(56K)The first of the second-generation ATS spacecraft, ATS-6 was also the last ATS project. A follow-on spacecraft was started, but was cancelled by Congress in 1973 as a result of budgetary pressures. A decision was made to let the commercial satellite communications industry support its own research and development. An attempt by NASA in 1974 to reinstate the ATS program was unsuccessful. The impressive ATS-6 project, launched on May 30, 1974, marked the end of an era and the beginning of a dry spell for NASA experimental communications satellites.
ATS-6 was a much larger spacecraft than its predecessors. It weighed 1336 kg (compared with 431 kg for ATS-5) and was 8.51 meters tall and 16 meters wide across its booms (ATS-5 was 1.4 meters in diameter and 1.8 meters tall). According to the ATS-6 Final Report, ATS-6 reflected many significant design firsts, including:
- Largest geosynchronous communications satellite launched to date
- First 3-axis stabilized communications satellite
- First spacecraft to use
- A 9.14-meter parabolic reflector
- A digital computer for attitude control
- Solid state high power rf transmitters
- Graphite composite material for primary structure
- Heat pipes for primary thermal control
- Monopulse tracking for attitude control
- Rf interferometer for attitude determination and control
ATS-6 demonstrated the technology for tracking and data relay satellites which led to the TDRSS program. A tracking and data relay satellite uses the GEO vantage point to look down on LEO satellites. Data is relayed from the LEO satellite through the GEO satellite and down to the ground. Without the space relay capability, NASA needed ground stations all over the globe to collect data from satellites as they passed overhead. Since a LEO satellite orbits the Earth in a matter of a few hours, it is only in view of a single ground station for typically twenty minutes at a time. ATS-6 tracked the Nimbus-5/6 and Geodynamics Experimental Ocean Satellite (GEOS)-3 satellites with a roll and pitch accuracy of better than 0.2 degree.
The large antenna provided the capability for small ground receivers to pick up a good quality signal. A demonstration in India in 1975 relayed TV signals from a 6 GHz uplink through ATS-6 and back down at 860 MHz direct to 10 ft antennas installed in 2000 villages. The 9 meter deployable antenna required tight pointing by the spacecraft, which is why 3-axis stabilization was used. Propagation studies were carried out at frequencies up to 30 GHz. L-band (1550-1650 MHz) relay links to aircraft were established and multiple aircraft tracking was demonstrated.
At first sight, one of the solar panels appears to be on backwards. Upon closer examination, it can be seen that the two panels are each half of a cylinder and that the two semicylinders have been displaced along their long axis only (the booms being aligned with the long axis of the semicylinders). This configuration, with sunlight incident normal to the long axis, allows the solar arrays to produce a constant power output independent of the orientation of the spacecraft around the boom axis. Rolling the spacecraft around the boom axis is equivalent to a spinning cylinder with the same fraction of solar cells being illuminated at all times. The initial solar array power for spacecraft use was 595 Watts.
ATS-6 was built by Fairchild Space and Electronics Company for NASA Goddard Space Flight Center. It was launched on May 30, 1974. The failure by May 1979 of three of the four orbit control jets led to the decision to power down the spacecraft on August 3, 1979 after it had been placed in an orbit with an altitude about 500 km below synchronous. The ATS-6 telemetry system was activated from November 1979 to February 1980 to collect particle data for correlation with similar data being collected by other satellites.