Out of interest are speaking from a position of authority as someone who’s implemented GPS receivers, or as someone who understands the theory of GPS receivers?
From my reading of the article, it sounded like the author was commenting on an interesting implementation detail of GPS receivers, and how they might deviate from their expected theoretical implementation. Rather than describing how an ideal GPS receiver is implemented.
There's pretty much never a reason (...normally, but see below) you'd use a separate satellite for the time -- you do need four satellites to solve for X/Y/Z/time, but you solve them together. Basically, there's only one combination of satellite positions and (pseudo)ranges that will produce a solution, and one of the outputs of that solution is the time (the inputs are just time offsets between the various signals, based on the receiver's non-precision local clock).
The see below part: There are timing receivers that will do a long "survey" to figure out their exact location (or as close to it as they can), and once they have that they can use a single satellite to determine the current time, since they already have most of the needed equations "solved" when the receiver already knows its own (static) location. This is sometimes preferable, depending on one's application, because it makes for less jumpiness in the time solution as new satellites go into and out of view (since the changing geometry of the constellation will make for slightly different solutions every time it changes)
Fair question - I’m familiar with the theory of operation and I’ve implemented toy receivers (on the software side). I will say that even in the basic designs I worked with it made sense (and was easier) to solve the system of equations and not do something special like assign a “time satellite”. It’s a much easier way to describe the system to someone without a linear algebra background, so I bet that’s the author’s intent.
As someone who did location work based on timing in another context - picking 3 satellites straight above seems weird - we had better accuracy when beacons were right angles - basically if you have a timing error and sats are near each other - the intersection of the two lines moves hugely
It's physically impossible to dedicate one satellite to time synchronization. The satellites broadcasts a time signal, but that's the time at the moment of transmission, not the moment of reception. Since the receiver by definition doesn't know where it is yet, it doesn't know the propagation delay from the satellite, and it can't calculate the time at the reception site (to the accuracy needed for positioning, anyway).
From my reading of the article, it sounded like the author was commenting on an interesting implementation detail of GPS receivers, and how they might deviate from their expected theoretical implementation. Rather than describing how an ideal GPS receiver is implemented.