(341K)The Syncom project objective was to demonstrate the technology for synchronous-orbit communications satellites. A synchronous orbit (also known as GEO for geosynchronous Earth orbit) is one in which the satellite makes one orbit per day. Since the Earth rotates once a day and the satellite goes around the Earth at the same angular rate, the satellite hovers over the same area of the Earth's surface all the time. The altitude of a synchronous orbit is 22,235 miles (19,322 nautical miles or 35,784 km). At lower altitudes, satellites orbit the Earth more than once per day. For example, the Space Shuttle at a nominal altitude of 180 miles orbits the Earth in an hour and a half. On the other hand, the Moon, at a distance of around 240,000 miles, takes a month to orbit the Earth.
About 42% of the Earth's surface is visible from a synchronous orbit. With three properly placed satellites the entire globe is covered. The idea for the synchronous communications satellite was invented by Arthur C. Clarke and published in the seminal article in Wireless World in 1945, but the first spacecraft to synchronous orbit, Syncom I, was not launched until February 14, 1963. Unfortunately, when the apogee kick motor for circularizing the orbit was fired, the spacecraft fell silent. Telescopic observations later verified that the satellite was in an orbit with a period of nearly 24 hours at a 33 degree inclination.
A key advantage of a synchronous satellite is that a ground station has a much easier tracking and antenna pointing job because the satellite is always in view. For lower altitude orbits, the spacecraft must be acquired as it comes into view rising above one horizon, must be tracked across the sky with the antenna slewing completely over to the opposite horizon where the satellite disappears until its next pass. For continuous coverage of a ground station by satellites, a second antenna might be needed to acquire and hand off to the next satellite. For continuous coverage of a satellite, ground stations have to be distributed around the globe so that the satellite is rising over the viewing horizon of one ground station as it is setting for another.
A particular type of synchonous orbit is the geostationary orbit. This is a synchronous orbit around the equator in which the satellite appears to be stationary over a point on the surface. This orbit was demonstrated by Syncom 3. The Syncom 2 orbit was inclined 33 degrees to the equator so that it appeared to move 33 degrees North and 33 degrees South in a figure 8 path over a 24 hour period as observed from the ground.
In the early 1960's, just reaching a synchronous orbit was a challenge. A major goal of Syncom was just to show that a synchronous satellite was possible. An observer at the time (1962) noted:
Nearly everyone agrees that for a short-range, experimental first venture, the medium altitude active repeater satellite, such as Telstar or Relay, is the best bet.... Lockheed...is confident that before the complex problems of operating fifty or more satellites at lower altitudes are solved...the U.S. will be able to put up a full-powered, simpler, high-altitude system. Other experts, however, say that the synchronous high-altitude satellite is still some order of magnitude beyond present technology. [Lessing , pp. 140-141]
In addition to communications experiments, the Syncom satellites contributed to determination of the Earth's gravitational field. The Syncom range and range-rate system was capable of measuring range at synchronous altitude to an accuracy of less than 50 meters. The high altitude minimized perturbations by local topology changes on the surface of the Earth.
The Syncom satellites carried redundant transponders with 2 W lightweight TWT amplifiers. The frequencies used were 7363 MHz up/ 1815 MHz down. The channel capability was for either several two-way telephone circuits (500 kHz) or one television signal (5 MHz). The spacecraft weighed 39 kg (86 lb) after the apogee motor burnout with an overall length of 1.17 m (less than four feet).
Syncom 1 was launched on February 14, 1963 into a nearly synchronous orbit, but failed during the apogee motor burn. The most likely cause was determined to be a failure of the high pressure nitrogen tank. Syncom had two separate attitude control jet propellants: nitrogen and hydrogen peroxide. One of Syncom's objectives was to demonstrate attitude control for antenna pointing and station keeping.
Syncom 2 was launched on July 26, 1963 after improvements in the nitrogen tank design were incorporated and it was a success. It transmitted data, telephone, fax, and video signals.
Syncom 3 launched Aug. 19, 1964 into a geostationary orbit. It had the addition of a wideband channel for television and provided coverage of the 1964 Olympics in Tokyo. Both satellites were turned over to the DoD in April 1965, and were turned off in April 1969.
The Syncom spacecraft were built by Hughes for NASA's Goddard Space Flight Center.