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tethering note
*GPS shuts off when user locks the screen manually or the device auto-locks
:You can use <tt>[UIApplication sharedApplication].idleTimerDisabled = YES;</tt> to keep the screen on, but this is bad for the screen itself and takes a nice toll on battery life. Instead, set up an <tt>AVAudioSession</tt> with the <tt>AVAudioSessionCategoryPlayback</tt> category, then set the <tt>kAudioSessionProperty_OverrideCategoryMixWithOthers</tt> property (so that the user's iPod can keep playing) and a more aggressive I/O buffer using <tt>kAudioSessionProperty_PreferredHardwareIOBufferDuration</tt>. Then, use <tt>AVAudioPlayer</tt> to play a "recording" of total silence on endless loop for when you need to keep GPS running. <em>Note</em>: accelerometer and WiFi still shut off when the screen is locked.
:Note also that to test this properly, you *must* untether your device, as when your device is connected to power, it never enters low-power mode (WiFi and GPS stay on, even if you don't play any sound).
*Calling <tt>[[AVAudioSession sharedInstance] setPreferredIOBufferDuration:</tt> always errors, regardless of what value you use!
:See [http://stackoverflow.com/questions/1714507/recording-audio-on-iphone-error-with-setpreferrediobufferduration/2530737#2530737 my response on StackOverflow]. Basically this is a bug in the interface and you '''have''' to use the plain C method instead of the Objective-C one to get this to work.