The 20,000km view: How GPS works

Accepted Session
Short Form
Scheduled: Thursday, June 26, 2014 from 10:00 – 10:45am in B201


GPS is more than just letting your phone tell you where you are. I believe GPS is a contender for "most amazing piece of engineering in the history of humanity", and I'll show you why.


The Global Positioning System, or GPS, dates back to 1973, four years after the first moon landing. By necessity, the design can work with electronic components much simpler than those demanded for a modern wireless protocol.

GPS satellites orbit the Earth at an altitude of 20,000km (that’s half the Earth’s circumference), transmitting a radio signal with an effective power output of about 500 watts. Imagine hanging up five 100W lightbulbs. Now imagine trying to see those bulbs at twice the distance from Portland to Greece. With brighter lights flashing all around you. That’s what the tiny GPS receiver in your phone has to deal with.

At that altitude and orbital velocity, the GPS satellites are affected by both general and special relativity. That’s right: when you ask your phone for directions from your current location, you’ve validating Einstein’s theory of relativity.

I’ll focus on the technologies involved in GPS from the radio up, but no discussion of GPS would be complete without considering the history and politics of this cold-war era invention.

You don’t need to know anything about radio or signal processing for this talk, but I will introduce some fundamentals of those fields.

Finally, in case you’re inspired to experiment with GPS yourself, I’ll briefly present open-source and open-hardware tools you can use to learn more, and our local and open GPS-related projects that could use your help.


GPS, engineering, radio, software-defined radio, signal processing, science, relativity

Speaking experience

I gave a two-hour version of this talk for the Portland Area Robotics Society last June, and an Open Source Bridge unconference presentation on the subject in 2012.

I gave my first conference talk at the XFree86 Technical Conference in 2001. Since then I have presented at various conferences including the USENIX Annual Technical Conference, the X Developers' Conference, the Desktop Developers' Conference, OSCON, and of course, at Open Source Bridge in 2010 and 2011.


  • Jamey is climbing the halls


    Jamey Sharp was placed on Ritalin, briefly, in fifth grade. His interests and activities have been varied ever since. His biggest projects have been the Portland State Aerospace Society, a student rocketry club at Portland State University; XCB, a new low-level binding to the X protocol, in the process of replacing Xlib; and Comic Rocket, because his other projects didn’t leave him enough time to read his favorite webcomics without tool support.

    Jamey’s interests span computer science fields including cryptography, combinatorial search, compilers, and computational complexity; systems-level programming, such as file format and network protocol implementations, Linux kernel development, and boot-loader hacking; computer architecture and its impact on software design; and functional programming, preferably in Haskell.