I'm not sure this is technical enough, but this gist is as follows. GPS signals are extremely weak. They are below the background noise floor (you can't see them with a spectrum analyzer) and can only be received by convolving with a pseudorandom code (PN code). Because they are so weak, an adversary can overpower them and cause receivers to "lock on" as long as rogue signal is well designed. GPS signals are fairly simple. They mainly encode precise time information from atomic clocks orbiting the earth. By receiving 4 time signals from 4 atomic clocks in known orbits, one's position can be precisely known.

This is a hard attack, but not not impossible. The Iranians are rumored to have used this technique to down a stealth drone the US was using for spy purposes.

Here is some slides from Defcon 23 : https://media.defcon.org/DEF%20CON%2023/DEF%20CON%2023%20pre...

They did a record, replay with a USRP B210 and GPS antenna.

And here is a project that generates GPS baseband signal data streams for bladeRF, HackRF, and USRP.

https://github.com/osqzss/gps-sdr-sim

Here's an article on GPS spoofing and some potential countermeasures:

http://spectrum.ieee.org/telecom/security/protecting-gps-fro...