by Scott Leonard
DAHLGREN, Va. — When the Soviet Union launched Sputnik I, the first man-made satellite, in October 1957, the space race began.
The United States didn’t have the capability to detect satellites. But, by 1961, the Naval Research Laboratory’s innovative research made such detection a reality with its Naval Space Surveillance System, commonly known as the Fence in Dahlgren.
The Fence is a specially designed multi-static radar interferometer system located at nine stations on a great-circle arc across the southern United States on the 33 degree parallel. It was one of the military’s first tracking assets, and the use of its technology was instrumental in the development of the current space surveillance network world-wide.
The Fence can detect basketball-sized objects in orbit around the earth out to an effective range of 15,000 nautical miles. The joint-service Space Surveillance Network, of which the Fence is a part, consists of approximately two dozen radar and optical sensors located around the world. The Air Force operates most of the SSN sites, including phased-array radars, mechanical radars, optical telescopes and passive radio-frequency receiver sites. The Air Force assumed operations of the Fence in 2004, effectively changing its name to the Air Force Space Surveillance System, or AFSSS.
Contributions to Space
The SSN was never designed as the “system of systems” that it has become today. It consists mainly of sensors that developed into service over the years to accomplish the space surveillance mission. Of the approximately 20 SSN radars, only three were designed to monitor orbiting space objects: the phased-array radar at Eglin AFB, Fla., Satellite-Based Surveillance System and the Fence. All the other radars have primary missions other than space surveillance, such as missile warning, launch and test range support, intelligence and research. The Fence produces approximately 50 percent of the observations accumulated by the SSN. All the other radars collectively provide the remainder. The Fence is the most economical of all the sensors, besides being the most productive: the cost per observation is considerably lower than any other radar. After 50 years and running, the Fence’s reliability is top among all sensors maintaining a 99 percent overall availability.
Over time, the Fence observes at least 80 percent of all near-Earth objects and 45 percent of all deep-space objects. The Fence is closer to being an “AWACS of Space” than is any other SSN asset. In particular, the Fence’s un-cued detection of deep-space satellites is a unique capability in the world, and allows the system to reliably observe a type of object that is notoriously difficult for other sensors to acquire.
Satellites in rapidly decaying, highly eccentric orbits have a complicated motion that is hard to predict, so with a narrow-beam or narrow-field-of-view, other sensors often have little chance of finding these elusive targets. In comparison, the Fence is virtually guaranteed to observe such an object if it passes anywhere through the field of regard. Objects in such orbits are a special concern because they eventually decay down to shuttle and International Space Station altitudes, where they can become collision hazards as well as re-entry hazards.
What’s on the Horizon
Today, the AFSSS continues to function as a unique national asset. It serves as one of the first indicators on satellite maneuvers and breakups in orbit, as well as the primary provider of radar data for NASA’s manned spaceflight program, the International Space Station and shuttle. There is a plan to replace the AFSSS with modernized S-Band radar tracking smaller objects, greater numbers of objects, and increased radar accuracy. But for now, the AFSSS remains vigilant providing millions of satellite observations every month.
Historical Timeline of the AFSSS “Fence”
Oct. 4, 1957–Sputnik I (first artificial satellite) launched
1958–NRL started development on surveillance systems
1959–Six-station Fence network operational 24/7
1961–Naval Space Surveillance System (NAVSPASUR) established
1963–NAVSPASUR was designated as the “back-up” for NORAD in Cheyenne Mountain, Colo.
1968–Fleet support products created from Fence data to track satellites
1983–Naval Space Command established
1984–New mission as the Alternate Space Control Center (ASCC)
1993–Naval Space Command assumes Fence operations and space surveillance mission
2002–Naval Space Command and Naval Network Operations Command merge into the Naval Network and Space Operations Command (NNSOC)
2004–Air Force takes operational responsibility of the “Fence” from the Navy, and renamed AFSSS
2004–20th Space Control Squadron, Detachment 1 was established
2007–AFSSS detects Chinese anti-satellite test debris, Dahlgren analysts find more than 3,000 pieces
2009–AFSSS identifies satellite collision between Iridium and Russian satellite; Dahlgren analysts find more than 2,500 pieces
2010–ASCC changed to Distributive Space Command and Control as a detachment under 614th Air and Space Operations Center at Vandenberg AFB, Calif.
2011–AFSSS reached its 50th anniversary milestone
(Mr. Leonard has worked in space programs for 21 years. Dr. Paul Schumacher from Air Force Research Lab in Maui, Hawaii, and Carroll Hayden from ITT Sensor in Fredericksburg, Va., contributed to this article.)