An iOS GPS application that shows satellite locations and lock-on details?
There is no such interface available for iOS, since everything is enclosed within the CoreLocation framework (no way to access NMEA data).

You can monitor some variables (like horizontalAccuracy, verticalAccuracy) but that's it. The easiest way to obtain this information is to run some GPS application that displays this information, such as "GPS Status-".

(Be aware of multiple apps with very similar 'GPS Status' names are not all identical.

At the time of this edit, "GPS Status-" by Fawkes Wei is much more positively reviewed and kept up to date (last update 25 May 2019) than the "GPS Status" by PocketGPSWorld.com Ltd (last update 5 June 2011).)