User:DNJH/Intelsat III

INTELSAT III Satellite

The Intelsat III series of satellites were the 3rd generation of geostationary communications satellites for the INTELSAT Corporation. Designed and built by TRW in 19xx-19xx, there were eight III-series satellites built INTELSAT III-F1, III-F2, III-F3, III-F4, III-F5, III-F6, III-F7, and III-F8. The "F" designated the flight number (number in the series).

Design

The Intelsat III satellite was very similar in design to the Intelsat I and II satellites, the Syncom satellites, and Comsat 1. They were a larger version of these satellites.

basic deisgn of this era in satellite designhe entire spacecraft was spun at 30 revolutions per minute (rpm) to impart gyroscopic stability to the satellite in the earth's gravitation field. The early satellites like A section of the spacecraft supporting the communications payload and antenna was de-spun to allow the antenna to point at the desired location on the earth.

The Intelsat VI series combined two design features of previous HAC satellites, larger solar array and wide body design. The HS376 extended power spinner satellite had a extra concentric cylindrical solar array which deployed after launch to increase the power generating capability of the satellite, and allow for a larger communications payload. The U.S Government's Wide-body Spacecraft was a larger diameter satellite designed to be launched by the Space Transportation System (STS, US Space Shuttle). Thus the Intelsat VI satellite were of a wide body spinning design with a larger solar array, due to the deploy-able array. The later HS393 series of satellites also used the wide body and extended solar array design.

This resulted in a spacecraft that was 3.6 meters in diameter and approximately 5.3m tall as configured for launch on an Ariane 4 rocket. When the spacecraft had arrived at its assigned orbital location, the concentric solar array would be extended (deployed), along with deployment of the communications antenna. The spacecraft would then be 11.7m in length.

The Intelsat VI series of satellite were designed to be launched by either Ariane 4 rockets or the U.S. Space Shuttle.

Propulsion

A liquid bi-propellant propulsion subsystem was used on the INTELSAT VI series satellites, and used nitrogen tetra-oxide and mono-methyl hydrazine. Four radial thrusters, rated at 22 Newtons (N)) are used for east-west station keeping, and spin-up/spin-down control. Two 22N axial mounted thrusters provide north-south station keeping and attitude control. Two 490N apogee thrusters were used to provide the apogee boost to the satellite and support re-orientation maneuvers.

Power Subsystem

The solar array on the INTELSAT VI was sized to provide about 2600 Watts of power at the beginning of the satellites life. The INTELSAT VI satellites used nickle hydrogen pressure vessel batteries to support operation when the spacecraft was in eclipse behind the earth.

As noted in the introduction, the INTELSAT VI series of satellites were designed with a cylindrical spacecraft body which was covered by photovoltaic(PV) solar cells. Since the satellite was rotating at 30 rpm, a flat panel solar array on a side of the spacecraft would be exposed to the sun intermittently and not generate continuous power. With a cylindrical array part of the solar array would always be in sunlight and would generate power for the spacecraft to operate.

Communications Payload

The communications payload basically consists of the receivers, filters, amplifiers and interconnection cables or waveguide used to receive radio signals from earth transmitters, and convert them to suitable downlink frequencies, and retransmit the signals back to the earth.

The INTELSAT VI satellite used C-band at 6 GHz uplink frequency/4 GHz downlink frequency, and Ku-band at 14 GHZ uplink/11 GHz downlink, and had 50 communications transponders which were designed to carry 33,000 telephone circuits, the equivalent of 33,000 two way telephone calls, as well as four television channels. The INTELSAT VI satellites used a RF switching network to allow static connections between the uplink channels and downlink channels. The satellite also used a Time Division Multiple Access (TDMA) dynamic microwave switching network on channels 1-2 and 3-4 to allow the dynamic cross connection of the channels for TDMA type signals.

Antennas

The antenna system and coverages were designed to be identical for all of the INTELSAT VI satellites. This provided simplicity of design and manufacturing for the five satellites in the series, since all the antenna components could be made identical for each of the five satellites. It also allows for any of the VI series satellites to replace another satellite in case of an on-orbit failure.

A 2.0 m diameter reflector antenna was used for receiving C-band signals transmitted up from the earth. The satellite had two C-band "hemi" beam coverages which were designed to cover the landmass areas as seen from any of the orbital locations. Four beams were designed to provide smaller zone coverage for specific areas of the earth depending on the orbital location. Both the "hemi" and zone beams used an antenna reflector 3.2m in diameter with a 4.2m focal length. A 149 element feed horn array and four switching networks (three were switchable in orbit) allowed the zone coverage to be changes to match the orbital location.

The satellite had a C-band global coverage horn, which provided coverage of the entire earth, for receive and transmission of two channels or repeaters.

The satellite also had two Ku-band steerable spot beams which could be moved to cover any specific area on the earth, and could be re-pointed as needed. The Ku-band spot beams provide both receive and transmit capability.

TC&R

The Telemetry, Tracking and Control (TT&C), or Telemetry, Command and Ranging (TC&R) subsystem is used to receive spacecraft control commands sent from ground control stations, send telemetry from the satellite subsystems to ground receivers, and support tracking and ranging of the satellite by ground stations.

The INTELSAT VI satellites used C-band for the TC&R subsystem, and a pair of omni-directional antennas were mounted on a deploy-able boom.