iSextant is a sun sign reduction program, that can be used to confirm your location in the event of GPS signal problems.
It is important to note that in increasing world orbital use/misuse we are always closer to the http://en.wikipedia.org/wiki/Kessler_syndrome where there is an increasing chance of GPS outage.
iSextant can also be used as an aid to learning astro-navigation.
It requires that you take a sun sight using a Sextant and note various other conditions, such as pressure and temperature. Using this information along with an assumed position the program with give you GHA, Declination of the Sun followed by an Azimuth and Intercept from your assumed position.
For more information about astro-navigation please refer to http://en.wikipedia.org/wiki/Astro_navigation.