ATS-6 - AbsoluteAstronomy.com

ATS-6 was a NASA experimental satellite, built by Fairchild Space and Electronics Division

Fairchild Industries

Fairchild Industries was created from a name change from Fairchild Hiller Corporation, division and subsidiaries: Fairchild Aircraft Marketing Company, Fairchild Aircraft Services Division, Fairchild Republic Division, Fairchild Space and Electronics Division, Fairchild Stratos Division, Burns Aero...

It has been called the world's first educational satellite as well as world's first experimental Direct Broadcast Satellite

Direct broadcast satellite

Direct broadcast satellite is a term used to refer to satellite television broadcasts intended for home reception.A designation broader than DBS would be direct-to-home signals, or DTH. This has initially distinguished the transmissions directly intended for home viewers from cable television...

as part of the Satellite Instructional Television Experiment

Satellite Instructional Television Experiment

The Satellite Instructional Television Experiment or SITE was an experimental satellite communications project launched in India in 1975, designed jointly by NASA and the Indian Space Research Organization . The project made available informational television programmes to rural India...

between NASA

NASA

The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

and ISRO. It was launched May 30, 1974 and decommissioned July 1979. ATS-6 carried no fewer than 23 different experiments, and introduced several breakthroughs. It was the first 3-axis stabilized spacecraft in geostationary orbit

Geostationary orbit

A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...

. It was also the first to use experimentally with some success electric propulsion in geostationary orbit. It also carried several particle physics experiments, including the first heavy ion

Heavy ion

Heavy ion refers to an ionized atom which is usually heavier than helium. Heavy-ion physics is devoted to the study of extremely hot nuclear matter and the collective effects appearing in such systems, differing from particle physics, which studies the interactions between elementary particles...

detection in geostationary orbit.

During its five-year life, ATS-6 transmitted educational programming to various countries, including India

India

India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

, the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

and other regions. The vehicle also conducted air traffic control

Air traffic control

Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...

tests, was used to practice satellite-assisted search and rescue techniques, carried an experimental radiometer subsequently carried as a standard instrument aboard weather satellites, and pioneered direct broadcast TV.

ATS-6 was a precursor to many technologies still in use today on geostationary spacecraft: large deployable antenna, 3-axis attitude control with slewing capabilities, antenna pointing through RF sensing, electric propulsion, meteorological radiometer in geostationary orbit, and direct to home broadcasting. It is also possible that ATS-6 was a forerunner of the large ELINT satellites such as Mentor

Mentor (satellite)

MENTOR, sometimes called Advanced Orion by observers, is reportedly a code-name for a class of United States spy satellites that collect signals intelligence from space...

Launch

ATS-6 was launched on May 30, 1974, by a Titan III-C launch vehicle. The spacecraft was inserted directly in the geosynchronous orbit

Geosynchronous orbit

A geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...

. This reduced the on-board fuel requirements to less than 40 kg (for a total mass at launch of nearly 1400 kg). The highly accurate orbit insertion further lowered the amount of fuel required for final positioning to 9 kg. This enabled a life extension from the original 2 year to 5 years, even accounting for the premature failure of the electric propulsion subsystem (the station-keeping fuel requirement being around 1.6 kg/year).

Structure, power subsystem and Antenna

One of the major innovations of ATS-6 was an in-flight deployable antenna of more than 9 m in diameter. The antenna reflector was furled during launch under the launch vehicle fairing, and was deployed in orbit much like an umbrella. The antenna reflector was built from 48 aluminum ribs, supporting a metallized Dacron mesh. The antenna feeds (in C, S, L, UHF and VHF bands) were placed on the spacecraft body, facing the antenna reflector, and linked to the antenna and the solar panels masts by a carbon-fiber reinforced plastic (CFRP) truss. The solar panels were rigidly mounted on two deployable masts. They were of hemi-cylinder shape, thus providing a relatively constant power (595 W beginning of life). Electric power was supplied during eclipse

Eclipse

An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...

s by two Nickel cadmium batteries of 15-A·h capacity, powering a regulated 30.5-V bus. The satellite dimensions in orbit were 15.8-m width by 8.2-m height.

This deployable antenna parabola was designed and developed by Lockheed Missiles and Space Company (LMSC), now Lockheed Martin, under subcontract to Fairchild Aerospace, after several years of small study contracts at LMSC. The program manager at LMSC was GKC (Colin) Campbell. The deployment of the reflector was initiated by pyrotechnically operated SQUIB cable cutters. Deployment time was on the order of 2.5 seconds producing 2500 Ft Lbs of torque at the spacecraft interface. The reflector surface was designed for optimal operation at S-Band frequencies. It weighed 182 lbs at launch and stowed into a toroidal volume (doughnut shaped) approximately 6 feed in diameter and 10 inches thick. Three models were fabricated, the STM or structural test model, the F reflector and the G reflector. The STM was destroyed by Fairchild shortly after the program was finished and the F model was launched with the spacecraft in 1972. The G model sat unprotected in the Farchild parking lot for several years before it was donated to the Smithsonian. Bill Wade, the assistant program manager and test manager on the program supported The Smithsonian in the restoration by providing a complete set of drawings and specifications and visited the Silver Hill facility to provide technical guidance.

At the time of launch it was the largest parabolic surface launched into orbit.

Three-axis stabilisation

ATS-6 has been the first geostationary satellite with three-axis stabilization and pointing., This subsystem was capable of a highly accurate pointing (better than 0.1° through the inertial measurement units, down to 0.002° by using a radio-frequency interferometer.). Furthermore, the satellite was able to follow low earth orbit satellites through slewing,, by tracking the low earth-orbit satellite through an S-band RF sensing. The system was also able to perform orbitography of the tracked satellite, and was a precursor to the operational system TDRSS.
This highly advanced (for the time) pointing subsystem used earth and sun sensors, a star tracker pointed to the pole star, Polaris

Polaris

Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....

, and three inertial sensors. The sensor measurements were fed to two digital computers (nominal and redundant), as well to a back-up analog computer

Analog computer

An analog computer is a form of computer that uses the continuously-changeable aspects of physical phenomena such as electrical, mechanical, or hydraulic quantities to model the problem being solved...

. It was also possible to orient the satellite by using radio-frequency sensors. Actuators were three momentum wheels, and hot gas (hydrazine mono-propellant) thrusters. One of the momentum wheels having failed in July 1975, an alternative scheme was developed, allowing station-keeping with the two remaining wheels and thrusters.

Radiometer

A radiometer was on-board ATS-6, mounted on the earth-facing panel . This instrument was (for the time) of very high resolution. It operated on two channels: infra-red (10.5 to 12.5 µm) and visible light (0.55 to 0.75 µm). Images taken with the radiometer covered the whole earth disk, with a resolution of 1,200 lines of 2,400 pixels each (11 km square pixel in infra-red, and 5.5 km square in visible light). The IR detector was passively cooled at 115K, and the visible light detector maintained at 300K. A complete image of earth's disk was transmitted to ground every 25 minutes. Several hundreds images were taken and transmitted, until a mechanical component of the radiometer failed, two and a half months after launch.

Telecommunication experiments

The main mission of ATS-6 was to demonstrate the feasibility of direct-to-home (DTH

Direct broadcast satellite

) television broadcasting. To this end, in addition to the high-gain antenna, the spacecraft payload was able to receive in any of the VHF, C, S and L-bands, and to transmit in S-band (2 GHz) through a 20-W solid state transmitter, in L-band (1650 MHz) at 40W, in UHF (860 MHz) at 80W (which was used for the SITE experiment (see below), and with a TWTA-based transmitter of 20 W in C-band (4 GHz). The antenna produced two spots on earth of 400,000 km² each, in which the TV broadcast could be received with 3 meters diameter antennas. This payload was first used over the United States for tele-education and tele-medicine experiments, from August 1974 to May 1975. The spacecraft was then moved over the geo-stationary arc from 94 °W to 35 °E, in collaboration with the Indian Space Agency (ISRO), who had deployed in India more than 2500 receive ground stations. A tele-education programme was started (Satellite Instructional Television Experiment

Satellite Instructional Television Experiment

or SITE ) and run for one year. During the experiment, a receive station was offered by the Indian Government to Arthur C. Clarke

Arthur C. Clarke

Sir Arthur Charles Clarke, CBE, FRAS was a British science fiction author, inventor, and futurist, famous for his short stories and novels, among them 2001: A Space Odyssey, and as a host and commentator in the British television series Mysterious World. For many years, Robert A. Heinlein,...

, who was living in Sri Lanka

Sri Lanka

Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka is a country off the southern coast of the Indian subcontinent. Known until 1972 as Ceylon , Sri Lanka is an island surrounded by the Indian Ocean, the Gulf of Mannar and the Palk Strait, and lies in the vicinity of India and the...

. This experiment was highly successful, and encouraged ISRO to start building an operational program, with the Indian spacecraft INSAT IB

Indian National Satellite System

INSAT or the Indian National Satellite System is a series of multipurpose Geo-stationary satellites launched by ISRO to satisfy the telecommunications, broadcasting, meteorology, and search and rescue operations....

(launched 1983). After the SITE experiment, the satellite was brought back over the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, and served notably as a data-relay and tracking satellite for low-orbit spacecraft such as Nimbus 6

Nimbus program

The Nimbus satellites were second-generation U.S. robotic spacecraft used for meteorological research and development. The spacecraft were designed to serve as stabilized, Earth-oriented platforms for the testing of advanced systems to sense and collect atmospheric science data...

, and for the Apollo-Soyuz flight

Apollo-Soyuz Test Project

-Backup crew:-Crew notes:Jack Swigert had originally been assigned as the command module pilot for the ASTP prime crew, but prior to the official announcement he was removed as punishment for his involvement in the Apollo 15 postage stamp scandal.-Soyuz crew:...

Electric propulsion

ATS-6 was equipped with two electric thrusters based on the acceleration of cesium ions, that were to be used for North-South Station Keeping. This subsystem development followed earlier failed attempts on the previous ATS spacecraft. Each of the thrusters had a mass of 16 kg, used 150 W of electric power, and produced a thrust of 4 mN, with a specific impulse of 2500s. The on-board supply of cesium would have been sufficient for 4400 hours of thrust. Unfortunately, both thrusters failed prematurely, one after 1 hour of operation, one after 95 hours. However, some of the experiments objectives could be met, such as the measurement of the effective thrust, the absence of any interference with the radio-frequency payloads (from 150 MHz to 6 GHz), no cesium redeposition on the critical parts of the payload (such as the radiometer), and the correct neutralisation of the spacecraft versus its environment.

Particle physics experiments

Several particle physics experiments were on-board ATS-6. The most significant measured low energy protons (from 25 keV to 3.6 MeV) , as well as detected heavy ions (up to 6 MeV). This latter experiment allowed to detect the first heavy ions (Z > 6) with an energy E > 4 MeV, in geostationary orbit.

Propagation experiments

Finally, ATS-6 embarked several beacons, which allowed to measure electromagnetic propagation properties of the atmosphere at 13, 18, 20 and 30 GHz.

External links

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