Karl’s tablet is an Amazon Kindle Fire 7 inch model. My understanding is that the “Fire OS” is simply Amazon’s GUI on top of Android 4.x.
That being the case, Digital mode and SOTA apps written for Android should work on the Fire, however Amazon does not allow access to Google Play where Android apps are normally loaded from. The Android developer could publish his apps to the Amazon store, but most don’t do this yet. The other option is to download the app to another Android device such as a smartphone and then transfer the APK program files across to the Fire’s disk and install from there. Here’s an article on doing this:
If you don’t have another Android device the only other option is to “root” the Fire and install Google Services so that you can access the Google Play store directly - that unfortunately can void the warrenty on the Fire however.
One app you might like to try Karl is Rucksack Radio Tool, from Joerg DL1DLF, which is not on Google Play but you can install directly from the developers website by going to it using the browser on the fire. Here is the URL you need: http://www.dl1dlf.de/Rucksack_Radio_Tool_Latest.apk
To read about the app, go here: http://www.dl1dlf.de/rucksack_radio_tool_english