[Swordmaker]

GPS phones aren't necessary because the cell phone tower positions are known. Your cell phone periodically checks in with nearby cell phone repeaters to say: "Hi, here I am if I get a call", and the repeaters can measure the signal strengths to triangulate. All that's needed is to subtract the time between two repeater check-ins to determine an approximate travel speed and direction. GPS phones though would add the capability of tracking side-street traffic flow.

The 2G iPhones (the first model) already use that technique for a quasi-GPS location... but the accuracy is far less than what would be needed to determine travel speed and direction. In many areas, it is capable of locating you with in a one mile radius... in others, it's capable of getting it down to about 500 feet.

[Star Traveler]

Swordmaker, you said — In many areas, it is capable of locating you with in a one mile radius... in others, it’s capable of getting it down to about 500 feet.

—

Yeah, I’m not sure how good this would be, considering the accuracy that would be needed. In Texas and Oklahoma there are a lot of frontage roads on each side of the highways/freeways. And I’m not sure that they can distinguish between the frontage roads and the highways.

Heck, sometimes with some jam-ups on the freeway, I’ll make better time on the frontage road, driving at 50 MPH, while the freeway is stalled at around 10-15 MPH... LOL...

[Reeses]

Cell phone repeaters are ideally suited to become ground-based super GPS satellites. By measuring the phases of several inbound radio frequencies from several repeaters a cell phone could calculate its position to within a millimeter, something that can’t be done with regular GPS satellites. There’s a lot of new applications that could come out of that, such as auto-steering cars and landing airplanes.