Question regarding GPS/Location in iPhone

Currently I'm doing an location app in iPhone but my question is....

What is the service does iPhone using?

According to Apple Developer Manual "The framework uses information obtained from the built-in cellular, Wi-Fi, or GPS开发者_如何学JAVA hardware to triangulate a location fix for the device."

So it will be either built-in cellular, WiFi or GPS hardware, how am I ensure that it always uses GPS hardware to get the location?

Because my app requires accurate location.

To limit the supported devices to the ones with GPS you can use the UIRequiredDeviceCapabilities in your app's plist. The only way (to my knowledge) to force the device using the most precise location determination (in this case GPS) is to set the desiredAccuracy and distanceFilter of CLLocationManager accordingly.