In this lesson from Yale's Rocket Science for Everyone, astronomer Marla Geha explains Medium Earth Orbit, the region of space situated between Low Earth Orbit and geosynchronous orbit, and its role in enabling GPS navigation. The lecture describes how a constellation of satellites in this orbital band allows a phone or receiver to determine its position anywhere on Earth. Following the course's earlier lesson on Low Earth Orbit, this lecture continues building a comparative picture of the different orbital regions used for different purposes, from the International Space Station's low orbit to the navigation satellites in medium orbit covered here.