The sea and wind can at the same time convey my neighbor’s vessel and my own. Jean-Baptiste Say, Economist

Before I begin the next big chapter in my life, I think it’s a good time to explain a bit about this program called GPS. Many readers may feel that GPS is simply the device on your phone or in your car that tells you where you are and how to get to your destination, but of course it is much more than that.

The Navstar Global Positioning System is a joint program developed and operated by the Department of Defense to provide worldwide positioning, velocity, and timing services. While its primary purpose was for military applications, it soon became obvious that it was in the national interest to provide GPS services for civilian applications as well. The concept of instantaneous world-wide space-based navigation was an evolving idea embraced by many in the aerospace industry and government. One of the key people advocating for and leading the development effort was Brad Parkinson, a colonel in the US Air Force who worked with a team of experts starting in 1973 to devise a space based navigation system that would work anywhere in the world and in any weather. It had to be fast and accurate. The US Navy had already fielded a system called Transit that could provide position at sea, but it relied on the Doppler ranging technology which was slow, inaccurate, and not adapted to rapidly moving vehicles. The GPS system promised to overcome the limitations of Transit by providing more satellites in the sky and at higher altitudes and using a signal structure and atomic clocks to provide high accuracy position available anywhere.

GPS is comprised of three segments: the space segment, that is, the satellites in space; the ground segment, comprised of the master control station and network of tracking stations to control and monitor the satellites; and the user segment, the operational equipment that determine positioning and timing. Today the space segment comprises nominally 24, but in actuality 32, satellites in space, revolving around the earth at a distance of about 12,500 miles. They are placed in an orbit that is “half-synchronous”, meaning they revolve around the earth twice a day, unlike geo-synchronous satellites, such as weather and television satellites, that move at the same rate as the earth, once a day. 


The control segment consists of the Master Control Station, located at Schriever Space Force Base in Colorado Springs, Colorado, and a network of remote stations around the globe. There are two kinds of remote stations. Monitor Stations passively listen to the GPS signals from the satellites, and send their data back to the Master Control Station. Ground Antennas are active antennas, receiving and sending signals to each of the satellites by means of large swiveling antenna dishes that point at the satellite, sending the satellite commands and receiving data signals (called telemetry) that report back the state of health and status of the satellites. At the Master Control Station, personnel receive the state of health signals and monitor the performance of the satellites, and send commands to control each satellite. They also upload to the satellites data necessary for the GPS mission.

The satellites revolve around the earth, broadcasting their signals for use by those on the ground to determine user’s position and time. The positions reported are quite accurate, within just a few feet of the actual location. The time accuracy is even better with timing reported to better than a hundred billionth of a second of the true time.

That is a high level overview of GPS. I will include more about GPS as I progress in this book. My career started working with the control segment, and later involved work in the user segment. I never directly worked with the satellites in the space segment, but in the course of my career, I became very familiar with the various satellite types, their operation, and their capabilities. Anyone seeking further information on GPS can visit the website gps.gov.