I am tesing AGPS for Iphone 3g ,4 4S and 5. It works good with Iphone 3g and 4 but 4S and Iphone 5 sends back :,Assistance Data Ack with reference NO:0. According to 3gpp standards If the MS has not been able to decode the Reference Number (e.g. IE missing), it shall use '0' as the Reference number in the response. I tried sending Measure Postion request and Position Capability Request but same packect in response. So Looks like there is some chnage in chip after A4.