Cell phone GPS: WaPo: This little blue dot on your phone is a revolutionary invention

WaPo: This little blue dot on your phone is a revolutionary invention by Katherine Dunn (“U.S. policy used to jam up GPS. Now, those signals beam into your pocket.”):

As Taiwanese manufacturers scrambled to fabricate GPS chips in the early and mid-2000s, and as satnav companies rushed to install them in their receivers, a GPS chip start-up in Silicon Valley called Global Locate was finessing another technical challenge. Making mobile phones trackable when they called 911 was one thing. But an engineer at Global Locate named Frank van Diggelen wondered: What if GPS chips in mobile phones could be useful outside of emergencies, for directions, in the same way as TomTom receivers in cars? This was a great idea in theory. But the chips were still too slow, and too power-hungry, to be much use as an everyday tool.

There was another concern, one based on human impatience: If it took too long for a receiver to “lock on” to GPS and determine its location — which was highly likely, since the GPS antenna in a phone is tiny, the signal is weak and the user is often inside a building — people tended to just give up. Van Diggelen figured he had about a couple of seconds to make a human stay.

The system van Diggelen and his colleagues designed to address both needs — mobile phone traceability and navigation — is called “assisted GPS.” To find its location, a phone’s chip first jumps back to the location of a landmark it most recently interacted with: a mobile phone mast, for example, or a WiFi hotspot. The chip then cross-references the longitude and latitude of that location with the so-called ephemeris data, the guide produced by the U.S. government on where all the GPS satellites are expected to be and when.