Determine the position of the GPS satellite relative to and from the center of the earth and ratio of clock rates on a GPS clock in orbit and one on surface of earth.
Find out how high up a GPS satellite is relative to the center of earth and from that discover the ratio of clock rates on a GPS clock in orbit and one on the surface of the earth. Here is a quote from Wikipedia, which will help with this and with the next question: "Orbiting at an altitude of approximately 20,200 kilometers (about 12,550 miles or 10,900 nautical miles; orbital radius of approximately 26,600 km (about 16,500 mi or 14,400 NM)), each SV makes two complete orbits each sidereal day, repeating the same ground track each day." (Here, SV means "space vehicle" and refers to a satellite).