10.1 Program Description
The International Telecommunications Satellite Consortium (Intelsat) is a partnership initially established between 14 member nations in 1964 for the purpose of providing global commercial tele-communications via satellite. Since that time, the organization has expanded to include 79 member nations (as of May 1971), and new applications for membership continue to be received.
From the time it was established in 1964 to the present, Intelsat has produced four generations of satellite and ground systems. Development of the initial satellite, nicknamed Early Bird and later designated Intelsat I, was initiated by Comsat in November 1963. Just a year earlier, in August 1962, the U. S. Congress had passed the Communications Satellite Act, which authorized the creation of a private corporation (Comsat) to instigate the development of a global commercial communications satellite system. At the time when Early Bird’s development began, the Syncom II satellite had just completed demonstrating that reliable communications could be provided through lightweight synchronous satellites. As a result, the Syncom satellite design formed the basis for the Early Bird spacecraft. Early Bird was launched in early 1965, as indicated in Table 10-1, and by April 22, 1965, had successfully achieved synchronization into the desired geostationary orbit with the satellite located over the Atlantic Ocean. After a period of satellite performance testing, system parameter evaluation using the operating ground stations, terrestrial and satellite circuit lineup, and public demonstrations, commercial operation was initiated on June 28, 1965. The satellite successfully provided commercial communications service between the United States and Europe until it was retired in early 1969. It was reactivated for a brief period later in 1969 when temporary difficulties were encountered with the antenna system of a third generation Intelsat satellite.
The second generation of Intelsat spacecraft was designated Intelsat II. Even when the Intelsat I system was under development, it was realized that many of the inherent advantages of space communications could not be exploited. Specifically, its antenna characteristics were such as to embrace only the north-eastern part of North America and the western part of Europe, and it did not allow for simultaneous intercommunication among numerous earth stations. By late 1965, it was recognized that the constraints imposed by these deficiencies would be incompatible with a NASA requirement for multichannel communications in late 1966 among its tracking stations at Carnarvon, Australia, Ascension Island, Canary Island, tracking ships in the Atlantic, Pacific, and Indian Oceans, and the Manned Space Flight Center in Houston, Texas. In the past, these circuits had been carried by HF radio, but for manned space flights, improved communications were desired. Consequently, in the fall of 1965 the development for Intelsat II had begun with the primary goal of satisfying the NASA requirements; excess capacity was to be used for other commercial traffic. Because of the urgency to satisfy the NASA requirement, the Intelsat II design evolved directly from that of Intelsat I.
Four Intelsat II satellites were produced and launched as indicated in Table 10-2. The first launch occurred in October 1966; but when the satellite’s apogee motor malfunctioned, the spacecraft was left in a highly elliptical inclined orbit, making it unusable for full-time commercial operations. Subsequent launches in January and September 1967 successfully placed two satellites into operational service over the Pacific Ocean. A March 1967 launch successfully supplemented the Intelsat II satellite in operation over the Atlantic Ocean. With these three satellites in place, commercial service was available over both the Atlantic and Pacific Oceans. The Intelsat II satellites continued to meet international commercial communications requirements successfully until third generation replacement satellites allowed them to be retired to the active reserve. The development of Intelsat III was initiated in 1964 with a design study and followed 2 years later with the award of a contract for the design, development, and fabrication of the necessary spacecraft. Eight Intelsat III satellites were launched between September 1968 and July 1970 as indicated in Table 10-3. Launch failures in September 1968, July 1969, and July 1970 made three of these satellites unusable. A successful launch in December 1968 placed a spacecraft in service over the Atlantic. Some difficulties were encountered when this satellite’s mechanically despun antenna started sticking in mid-1969. This occurrence made necessary the aforementioned reactivation of Early Bird. The antenna problem was resolved by August 1, 1969, and commercial operations were resumed until a subsequent January 1970 Intelsat Il launch allowed the satellite to be placed in the retired reserve. In April 1970 another successful Intelsat III launch supplemented the operational capability available over the Atlantic. An Intelsat III satellite was first placed into operation over the Pacific in February 1969. This satellite was supplemented by a second spacecraft in May 1969. When the first Pacific Intelsat III lost 6 dB of transponder gain due to an RF receive amplifier malfunction, it was relocated over the Indian Ocean where the traffic requirements were lighter. As a result, four Intelsat III satellites were, as of May 1971, providing global commercial service over the Atlantic, Pacific, and Indian Oceans.
Development of the fourth generation of Intelsat spacecraft, Intelsat IV, began in the latter portion of the 1960s. These satellites, manufactured by Hughes Aircraft Corporation, have been designed to provide a substantially greater capability to meet the increased global communication needs of the 1970s. The first, in an expected series of eight satellites, was successfully launched by an Atlas Centaur rocket into a geostationary orbit on January 25, 1971. It was positioned over the Atlantic at 24 5°W longitude. Subsequent Intelsat IVs will be launched for service over the Atlantic, Pacific, and Indian Oceans with an additional two satellites as spares in orbit. Each satellite will have a design life expectancy of about 7 years.
Because the Intelsat program has been a commercial venture, the number of major innovations in equipment and techniques employed has been limited. The intent has been to minimize the risk of spacecraft failure and the exceptional reliability record amassed by the system testifies to the success of this policy. Nevertheless, the Intelsat program has made significant contributions to satellite communications.
Numerous subjective tests with Intelsat I demonstrated conclusively that the round trip time delay and echo due to two-wire user terminations were not insurmountable obstacles to the utilization of synchronous satellites for commercial communications. This was in confirmation of preliminary indications obtained on Project SYNCOM.
The Intelsat II spacecraft demonstrated that tunnel diode amplifiers of operational reliability were available for use as RF receive preamplifiers. Utilizing these relatively low-noise, high-gain preamplifiers allowed direct RF to RF conversion in a single stage to be employed. Sufficient spacecraft power and high performance earth terminals allowed these transponders to be designed for linear input/output power transfer characteristics. Additionally, Intelsat II and an expanded ground complex demonstrated the feasibility of extensive multiple accessing of a single satellite transponder by a group of operational ground terminals.
When the wideband Intelsat III satellites were placed into operation, it was necessary to introduce a third generation of earth stations to the system in order to take full advantage of the expanded capabilities of the space subsystem. These terminals employed newly developed 500-MHz bandwidth cooled parametric amplifiers, as well as 500-MHz bandwidth high power traveling-wave tube transmitters, capable of over 6 kW of multi-carrier power.
The more recent Intelsat IV spacecraft have contributed to satellite communications technology by demonstrating fixed narrow beamwidth (i. e., 4.5°) antennas mounted on a mechanically despun platform and a highly channelized satellite repeater (i. e., 12 inde- pendent transponders). The narrow-beam antennas provide coverage to a fixed, relatively restricted area of the earth but the high antenna gain available significantly increases satellite EIRP. The large number of transponders, each having a 36-MHz bandwidth, allows users with substantially different communication requirements to operate independently of each other in separate satellite channels (e. g., television distribution in one channel, high-capacity telephone trunks in another, and low-duty cycle individual voicelinks in still another). Additionally, a fully variable, demand-access, satellite system will be demonstrated for the first time during the period when the Intelsat IV satellites are being placed into operational service. Comsat has developed a single-carrier-per-voice channel PCM-PSK-FDMA demand-access system nicknamed SPADE that will be employed.