Evolution of the Air Force Satellite Control Network

The master control room of the Air Force Satellite Test Center in Sunnyvale, California, just after its completion in 1961. (Photo courtesy of US Air Force)

Evolution of the Air Force Satellite Control Network

Jeffrey Fedor, Ronald Nishinaga, Carl Sunshine, and Patrick James

Space systems must endure a physically stressful journey from the launchpad to their final destinations. Adequate testing can help ensure they survive the trip.

The Air Force Satellite Control Network (AFSCN) is a global, robust, continuously operational network of ground stations, operational control nodes, and communications links that support launch, command, and control of various space programs managed primarily by the Department of Defense and other national security space organizations. In addition, it supports certain nonmilitary U.S. space assets, such as the weather satellites operated by the National Oceanic and Atmospheric Administration (NOAA), and select programs of allied nations. The Air Force Space and Missile Systems Center (SMC) manages acquisition, including development and sustainment of network capabilities.

The mission of AFSCN is to provide telemetry, tracking, and commanding as well as communications, selected mission-data dissemination, and data processing for operational missions and programs relating to research, development, testing, and evaluation of space systems. The history of AFSCN demonstrates the challenges of modernizing a fielded system that is in constant operational use. For more than four decades, program managers have been challenged by the need to insert more operationally effective and economical computing systems, communications equipment, and standards. Today, the network is undergoing a major upgrade of its remote ground stations while planning for increased interoperability with defense, civil, and commercial space control assets. The Aerospace Corporation has been a vital partner in the planning and evolution of the AFSCN since its early years and is helping to transition the network through its next phase of development.

The AFSCN currently has facilities in Colorado Springs, Colorado; Sunnyvale, California; Vandenberg, California; New Boston, New Hampshire; Thule, Greenland; England; Guam; Diego Garcia in the Indian Ocean; and Oahu, Hawaii. Throughout the years, remote tracking stations have been installed and decommissioned in various locations to suit changing requirements. Stations have also operated in Annette Island, Kodiak Island, and Donnelly Flats, Alaska; Ft. Dix, New Jersey; Camp Roberts, California; and the Indian Ocean, Seychelles.

Overview

The AFSCN consists of two operational control nodes, numerous satellite operations centers, eight geographically dispersed remote tracking stations, 15 antennas, and the wide-area network that connects them.

The two operational control nodes, located at Schriever Air Force Base in Colorado Springs, Colorado (primary) and Onizuka Air Force Station in Sunnyvale, California (secondary), provide the communication relays and resource management that allow the satellite operations centers to interact with the remote tracking stations.

The satellite operations centers command and monitor space vehicles. Many are housed within the operational control nodes, but some are geographically separated. Each is responsible for the day-to-day operations of one or more satellites in a constellation or family. Some are common to many programs and involve the satellite bus for activities such as stationkeeping, power management, and attitude control. Others are focused on specific missions and perform functions related to operating and managing the unique mission payloads. Specific tasks of the operations centers include receiving and interpreting data from the satellites, locating and tracking satellites, transmitting commands to the satellites, and isolating and correcting both spacecraft and ground system failures. Communications with remote tracking stations rely principally on Defense Information Systems Network and commercial communication assets.

The remote tracking stations communicate with space vehicles using the highly reliable Space Ground Link Subsystem (SGLS) in the upper L and S bands (1755–1850 megahertz uplink, 2200–2300 megahertz downlink), which are relatively unaffected by adverse weather conditions. Several stations have extra high power to ensure communications even when satellite subsystems are degraded or when the vehicle is tumbling and communications must rely on omnidirectional satellite antennas with lower gain. Communications are only possible when ground antennas have a direct line of sight to a satellite. Sight times vary with orbital altitude and can be quite brief—as short as a few minutes.

Typical AFSCN daily contacts.

Operations

Typically, a satellite operations center uses AFSCN to connect to a specified remote tracking station for a satellite contact of 15–20 minutes, during which commanding, telemetry monitoring, tracking, or mission data recovery functions may be performed. National systems programs represent just over 40 percent of these daily contacts. Other primary clients include the Defense Meteorological Satellite Program, the Defense Support Program, and the Defense Satellite Communication System, which collectively account for more than 30 percent of daily AFSCN usage.

Requests for service are generated by the satellite operations centers and sent to the network operations center at Schriever Air Force Base, generally 10 days before the satellite pass. These are reviewed and reconciled according to an established priority scheme, leading to an overall schedule for all AFSCN operations, which is sent back to the satellite operations centers and to each remote tracking station. At the allocated time, each operations center connects directly to the assigned tracking station and directs the contact activity. In case of trouble, assistance is available from a network control center.

As part of the contact process, the communications links and station configuration are first checked during a "prepass" interval. Then, the remote tracking station is directed to point its antenna at the space vehicle. It tracks the satellite, transmitting commands and ranging signals while receiving telemetry. After the satellite pass, the satellite operator frees up system resources, and satellite operations center personnel add the contact data to the satellite history to identify trends and update orbits.

AFSCN allows real-time exchange of space vehicle data and station status and commands between the satellite operations center and the remote tracking station. Some contacts require only a subset of these functions, while other activities, such as satellite troubleshooting or early orbit checkout, may require more extensive contacts and have high-priority access to AFSCN assets. Some missions require prolonged or continuous coverage of their satellites, and in these cases, dedicated networks managed from the satellite operations centers are used in parallel with AFSCN. For example, the Global Positioning System uses AFSCN for launch, early orbit checkout, and anomaly resolution and a separate dedicated satellite control ground system for daily operations.

Aerospace and AFSCN: The Early Years

Aerospace has been involved with AFSCN almost since its inception, providing technical support throughout its long evolution. As early as 1961, the Air Force asked Aerospace to establish a satellite control office to provide general systems engineering and technical direction for its Satellite Control Facility in Sunnyvale (now known as Onizuka Air Force Station). At that time, the facility was less than three years old, but already experiencing growing pains (see sidebar, AFSCN Historical Timeline). Deficiencies in equipment and difficulties in coordinating contractors were hampering operations and preventing needed growth. Aerospace set up an office in Palo Alto, California, which soon became part of a larger satellite control office in El Segundo.


Interim Satellite Control Center in Palo Alto, California, as it looked in 1959. (Photo courtesy of US Air Force)	The Air Force remote tracking station at Kaena Point on the island of Oahu, Hawaii, soon after its construction in 1959. (Photo courtesy of US Air Force)

In those early days, AFSCN was clearly an ad hoc effort. No two tracking stations shared the same configuration, and each satellite program employed its own system for telemetry, tracking, and control. Aerospace immediately helped plan and implement the first major upgrade, known as the Multiple Satellite Augmentation Program. This project involved standardization of support equipment, simplification of remote tracking stations, and augmentation of data processing at the Satellite Control Facility. By the time this upgrade was complete, the number of AFSCN remote tracking stations had grown from the original three to six, and of these, three could control two satellites at once. In addition, the Satellite Control Facility had the processing capacity to control six satellites simultaneously.

Aerospace quickly followed up supporting a second modernization effort, the Advanced Data System project, which entailed writing new software, installing a new communications system, and standardizing satellite transmission frequencies and beacons. Part of these efforts involved implementation of the Space Ground Link Subsystem, which integrated the separate tracking, telemetry, and commanding subsystems. Thus, a single autotracking antenna at each tracking station could send and receive encrypted data for one satellite, and within a matter of minutes, be ready to interact with another. By the end of the decade, it was deployed in every tracking station, enabling real-time management of space assets.

"Without the system of satellite command and control, there could have been no military space program, and more importantly, no system of satellite-based reconnaissance."

—Forrest S. McCartney, Lt. Gen, USAF (Ret.), satellite operator 1959–1961

Continued Support

Since then, Aerospace has helped transition AFSCN through numerous hardware and software upgrades. One of the most significant was the Data Systems Modernization program, which introduced centralized database-driven computer hardware and software to replace the dispersed Current Data System. Using this new architecture, all data would be sent for processing to Sunnyvale, which would also maintain centralized control of operations such as satellite pointing. Thus, the remote tracking stations essentially became remote relay stations, requiring only limited personnel for basic maintenance. The program was initiated in 1980, and by February 1992, the new system was able to perform all of the functions needed to support the satellites then in orbit. The Data Systems Modernization was more reliable than the old system, cheaper to maintain, and faster in its operation, allowing it to support a steadily increasing satellite support workload.

In 1985, during the course of the Data Systems Modernization effort, satellite control operations started a gradual transition from Sunnyvale to a more secure and modernized facility at Schriever Air Force Base in Colorado Springs. But before this newly built Consolidated Space Operations Center (CSOC) could become operational, it needed a communication system to connect it with remote tracking stations throughout the world. The new communication system was experiencing development problems, and it looked as though the Air Force would not be able to start satellite control operations on schedule. To prevent this delay, Aerospace worked with the Air Force to develop a communication system, called Backhaul, that would connect the Consolidated Space Operations Center to the Satellite Control Facility at Sunnyvale. It could then use the existing communication links from Sunnyvale to the remote tracking stations. Aerospace conducted architecture studies, developed system specifications, and supported the design, integration, and testing of this system. As a result, Air Force operators were able to begin satellite operations through the Backhaul link in 1988. Moreover, the experience gained in implementing Backhaul proved useful when the new communication system connecting the Consolidated Space Operations Center to the remote tracking stations was completed several years later, through a project called Wideband Direct Connect.


The operations center, known as the Blue Cube, at Onizuka Air Force Station, in Sunnyvale as it appeared in 1993. The Blue Cube was constructed during 1967–1968. The Sunnyvale complex was known as the Air Force Satellite Test Center. (Photo courtesy of US Air Force)	The Consolidated Space Operations Center, now the headquarters of Air Force Space Command's 50th Space Wing, shown soon after its completion. SMC developed, built, and tested this satellite control node and support facilities before turning it over to Air Force Space Command in 1993. (Photo courtesy of US Air Force)

During the 1990s, Aerospace assisted the Air Force in developing and enhancing four mission control complexes as well as a range control complex at the Consolidated Space Operations Center. Aerospace also established the acquisition program baseline and developed the requirements definition and rationale for transferring responsibility of the Consolidated Space Operations Center from the Air Force Materiel Command to Air Force Space Command in 1993. As the network evolved, Aerospace continued to play a major role in defining requirements for engineering upgrades.

For example, Aerospace took part in the multiyear Automated Remote Tracking Station project, which was completed in 1994. Aerospace helped the Air Force modernize the tracking stations, installing four new 10-meter fixed antennas and two transportable antenna systems. Automated equipment was installed at all the existing tracking stations and in new stations in Colorado Springs and on the island of Diego Garcia in the Indian Ocean. The new equipment improved reliability, increased the operational capacity of the tracking stations, and automated many critical functions. These enhancements enabled the Air Force to reduce the number of operators at each station from five to two per shift, thereby trimming the cost of operations and maintenance.

tracking station with phased-array antenna

Conceptual rendering of a remote tracking station featuring a phased-array antenna incorporated into a geodesic dome. (Courtesy of Ball Aerospace)

Recent Changes

In 1997, Air Force Space Command established a new policy, directing that each future space mission should develop its own satellite operations center. AFSCN would continue to provide a shared worldwide communication and tracking station network to support the communications needed for satellite control, but the AFSCN program would cease active sustainment of the common command and control subsystem when all programs transitioned to their unique system. In support of this move, Aerospace assessed plans for future command and control segment requirements to ensure a smooth transition to self-sufficiency for each of the satellite programs involved.

Still, work on the core network continued, and Aerospace prepared an AFSCN evolution plan that was approved by the Air Force to chart the future direction. This study supported guidance from the National Security Space Architect and an interagency team to move U.S. Government satellite control networks toward common technical standards and a more interoperable set of networks.

In March 2004, Aerospace engineers joined an Air Force-contractor team to resolve tracking problems associated with a new antenna deployed at the Telemetry and Command Station in England. The contractor had spent two years trying to isolate and resolve unstable performance of the antenna as it tracked satellites. Aerospace experts in antenna receiver, control systems, and digital software design obtained measurements and performed analysis to determine the causes of the tracking problems. They evaluated the new reflector system's autotrack performance and were able to isolate the problematic subsystem and to develop test scenarios for providing additional diagnostic data. Performance assessments were provided immediately after each mission, allowing the team to efficiently search for the cause of the problem. An improper configuration value for the antenna controller software was identified as the final cause of the instability; once this error was corrected, the antenna control system was able to automatically track all satellites with 100 percent success during extensive operational tests.

Future Directions

evolution of AFSCN from initial point-to-point architecture

Aerospace is involved in the continued evolution of AFSCN from its initial point-to-point architecture using proprietary data-transfer protocols to an interoperable network architecture using standard protocols. Aerospace defined a five-year demonstration program to show how DOD satellite contact requirements could be satisfied using open standards and protocols.

Today, Aerospace is involved in the continued evolution of AFSCN from its initial point-to-point architecture using proprietary data-transfer protocols to an interoperable network architecture using standard protocols. New data formats and protocols are under consideration for commanding, telemetry, scheduling, tracking, and remote ground station configuration. Aerospace defined a five-year demonstration program, now nearing completion, to show how DOD satellite contact requirements could be satisfied using standards and protocols from the Internet and the Consultative Committee for Space Data Systems, with its related Space Link Extension. These demonstrations have included the use of NASA, NOAA, and commercial ground sites to contact DOD research and test satellites. Further work is needed to enhance information assurance methods and obtain agreements that would allow potential operational use by more missions.

Aerospace is also involved in the development and assessment of future remote tracking station enhancements in several areas. Greater automation will allow unattended operation and faster switching between contacts. The use of digital signal processing will accommodate a wider range of more bandwidth-efficient ground-to-space link modulation waveforms. Replacement of traditional dish antennas with phased-array antennas will allow multiple simultaneous contacts and lower maintenance cost and downtime through the elimination of moving parts.

Conclusion

During the past 50 years, the AFSCN evolved to meet satellite command and control needs of national security space systems. Its evolution is on track toward a more standards-based system that can operate with other DOD and civil satellite control systems and become a critical element of the U.S. Government interoperable satellite control network.