The Physics of GPS

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

even though the concepts are straightforward, the implementation requires great care in order to maintain and extract the required precision. Throw that tech into space takes everything to an even higher level requiring radiation hardening, weight management, and long term reliability. You can't send repair crews to fix them if they break. As an engineer, I am in awe of those who design and build these things.

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

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.

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.

Just receiving on a radio is often a surprisingly expensive operation, especially for a small battery powered device that is otherwise well optimized, and especially when you need to receive continuously like with GPS.

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.

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.

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.

I've noticed very similar degradations in performance moving from a Pixel 3XL to a 7A.

When disabling "Android intelligence" (iirc this is what runs WiFi location scanning in the background) from running in the background, the 7A would take much, MUCH longer to get a GPS lock on a window sill, vs the 3XL right next to it... as well as lower SNRs and fewer satellites seen and used.

I don't get it.

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

I guess we are using mems pressure sensors for altitude these days.

> 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'.

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.

> the cell can't know when the signal was transmitted without some prior time or location knowledge?

The signal that GPS satellites broadcast includes the current time, so knowing when the signal was transmitted is easy.

Knowing when it was received is harder, you would need a calibrated, synchronized clock. But you need to receive multiple signals to figure out your clock, so keep reading for that.