GPS Satellite Question

[Tartar Horde]

I might be wrong but I believe the satellites that control the planet wide GPS system are military and so might be on separate power systems enabling them to function after the civilian grid is down. Loss of GPS is unthinkable for modern warfare and travel so I reckon they would make every effort and sacrifice to keep it working. The satellites are powered by solar but rely on electrical systems to communicate, and atomic clocks to regulate and syncronise.