Space system profile: The Space Fence

The Space Fence is the oldest and most basic of the numerous systems the US uses to track objects in space. The system grew out of an effort by the Naval Research Laboratory (NRL) to detect and track satellites that did not emit signals as part of their normal operations. It was formally ushered into existence as the Naval Space Surveillance System (NSSS) in 1961. Its nickname, the “Space Fence,” was derived from its system concept, continuous wave multistatic radar. Systems of this type use high-power transmitters to emit a fan-shaped beam of energy, which is known as a “fence.”

The system is composed of three transmitters and six receivers interspersed across the southern US at the 33rd parallel. While it only costs about US$33 million a year to run, much of which is needed for electricity to power the transmitters, the Space Fence is capable of detecting nearly 60 percent of the orbiting object catalog. The objects it cannot detect are those which do not cross over the US near the 33rd parallel and those too small to detect (smaller than 30 centimeters [cm] in diameter).

While this venerable system is still quite useful, it does have its limitations. As noted above, some orbital trajectories are beyond its reach, and it cannot reliably detect objects above 15,000 nautical miles (nm), though from time to time it picks up objects at a range of 20,000 nm or above. This allows it to reliably detect objects in Low Earth Orbit (LEO) and Medium Earth Orbit (MEO), but not Geosynchronous orbit, which is home to a number of critical communications satellites.

The original mission of the NSSS was to track potentially hostile satellites that could compromise the positions of naval assets, but it has since played a part in general space situational awareness: detecting breakups of existing objects and debris from new launches and maneuvers.

The Space Fence is also particularly useful when examining the orbits of previously undetected objects. Because about 97 percent of the fence’s observations are automatically associated with known objects, a much higher percentage than is the case with the other satellite tracking systems in the Space Surveillance Network (SSN), it is uniquely suited to analyze these rogue objects, or “uncorrelated targets.”

In late 2003, the Navy began transferring the Space Fence to the US Air Force (USAF), which renamed the system the Air Force Space Surveillance System (AFSSS). The system was operated by the Navy from its traditional command post in Dahlgren, Va, until the end of fiscal year 2004 (FY 04), when operations were transferred to the Air Force’s 20th Space Control Squadron, based at Eglin Air Force Base, Fla. The new command post is being updated to allow it to interface with the Air Force Integrated Space Command and Control Architecture at Cheyenne Mountain.

This US$1.5 billion, 15-year program, which is run by Lockheed Martin, will integrate upwards of 40 systems with an end goal of producing real-time data of the global space operational picture.

A plan to upgrade the system’s transmitters and receivers by employing the S-band frequency range was approved in 1997; the new system should be able to detect objects as small as five cm in diameter, a significant improvement over its current detection threshold. Upgrades were originally scheduled to begin in 2002 and end in 2010; this schedule has been delayed by almost a decade due to issues related to the transfer of operations between armed service branches and delays in developing other programs which could have replaced the system. The current schedule calls for releasing a draft request for proposals in February 2008, with a formal request to follow in early 2009. The installation of the S-band system is now supposed to take place in 2013, but if the cuts that have plagued the system of late persist, this could very well be delayed.

In the FY 08 budget, the Space Fence is the recipient of requests from two different areas. One area is the Research, Development, Testing, and Evaluation request, devoted to upgrading the system. The budget contains specifics about these plans.

“By using higher radio frequencies in conjunction with radar transmitters and receivers co-located at sites dispersed worldwide, the Space Fence will provide timely detection of smaller orbiting objects, primarily those in Low Earth Orbit. As a result, it will expand the detection and tracking capacity of the Space Situation Awareness network by an order of magnitude, from 10,000 to 100,000 objects, while working in concert with other network sensors.”

The request for FY 08 for this project totals a little over US$4 million, down US$9.91 million from the FY 07 budget plan for US$13.910 million in FY 08. Though in FY 09 this will increase fourfold and in FY 10 the planned request is more than US$65 million, these figures are again lower than the amount projected by the FY 07 budget request. The latter had shown a planned FY 10 budget of US$106.53 million.

The Space Fence also receives funding under the Air Force’s Operations and Maintenance (O &M) budget area which, though not broken down for the fence alone, should be in line with previous requests (the yearly O &M budget is about $30 million).

The Air Force is also seeking funding for a Space-Based Satellite Surveillance System (SBSS), a constellation of satellites that would perform a mission similar to that of the Space Fence. When justifying the overlap, the Air Force “characterized the Fence and the Space Based Space Surveillance System as complementary and said they would be used to ‘emphasize distinct requirements sets.’”

The Space Fence is a shining example of efficiency, effectiveness and longevity in a field where these characteristics are typically not associated. However, the Air Force briefly considered shutting the system down when it gained operational control a few years ago, causing the planned system upgrades to be delayed. Furthermore, systems like the SBSS have been proposed that would make the fence obsolete, albeit at a high cost. Though the future of the Space Fence is secure for the time being, the system’s accomplishments to date should not be overlooked.