GPS - look into Gold Codes [1] and auto-correlation. GPS arrives on your device at -128dBm, give or take, while thermal noise is -111dBm at the ~2MHz sampling bandwidth used. We use the processing gain from correlating against the ~1Mchip/s gold code to extract the lower bitrate signal, so you're pulling signals from approximately 20dB below the relevant thermal noise floor.

[1] https://en.wikipedia.org/wiki/Gold_code

DSSS, direct sequence spread spectrum, used for encoding GPS signal

Spread spectrum, it's pretty magical.

Thanks @FrankPetrilli.

I found this an interesting writeup on the technologies needed to receive GPS signals:

http://www.aholme.co.uk/GPS/Main.htm

The combination of RF electronics and then doing the signal processing in an FPGA for the timing critical stuff and the actual DSP with a raspberry pi, very interesting.