If you have ever used a smartphone map or watched a delivery vehicle move across a tracking app, you have used GPS. What many people do not realize is that GPS — the US's Global Positioning System — is only one part of a broader family known as global navigation satellite systems (GNSS). Four global satellite systems circle Earth. They guide aircraft, ships, cars and trucks, or tourists looking for a place to eat. But they also play a central role in war. At its core, satellite navigation is a