Finally: how satnav and GPS find routes

This may only scratch an itch of mine but it’s a big itch and I’ve had it a long time. It’s very easy to understand how GPS works – it’s triangulation – even when you know exactly where you are on the globe, how in the hell do you work out the best route to Tesco?

I think about this a lot and I’ve asked people about it who ended up explaining GPS like I hadn’t just said I got that bit. Usually I don’t get anywhere. But noodling around this evening, look what I found: the explanation.

There’s this guy, right, and he was preparing to demonstrate some fancy computer.

For a demonstration for noncomputing people you have to have a problem statement that non-mathematicians can understand,” [Edsger W. ] Dijkstra recalled in an interview not long before his 2002 death. “They even have to understand the answer. So I designed a program that would find the shortest route between two cities in the Netherlands, using a somewhat reduced road-map of the Netherlands, on which I had selected 64 cities.”

“What’s the shortest way to travel from Rotterdam to Groningen?,” Dijkstra said. “It is the algorithm for the shortest path, which I designed in about 20 minutes.”

The Simple, Elegant Algorithm That Makes Google Maps Possible – Michael Byrne, Motherboard (22 March 2015)

Read the full piece for some basic maths on an admittedly simplified version of how it’s all done. But it’s how it’s all done. I am so pleased right now.

Weekend read: trying to kill GPS with an axe

Nearly a quarter of a century ago, two men attempted to axe the GPS programme – entirely literally:

On May 10, 1992, the activists Keith Kjoller and Peter Lumsdaine snuck into a Rockwell International facility in Seal Beach, California. They used wood-splitting axes to break into two clean rooms containing nine satellites being built for the U.S. government. Lumsdaine took his axe to one of the satellites, hitting it over 60 times.

They were arrested and faced up to 10 years in prison for destroying federal government property, causing an estimated $2 million in damage. Ultimately, Kjoller and Lumsdaine took guilty pleas and were sentenced to 18 months and two years in prison respectively for an act of civil disobedience they named “The Harriet Tubman-Sarah Connor Brigade.”

Acting in a tradition of civil disobedience established by the Plowshares movement while citing the leader of the Underground Railroad and the heroine of the Terminator series, the Brigade’s target was the Navigation Satellite Timing And Ranging (NAVSTAR) Program and the Global Positioning System (GPS). Back then, GPS was still a fairly obscure and incomplete military technology, used in some civilian applications (the first civilian GPS device, the Magellan NAV 1000, came on the market in 1988) but far from a mainstream resource. Today, GPS feels almost more intimate than industrial or weaponized.

The Failed Attempt to Destroy GPS – The Atlantic

Read the full piece to find out why they tried and why at least one of them doesn’t regret it.