For wider coverage areas, you could use balloons, or simply put GPS stations on commercial jetliners. They don't have to have fixed locations.
Considering that even after there's a hole in the coverage, where and when it will show up is going to be predictable, temporary fill ins could be deployed in that fashion.
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Considering that even after there's a hole in the coverage, where and when it will show up is going to be predictable, temporary fill ins could be deployed in that fashion.