Satellite Sub-systems

Transcription

1 Satellite Sub-systems Although the main purpose of communication satellites is to provide communication services, meaning that the communication sub-system is the most important sub-system of a communication satellite, for communication satellites to function properly, they must include many important subsystems other than the communication sub-system. The following is a description of the main sub-subsystems of a communication satellite. 1) Attitude and Orbit Control Sub-system The attitude of a satellite is the direction at which the satellite points to with respect to Earth. Communication satellites, and most other types of satellites, must point to Earth or point in some other specific direction. A communication satellite must point towards Earth because it has high gain antennas (directional antennas that must be pointing towards the Transmitter/Receiver to be able to receive/transmit). Other objects may need to avoid pointing towards Earth such as the Hubble Space Telescope, which must not point towards Earth, the Sun, or the Moon to avoid having its very sensitive cameras and its cooling system over-exposed to the high intensity light from these objects. On the other hand, spy and weather satellites with cameras must point towards Earth to take photographs. So, it is very important that the direction to which a satellite is pointing is knows and controlled. It would be a huge waste to use rocket fuel to control the satellite attitude of a satellite because this is done very often within the course of a single orbit. So other means of turning the satellite are used. Satellites are usually equipped with what is known as momentum wheels to rotate the satellite about its axis. When the momentum when is spun in a specific direction about some axis, the satellite will spin the opposite direction. Some satellites need to be stabilized in several dimensions while others need to be stabilized in only a single dimension: o 3-axis stabilized satellites: contain three momentum wheels that are fixed in 3 dimensions, one for each dimension, or a single momentum wheel that is installed on gimbals is used instead. This type of satellites has to be stabilized in 3 dimensions for its antennas to face Earth and its solar cells to face the Sun for maximum power generation. o Spinner satellites: are stabilized only in one dimension. The other two dimensions are stabilized by having the satellite spin around an axis at rates from 30 to 60 rotations per minute. In this case: Satellite antennas must either have circular symmetry such as omnidirectional antennas (for example, monopoles) Directional satellite antennas must be de-spun (antennas are rotated at an opposite direction to the satellite rotation for the antennas to appear to be stationary. Bearings are used in this process. Satellite orbital control is achieved by rocket engines that use fuels such as hydrazine (N2H4) or arc jets (ion thrusters).

2 o Orbital control is very important for GEO satellites to prevent loss of signal at Earth stations. Also important for GPS satellites to keep them in the desired orbit. 2) Telemetry, Tracking, Command, and Monitoring (TTC & M) Sub-system Telemetry and Monitoring: is the process of measuring different parameters of the satellite and determining the condition of a satellite and its health, which include 100s of parameters obtained from sensors onboard the satellite and sending them back to the Earth station to take actions. Some of these parameters include: o Pressure in fuel tank indicating amount of fuel remaining, o Power provided by solar cells, o Power consumed by different communication parts, o Temperature of different parts of the satellite, o Position of switches that control different devices on the satellite, o Attitude information Tracking: is the process of determining exactly where the satellite is located, where it is heading, its speed, and its acceleration. o Velocity and Acceleration Measurements: Velocity may be measure by integrating acceleration, which may be obtained using accelerometers (acceleration sensors), and then adding the initial value of the velocity to get a very good estimate of the satellite velocity at any time, o Beacon Signal Doppler Shift: Satellites usually have beacons (transmitters that transmit signals with specific know frequencies). Earth stations monitor the Doppler shift of beacon transmissions to determine the speed of the satellite. o High Gain Antenna Tracking: Earth stations usually use very high gain antennas that have very narrow beam-widths. So, the position of a satellite can be determined by pointing a very high gain antenna towards it and moving the direction of the antenna to track the satellite movements, o Echoing Delay Measurement: If the processing delay of a satellite is known, Earth stations can monitor the pulse echoing delay echoing delay to determine how far a satellite is from the Earth station. Command: is the process of ordering the satellite to perform some operation. o Command Channel: to avoid having any un-authorized tampering with the satellite, secure channels (encryption of commands and responses) are used for sending commands to the satellite and receiving responses. o Types of Commands: The usual commands to a satellite may be to: Fire its rocket thrusters to correct its orbit, Correct its attitude, Shutdown or turn on some parts of its communication systems, Change the position of some switches, Extend solar cells, o Redundancy of Commands: to avoid having any command being misinterpreted by the satellite, which may cause a disaster, the commands are

3 echoed back the Earth station before being executed, and the execution requests are also echoed back to the Earth station after being executed. This is shown in the following figure: Earth Station Satellite Fire Rocket Thruster Command received Echo Received Fire Rocket Thruster? Execute Last Command Execution request received Execution Confirmation received Send Confirmation of Execution 3) Power Sub-system Batteries: Because satellites sometimes pass through the shadow of Earth and to remain operational, satellites are equipped with batteries to store excess energy from the solar cells during the period in which the solar cells are illuminated by the Sun and provide the satellite with energy when satellite is in the dark: o How Many Batteries are Needed: Batteries need to support the operation of a GEO satellite during the maximum eclipse duration of 70 minutes every day. The longest eclipses occur in the Fall and Spring (around 21 September and 21 March of each year). Shorter eclipse periods occur before and after these dates. During a big part of the year, no eclipses will occur. o If Batteries cannot Support the Whole Satellite: Parts of the communication system of a satellite may be shut down if batteries cannot support the whole satellite for the whole period eclipse period, and then started again once the satellite goes out of Earth s shadow. o Ratings of Batteries: Batteries used on Satellites usually have ratings of V and Amp-hour. o Efficiency of Batteries: The efficiency of any batteries deteriorates with time, so when designing the satellite, the amount of batteries to be included is usually based on the end of life efficiency of the batteries. That is, how much energy would the batteries be able to store near the end of life of the satellite. Solar Cells: Solar cells are the element that provides the whole communication satellites with power to perform all of its tasks (other than orbit control). The sun provides an amount of energy equal to approximately kw of power per square meter at Earth s distance from the sun. Although Earth s atmosphere absorbs a significant amount of this power (mostly around the ultra violet region of the spectrum), since satellites are in space, this is the amount of energy that they can theoretically get from the sun per square meter:

4 o How Efficient are Solar Cells: The best solar cells in the market can only extract a fraction of the incident solar power and convert it to electric power. Typically, the best solar cells have an efficiency of only 20 to 25%. o Change of Solar Cell Efficiency with Time: Solar cells may have efficiencies as high as 25% when they are first manufactured. However, with time, this efficiency drops because of many factors including aging and the small scratches that occur because of small meteors that constantly hit them. Therefore, it is expected that the efficiency of a solar cell may drop from 25% to 15%, for example at the end of the satellite life. Therefore, when building a satellite, it makes sense to have extra solar cell area to compensate for the expected drop in efficiency near the end of life of the satellite. o Effect of Solar Cell direction on Converted Power: Solar cells convert solar power to electricity. The more solar light a solar cell blocks the more the electric power that will be generated. Therefore, solar cells that are directed such that solar rays hit them perpendicular to their plane will generate the maximum amount of power. If the incident light and the line perpendicular to the solar cell are parallel, the solar cell will produce the maximum amount of power. If the angle between these lines is θ, the amount of converted power will be equal to a fraction of cos(θ) of the maximum power. If solar light is parallel to the surface of the solar cell, (i.e. θ = 90 ), the produced power will be 0. o Power needs of a Communication Satellite: Commercial communication satellites typically need an amount of power from 5 to 10 kwatts. o Total Power Generated by Solar Cells on Satellites: 3-axis stabilized satellites have flat solar cell panels that are fixed on rotating shafts to allow the satellite to always point its solar cell panels directly to the sun. Spinner satellites, however, have their solar cells covering its cylindrical surface, so solar cells have a cylindrical shape and only one half of the total area of solar cells will be illuminated by the sun. In addition, not all of the illuminated region will produce maximum power. Assuming that the solar cells on a satellite are directed as to generate the maximum power, if the total area of solar cells on the satellite is A m 2 and they have efficiency η : P 3-axis stabilized = (1,361 η A) Watts P spinner = (1,361 η A) / π Watts o Spinning Effect on Solar Cell Efficiency: Because solar cells of spinner satellites are not fully exposed to solar power at all time, they remain at a lower temperature than solar cells of 3-axis stabilized satellites. The lower temperature of solar cells usually results in a higher efficiency. So, the efficiency of solar cells of spinner satellites are usually higher than the efficiency of solar cells of 3- axis stabilized satellites. 4) Communication Sub-system Clearly, the communication sub-system is the most important system of a communication satellite. All other sub-systems are there to serve this sub-system. The basic building block of the communication sub-system is called transponder, which is basically a block that receives a signal from an Earth station, changes its frequency and amplifies it, and then transmits it to another Satellite or back to Earth:

5 o Number of Transponders in a Communication Satellite: Typically, a communication satellite contains from 10 to 50 transponders depending on how crowded the coverage region of the satellite (in terms of customers requesting bandwidth on satellites) and many other factors. o Operation of Transponders: A transponder is simply a device that receives a signal from an Earth station, amplifies it, and then sends it back to Earth. o Difference between Uplink and Downlink Frequencies: It would be great if the same RF frequency is used for the uplink (the signal transmitted from Earth to a satellite) and the downlink (the signal transmitted from the satellite to the Earth station) because this would reduce the components in the transponder to a minimum. However, the use of the same frequency for the uplink as well as the downlink would cause a big problem in the amplifiers. Even if very highly directional antennas are used in the satellite for the uplink and downlink, there would always be some signal leakage from the transmitting antenna on a satellite to the receiving antenna. Since the signals received by satellite are so weak, transponders use filters of very high gains. Even if a small amount of transmitted signal leaked to the receiving antenna, it would be highly amplified, which would cause even more leakage of the transmitted signal back to the receiving antenna. This would cause oscillation in the amplifier and it would saturate, making the amplifier useless. The only way to avoid this is to make sure that the transmitted signal has a different frequency from the received signal. The uplink/downlink frequencies of several frequency bands used in satellites are: C-band (6 GHz / 4 GHz), Ku-band (14 GHz / 11 Ghz), and Ka-band (30 GHz / 20 GHz) o Bandwidth of Transponders: Transponders have different bandwidths usually related to the band they operate at. Typical transponder bandwidths are 36 MHz (C-band), 54 MHz (Ku-band), and 72 MHz (Ka-band). Knowing that the bandwidth of a standard definition TV channel is around 5 MHz, this means that a transponder may process from around 7 to 15 channels, or so. o Frequency Reuse in Satellites: By using many techniques that allow a single satellite to reuse the allocated bandwidth for it several times, a satellite with bandwidth of 500 MHz, for example, may be able to effectively have a bandwidth of up to 7 times that much or so. One of the methods used to achieve this frequency reuse is using two orthogonal polarizations. o Frequency Conversions in Transponders: Transponders come in two flavors: single conversion and double conversion transponders. In single conversion transponders, the frequency of the incoming signal (at Uplink RF frequency) is directly shifted to the frequency of the outgoing signal (at Downlink RF frequency). In double conversion transponders, the frequency of the incoming signal (at Uplink RF frequency) is first converted to a low intermediate frequency (IF frequency), then the frequency is shifted to the frequency of the outgoing signal (at Downlink RF frequency). o Power Transmission of a Transponder: The typical power transmitted by a transponder is around 200 Watts or so. This is equivalent to around 20 Watts per TV channel or so. o Single Conversion Transponders: The following shows a block diagram of a typical narrowband single conversion transponder. The bandwidths of the amplifiers and filters are approximate values of what may be found in a C-

6 band transponder. (LNA = Low Noise Amplifier, LPA = Low Power Amplifier, HPA = High Power Amplifier, BPF = Band Pass Fitler) o Double Conversion Transponders: The following shows a block diagram of a typical narrowband double conversion transponder. The bandwidths of the amplifiers and filters are typical values that may be found in a C-band transponder. LNA BPF Mixer BPF Mixer BPF LPA HPA Wideband Center Freq. = GHz BW = 36 MHz X Center Freq. = GHz BW = 36 MHz X Center Freq. = GHz BW = 36 MHz Local Oscillator 1 ~ Freq. = GHz Local Oscillator 2 ~ Freq. = GHz Uplink Center Frequency = GHz Bandwidth = 500 MHz Downlink Center Frequency = GHz Bandwidth = 36 MHz