History of U.S. Military Satellite Communication Systems

A History of U.S. Military Satellite Communication Systems

Donald H. Martin

Twenty-five hundred years ago the Chinese general Sun Tzu wrote, "If you know the enemy and know yourself, you need not fear the result of a hundred battles." But how are U.S. soldiers, operating covertly in unfamiliar and hostile territory, to know where their allies are, where their enemies are, and what each is doing? How are they to receive commands and report status? The answer is satellite communications.

Satellite communication has been a vital part of the United States military throughout the space age, beginning in 1946, when the Army achieved radar contact with the moon. In 1954, the Navy began communications experiments using the moon as a reflector, and by 1959, it had established an operational communication link between Hawaii and Washington, D.C.

As the U.S. space program grew in the 1960s, the Department of Defense (DOD) began developing satellite communication systems that would address the special requirements of military operations. In addition to protection against jamming, these needs included the flexibility to rapidly extend service to new regions of the globe and to reallocate system capacity as needed.

The goal of these systems has been to provide communications between, and to supply information to, military units in situations where terrestrial means of communication are impossible, unreliable, or unavailable. This goal was partly realized with the earliest DOD communication satellites, and as satellite and communications technology has improved, the goal has been realized to a much greater extent.

Early DOD satellite communication experiments led to initial operational systems, which evolved to a complete military satellite communications (milsatcom) architecture encompassing DOD's unique requirements. Within this milsatcom architecture, different systems were developed for three broad populations of users: wideband, tactical, and protected. Each is characterized by its own satellite designs, Earth terminals, and applications.

The Aerospace Corporation has been a key player throughout this history. From the early days of the space age, the company has taken a significant role in the development and deployment of military communication satellites. Aerospace participates in all planning efforts for these satellite systems, including studies of requirements, surveys of current and projected technologies, and analyses of multiple alternatives to satisfy requirements (see Milsatcom Timeline).

Aerospace assists DOD in the definition of technical requirements for satellite systems. As satellite hardware is designed, built, and tested, Aerospace reviews the designs, analyses, and test plans; observes testing; and studies test results. It also assists with launch preparations and support of on-orbit operations.

The First Satellite Communication Programs

The first U.S. military communication satellites were of an experimental nature and used low-altitude orbits. They were developed to provide basic experience with satellites and to explore what satellite communications could do. Later systems would see actual military field use.

SCORE

The first artificial communication satellite, Project SCORE (Signal Communication by Orbiting Relay Equipment), was launched in 1958, primarily to show that an Atlas missile could be put into orbit. The secondary objective was to demonstrate a communications repeater built into the missile. A repeater receives a signal, amplifies it, and then retransmits it.

The Army Signal Research and Development Laboratory created the repeater by modifying commercial equipment. Two redundant sets of equipment were mounted in the nose of the SCORE missile. Four antennas were mounted flush with the missile surface, two for transmission and two for reception.

SCORE's other equipment included two tape recorders, each with a four-minute capacity. Any of four ground stations in the southern United States could command the satellite into playback mode to transmit the stored message or into record mode to receive and store a new message. One was a Christmas message from President Dwight D. Eisenhower.

Courier

The objective of the Courier program was to develop a satellite of higher capacity and longer life than SCORE for use in communication tests and assessments of traffic-handling techniques. Courier's primary operating mode, like SCORE's, was store-and-dump (storing data onboard to be later "dumped," transmitted to a ground receiving station when one is in sight of the satellite) using tape recorders. Unlike SCORE, however, Courier was a self-contained satellite.

The first Courier launch was unsuccessful because of a booster failure; the second, in October 1960, succeeded. Communication tests were performed by ground terminals in New Jersey and Puerto Rico. The satellite performed satisfactorily until 17 days after the launch, when a command-subsystem failure stopped communications.

Advent

Courier was a relatively simple satellite. Since it was designed for experimental use, the Advanced Research Projects Agency undertook the Advent program in 1960, concurrent with the Courier program, to provide an operational military communication satellite. The concept for Advent was far more sophisticated than the technology available at the time; hence a number of problems occurred in development, and it was canceled in 1962.

A few of West Ford's 480 million copper wires. (IEEE)

West Ford

West Ford grew out of a Lincoln Laboratory study on secure, survivable, reliable communications. The West Ford "satellite" consisted of 480 million thin copper wires, each about 1.5 centimeters long. The 19.5 kilograms of wires were dispensed from an orbiting container in 1963. During the first few weeks after launch, voice and data were transmitted from Pleasanton, California, reflected by the wires, and received at Westford, Massachusetts, the source of the project name. Four months later, when the wires were further dispersed, they could reflect only very low-rate data from Pleasanton to Westford. Because of this low capacity and the growing use of active satellites, no more experiments like West Ford were attempted.

Early Operational Satellite Communication Programs

Policy debates in the early 1960s addressed the question of whether military and civilian communication satellite systems should be separate or combined. Aerospace participated in 1964 congressional hearings that resulted in a government policy to establish and maintain separate military satellite communication systems to satisfy unique and vital national-security needs that commercial systems could not satisfy. The government was still able to use commercial satellites if those satellites provided links of the required type and quality in a timely manner at reasonable cost.

Lincoln Experimental Satellites

After the West Ford program, Lincoln Laboratory continued its investigation of space technology for application to military communications, developing the Lincoln Experimental Satellites (LES) series.

Aerospace and Air Force program managers with a model of an IDCSP satellite in 1966. The polyhedral body is covered with solar cells, and the antennas extend to the right of the body. (The Aerospace Corporation Archives)

LES-1 through LES-4 carried equipment for communication and propagation experiments. The primary experiment on LES-1, -2, and -4 was a repeater and an eight-horn electronically switched antenna. Frequencies were in the 7900-to-8400-megahertz range for the uplinks (the Earth-to-satellite transmissions) and the 7250-to-7750-megahertz range for downlinks (satellite-to-Earth transmissions)—a combination used by later military communication satellites that is called X-band (see sidebar, Frequency Selection).

The LES-3 frequency was in the portion of the spectrum that DOD called ultrahigh frequency (UHF), commonly used for DOD tactical communications among small terminals. LES-3 was intended to investigate the extension of UHF communications to links with satellites. It transmitted a signal to be used in atmosphere-propagation measurements.

The first four LES satellites were launched in 1965. Although not all reached their intended orbits, they were used for more than a year. They demonstrated payload operations in space, supported propagation measurements, and helped improve ground equipment for both communications and satellite control (see sidebar, A Communications Satellite Payload).

The LES-5 and -6 satellites had communications equipment that operated in the UHF band. Both had transmitter designs with significant improvements over those of prior satellites.

LES-5 was launched in 1967. Airborne, shipborne, and fixed and mobile ground terminals were involved in a large number of successful tests with LES-5, which operated until 1971. LES-6, used in similar tests, was launched in 1968. These satellites clearly demonstrated that reliable communications could be extended to military units equipped with small terminals.

Eight IDCSP satellites mounted on a launch dispenser in preparation to be launched from Cape Canaveral, Florida, on a Titan IIIC launch vehicle. (U. S. Air Force)

Initial Defense Communication Satellite Program

When the Advent program was canceled in 1962, a recommendation was made for another program that would be operational, not experimental. As a result, the Initial Defense Communication Satellite Program (IDCSP) was created. Its design principle was simplicity. Each IDCSP payload had a single repeater with a capacity of about 10 voice circuits or 1 megabit per second of data when communicating with large terminals on Earth.

Seven IDCSP satellites were launched in 1966 with additional groups of three to eight satellites launched in 1967 and 1968. Twenty-eight satellites were placed into orbit, operating for periods ranging from one to ten years. The IDCSP satellites drifted in orbits slightly below geostationary altitude. In contrast, almost all subsequent DOD communication satellites operated in the geostationary orbit (see sidebar, The Geostationary Orbit).

In 1967, increasing military activity in Vietnam led to the establishment of an operational communication link that used IDCSP. In this link, digital data were transmitted from Vietnam to Hawaii through one satellite and on to Washington, D.C., through another. In 1968, the system was declared operational, and its name was changed to Initial Defense Satellite Communication System.

Tactical Communication Satellite

The IDCSP satellites and the advanced satellites that followed them were for strategic communications between large-antenna, fixed or transportable ground stations and large shipborne equipment. The Tactical Communication Satellite (Tacsat), following the Lincoln satellites, was designed for a complementary function: operation with small land-mobile, airborne, or shipborne tactical terminals.

The Tacsat communication payload was designed with both UHF and X-band capabilities to permit operation with a wide variety of terminals. The requirement to operate with small terminals called for the use of high-power transmitters, which necessitated a very large, cylindrical body to provide the required solar-cell area. The need for the large body in turn required the development of a new stabilization technique, which was refined and subsequently applied to many commercial communication satellites.

The Tacsat satellite; the large cylinder was covered with solar cells, and the five helixes were the UHF antenna.

Tacsat was launched in 1969. On-orbit testing was done with a variety of terminals, including large ground stations, mobile ground stations, aircraft, and ships. Tacsat was used for operational support of Apollo recovery operations; it connected the aircraft, the aircraft carrier, and the ground stations. Military use, especially of the UHF band, was extensive. Operations continued until an attitude-control failure in 1972.

Milsatcom Architecture

By the early 1970s, DOD had determined the need for a milsatcom architecture to guide the development of technology and programs that would be responsive to military users' requirements and realizable within the DOD budget. In 1973 the Defense Communications Agency (DCA), now the Defense Information Systems Agency, was assigned responsibility for developing this architecture.

The first comprehensive milsatcom architecture, published in 1976, has been refined several times since then. It has three segments: wideband, mobile and tactical (or narrowband), and protected (or nuclear-capable). Within each segment is a mix of users similar enough to be supported by a common satellite system.

Aerospace participated from the beginning in the development and refinements of this architecture. The DCA office that directed the architecture work was headed from inception until 1976 by an engineer on leave from Aerospace. Additionally, Aerospace simulators have been used and refined over many years for studying the performance of architectural options in various military scenarios.

Wideband Systems

Users of the wideband segment primarily have fixed and transportable land-based terminals; a few have terminals on large ships or aircraft. Their data rates vary from moderate to high, and their connectivity may be point-to-point or networked at distances ranging from in-theater to intercontinental. The wideband systems are the Defense Satellite Communication Systems (DSCS) II and III and the Global Broadcast Service (GBS) payload on the UHF Follow-On (UFO) satellite.

Defense Satellite Communication System II

The IDCSP satellites were the DSCS Phase I space segment. They demonstrated that satellite communications could satisfy certain DOD needs, so in 1968, DOD decided to proceed with the development of satellites for DSCS Phase II.

A DSCS heavy terminal with an 18.3-meter-diameter antenna, used at major communication nodes. (U. S. Department of Defense)

DSCS II satellites had a command subsystem, attitude control and stationkeeping capabilities (the ability, on command from Earth, to adjust satellite orientation or or-bital position), and multiple communication channels with multiple-access capability. IDCSP had none of these features; however, the DSCS II design was compatible with modified Phase I ground terminals as well as new terminals specifically built for Phase II.

The DSCS II communication payload had four channels with various combinations of bandwidth and antennas. The combinations provided the flexibility to handle a wide variety of links and to communicate with many sizes of terminals. Initial terminals constructed at major nodes had 18.3-meter-diameter antennas.

The DSCS II program began with six satellites launched in pairs, the first in 1971. Major technical problems caused the satellites to fail in 1972 and 1973. Analyses of these problems, with significant Aerospace contributions, provided the basis for design modifications for the next satellites. By 1989 a total of 16 satellites were launched to establish and maintain an orbital constellation with at least four active and two spare satellites. All are now out of service and have been moved above the geosynchronous orbit to prevent interference with active satellites.

DSCS III

The DSCS program was planned for long-distance communications between major military locations. During the system's evolution, both the number and variety of terminals increased as more DOD units sought to benefit from satellite communications. By the 1990s, a majority of DSCS terminals fell into the categories of small, transportable, or shipboard. Phase III DSCS satellites were developed to operate in this diverse environment.

The January 2000 launch of a DSCS III satellite on an Atlas IIA launch vehicle from Florida. (International Launch Services. Photo by Carlton Bailie)

The primary DSCS III communication payload operates in the X-band and has eight antennas that can be connected in various ways to the six transponders ("transmitter/responders"). Each transponder can be configured to serve a specific type of user requirement. Besides Earth-coverage antennas, the satellite has one multibeam receiving antenna, which can form a beam of variable size, shape, and location, and two similar multibeam transmitting antennas.

DSCS III development started in 1977. The first satellite was launched in 1982, and 11 additional satellites have been launched since then. All are operational. Five occupy the prime operating locations of the DSCS constellation, which are spaced around Earth in geostationary orbit. The others, spares for the five primary satellites, augment the capacity and coverage of the constellation.

To satisfy increasing user needs, the last four DSCS III satellites were enhanced to improve their communications capacity by 200 percent, with up to a 700-percent increase in capacity to tactical users (those with small terminals) in certain scenarios. Aerospace played a key role in identifying and analyzing options for this enhancement program—an important activity, since the number of tactical users increased greatly over the past two decades. Many use truck- or trailer-mounted terminals with 2.4-meter-diameter antennas. Such terminals do not operate while in motion, but can be set up at an unprepared site by two or three people in less than 30 minutes.

Global Broadcast Service

The Global Broadcast Service (GBS) is another part of the milsatcom architecture's wideband segment. Its mission is to deliver high-rate intelligence, imagery, and map and video data to tactical forces using small, portable terminals. Phase I of GBS used a commercial satellite and a limited number of commercial receive terminals. Phase II uses the GBS payload on UFO satellites 8 through 10. This payload uses 30-gigahertz uplink and 20-gigahertz downlink frequencies, often called Ka-band. Phase III will be developed in the future.

The UFO satellite in orbit; the large structure on the front of the satellite body is the UHF transmit antenna, the small square toward the bottom of the body is the UHF receive antenna, and the equipment on the top is the GBS antenna apparatus.

Information to be disseminated through GBS is assembled into a broadcast stream transmitted to the satellite and rebroadcast to a large number of users in one of several spot-beam coverage areas. (Spot-beam antennas focus on a limited area of Earth.) Each user, typically a small military unit, has an easily moveable set of receiving and display equipment.

Mobile and Tactical Systems

Users in the mobile-and-tactical segment of the architecture are characterized by small terminals with relatively low-gain antennas; they are located on ships, aircraft, and land vehicles. Data rates are low to moderate, and connectivity is typically in networks at distances ranging from in-theater to transoceanic. Systems in this segment are the Fleet Satellite Communications System, the Leasat program, and the UHF Follow-On (UFO) program.

Fleet Satellite Communications

Tacsat and LES-5 and -6 were experimental satellites that demonstrated UHF (225- to 400-megahertz) links with mobile terminals. These satellites were used for numerous tests, and Tacsat and LES-6 provided a limited operational capacity for DOD. The Fleet Satellite Communications (FLTSATCOM) system was DOD's first operational system (dedicated to supporting military operations) for tactical users.

A ground mobile forces satellite terminal with the 2.4-meter antenna; communications electronics are inside the truck. (U. S. Department of Defense)

FLTSATCOM served Navy surface ships, submarines, aircraft, and shore stations. The largest antenna, used for UHF transmissions, was a 5-meter-diameter paraboloid that had a solid center section and a deployable outer mesh section. The separate UHF receiving antenna was a single helix deployed to the side of the large paraboloid. Most links were in the UHF band.

The first FLTSATCOM was launched in 1978, the last in 1989. For this program, as for others, Aerospace performed a structural dynamics analysis of the satellites as subjected to launch vehicle loads.

One FLTSATCOM satellite was damaged during ascent and could not be used. The others operated as expected and greatly exceeded their five-year design lives. They have been removed from service and replaced by UFO satellites.

Leasat

In 1976 and 1977 Congress directed DOD to increase its use of leased commercial satellite services, and specifically applied this direction to the tactical satellite system that would follow FLTSATCOM. The result, the Leasat program, primarily served the Navy, plus Air Force and ground forces mobile users. FLTSATCOM terminals were used with Leasat. Leasat had four types of communication channels, with characteristics very similar to the FLTSATCOM channels, and four communications antennas: two X-band and two UHF; all were Earth-coverage.

The contract for Leasat development was awarded in 1978 and called for five years of communication service to be provided at each of four orbital locations. The first two launches took place in 1984, the last in 1990. Leases on satellites 2, 3, and 5 were extended into 1996. The Leasats have been removed from service and replaced by UFO satellites, with the exception of one now in use by the Australian Defence Forces.

UFO

The UFO satellites replaced the Navy's FLTSATCOM and Leasat satellites. The Navy's requirements for UHF capacity had grown considerably since the first FLTSATCOM launch in 1978. At that time, four operational satellites plus one orbiting spare were planned. The 1991 constellation was double that size and included six FLTSATCOMs and four Leasats. These satellites were still functioning properly at the end of 1995, but by 1999, all had been removed from service.

The Army's Enhanced Manpack UHF Terminal, which is capable of being carried, set up, and used by a single soldier, communicates via the UFO satellites. (U. S. Army)

The Navy replaced the older satellites with a constellation of eight UFOs, plus one spare. The UFO satellites have more channels than the earlier satellites and are designed to be compatible with the more than 2000 UHF terminals used with FLTSATCOM and Leasat. Satellite capability increases have led to a reduction of terminal sizes. Some terminals can be carried, set up, and used by one person, thus minimizing the burden on military users and increasing the number of military units that can benefit from satellite communications.

The UFO contract was awarded in 1988; a total of ten were ordered by 1994. A contract for an eleventh satellite was signed in 1999. The first UFO was lost as a result of a launch vehicle problem, but the next nine, successfully launched between 1993 and 1999, are in use. Satellite 11 will be launched in 2003. Besides performing its usual systems engineering tasks throughout the UFO program, Aerospace also developed a telemetry analysis workstation and installed it in a satellite control center.

Protected Systems

Mobility characterizes users of the protected segment of the milsatcom architecture, whether they are on ships, aircraft, or land vehicles. They accept very low to moderate data rates in exchange for considerable protection of their links against physical, nuclear, and electronic threats. Systems in the protected segment of the milsatcom architecture are the Milstar system and the Air Force Satellite Communications (AFSATCOM) and extremely high frequency (EHF) payloads.

AFSATCOM

AFSATCOM served Air Force strategic aircraft, airborne command posts, and ground terminals. AFSATCOM satellite payloads are on the FLTSATCOM satellites and on several satellites situated in high-inclination orbits to provide coverage of the north polar region, which is not visible from equatorial satellites. AFSATCOM also uses a single-channel transponder on DSCS III satellites.

AFSATCOM uses a mix of UHF and X-band frequencies with antijamming protection for most of its uplinks by frequency-hopping (rapid switching of frequencies during transmission). A portion of the Milstar communications payload continues the AFSATCOM mission.

FLTSATCOM EHF

FLTSATCOM satellites 7 and 8 contained an EHF payload (44-gigahertz uplink and 20-gigahertz downlink) called the FEP (FLTSATCOM EHF Package). FEP was developed to demonstrate operational capabilities of EHF terminals and prove key functions of the Milstar system. This payload had both Earth-coverage and spot-beam antennas, and it processed received signals before their downlink transmission. Both links were frequency-hopped.

Milstar Block I and II

The Milstar system is designed to emphasize robustness and flexibility. The term "robustness" here refers to the ability to operate under adverse conditions, including jamming and nuclear attack. Aerospace played a key role in assessing Milstar system performance against a DOD-validated threat model. "Flexibility," in this context, is the ability to provide worldwide unscheduled access and worldwide connectivity to terminals on all types of platforms.

The Milstar program includes two Block I and four Block II satellites. These blocks are also known as LDR (low data rate) or Milstar I, and MDR (medium data rate) or Milstar II. The block change resulted from a 1990 program restructure in response to global political changes.

The Milstar satellite in orbit with the two equipment wings and two longer solar-array wings deployed; various antennas are visible on the equipment wings. (Lockheed Martin Missiles and Space)

Relaxation of survivability requirements and improvements in satellite electronics allows the MDR satellites to provide robustness and flexibility for 32 MDR channels, for single-user data rates up to 1.5 megabits per second, in addition to the 192 LDR channels, for single-user data rates up to 2.4 kilobits per second. Aerospace was a leader in the development of the standard for the LDR and MDR waveforms.

The Milstar system has three segments: mission control, terminal, and space. The mission control segment plans mission activities, allocates system resources, tests and controls the satellites, and resolves satellite anomalies. It includes a fixed site as well as mobile units. Intersatellite crosslinks enable monitoring and control of all Milstar satellites from a single location.

The terminal segment, developed by the Air Force, Navy, and Army, contains more than 1000 terminals of many types; some are vehicle-transportable or human-portable, while others are located at fixed sites or on airborne command posts or other aircraft, ships, or submarines. Antenna diameters vary from 14 centimeters for submarine terminals to 3 meters for fixed command-post terminals.

The space segment consists of the Milstar satellites. Each has a central bus, two payload wings, and two solar arrays. At the outer end of each wing is a crosslink antenna. Other antennas are mounted on the wings. Features that support the system's robustness include frequency-hopping, extensive onboard processing, and crosslinks. Features that support flexibility include multiple uplink and downlink channels operating at various rates; multiple uplink and downlink beams, including agile beams; and routing of individual signals among uplinks, downlinks, and crosslinks.

The Block I satellites were launched in 1994 and 1995. Aerospace had a lead role in the analysis of one launch-vehicle failure and another anomaly just prior to the first launch, and provided launch-readiness verification for the Milstar 1 launch vehicle. The first Block II satellite was lost because of a launch problem. The second was launched in 2001; it and the Block I satellites are in operation. The third was launched in January, 2002. The last one is scheduled for launch in November, 2002.

UFO and Interim Polar EHF

Beginning with the fourth satellite in the UFO series, an EHF communications payload compatible with Milstar terminals was added to that satellite. An enhancement to the EHF payload beginning with UFO 7 doubled its communication capacity.

The EHF payload accommodates multiple uplinks distributed between the Earth-coverage antenna and the deployed steerable spot-beam antenna. Each uplink is time-shared by multiple users. The downlink (at 20 gigahertz) is a combination of all the uplinks (at 44 gigahertz). Both links are frequency-hopped.

The Interim Polar Program adapted the UFO/EHF payload for use on host satellites in high-inclination orbits. These payloads communicate with military forces operating above 65 degrees north latitude, where visibility to geostationary-orbit satellites is poor or impossible. The first launch with an interim polar payload was in 1997. Two launches remain, in 2003 and 2005.

Summary

U.S. military satellite communications have improved and expanded greatly over the past four decades, from SCORE through DSCS III, UFO, and Milstar. Capabilities have grown dramatically with the development of satellite and electronics technologies. Higher-power and wider-bandwidth satellites have enabled increased information transmission to an ever-wider assortment of terminal types deployed with an increasing number and variety of military units.

Throughout this history, and now, Aerospace has been involved in every phase of development and deployment of DOD satellite communication systems, from concept development and requirements definition through design and test reviews to launch preparations and on-orbit testing and operations. Aerospace regularly applies lessons learned in the course of one program to all DOD satellite programs.

As military satellite communication systems improve, they continue to provide information superiority to the U.S. military. This enables our military forces to remain dominant in the increasing speed and diversity of their actions during times of peace as well as times of conflict.