If you want to learn more about what makes GPS tick, here’s a home made receiver that was build from the ground up. A stunning project with an incredibly detail writeup.
One of the things that really caught me by surprise is that the GPS signal is way too weak to be detected out of the noise in isolation. It’s only possible to receiver it if you know what to expect:
“All GPS satellites transmit on the same frequency, 1575.42 MHz, using direct sequence spread spectrum (DSSS). The L1 carrier is spread over a 2 MHz bandwidth and its strength at the Earth's surface is -130 dBm. Thermal noise power in the same bandwidth is -111 dBm, so a GPS signal at the receiving antenna is ~ 20 dB below the noise floor. That any of the signals present, superimposed one on another and buried in noise, are recoverable after bi-level quantisation seems counter-intuitive! I wrote a simulation to convince myself.
GPS relies on the correlation properties of pseudo-random sequences called Gold Codes to separate signals from noise and each other.”
What's even more amazing, is that differential GPS works at all, given that it relies on measuring the timing of the underlying carrier signal. (As opposed to the pseudo code.)
It takes the advantages of digital signaling, and throws them out, instead measuring the analog carrier waves directly. That this can be done at such low signal levels is unbelievably impressive.
(The statistical methods involved in getting an initial signal lock are nearly as impressive.)
And the whole thing was a 1980s hack for land surveyors to get better than 100m accuracy while systematically eliminating uncertainty due to Selective Availability (SA).
It's a hack that improved the overall accuracy of the system from ~100m to sub ~1cm, and eventually sub ~1mm accuracy, when used with some other methods.
http://www.aholme.co.uk/GPS/Main.htm
One of the things that really caught me by surprise is that the GPS signal is way too weak to be detected out of the noise in isolation. It’s only possible to receiver it if you know what to expect:
“All GPS satellites transmit on the same frequency, 1575.42 MHz, using direct sequence spread spectrum (DSSS). The L1 carrier is spread over a 2 MHz bandwidth and its strength at the Earth's surface is -130 dBm. Thermal noise power in the same bandwidth is -111 dBm, so a GPS signal at the receiving antenna is ~ 20 dB below the noise floor. That any of the signals present, superimposed one on another and buried in noise, are recoverable after bi-level quantisation seems counter-intuitive! I wrote a simulation to convince myself.
GPS relies on the correlation properties of pseudo-random sequences called Gold Codes to separate signals from noise and each other.”