Earlier quoted context omitted.
There's a whole ecosystem of software for building tile databases and viewing them on phones/tablets: http://mobac.sourceforge.net/ There's also a bunch of "WMS Viewers" that you can point at National Map endpoints (which shouldn't have any licensing issues): http://raster.nationalmap.gov/arcgis/rest/services I've never had a use for them so I don't know if one works well and has nice caching features.
Interesting but it looks like the one iOS client for Mobac is geared towards vector maps, not the satellite imagery.
https://galileo-app.com/manual.html#offline_maps
I guess it would at least be fine for visualization.