Do we need satellites for GPS to work, or can a ground based technology do the same thing?

You can triangulate from cell towers very accurately if the terrain is flat. You run into problems if there are hills.

… or other things that interfere with radio signals, like tall buildings made of radio-blocking materials like concrete or metal.

That makes them fairly useless in cities or anywhere near the Atlantic coast (or rather, the Appalachian Mountains)

On many phones, “GPS” uses cell towers and WiFi as well as actual GPS.

Before GPS, there was LORAN, using radio towers for air and sea navigation.

It’s technologically possible to do what you suggest from the ground, in fact, that’s how most coastal navigation is still done by most sailors: simple triangulation. Electronics aren’t even needed.

However, to attempt to do it on the nearly universal scale that GPS works over would be hugely, unimaginably expensive: to build towers that can be “seen” three (or more) at a time from every point on land where humans are likely to be? You figure the cost!

Its not impossible. Mathematically its possible and the shorter distance has to travel would probably increase accuracy. However, the cost to get the necessary redundancy, etc… would be prohibitive.

Draw a circle on a sheet of paper and pretend thats the Earth. Now draw a tower on it. How much of the circle can it “see”? Shouldn’t be very much. Now draw a satellite higher up. How of the earth can it “see”? It should be more. (See this picture to see what I mean). Therefore, you need more towers than satellites. One tower is probably cheaper than one satellite but overall the cost is less for satellites (which probably serve multiple functions anyway).

and then there are the large spans of ocean that need GPS service and is strangely absent of phone towers. Hubby works with GPS, specifically for the maritime industry. Gotta have satellites for the dynamic positioning systems to work so they don’t spill the oil everywhere.

Oh.. update after talking to hubby about this. (Man, he is an interesting guy.) I can categorically state that GPS, as we know it, would NOT work without satellites, including land areas. Triangulation using cell phone towers is vague. GPS can give an accuracy of 7 to 10 feet under normal circumstances. Triangulation uses city blocks to describe locations, at it’s best, and it is completely reliant on the location and density of cell phone towers.

To give you an idea of how accurate the system can be using the GPS satellites, with some proper surveying and computer software, my husband regularly gives ships and rigs a location accuracy within 10cm.

@cazzie But isn’t that because cell towers don’t do exact triangulation? On land, I think it would be possible to do via SOME kind of tower, they’d just need to be more accurate than cell towers.

The way they triangulate from your cell phone in relation to the nearest cell phone towers is the strength of the signal coming from your phone to the tower. Your phone is giving a signal all the time and is registered by the towers. The closest one reads that you are nearest to it and services your call, the others simply register the strength of the signal and that is how they find people, but, like I said, it’s vague tracking.

Now, listen carefully…. I get sick of seeing this on TV.. Cell phones transmit. GPS equipment doesn’t. (or at least it doesn’t need to for it’s most basic use) It only receives the signal from the satellites. It doesn’t send a signal registering your whereabouts to the satellite and the satellite is a honey badger… it don’t care. What does transmit is when you’re using it in relation to an app on your cell phone. The cell phone can take that information and other searches you’ve done on your phone and transmit it to a tower and that information can (and has been) collected. I’m sure there will be an upgrade soon so that the car GPS information you use is transmitted and downloaded into some ‘big brother’ collection point so they can sell you more fast food, designer shoes and tell you where to get your hair done, and,of course, so the police can follow your every move.

Welcome to the 21st century and you can thank the people at Apple. The collection of GPS location searches and data was their idea first.

We don’t need anything on land. It would extremely redundant. GPS is only one of the several global satellite networks buzzing around. Here’s a link of some of the latest info.
http://en.wikipedia.org/wiki/Satellite_navigation

Also, basing the stuff on land is very problematic. Signal interference would be a big problem, as would finding decent surveyors to place them and set them up correctly (you wouldn’t believe the problems….). The things would also be struck by lightening and they’d have to be completely replaced.