In spread spectrum transmissions a secret code is used to determine the frequency sequence used to spread the signal. In the world of GPS, this "secret" code is called a C/A code. C/A codes are made to look like random noise and hence are known as pseudo random codes (PN codes) but I'll be referring to these as C/A codes in the context of consumer GPS. As part of the original design of the Navstar Global Positioning System, 32 C/A codes were defined and published. Each of these 32 unique C/A codes is assigned permanently to a satellite during its operational lifespan. Every GPS receiver in the world has a copy of these 32 C/A codes and is therefore able to de-spread and obtain the data stream from the transmission.

If you happen to be listening to the sequence of frequencies exactly as described by a particular C/A code, a mountain of information appears from an ocean of noise. If you try to decode the noise using the wrong code or the right code at the wrong synchronization, nothing arises and no data is found - only noise.

Multiple Transmissions, Multiple Satellites

Many completely different data streams can overlap on the same frequency, but can still be recovered individually and separated. The GPS system transmits on a single frequency, but C/A codes are used both to encrypt data and to wobble the signal to one side and the other of that frequency. With so many GPS satellites in the constellation, it is remarkable that all the satellites are transmitting data simultaneously on the same frequency. It's why GPS gadgets are capable of listening to a dozen distinct satellites with only a single antenna.

Thanks to spread spectrum technology, each satellite uses its unique C/A code to scramble the data stream. The data is multiplied into this C/A stream and modulated, resulting in the data being spread out randomly across a 1 MHz band around the GPS L1 carrier (1575.42Mhz). Think of the data being transmitted by the satellites as encrypted TCP/IP packets, each stream mixed among others, then each unique C/A code is the key to not only plucking the correct packets out of the ether, but also carries the sequence of packets to allow the stream of data to be reconstructed. In a sense, a GPS receiver sniffs the network and uses a dictionary of 32 possible passwords to eventually unlock the data.

Patent 2,292,387 And GPS

Hedy Lamarr and George Antheil imagined using a piano roll as the "C/A" code. While seeming a little quaint, if you examine such a piano roll, what you are looking at is a time-encoded 88-bit pseudo random sequence.

Here is an actual drawing from the patent application showing a piano roll-like approach to encoding.