Intelsat 1 – The Early Bird
Source: Intelsat |
Intelsat 3
Source: Intelsat |
Intelsat I – The Early Bird
Launched on 6 April 1965 by a Delta D rocket, Intelsat I was a larger, commercial version of Hughes experimental Syncom satellites, basically an up-rated model of Syncom 3 with a pre launch weight of 328.5 lbs (149.0 kgs) accommodating higher capacity transponders designed to carry commercial traffic.
Its two 6 Watt transponders operating at C-Band (6GHz uplink-4GHz downlink) each with 50 Mhz bandwidth could carry either 240 voice circuits or one TV channel but not simultaneously.
Primary power was provided by a larger array of solar cells delivering 45 Watts, increased from Syncom’s 29 Watts..
Intelsat III
Launched 19 September 1968 by a more powerful Delta M rocket, Intelsat III was designed by TRW but bore a striking resemblance to Hughes Intelsat I. It was double the weight of Intelsat 1 weighing 646 lbs (293 kgs) including propellant and incorporated several performance improvements, the most important of which was the technology enabling the use of high gain antennas.
Previous spin-stabilised satellites used antennas with radiation patterns which were omnidirectional about the satellite’s spin axis in order to maintain communications with the Earth as the satellite body rotated, but this was very wasteful since most of the satellite’s RF power was radiated into space.
The De-Spun Antenna: To enable the use of a high gain directional antenna, the antenna must be kept in a fixed direction pointing towards the Earth at all times otherwise communications would be intermittent if the narrow beamed antenna rotated with the satellite body.
Intelsat III was the first communications satellite to solve this problem which it did by de-spinning the antenna using a motor to rotate the antenna at the same speed as the satellite spin but in the opposite direction so that it appears stationary. Infra-red solar sensors which detected the Earth’s horizon were used to synchronise the motor speed with the satellite’s speed of rotation. It used a de-spun directional horn antenna, 34 inches (86 cms) tall, with a gain of 15.6 dB replacing the previous slotted dipoles which had a gain of only 4 dB, increasing its effective radiated power and greatly enhancing the transponder’s signal to noise ratio and thus its usable bandwidth. See Signals and Noise.
Instead of the hydrogen gas jet thrusters as used in Intelsat I, Intelsat III used hydrazine propellant for station keeping.
Primary power was increased to 178 Watts peak by increasing the number of solar cells with the energy being stored in a 9 AmpHour Nickel Cadmium battery.
This extra power was needed to drive the de-spinning motor but it also allowed the use of higher power TWT amplifiers in its C-Band (6GHz uplink-4GHz downlink) transponders. The up rated power output of 12 Watts per channel improved the signal to noise ratio even further.
These potential improvements in noise performance enabled the bandwidth of each channel to be increased to 300 MHz, sufficient to carry 1,500 telephony circuits or four television channels.
Molniya Satellite
Molniya 1 Satellite
Public Domain |
Launched on 23 April 1965, the Molniya 1 (Russian “Lightning”) was Russia’s first communications satellite. It’s design concept and its orbits differed in almost every aspect from the early spin stabilised and later three axis stabilised systems developed in the USA. It was also much heavier than contemporary US systems with a launch weight of 3630 lbs (1650 kgs), more than 10 times the weight of the Early Bird satellite launched the same year. Its transmitter power outputs of 40 Watts and 20 Watts were also 10 times greater than the Early Bird’s.
Electronic systems were contained in a cylindrically shaped body with conical ends 14.4 feet (4.4 m) tall and 4.6 feet (1.4 m) in diameter. An active liquid cooling system kept the components at a stable temperature during the day-night cycle.
The Molniya Orbit
Molniya satellites used a highly elliptical orbit which enabled them to concentrate their signal coverage into a footprint spanning the whole of Russia and its immediate neighbours including much of the Arctic polar region.
See more about the Molniya Orbit and its benefits.
Applications
Molniya’s body was designed with the flexibility to house different civil and military applications. The first examples carried out experimental TV transmissions with the uplink transmitting TV signals from Moscow to the satellite and the downlink transmitting to 20 ground stations in cities in Siberia and the Russian Far East including Norilsk, Khabarovsk, Magadan and Vladivostok.
In 1967 Orbita, the world’s first national satellite television network was set up with Molniya satellites relaying the Moscow transmissions to ground stations across the country which received the signals through 40 to 50 foot (12m to 15m) parabolic antennas and converted the signals to frequencies suitable for reception by domestic TV receivers and re-broadcast the programmes to local communities through conventional TV transmitters.
The other major early application was long range military communications.
Later examples were used for multi channel telephony, mobile radio systems, monitoring weather systems, Earth observation and photography.
On Board Power
One of Molniya’s design goals was to minimise the use of propellants for attitude control and the weight penalty they incurred by maximising the use of renewable solar power where possible for this purpose. It therefore incorporated six large windmill-type solar panels, fixed to the satellite body, with a span of 26.90 ft (8.20 m) which provided up to 1 kW of electrical power.
Molniya’s lifetime was limited by the vulnerability of its solar cells and other electronic components to electromagnetic radiation as it passed four times per day through the inner and outer Van Allen radiation belts.
Stabilisation System
As well as the use of very large solar panels, a second design goal was the use of high gain antennas but neither of these goals could easily be accomplished with a spinning satellite. The body of the Molniya satellite was therefore deigned to be static to avoid these limitations. Nevertheless, it still used gyroscopic stabilisation, but instead of spinning the satellite body, it used an internal gyroscope aligned with the satellite’s axis which achieved the same effect.
Attitude Control
The Molniya satellite used a three axis attitude control system. Attitude sensing for the satellite body was by means of a Sun sensor mounted near the centre of the solar arrays. Two pairs of small reaction thrusters, one pair on each of the two axes orthogonal to the main satellite axis, were used in a control system to adjust the orientation of the body so that its axis and thus its solar panels were pointing directly towards the Sun to optimise the capture of solar energy. The pointing accuracy of 10 degrees was low but sufficient.
Two more jet thrusters were used to control the angular position of the satellite body around the gyro spin axis and to damp any tendency for the body to spin about its axis.
The Antennas
Molniya’s two antennas spaced 180 degrees apart were electrically steerable but only one was employed at any one time and because of the satellite’s highly elliptical orbit, the active antenna was only used eight hours per day. The second antenna was kept on standby.
During communications sessions the antenna pointing system used two optical horizon sensors to detect the position of the Earth and an electrical motor control system to point the active antenna towards the Earth’s centre. Because the angle between the Sun line and the Earth line varied as the Molniya moved around its orbit, the antenna positioning system was in constant action while the satellite was communicating with the ground stations, but this usually only happened for eight hours per day while it was passing over Russia. The rest of the time it the transmitter and the antenna control motors were switched off to conserve power.
Antenna gains were approximately 18 dB
Transponders
A variety of electronic component technologies were used in the transponders, mostly solid state but also metal-ceramic triodes. klystrons, magnetrons and traveling wave tubes. Because of the harsh operating environment most of the electronic systems were duplicated with one system operational and redundancy provided by one system and sometimes two systems on standby.
For government and military applications the transponder uplink frequency was 1.0 GHz and the downlink frequency was 800 MHz with a transmitter power of 40 Watts
TV channel frequencies were 4.1 GHz for uplinks and 3.4 GHz for downlinks transmitting with 40 Watts power. Data and telephony were transmitted with a power of 20 Watts.
The telemetry was carried at 1.0 GHz.
See Molniya History
ATS-6 (Applications Technology Satellite-6)
ATS-6 (Applications Technology Satellite-6)
Source: NASA |
Launched on May 30, 1974, the ATS-6 was the first geostationary satellite to use three-axis stabilisation for attitude control and the first to provide Direct to Home (DTH) television broadcasting, also called Direct Broadcasting Satellite (DBS).
The ATS-6 project benefitted from the use of a much larger launch vehicle, a Titan IIIC, which could carry a much greater payload. The satellite’s weight at launch was 2945 lbs (1336 kgs), nearly ten times the weight of the Early Bird, and it was 28 feet (8.51 m) tall and 59 feet (16 metres) wide across the two booms holding its solar arrays. The Titan’s guidance sytem enabled the satellite to be inserted directly in the geosynchronous orbit which reduced its on-board fuel requirements to less than 40 kgs.
Three Axis (Body) Stabilisation Benefits
The major advance of ATS-6 was its three axis stabilisation system, the enabling technology which made many new applications possible. Spin-stabilisation had been used in previous satellites but their spinning bodies imposed severe restrictions on the size and shape of the solar arrays and antennas which they could support. The gyro controlled three axis or body stabilisation transformed the satellite into a stable, fixed platform which no longer needed to be spinning enabling many new benefits to be realised.
- It provided more accurate attitude control.
- The antenna no longer needed to be de-spun saving energy and complexity.
- Large high gain directional antennas could now replace the omnidirectional antennas previously necessary with spinning satellites, avoiding the wasteful loss of the transmitter energy into space and focusing it all onto the Earth into defined footprints. At the same time the use of higher gain antennas increased the satellite’s effective radiated power.
- Larger flat solar arrays could also be deployed with every solar cell normal to the Sun’s radiation receiving the maximum possible uninterrupted solar energy from the number of cells used.
-
This was a major improvement on the spinning satellite’s solar arrays which suffered from three drawbacks:
- Their capacity is limited by the quantity of solar cells which could be mounted on its curved surface
- Most of the cells are inclined to the direction of Sun’s rays capturing less of the available energy.
- Because of the satellite’s rotation, only 50% of the cells are exposed to the Sun at any one time.
To take full advantage of this opportunity however, the orientation of the flat solar array panels must be controllable to keep their surfaces normal to the Sun’s radiation.
- With more available solar power came the possibility of higher power transmitters, more equipment and more capabilities.
- With more transmitter power and a high gain antenna, signals could be received by smaller antennas and less sensitive receivers on the ground.
The ATS-6 design made the best of these opportunities.
Attitude Sensing
The attitude control system consisted of a monitoring system which sensed the satellite’s actual attitude and compared it to the desired attitude to provide an error signal which was used in a feedback control system to drive the error to zero. Precision attitude sensing was not only required for station keeping, but also for pointing and slewing.
Pitch and roll attitude sensing was by means of radio frequency interferometry and Polaris star tracking was used to sense the yaw attitude. (See diagram of ATS-6 Attitude Axes). The pitch and roll RF interferometers used separate C band radio uplink transmitters each transmitting continuously to three horn antennas arranged along each of two orthogonal baselines parallel to the satellite pitch and roll axes. One antenna in each trio was used as a measurement reference on each baseline, with the remaining two horns, spaced at 1.66 and 19.95 wavelengths apart to provide coarse and fine phase measurements relative to the respective reference antenna. The measured phase difference associated with each axis was digitised and transmitted back to ground control. The angular resolution was 0.017 degree in the coarse mode and 0.0014 and degree for the vernier mode.
The star tracker is an optical system which senses the satellite’s yaw, that is its angular deviation from its desired attitude, by measuring the displacement of the image of a chosen navigation star from its position on a reference star map. As with the pitch and roll measurements, the result is digitised and transmitted to ground control. The yaw attitude could be determined within 0.5 degree.
The Antenna
The 30 foot diameter (9.14 metre) parabolic antenna reflector provided gains from 34 dB to 46 dB in the range UHF to C band depending on the frequency. Coupled with a 80 Watt UHF transmitter transmitting at 860 MHz, it provided the capability for direct TV broadcasting with reception by domestic receivers on the ground using small 10 foot (3 metres) antennas.
Tracking, Pointing and Slewing
The combination of precision sensing and three axis attitude control enabled ground control to perform accurate pointing and slewing of the satellite and with the aid of its high gain antenna ATS-6 became the first satellite capable of tracking sub synchronous S-Band satellites. This was the precursor to the NASA’s Tracking and Data Relay Satellites (TDRSS) program. Using its GEO vantage point the ATS-6 could look down on LEO satellites and relay data from a LEO satellite through the GEO satellite and down to the ground. This reduced the need for NASA to maintain ground stations all over the globe to collect data from LEO satellites such as the Hubble Space Telescope (HST) and the International Space station (ISS) as they passed overhead. Similarly propagation studies demonstrated the feasibility of multiple relay links to aircraft.
The Solar Energy
The two solar arrays contained a total of 21,600 solar cells delivering an instantaneous power of 595 Watts at the beginning of life with the energy being stored in 15 AmpHour Nickel Cadmium batteries supplying a 30.5 Volt bus. The satellite did not have the capability to orient the direction of the solar array independently of the antenna so its solar panels were half cylindrical in shape with one array pointing North and the other pointing South to ensure that a sufficient number of solar cells were normal to the Sun as the Sun’s apparent direction moves from 23.5 degrees North to 23.5 degrees South between summer and winter solstices. See Solar Energy Reception and ATS-6 Attitude Directions. This arrangement was also necessary to maintain the maximum possible electrical power levels when the satellite had to execute a roll manoeuvre as part of its tracking facility.
Thermal Management
One of the downsides of a static satellite is that it is subject to uneven solar heating with the fixed, Sun facing side possibly reaching very high temperatures while the opposite side, receiving no solar energy, remains very cold. In the vacuum of space this temperature difference can be very high. Spinning satellites do not suffer from this problem.
ATS-6 incorporated heat pipes and phase change materials to equalise the temperature distribution across the satellite body to alleviate this problem.
The Transponders
ATS-6 could receive in any of the VHF, L, S and C-Bands, and transmit using solid state transmitters with outputs of 80 Watts in UHF (860 MHz), 40Watts, in L-band (1650 MHz), 20 Watts in S-band (2 GHz) and, using a TWTA transmitter, 20 Watts in C-Band (4 GHz).
The transponder provided cross connections at the 150 MHz intermediate frequency (IF) so that any receiver could be connected to any transmitter.
Applications
ATS-6 carried out 23 different experiments and was the first satellite to provide DBS broadcast television to simple home receivers which it demonstrated by transmitting educational programmes to India, the USA and other countries. It was also the first GEO satellite to demonstrate electric propulsion. Tests included monitoring the space environment and it was used to carry out particle physics experiments and to measure the affect of radiation on the life of solar cells. For other experiments it carried a high resolution scanning bolometer (radiometer). Operating on two channels: infra-red (10.5 to 12.5 µm) and visible light (0.55 to 0.75 µm), it was able to scan the Earth, measuring its infra-red radiation (temperatures) and cloud patterns, techniques which were subsequently used by weather satellites.
ATS-6 was also used to also carry out air traffic control tests and to practice satellite-assisted search and rescue techniques and it played a major role in the Apollo/Soyuz docking in 1975 when it relayed signals to the Houston Control centre.
See ATS-6 History
Intelsat V
Intelsat V
Source: Ford Aerospace |
Launched in December 1980 Intelsat V was the first commercial Direct Broadcast TV satellite. This was made possible by adopting three axis stabilisation using momentum wheels as pioneered by the ATS-6 satellite. Weighing 4250 lbs (1928 kgs) at launch it was stabilised to within 0.5 degrees and propulsion was by means of hydrazine thrusters.
Because it did not rely on a spinning body for stabilization, Intelsat V could be made in any convenient shape, in this case a box, onto which various appendages housing subsystems could be mounted.
An antenna farm was located on the side of the box facing the Earth with antennas optimised for global, hemispherical, zone and spot footprints with linear and circular polarisation and different frequencies to avoid interference.
Two great fields of solar panels spanning 52.1 feet (15.9 metres), delivering 1800 Watts of power, extended from the adjacent sides of the box and were kept pointing towards the Sun by electric motors as it orbited the Earth and during the Sun’s apparent North – South seasonal excusions. Energy was stored in Nickel Cadmium and Nickel Hydrogen batteries.
Communications were provided by 21 C-Band (6GHz uplink-4GHz downlink) and 4-Ku-Band (14 GHz uplink 11 GHz downlink) transponders carrying 12000 voice circuits and 2 TV channels.
As in ATS-6, it used passive thermal management.
The Intelsat V configuration became the template adopted by many subsequent satellite designs.
Intelsat V was designed and manufactured by Ford Aerospace led by Robert E. Berry.
See also Satellite Technology and GPS Satellite Navigation