One of the more interesting facets of GPS is that, at least from the receiver’s point-of-view, it’s a fairly passive system. All of the information beamed down from the satellites is out in the ether, all the time, free for anyone on the planet to receive and use as they see fit. Of course you need to go out and buy a receiver or, alternatively, possess a certain amount of knowledge to build a circuit that can take those signals and convert them into something usable. Luckily, [leaning_tower] has the required knowledge and demonstrates it with this DIY GPS receiver.
This receiver consists of five separate circuit boards, all performing their own function. The first, a mixer board, receives the signal via an active antenna and converts it to a lower frequency. From there it goes to a second mixer and correlation board to compare the signal to a local reference, then a signal processing board that looks at this intermediate frequency signal to make sense of the data its seeing. Finally, an FPGA interfacing board ties everything together and decodes the information into a usable form.
Dealing with weak signals like this has its own set of challenges, as [leaning_tower] found out. The crystal oscillator had to be decapped and modified to keep from interfering with the GPS radio since they operated on similar frequencies. Even after ironing out all the kinks, the circuit takes a little bit of time to lock on to a specific satellite but with a second GPS unit for checking and a few weeks of troubleshooting, the homebrew receiver is up and running. It’s an impressive and incredibly detailed piece of work which is usually the case with sensitive radio equipment like GPS. Here’s another one built on a Raspberry Pi with 12 channels and a pretty high accuracy.
