The United States Space Surveillance Network (SSN) detects, tracks, identifies, and catalogs artificial objects orbiting Earth. Examples include satellites, abandoned launch vehicle stages, and other space debris. The SSN is the responsibility of United States Space Command and is operated by the United States Space Force. Its functions include:
Predict when and where a decaying space object will re-enter the Earth's atmosphere; Prevent a returning space object, which to radar looks like a missile, from triggering a false alarm in missile-attack warning sensors of the U.S. and other countries; Chart the present position of space objects and plot their anticipated orbital paths; Detect new artificial objects in space; Map objects traveling in Earth orbit; Produce a running catalog of artificial space objects; Determine ownership of a re-entering space object; Command and control of future U.S. anti-satellite weapon (ASAT) systems.
The SSN includes dedicated, collateral, and contributing electro-optical, passive radio frequency (RF) and radar sensors. It provides space object cataloging and identification, satellite attack warning, timely notification to U.S. forces of satellite fly-over, space treaty monitoring, and scientific and technical intelligence gathering. The continued increase in satellite and orbital debris populations, as well as the increasing diversity in launch trajectories, non-standard orbits, and geosynchronous altitudes, necessitates continued modernization of the SSN to meet existing and future requirements and ensure their cost-effective supportability.
History
1957–1963
The first formalized effort by the US government to catalog satellites occurred at Project Space Track, later known as the National Space Surveillance Control Center (NSSCC), located at Hanscom Field in Bedford, Massachusetts. The procedures used at the NSSCC were first reported in 1959 and 1960 by Wahl,Wahl, E[berhart] W., Program Development in Orbital Computation at the U.S. National Space Surveillance Control Center. [Proceedings of the Second Symposium (International) on Rockets and Astronautics]. [Tokyo: May 1960.] who was the technical director of the NSSCC. In 1960, under Project Space Track, Fitzpatrick and Findley developed detailed documentation of the procedures used at the NSSCC. Project Space Track began its history of satellite tracking from 1957 to 1961.
Early Space Track observations of satellites were collected at more than 150 individual sites, including radar stations, Baker–Nunn cameras, telescopes, radio receivers, and by citizens participating in the Operation Moonwatch program. Individuals at these Moonwatch sites recorded satellite observations visually, but there were numerous observation types and sources, some automated, others only semi-automated. The observations were transferred to the NSSCC by teletype, telephone, mail, and personal messenger. There, a duty analyst reduced the data and determined corrections that should be made to the orbital elements before they were used for further predictions. After this analysis, the corrections were fed into an IBM 709 computer that computed the updated orbital data. The updated orbital data were then used in another phase of the same computer program to yield the geocentric ephemeris. From the geocentric ephemeris, three different products were computed and sent back to the observing stations for their planning of future observing opportunities.
Missile Warning and Space Surveillance in the Eisenhower Years The launch of Sputnik 1 by the Soviet Union led to a US government-perceived need to better track objects in space using the Space Tracking System. The first US system, Minitrack, was already in existence at the time of the Sputnik launch, but the US quickly discovered that Minitrack could not reliably detect and track satellites. The US Navy designed Minitrack to track the Vanguard satellite, and so long as satellites adhered to the international agreement on satellite transmission frequencies, Minitrack could track any satellite. However, the Soviets chose not to use the international satellite frequencies. Thus, a major limitation of this system became visible. Minitrack could not detect or track an uncooperative or passive satellite.
Concurrent with Minitrack was the use of the Baker-Nunn satellite tracking cameras. These systems used modified Schmidt telescopes of great resolution to photograph and identify objects in space. The cameras first became operational in 1958 and eventually operated at sites worldwide. At their peak, the Air Force ran five sites, the Royal Canadian Air Force ran two, and the Smithsonian Institution's Astrophysics Observatory operated a further eight sites. The Baker-Nunn system, like Minitrack, provided little real-time data and was limited to night-time, clear weather operations.
Beyond the problems in acquiring data on satellites, it became obvious that the US tracking network would soon be overwhelmed by the tremendous number of satellites that followed Sputnik and Vanguard. The amount of satellite tracking data accumulated required creation or expansion of organizations and equipment to sift through and catalog the objects. The need for real-time detection and tracking information to deal with Soviet satellite launches led on 19 December 1958 to ARPA's implementation of Executive Order 50-59 to establish a spacetrack network. This spacetrack network, Project Shepherd, began with the Space Track Filter Center at Bedford, Massachusetts, and an operational space defense network (i.e., a missile-warning network). ARDC took up the spacetrack mission in late 1959 and in April 1960 set up the Interim National Space Surveillance Control Center at Hanscom Field, Massachusetts, to coordinate observations and maintain satellite data. At the same time, DOD designated the Aerospace Defense Command (ADCOM), formerly Air Defense Command, as the prime user of spacetrack data. ADCOM formulated the first US plans for space surveillance.
During the period that intercontinental ballistic missiles were being developed as frontline weapon systems, numerous missile detection and warning sensors were fielded as operational sensors. Most of these contributed satellite observation data at one time or another. Among these were detection and tracking radars in Trinidad Laredo, Texas, Moorestown, New Jersey, Kaena Point, Antigua, Ascension Island, Naval Station San Miguel, and Kwajalein Atoll, the three BMEWS sites, the Pave Paws sites, the AN/FSS-7 missile warning radar sites, the passive electronically scanned array sites, Cavalier, ND, Eglin, FL, Maui Space Surveillance Complex, Globus II, San Vito dei Normanni Air Station, TOS/CROSS, and MIT Lincoln Laboratory.
Air Force Space Surveillance System
The Air Force Space Surveillance System (AFSSS) was a very high frequency radar network located at sites across the southern United States with a centralized data-processing site at the Naval Space Command in Dahlgren, Virginia. AFSSS began as the U.S. Navy Space Surveillance System (NSSS) in 1961. The system detected space objects from new launches, maneuvers of existing objects, breakups of existing objects, and provided data to users from its catalog of space objects. Orbital parameters of more than 10,000 objects were maintained in this catalog—which has now gained usage by NASA, weather agencies, and friendly foreign agencies. The information is essential to computing the collision avoidance information to de-conflict launch windows with known orbiting space objects.
NSSS was transferred to the Air Force in 2004 and renamed AFSSS. It was operated by the 20th Space Surveillance Squadron, Detachment 1. The 21st Space Wing closed the Air Force Space Surveillance System on 1 October 2013, citing resource constraints caused by sequestration. A new S-band Space Fence is under construction at Kwajalein Atoll.
US Space Catalog Since 1957, the United States Department of Defense (DoD) has maintained a database of satellites states known as the Space Object Catalog or simply the Space Catalog. Starting with the launch of Sputnik 1 as the second database entry and its booster listed as the first database entry. These satellite states are regularly updated with observations from the Space Surveillance Network, a globally distributed network of interferometer, radar and optical tracking systems. By the year 2024, the number of cataloged objects was past 31,000.
Different astrodynamics theories are used to maintain these catalogs. The General Perturbations (GP) theory provides a general analytical solution of the satellite equations of motion. The orbital elements and their associated partial derivatives are expressed as series expansions in terms of the initial conditions of these differential equations. The GP theories operated efficiently on the earliest electronic computing machines, and were therefore adopted as the primary theory for Space Catalog orbit determination. Assumptions must be made to simplify these analytical theories, such as truncation of the Earth's gravitational potential to a few zonal harmonic terms. The atmosphere is usually modeled as a static, spherical density field that exponentially decays. Third body influences and resonance effects are partially modeled. Increased accuracy of GP theory usually requires significant development efforts.
NASA maintains civilian databases of GP orbital elements, also known as NASA or NORAD two-line elements. The GP element sets are "mean" element sets that have specific periodic features removed to enhance long-term prediction performance, and require special software to reconstruct the compressed trajectory.
Shemya and Diyarbakir Radar Sites AN/FPS-17 and AN/FPS-80 radars were placed at Shemya Island in the Aleutian Islands off the Alaskan coast in the 1960s to track Soviet missile tests and to support the Air Force Spacetrack System. In July 1973, Raytheon won a contract to build a system called "Cobra Dane" on Shemya. Designated as the AN/FPS-108, Cobra Dane replaced AN/FPS-17 and AN/FPS-80 radars. Becoming operational in 1977, Cobra Dane also had a primary mission of monitoring Soviet tests of missiles launched from southwest Russia aimed at the Siberian Kamchatka peninsula. This large, single-faced, phased-array radar was the most powerful ever built.
The FPS-80 was a tracking radar and the FPS-17 was a detection radar for Soviet missiles. Both were part of the Ballistic Missile Early Warning System (BMEWS). The large detection radar (AN/FPS-17) went into operation in 1960. In 1961, the AN/FPS-80 tracking radar was constructed nearby. These radars were closed in the 1970s.
The Diyarbakır Air Station intelligence collection radar site ultimately consisted of one detection radar (FPS-17) and one mechanical tracking radar (FPS-79). The Pirinclik radars were operated by the 19th Surveillance Squadron. The FPS-17 radar reached initial operational capability (IOC) on 1 June 1955 and the FPS-79 in 1964. Both radars operated at a UHF (432 MHz) frequency. Although limited by their mechanical technology, Pirinclik's two radars gave the advantage of tracking two objects simultaneously in real time. Its location close to the southern Former Soviet Union made it the only ground sensor capable of tracking actual deorbits of Russian space objects. In addition, the Pirinclik radar was the only 24-hour-per-day eastern hemisphere deep space sensor. Radar operations at Pirinclik were terminated in March 1997.
AN/FPS-17