Ive been working with GNSS for the last decade and a bit, and this article and the comments here are a low point for HN in terms of people making comments about stuff they dont understand.
PSA: if you want to learn about corrected GNSS, stop reading this thread now. I started replying to some comments then gave up, most here are more wrong than right.
This place selects for confident contrarian takes based on superficial knowledge.
I get really annoyed by how in nearly any physics or cosmology post, half the comments are a bunch of people smugly declaring that dark matter is just an imaginary fudge factor added by lazy dishonest scientists.
Get a ublox F9P board and antenna. Couple hundred dollars.
Set it up to do RTK using public CORS base stations (will give you 2cm apx accuracy). You will get a kick out of it.
Then next level is grab the latest rtklib, and do PPP processing.
The RTK step is simple and will boost your motivation to learn more. PPP is also easy to setup, but there are many things you can take to make it more accurate. Each one you will learn about something new. Satellite clock corrections, position corrections, ionosphere and troposphere modelling, receiver dynamics, tidal loading, wind up, etc. List goes on a lot
Once you've done that you'll have a 'feel' for it. From there you'll find your own resources based on what most interests you
And yea, it is fascinating. I'm not bored of it after all this time. New stuff keeps coming out. SSR is super interesting right now
Not so much. Modern GPS will get a first fix pretty quickly regardless, you don't need to seed it.
AGPS is used when satellite coverage is poor, like urban canyons in a city, also a situation where there are a lot of cell towers and triangulation is somewhat useful
AGPS refers to a specific set of technologies, MSB where the almanac and time seed is provided over the cellular network, and MSA where the unit sends GPS data to the AGPS server and the server solves for location using whatever data it would like (which can include a multitude of corrections).
I think what you're referring to with cell tower triangulation (and also WiFi location, etc.) is generally referred to as "hybrid location service" and is usually done at a higher level (ie - in software) using AGPS as one input to a sensor fusion algorithm.
The time to first fix is mainly impacted by whether the receiver knows which satellites to listen to. This requires two things: having the ephemeris for the constellation (either 10min of listening to GPS signal, or getting that through AGPS), and it's approximate own position/time (done through cell antenna locations, and NTP) to know which satellites should be visible.
Once this is available, the receiver can then focus on the signals that are expected and will then be able to provide a 3D fix as soon as it receives something from 4 satellites, which would be in a few seconds.
In the unlikely event any Aussie or NZ farmers are reading this, it costs less than $1k to setup your own base station and transmit corrections via a number of RF links. One base station is good to a radius of about 60km, so not enough for large ranches but fine for crops. You can setup as many as you need, then no reliance on L band SBAS
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PSA: if you want to learn about corrected GNSS, stop reading this thread now. I started replying to some comments then gave up, most here are more wrong than right.