That was used to aid in the launch of four experimental satellites over the course of the 1960s and 1970s, the last of which was the first satellite to fly in the GPS 12-hour orbit and the first transmit GPS signals.
Orbiting the Earth in a path that was inclined at 45 degrees to the equator, passing as close to 590 miles to the surface of the Earth, Telstar could be used for just twenty minutes out of each two and a half hour long orbit to relay signals between the America, France, and the United Kingdom.