GPS

I think you're talking about receiving a full almanac and ephemeris, and the parent is talking about post-processing. See my parallel comment about PP.

Even sidestepping the internet just handing you a full alm+eph dump, modern standalone receivers can perform a cold-start much faster than their predecessors, because they have huge numbers of receiver channels available. The system operators cleverly offset the almanac being transmitted by each satellite, so if you can receive several satellites at once, you can start writing your almanac with several pencils on the page writing different paragraphs, as it were. Finish the page very quickly.

In the early 90s, it was common for a GPS receiver to have just 4 channels. So a blind search through all the satellite PRNs could take quite a while, and since the receiver didn't know where anything was yet, Murphy's law guaranteed that any satellite it did get a lock on would soon disappear over the horizon anyway. It took agonizingly long to get lucky and hit a bird just coming into view, so you could get whole messages from it and start filling in that table.

And of course any obstructions that limited your sky-view just made it worse.

By the late 90s, 12-channel receivers were fairly common, my first was one of these. This greatly increased the odds of getting useful satellites in a reasonable period of time, and on cold-start it would get a fix pretty reliably in 15 minutes, sometimes less.

In all cases, if the user could give the receiver a hint of the current time (within a few minutes) and location (within a few degrees), as soon as it got part of the almanac it could start figuring out which satellites must be behind the Earth right now, versus which ones would likely be overhead, and make much better use of its receiver channels to shorten the TTFF. Additionally, being able to estimate the Doppler shift greatly shortens the lock-on period.

Today's receivers don't even have discrete radio channels in the old sense, they just have a wide RF front end and then slice the data into digital correlator pipelines, achieving hundreds of virtual channels. True "all-in-view" reception is possible even with four full constellations aloft, and it's nearly magical how good they are. Cold-start times under a minute in some cases.

HOWEVER.

A survey receiver, whose data is being post-processed, need not even calculate its own position. (It probably does, since that costs nothing once the data has been received, but it's not strictly necessary.) It just records carrier-phase measurements and pseudoranges, along with clock and doppler info, in (or later converted to) a format called RINEX. The surveyor just keeps it in one place for a while, marks down "3:32pm-3:38pm, marker C", and then moves to the next point. Later back at the office (once the precise ephemeris comes out), the RINEX is crunched with that better data, and solutions are derived which allow the surveyor to say exactly where Marker C actually is.

This is better than doing it in real time, because the ephemerides available in real time just aren't that good. Only by measuring with a network of ground stations, can the better ephemerides be calculated, and then applied to the observations.

There's also RTK and correction networks, which deserve mention:

Real-Time Kinematic is called that because it tells you about distance and motion, the kinematics, _relative to a nearby base station_. If the base doesn't know where it is, the rover doesn't either. So the base is usually surveyed first, using the techniques outlined above, and then that surveyed position is combined with the kinematic differences, to derive the rover's precise position. It requires a data link between the base and rover, though that's gotten dramatically easier in the last few decades...

Correction networks do all of that, over a wide area, providing a "virtual reference station" nearby to wherever you need it to be. The corrections are transmitted typically over a separate data channel (often on leased L-band satellite time), and applied by the receiver. Some are available over the internet as well, if cellular signal is easy to come by wherever you happen to be. I don't know as much about these as I'd like to.

Yup! This issue, which is commonly described as the "wrong-side-of-the-street" or "wrong-city-block" error, is one of the biggest ones, and is where a lot of the innovation I mentioned is happening today.

This occurs because in "urban canyons", meaning streets with tall high-rises or sky scrapers, there is little or no line of sight to GNSS satellites (GNSS being the generic term for all satellite positioning systems, not just the American GPS). Consequently, what your phone picks up are signals reflected off of buildings, which exaggerates the distance between you and the satellites, and causes the positioning solution to be pushed away- onto the other side of the street or another city block.

One way Google/Android is tackling this is by using Google's trove of 3D building data, the same that is rendered in Google Earth when you use it in 3D. Your phone uses the building data to correct for reflections. Read on here (and note the authour!): https://android-developers.googleblog.com/2020/12/improving-...

And, the device can use various filters and smoothers to minimize sudden jumps, and normally does, but there are edge-cases (for example, an app may be requesting pure "unfiltered" GNSS location returned directly from the GNSS chipset, hence the jumps). But rest assured, we are working on this issue.

Anyway, thanks for the feedback. We're always doing our best to improve location accuracy and reliability for billions of users in all scenarios and environments, and it's no trivial feat!

I experienced this as a pedestrian in downtown Chicago (on iOS in my case). It made me curious about what was going on — were the buildings reflecting the signal or obscuring it somehow? It's a fascinating phenomenon.

Wow, this is a great resource! Major props to Bartosz Ciechanowski. It's rare to see the time aspects of GPS covered so well.

Ad the highest possible compliment, this is like Bret Victor quality!