I think I have been pretty open about the drawbacks and weaknesses of GPS … anyone who reads this blog a month or more certainly knows the term “urban canyon” and if you didn’t you will learn more today. The industry in general is pretty honest when making claims for performance .. especially claims of rapid ROI (Return On Investment) which can be verified easily. But one place even a few of the big boys fall down is failure to acknowledge the places where GPS doesn’t really work well, or work at all.
URBAN CANYONS: This is a generic term for any place where a GPS receiver can’t see an adequate expanse of sky because of crowded city buildings. No matter what the manufacturer tells you, all units will suffer from this effect. And the effect will vary significantly from day to day and hour to hour. I get quite a chuckle out of schemes to put GPS units on taxis in (as just one example) New York City, where the drivers are in strong opposition to the plan. Since New York has probably the largest and deepest concentration of “urban canons”, the units are going to perform abysmally a great deal of the time and the drivers are surely going to point this out.
By definition any other place the sky is obstructed is going to cause poor or no performance as well. Heavy canopies of tree foliage, tunnels and underground parking garages are the worst culprits. Just imagine a GPS receiver heading east along ah highway locked on to 4 satellites to the North, West and South … nothing to the East, where the receiver’s vehicle is headed, because there’s a huge mountain up ahead. Suddenly the road goes into a tunnel, and when the vehicle pops out on the East side of the tunnel perhaps none of the former “ok” satellites are going to be in view. I’m not sure if there’s a command in the processing chip of most receivers labeled “WTF?”, but that’s an accurate description of how the unit is going to feel.
Reception Problems: For reasons as far ranging as huge eruptions on the Sun’s surface to Homer Simpson’s unshielded sparkplugs on his rusted out 1982 Belchfire sedan, GPS signals are not guaranteed to get down from the satellites to your receiver 100% of the time. Typical GPS systems just “jump” from where they last knew their whereabouts to where they finally find themselves after signals resume … but that’s not what actually happened to the vehicle during the reception outage and would be really disconcerting to a person watching supposedly “live” real-time tracking of the vehicle.
One Smart Provider: ublox is a Swiss company who is one of the few manufacturers if GPS chip sets (the “engines” that power all GPS receivers) who is smart about the needs of the system. They have been steadfastly designing and rolling out products with a RD Dead Reckoning components .. sensors that take the speed of the vehicle as measured by the turning of the wheels and the compass course made good and continually update the supposed position based on actual vehicle behavior until the satellites one again provide an adequate update.
Before you invest in a system, ask your provider what’s going to happen when your units lose signal … because it is not a question of if you’ll enter and urban canyon, but rather when.