this is from the spektrum operators manual:

Spektrum™ telemetry technology makes it possible for you to get real-time information on your model's battery voltage, motor rpm, motor temperature and more. Many Spektrum transmitters feature built-in telemetry data receivers that let you view this information on their LCD screens. You also have the option of monitoring telemetry using the Spektrum STi™ app for the Apple® iPad®, iPhone® or iPod touch®.

Spektrum is releasing new telemetry sensors and accessories all the time. To give you a feel for what a difference Spektrum telemetry can make, here are some of the most commonly used telemetry features most Spektrum users take advantage of:

The Spektrum™ STi™ Telemetry Interface for iPhone®, iPad® and iPod touch®
The STi lets you transmit vital info from your model’s on-board Spektrum™ telemetry module directly to your iPhone®, iPad® or iPod touch®.* It can also be programmed to alert you when any telemetry value you’re monitoring exceeds limits you define. It’s a great way to keep tabs on your model if you’re flying with a DSM2®/DSMX® Spektrum or JR®/DSM® transmitter that doesn’t have built-in telemetry. Even with a telemetry-capable transmitter, you can use it to let someone else watch telemetry so you can focus completely on flying.

*Requires the free STi app. Compatible with iPad, iPad 2, iPod touch (4th gen), iPhone 4S, iPhone 4, iPhone 3GS, iPhone 3G.