We were looking forward to a lovely dinner at a restaurant in Rome. I checked Google maps on my phone for the directions. It was about a 15 minute walk and looked pretty straightforward. Every few blocks I checked the map because there were a lot of turns onto small streets.
After about 15 minutes, I noticed that the street corner I was approaching looked kind of familiar. Sure enough, we had been there earlier. While I was scratching my head, my wife asked a shopkeeper for directions. Ignoring my phone, we were able to follow the directions and arrive at the restaurant.
But it was too late; it was a very popular restaurant and they had given our reservation to someone else. But in Rome this is not a problem. There are so many restaurants that we had no trouble finding one nearby on the lovely Piazza Navona and had a thoroughly enjoyable dinner.
So what in the world had caused the directions on my phone to fail so badly?
I knew that mobile phones have a radio receiver that reads signals from GPS satellites (Global Positioning System) that are continually broadcasting their position and time. The phone’s position can be calculated by solving for four unknown quantities — latitude, longitude, altitude and the time it took for the signal to travel from the satellite to your location. If you remember some high school algebra, you know that solving for four unknowns requires four equations. So your mobile phone needs to get GPS readings from four satellites.
I also knew that GPS signals could be obstructed when you were surrounded by tall buildings, such as in Manhattan, downtown New York City. But Rome, and certainly where we were, does not have very high buildings. On the other hand, the streets are extremely narrow so there is not much sky visible. This was probably the reason I did not get the right directions on my phone.
What is the lesson to be learned from this?
Simple. If you suspect that your GPS may not be accurate, look up at the sky. If you can’t see lot of sky, your GPS may not work. However, this does not necessarily mean that your map directions will fail because mobile phones also use cell phone towers to determine location, but it may not be as accurate as GPS which determines location to within 5 meters.
Historical note: The first GPS system was developed for the U.S. military during the cold war. In 1983 a passenger plane on route from New York City to Korea entered prohibited airspace because of navigational errors and was shot down by Russia (the Soviet Union). After this, the U.S. made GPS available for civilian use to try and avoid such disasters. Not long after this, Russia also made their satellite system available for civilian use. Many smartphones will use the Russian satellites as well as GPS. Europe and China are also developing navigational satellite systems.
