The first time I read this write-up I found it to be such a great resource because of the range of topics it touches on, with each one an entry point to some new subfield I didn't know about (coming from a non-EE background).
DSSS? Time to go on a wikipedia dive into modulation schemes
Numerically-controlled oscillators? There's something I haven't encountered, and let's read about FPGAs while we're at it.
1-bit ADC? Crazy that that even works, but it uses a bunch of interesting DSP math that I never even knew I was missing!
Plus the entire GPS signal chain, from the atomic clocks to the demodulated bits and inverse trig problem... incredible to see it all compiled into one (so well documented) project. I still go back and read this sometimes when I've forgotten part of it because it's fun to rediscover all the gems that went into building this receiver.
DSSS is an exceptionally interesting topic, IMO. It uses linear feedback shift registers which some CS types may have encountered as a very cheap PRNG. But it also makes these interesting spectral properties, and it's orthogonal to time-delayed versions of itself. That's quite a lot of somewhat unexpected properties for a fairly simple shift register construction.
To shed some light on the connection between these properties: they arise from the fact that the coding sequence _is_ pseudo random noise. Specifically, white noise.
White noise has this nice property that its autocorrelation is ideally a discrete delta function, aka a 1 at t = 0 and 0 everywhere else. In the time domain, this means that only an exactly aligned code sequence matches itself – ideal for communication symbols.
Spectrally, white noise is flat and fills the entire available frequency spectrum. This produces the “spreading” of digital spread spectrum sequences (DSSS).
Any kind of white noise could be used for this, but as far as I remember, the Gold codes used in GPS are: easy to generate and maximally different from each other.
This article is amazing for its level of technical detail; something of which I would never be capable. But it reminds me of my first GPS, which I cobbled together from parts around 1999. A friend sold his minivan and for some reason yanked the GPS guts out of it because he thought I'd like to tinker with them.
Through a series of cable adapter, and some topographical bitmaps (maybe from Terraserver), I managed to get a working GPS on a U.S. Robotics Palm III.
It was a far cry from what we have today: locking in to the satellites took several minutes, I could only have one map at a time, there was no zooming, the display was 160x160px monochrome, and if I changed maps the whole system had to be recalibrated. But it was the future right inside my own car.
Now a 100x better GPS is available inside my watch for half the price.
However, I believe it's worth mentioning that if you buy a commercial navigation IC these days, it will support them all out of the box, often with the addition of inertial navigation, which is what allows the navigation system screen to update properly while you drive through tunnels without satellite signals.
Other ~viable (for various definitions of "good enough") navigation paradigms include celestial, visual/terrain, other signal sources such as cellular, various radio systems, AIS, etc.
The root link is kinda different, though... What you're liking to is taking off the shelf receiver stuff and accepting the data coming out of it. The root link goes into exactly how and why all of this works, and how the author worked with it. It's much, much lower level.
DSSS? Time to go on a wikipedia dive into modulation schemes
Numerically-controlled oscillators? There's something I haven't encountered, and let's read about FPGAs while we're at it.
1-bit ADC? Crazy that that even works, but it uses a bunch of interesting DSP math that I never even knew I was missing!
Plus the entire GPS signal chain, from the atomic clocks to the demodulated bits and inverse trig problem... incredible to see it all compiled into one (so well documented) project. I still go back and read this sometimes when I've forgotten part of it because it's fun to rediscover all the gems that went into building this receiver.