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. 5) Satellite Antennas Every satellite regardless of its purpose (weather, communication, spy, International Space Station, Hubble telescope, ) need antennas for sure. If a device has some communication equipment for it to communicate with Earth or some other object in space, it needs antennas to transmit/receive electromagnetic signals. In fact, the same thing applies to Earth stations that need to communicate with satellites. Antennas vary in shape, characteristics, and purpose of use. Some antennas are designed for broadcasting to a very wide area around the antenna, some antennas are designed to broadcast to a relatively small region in a specific direction, while some other antennas may be used for transmitting/receiving from almost a single point. Because of that, usually a satellite will carry several types of antennas at once. Antennas on satellites may be a combination of several types including: o Wire Antennas: These antennas are simply constructed from one of more pieces of wires (or some metallic rods). These antennas are characterized by being non direction (meaning that they transmit in all directions around the wire). If a wire antenna is placed in a vertical direction, the antenna transmits horizontally in all direction around the antenna. Simple wire antennas may be either mono-poles or dipoles. A monopole is a single wire while a dipole is made out of two wires as shown to the right. Wire antennas are usually used in circumstances where you would like to either transmit in all directions/receive from all directions or to transmit to/receive from a specific direction in which the position of the antenna may be random.

2 o Horn Antennas: These antennas are directional antennas meaning that they are able to concentrate transmitted power in case of transmission or receive from a specific direction in case of reception. Horn antennas are used in applications where some directionality is needed but is not suitable for highly directional applications because the directionality of horn antennas is limited. o Reflector Antennas: These antennas can be made to be highly directional with the ability to target a specific satellite for transmission from an Earth station, for example, among several adjacent satellites that are separated from the target satellite by a very small angle as seen from the Earth station. These antennas are composed of a horn antenna (as shown above) and a reflector that reflects the transmission from the horn antenna in case of transmission or collects the transmission into the horn antenna in case of reception. For efficient transmission/reception using a reflector antenna, the size of the reflector must be large enough to reflect most of the transmission of the horn antenna (i.e., all or most of the power transmitted by the horn antenna hits the reflector and reflects back). Reflector antennas are used in applications that require very high directionality either to be able to transmit high power, for example, even if the power being fed to the antenna is low or to be able to select a target satellite/earth station by the antenna among many adjacent ones. The reflector part of this antenna can either be spherical (usually used for achieving high gain and high directionality), or parabolic (sometimes used to shape the transmission pattern of the antenna to provide coverage to a specific geographical region for example). o Array Antennas: This type of antennas is the most sophisticated and most advanced of the above antennas. This is a directional antenna that is not only capable of transmitting to/receiving from a specific direction, but also is capable of changing the direction of its transmission/reception without changing

3 the physical direction of the antenna. Not only that, but this type of antennas is capable of transmitting to/receiving from different points simultaneously simulating the performance of multiple directional antennas at the same time and being able to track the position of different transmitters/receivers. This antenna is used in applications where it is desired to track a satellite electronically, for example, in the sky without the physical movement of the antenna parts, or when transmission to several points is desired at the same time. Important Antenna-Related Terms The following terms related to antennas are of importance and need to be understood very well: Radiation Pattern of an Antenna: This is the plot of the field strength in the far field (plotted in db below maximum value) in different directions around the antenna when the antenna is driven by a transmitter versus the rotation angle. The following plots represent the radiation patterns of several types of antennas: o Isotropic Antennas: These antennas are infinitesimally small spheres, which means that they are only theoretical and cannot be implemented in reality. They radiate power in all direction around the antenna equally as shown to the right. o Wire Antennas (Omni-Directional Antennas): The radiation pattern observed in the horizontal plane of a simple wire antenna that is placed vertically is a circle because it radiates equally in all directions. However, when observed in the vertical plain, it radiates maximum amount of power in the direction perpendicular to the antenna as seen to the right. Omni-Directional Antenna Top View Side View

4 Horn Antenna o Horn Antennas: The radiation pattern of these antennas is shown to the right. o Reflector Antenna: The typical radiation pattern of spherical reflector antenna is shown to the right. Reflector Antenna o Array Antenna: The radiation pattern of this type of antenna is not fixed and changes depending on the driving signals to the different elements of the antenna array. It may be something like the figure shown to the right at a specific time and direction of the different beams may be changing with time to track specific targets. Antenna Gain (G): Gain is a term that is usually associated with active electronic components such as amplifiers. When gain is used in the context of antennas, it refers to the ability of an antenna to concentrate its transmitted power in a specific direction. An isotropic antenna, which radiates equal power in all directions (around, up, down) as shown above has a gain of 1 (0 db). Any other type of antenna concentrates its transmission in specific direction(s) resulting in having a gain that is greater than 1 (> 0 db). The gain of an antenna is always considered to be the ratio of largest concentration of power it radiates among all direction over the power radiated by an isotropic antenna that is supplied with the same input power. Antenna Boresight: is the direction over which an antenna radiates maximum power. In any practical communication system, this direction for a receiving antenna is always

5 adjusted such that it points to the transmitting antenna while for a transmitting antenna this direction is always adjusted such that it points to the receiving antenna. Reciprocity: This basically states that an antenna that is operating as a transmitting antenna will have the same transmission gain, transmission radiation pattern, transmission boresight,. as a similar antenna that is operating as a receiving antenna will have when operating at the same frequency. Beamwidth (θ ): This is the angle over which an antenna concentrates its power. Since an the power transmission profile of an antenna usually drops slowly (like the passband of a real filter), the beamwidth is usually measured as the angle between the directions over which the transmitted power drops to one half of its maximum and this would be called the 3dB beamwidth. Typical Parameters of Different Antennas Depending on your application, you would usually pick an antenna that is suits your needs. The choice of the proper antenna for your application depends on which of them satisfy your requirements. The following table shows typical parameters including gain, beamwidth, of different antennas. (References: Antenna Type Typical Max Gain (G max ) Typical Beamwidth θ 3dB(min) Isotropic 1 (0 db) [Fixed] N/A Omni-directional 10 (10 db) N/A Horn 100 (20 db) 20 Reflector 10,000 (40 db) or more 1 or less Array 1000 (30 db) 5 What Antennas are Used in Satellite Systems It is rare that a satellite will carry only one type of antennas. Often, several types of antennas are used on a single satellite. So, what type of antenna is used for each application on a satellite? In the following discussion, we answer this question. Omni-directional: This type of antennas is very used for applications where you want to transmit to the satellite/receive from the satellite from any direction. This would be the case for the Telemetry, Tracking, Command, and Monitoring (TTC & M) Sub-system.

6 The reason is that if the satellite is not in the proper attitude, it would not be able to receive from Earth or transmit to Earth if it is using a directional antenna so a nondirectional antenna is used for this application to insure that the satellite would be able to communicate with the Earth station regardless of its attitude. Horn Antennas: This type is used to provide transmission coverage or (reception coverage) to a relatively wide region on Earth from a GEO satellite for example. Often, a GEO satellite is designed to provide TV broadcast, for example, to a continent, a geographical region, a large country, or so. To achieve this, one or more horn antennas are used. Reflector Antennas: This type is used to provide highly directed transmission/reception as when communicating to a spot on earth in the case of transmission/reception from an Earth station. Array Antennas: This type is usually used in LEO communication satellites, which are non-stationary with respect to Earth, to allow the satellites to track multiple users simultaneously.