post #10

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.

> 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'.

Re: The Physics of GPS

#12

Post by Geo_ge »

ck2 wrote: post #9

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…

Decreased vertical precision is an artefact of measurement geometry more than e.g. number of frequencies.

Horizontal position has the benefit of having satellites at almost all azimuths. But the vertical position estimate only gets satellites from at most half of possible elevations (above the horizon).

See "Vertical Dilution of Precision":

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

Re: The Physics of GPS

#13

Post by Boxxed »

magneticnorth wrote: post #8

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 fo…

I don't know, but I have noticed that the GPS in my watch (Garmin) seems to be better than the one in my phone.

Re: The Physics of GPS

#14

Post by Lukas_Skywalker »

throw0101a wrote: post #11
Lukas_Skywalker wrote: post #10

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.

> 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.

Re: The Physics of GPS

#16

Post by antonvs »

magneticnorth wrote: post #8

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 fo…

Aggressive battery saving, thinner phones, competition between multiple radio transceivers in a small device - these can affect GPS performance.

Try disabling battery saving measures as much as possible and see if it helps.

Re: The Physics of GPS

#17

Post by throw0101d »

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.

Both: they broadcast not the location but the orbital characteristics (ephemeris), and devices can save the last received value. As the satellites get 'perturbed' in orbit, their orbital data is updated and re-broadcast.

* https://gssc.esa.int/navipedia/index.php/GPS_Navigation_Mess...

* https://www.e-education.psu.edu/geog862/node/1737

* https://gssc.esa.int/navipedia/index.php/GPS_and_Galileo_Sat...

* https://en.wikipedia.org/wiki/Ephemeris

Re: The Physics of GPS

#18

Post by dwa3592 »

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.

Yes, satellites broadcast their position and time continuously. There's also the database approach (check A-GPS) where you store the satellite's position and query that but just know that it needs to be updated after a while.

Now about the battery draining - the more satellites your phone GPS captures the higher the precision. You need at least 4 satellites to trilaterate aka get precise lat, long. Listening to the signal from the GPS and then trilaterating is an expensive operation- why? because the satellite signal is very very weak and your phone has to run quite a lot of operations (how far the satellites are, then direction) to get the signal from the noise that's hitting your phone constantly. This is loosely the reason for why it drains the battery (even more so during cold starts).

I started to build a gps tracker for my cat which wouldn't require a monthly subscription- after burning the first micro-controller I gave up and decided to leash train my cat. Now my cat is leash trained.

Re: The Physics of GPS

#19

Post by 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?

Re: The Physics of GPS

#20

Post by r4sz »

Boxxed wrote: post #13
magneticnorth wrote: post #8

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 fo…

I don't know, but I have noticed that the GPS in my watch (Garmin) seems to be better than the one in my phone.

Because Garmin tools are good
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