LE/ESSE Payload Design

Transcription

1 LE/ESSE Payload Design 4.1 Communications Satellite Payload - Introduction Earth, Moon, Mars, and Beyond Department of Earth and Space Science and Engineering Room 255, Petrie Science and Engineering Building Tel: ext jjshan@yorku.ca Homepage:

2 Advantages of Satellite Communication Mobile/Wireless Communication, Independent of Location Wide Area Coverage: Country, Continent, or Globe Wide Bandwidth Available Throughout Independence From Terrestrial Infrastructure Rapid Installation of Ground Networks Low Cost Per Added Site Uniform Service Characteristics Total Service From a Single Provider Communications Payload - Introduction 2

3 Communication Satellite s Sub-Systems Communications Payload - Introduction 3

4 Antenna Antenna - Common name for a Satellite dish. SatCom antenna come in various sizes, configurations and materials. Ranging from 0.9 meters to 33 meters the antenna is the most impressive sub-system of an earth station. The main function of an antenna is a passive amplifier. The gain of an antenna is calculated as a relative function of its size and the wavelength of the signal to be amplified (dbi). Many factors govern the quality of an antenna. A SatCom antenna must remain pointed at the satellite under all conditions. The larger the antenna the smaller the main transmit lobe [beamwidth]. An antenna system comprises the following parts: The mechanical system which encompasses the reflector, back structure and pedastle. The primary source, comprising the illumination horn, the associated reflector sub-assemblies and the non radiating components. The most frequently used types of antenna are parabolic [axisymmetric or offset]. Typically antenna subsystems can achieve 66% efficiency, with the newer design, some antennas can reach 82% or more. Communications Payload - Introduction 4

5 Transponder Short-for Transmitter-responder. A term used for complete microwave channel of transmission. A communications satellite s channels are called transponders, because each is a separate transceiver or repeater. With digital video data compression and multiplexing, several video and audio channels may travel through a single transponder on a single wideband carrier. Original analog video only has one channel per transponder. Two payload configurations: single channel repeater and channelized repeater. Communications Payload - Introduction 5

6 What is a decibel? The decibel (db) is used to measure sound level, but it is also widely used in electronics, signals and communication. The db is a logarithmic unit used to describe a ratio. The ratio may be power, sound pressure, voltage or intensity or several other things. Decibels are useful because they allow even very large or small ratios to be represented with a conveniently small number. This is achieved by using a logarithm. db(w) power relative to 1 watt. dbi db(isotropic), the forward gain of an antenna compared to an idealized isotropic antenna. Some examples: 3 dbw means 3 db greater than 1 W. 6 dbw means 6 db less than 1 W. 0 dbw means no change from 1 W, in other words 0 dbw is 1 W. Communications Payload - Introduction 6

7 Microwave Link in Satellite Communications Frequencies above 1 GHz (but less than 60 GHz) Band is divided into separate halves, one for uplink and one for downlink. Line-of-sight propagation Low received signal level due to large distance Communications Payload - Introduction 7

8 Frequency Bands - I L- S- C- X- 1-2 GHz 2-4 GHz 4-8 GHz 8-12 GHz communications satellites, digital audio broadcasting (DAB), GPS, WorldSpace satellite radio. used by weather radar and some communications satellites, especially those used by NASA to communicate with the Space Shuttle and the International Space Station. Digital Audio Radio Satellite (DARS). In some countries, S band is used for Direct-to-Home satellite television. It is primarily used for satellite communications, normally downlink GHz, usually via twenty-four 36 MHz-wide transponders on board a satellite. The X band is used by some communications satellites and for radars, primarily for fire control, but also for longer-range ground and weather mapping. It s used primarily by military. K- Ku- Ka- Milli metre GHz GHz GHz GHz K band between 18 and 26.5 GHz is absorbed easily by water vapor. satellite communications. The band is split into multiple segments that vary by geographical region by the International Telecommunication Union (ITU) to 12.2 GHz band is allocated to the FSS (fixed service satellite, uplink 14.0 to 14.5 GHz). There are more than 22 FSS K u -band satellites orbiting over North America, each carrying 12 to 24 transponders, 20 to 120 watts per transponder, and requiring a 0.8-m to 1.5-m antenna for clear reception. The 12.2 to 12.7 GHz segment is allocated to the BSS (broadcasting satellite service). BSS/DBS direct broadcast satellites normally carry 16 to 32 transponders of 27 MHz bandwidth running at 100 to 240 watts of power, allowing the use of receiver antennas as small as 18 inches. K u band signals can be affected by rain attenuation. 20/30 GHz band is used in communications satellites (downlink GHz and GHz) Signals are extremely prone to atmospheric attenuation, making them of very little use over long distances. Even over relatively short distances, rain fade is a serious problem. Communications Payload - Introduction 8

9 Frequency Bands - II Ku band and under are the most popular: relative low cost of available equipment; more favorable propagation characteristics. Millimeter-wave bands are difficult: rain attenuation and hardware expense. C-BAND The frequency range allocated for C-band is 3.7GHz 6.425GHz. It is further divided into separate halves, one for ground-to-space links (Uplink) and one for space-to-ground links (downlink) as shown: C-band Uplink Frequencies: 5.925GHz 6.425GHz C-Band Downlink Frequencies: 3.7GHz 4.2GHz Ku-BAND The frequency range allocated for Ku-band is 11.7GHz 14.5GHz. Ku-band, like C-band, is further divided into separate halves, one for ground-to-space links (Uplink) and one for space-to-ground links (downlink) as shown: Ku-Band Uplink Frequencies: 14GHz 14.5GHz Ku-Band Downlink Frequencies: 11.7GHz 12.2GHz Communications Payload - Introduction 9

10 Path Geometry (GEO Link) 2 2 R0 = a + r 2ar cosϕ cosδ Communications Payload - Introduction 10

11 Examples To calculate the slant ranges from Anik F1 and F2 to an earth station at Toronto. Communications Payload - Introduction 11

12 Key Elements for the Uplink and Downlink Uplink: Ground-to-Space link Downlink: Space-to-Ground link Communications Payload - Introduction 12

13 Uplink Components The major components of a typical earth station uplink are the modem, upconverter, and high-powered amplifier. The following is a brief description of each. Modem: Modulates a baseband signal to an Intermediate Frequency (IF). Usually 70MHz or 140MHz. Upconverter: Converts IF to RF. High Power Amplifier (HPA): Increases the power of the RF signal to achieve satisfactory uplink operations. Communications Payload - Introduction 13

14 Downlink Components On the downlink side you would typically have a low noise amplifier, downconverter, and modem. A low noise block downconverter can be used in place of a low noise amplifier and downconverter. The following is a brief description of each. Low Noise Amplifier (LNA): Amplifies the RF signal received from the satellite. Downconverter: Converts the RF into IF that is then sent to the modem. Low Noise Block Downconverter (LNB): Amplifies and converts the RF signal from the satellite into IF. Essentially it is a Low Noise Amplifier (LNA) and downconverter that have been incorporated into a single unit Modem: Demodulates the IF signal and extracts the data for use. Communications Payload - Introduction 14

15 Polarization of EM Wave Linear polarization: vertical, horizontal Circular polarization: left hand or right hand Use of Linear Polarization Frequencies above S-band High rain attenuation regions Simple microwave components Use of polarization as measurement tool User of Circular Polarization Frequencies below C-band Simplify antenna alignment (no polarization adjustment required) No correction for day/night variation Communications Payload - Introduction 15

16 Footprint A footprint is the term used to describe the area of the earth on which the satellite beams its signals. Each satellite has a different footprint and these days most satellites have several beams each with their own footprint or area of coverage. The footprint of a satellite will dictate its connectivity. The transmitting and receiving antennas on the satellite are designed to cover specific regions of the earth s surface. This is done for several reasons. First, it concentrates the power radiated from the satellite into the desired region. Second, it increases the sensitivity of its receiving antenna minimizing interference with other adjacent satellite signals. Communications Payload - Introduction 16

17 Gain and Effective Area of a Circular Aperture D - Diameter A - Area of antenna η - Efficiency λ - Wavelength G - Gain A E - Effective area Communications Payload - Introduction 17

18 Antenna Capture Area Receive antenna captures power through its effective area Transmit antenna is a reciprocal device (yielding the same performance as in receive) Communications Payload - Introduction 18

19 The Power Balance Equation The Power Balance Equation simplifies the analysis of microwave links: The Power Received = the Power Transmitted plus all gains, minus all losses. P r = P t + G t - L t - A 0 + G r - L r Typically expressed in dbw and db in a table called the Link Budget Communications Payload - Introduction 19