iPhone GPS Performance

Question

I'm writing an iPhone App that relies on getting the device location. Management have tasked me with producing data on how well the GPS performs in cities (tall buildings) and within buildings.

Do any developers have data on reliability of GPS and the fall back to cell/wifi tower triangulation?

Management-friendly info would be best but raw data that I can translate would also be fine.

Answer 1

To determine an object's location, the GPS system must receive a radio signal from at least three satellites.

GPS accuracy is affected by a number of factors, including satellite positions, noise in the radio signal, atmospheric conditions, and natural barriers to the signal. Noise can create an error between 1 to 10 meters and results from static or interference from something near the receiver or something on the same frequency. Clouds and other atmospheric phenomena, and objects such a mountains or buildings between the satellite and the receiver can also produce error, sometimes up to 30 meters.

From here:

Multipath and masking effects of urban canyons degrade the accuracy of GPS ranging and increase geometric dilution of precision in receivers that operate in dense urban areas. In the case of GPS applications designed for vehicles, the effects of these phenomena on accuracy can be reduced, thanks to the velocity of the user that contributes in averaging multipath and thanks to the use of map matching. But pedestrians do not benefit from the same circumstances, and GPS-based positioning for pedestrians in dense urban areas suffers from inadequate accuracy and integrity. Tests performed in downtown urban areas over a variety of mass market terminals with integrated GPS receivers show 95 percent circular error probable (CEP) performances between 50 and 100 meters.

Wiki article: Global Positioning System