Poor GPS signal on iphone 4, Possible Fix for some.
Like many others I suddenly starting to have problems with my iPhone 4 GPS signal, as every time I turned it on it said poor GPS signal and turned the map grey. I couldn't understand this because I have previously had no problems with any of the navigation apps installed. Anyway, after a long process of elimination, I discovered that my problem was due to my FM transmitter that I use for my music. For some reason. my iPhone GPS did not seem to like the frequency that my FM transmitter was working on (104.8 FM) I discovered that if I unplugged the FM transmitter, I would get my GPS signal back with no problems.
However, I was now without my particular means to play my music which really annoyed me. I then took a wild guess regarding the possibility that it may just be the frequency of the FM transmitter and not the device itself. I then changed the FM transmitter frequency to 98.4 FM and everything worked like a charm. I am not suggesting for one moment that looking at the frequency of any devices including audio systems will work for everyone but it certainly worked for me. I am now sure that frequency thing was definitely my problem because I also tested the theory with my wife's iPhone using TomTom and the same problems and fix resulted.