Now that we’ve got this fun exhibit “GPS Adventures ” here at Pacific Science Center I’m getting some questions about the GPS satellites themselves.
How many GPS Satellites are there?
This is a more fascinating question that you might think. Everywhere I look the answer is “more than 24.” I find that confusing. Do you mean 25? 30? Can’t you count them? Well, actually, not so much.
The way the technology is set up means that there need to be at least 24 operational GPS satellites at any given time. So, when one breaks there needs to be another already up there to take its place. I counted 25 on NASA/JPL’s JTrack list today. There have been as many as 30 in the past, and they’ll launch more as those 25 get older and need to be replaced.
What kind of orbit are those satellites in?
A group of satellites is called a constellation. The GPS Satellites all orbit about 11,000 miles above the earth’s surface in a Middle Earth Orbit (MEO) which is between a Low Earth Orbit – like the Shuttle – and a Geosynchronous Orbit – like the satellites that broadcast satellite TV. Satellites in MEO orbit the Earth about twice a day.
How many satellites can you “see” at once?
Always a minimum of five. You only need three for basic positioning, four for better positioning, and five just means if one signal drops for you there’s another waiting.
What is the resolution of GPS?
General GPS (that which you might already have in your car) will pinpoint your location to within a meter or so. Advanced GPS will pinpoint your receiver to within a centimeter! Those advanced GPS technologies are also called “Differential GPS,” “Carrier Wave GPS,” or “Augmented GPS.”
Come to GPS Adventures – If you’re interested in GPS Satellites and GPS technology check out the first and fourth rooms of the maze – they’re chock full of information!  (Don’t want to get stuck in the maze with a gaggle of kids? Come to our next Science with a Twist – and experience the exhibit with a 21-and-up crowd. )
Check out the back of this month’s starmap – all about satellites you use. 
Where’d I Get My Info?
~ A l i c e !