Satellite Communications

Transcription

1 Satellite Communications Chen, Zhi Ning Contacts: URL: EE5404 Introduction 1

2 About This Course: Introduction to satellite communication systems Description of satellite system /Types of orbits Satellite launchers / Radio regulations Various satellite applications Link analysis, propagation and interference Calculation of link budget using G/T, C/N ratio, EIRP Effects of various propagation phenomenon like rain, ionosphere Terrestrial and satellites interference Earth station technology Earth Station design/ Antenna RF sub-system / Communication sub-system Network interface/ Monitoring and control Satellite communication payload Mission and characteristics Conventional, multibeam and regenerative transponder Antenna coverage EE5404 Introduction 2

3 To discuss the important issues of SCS at a system level. Target & Main Reference Books To learn the elements of satellite communication systems (SCS) To master the basic principle of the relevant designs of SCS [1] Satellite Communication Systems-Systems, Techniques and Technology (3rd Edition) By G. Maral & M. Bousquet November 2000, John Wiley & Sons [2] Digital Satellite Communications (2nd Edition) By Tri T. Ha 1990, McGRAW-HILL [3] Satellite Communications (3rd Edition) By D. Roddy 2001, McGRAW-HILL [4] Satellite Communication Systems Engineering (2nd Edition)By W. L. Pritchard, H. G. Suyderhoud, & R. A. Nelson 1993, P T R Prentice Hall EE5404 Introduction 3

4 Contents Introduction to satellite communication systems Satellite orbits (Geostationary Orbit) Propagation and Antennas Link analysis and interference Earth station technology Satellite communication payload Satellite communication techniques Satellite multiple access techniques Emerging satellite technologies EE5404 Introduction 4

5 1 Introduction 1.1 Satellite Communication Systems & Applications basic elements and applications 1.2 Some Issues in System Design EE5404 Introduction 5

6 1.1 Satellite Communication Systems & Applications An SCS is a type of wireless communication systems. Features: Providing coverage over large geographical areas Resulting in interconnectivity between various communication resources Realising seamless coverage to combine satellite mobile communication systems with terrestrial mobile systems It is important to note the common and different characteristics between the SCS and other wireless communication systems. EE5404 Introduction 6

7 Basic Systems System Technology Space segment Ground segment missile + microwave Different from other terrestrial wireless communication systems, the SCS basically comprises Space and Ground segments. The communication will be built up between the space and ground segments. The characteristics of each segment depend on their applications such as fixed, mobile or direct broadcasting systems. With the missile technology, the space segments can be sent to a proper position in space. To reduce sizes of the segments, especially the space segments, the segments are made by microwave technology. The microwave technology also allows the use of high frequency to lower path losses. EE5404 Introduction 7

8 Satellite Basic Systems Main satellite services: Fixed satellite service, FSS Mobile satellite service, MSS Direct broadcast system, DBS downlink TT&C Ground Station uplink Telemetry Tracking and Command (TT&C) To control satellite and monitor satellite s performance. A typical FSS system Earth Station EE5404 Introduction 8

9 Satellite Basic Systems Gateway Mobile Satellite System (MSS) In an MSS, the ground segments consist several types of mobile terminals, which can communicate each other directly and via a satellite. Also, they can be connected to fixed telecommunication networks through a satellite. Usually, the MSS can be categorized into three classes: maritime, aeronautical and land. Aircraft Fixed Network Ship EE5404 Introduction 9

10 satellite Basic Systems Direct-to-home Reception Gateway Program Source Direct Broadcast System (DBS) A large earth station transmits the programmes to a high-power satellite. The latter broadcast the programmes to small terminals on the earth. Community Reception EE5404 Introduction 10

11 Basic Elements Geostationary orbit is 35,786km over equator. The satellites orbiting the Earth in such an orbit rotate in unison with the Earth.They appear almost stationary to the users on the Earth. So, this minimizes the operational requirements of earth stations by simplifying tracking systems of ground terminals. Molniya Orbit Space Segment Ground Segment Any satellite communication system consists of at least one satellite and several terminals on the earth. The basic elements include the satellite, earth stations, and the orbit, where the satellite is moving on. EE5404 Introduction 11

12 Basic Elements Space Segment Satellites Receiving signals from an earth station Changing frequency of the carrier Amplifying the signals Transmitting the signals back to other earth stations Payloads Platform receiving antennas transmitting antennas all the electronic equipment Electric power supply Temperature Control Altitude and orbit control Propulsion equipment TT&C equipment EE5404 Introduction 12

13 VSAT: very small aperture terminals Basic Elements Ground Segment Terrestrial Networks All Earth Stations Connecting the end-users to earth stations Transmitting signals to satellites Receiving signals from satellites Changing frequencies of the signals FSS: several kinds of fixed earth stations large station(11-30m antennas) or VSAT (1-3m antennas) Earth Stations OF MSS: several kinds of mobile terminals connected to the fixed telecommunication networks via fixed earth stations (maritime, aeronautical, and land base) DBS: gateways plus small terminals with cm antennas EE5404 Introduction 13

14 Basic Elements HPA upconverter modulator Earth Station feed system baseband processing interface drive motors LNA tracking system downconverter demodulator from/to terrestrial systems A typical earth station usually include the elements used in any wireless communication systems. They are roughly classed into two parts, transmitting and receiving parts. EE5404 Introduction 14

15 Multipoint Data Transmission Network Satellite Broadcasting Network Data Collection Network 1.2 Some Issues in System Design Services Voice, data,video Cost Technology network Signal Quality Link Design System Design Selection of radio frequency To be discussed Application Propagation Technology Regulation Spectrum Modulation Coding... The target of system design is the high signal quality. The important issues include kinds of services, cost of system, status of technology, network in use. The most important consideration is the selection of the radio frequency. The selection of the RF is strongly dependent on these factors. EE5404 Introduction 15

16 Frequency Allocations for Satellite Electromagnetic Spectrum for RF Frequency Wavelength, m Designation 3 Hz-30 khz Very low frequency (VLF) khz Low frequency (LF) MHz Medium frequency (MF) 3-30 MHz High frequency (HF) MHz 10-1 Very high frequency (VHF) GHz Ultra high frequency (UHF) 3-30 GHz Super high frequency (SHF) GHz Extremely high frequency (EHF) GHz Infrared, visible light, ultraviolet This is electromagnetic frequency spectrum. Of them, the bands of 3-300GHz is used for various radio applications, such as radar and communications. EE5404 Introduction 16

17 Frequency Allocations for Satellite Frequency spectrum for Satellite Communications Band Frequency range, GHz L 1-2 S 2-4 C 4-8 X 8-12 Ku K Ka Millimeter Particularly, these frequency bands are allocated for satellite systems. EE5404 Introduction 17

18 Frequency for uplink is always higher than frequency for downlink! Why? Frequency Allocations for Satellite Frequency Allocations for Satellite Services FSS - C band: ~6 GHz uplink, ~4 GHz downlink - INTELSAT - X band: ~ 8 GHz uplink, ~7 GHz downlink- government - Ku band: ~ 14 GHz uplink, ~12 GHz downlink- EUTELSAT, Telecom I & II -INTELSAT - Ka Band: ~ 30 GHz uplink, ~20 GHz downlink- experimental - Each allotment consists of an orbital position/bandwidth in the bands mentioned/service area VSAT Very Small Aperture Terminals (C and Ku band) MSS -L band ~ 1.6 GHz uplink, ~1.5 GHz downlink for the mobiles -Ku band for Network Control Centre and Hubs. DBS - Ku band: ~12 GHz downlink EE5404 Introduction 18

19 International Organizations for Satellite Communications International Telecommunication Union (ITU) - United Nations (UN) organization for telecommunications CCIR= International Radiocommunications Consultative Committee CCITT= International Telegraph and Telephone Consultative Committee Technical & Operational matters, produce Reports & Recommendations. WRAC= World & Regional Administrative Radio Conferences Discuss certain topics and carry out total & partial revision of the administrative regulations like Radiocommunications Regulations (RR) IFB= International Frequency Registration Board Register frequency assignments to radio stations given by the countries and verify that the they conforms to the international frequency assignments ITU divided the Earth into 3 regions for frequency allocation 1. Region 1- Europe, Africa, ex-soviet Union, Mongolia 2. Region 2- North & South America, Greenland 3. Region 3- Asia(excluding region 1 Areas), Australia & South West Pacific (Singapore) EE5404 Introduction 19

20 Conclusions Brief introduction of SCS Basic elements of SCS Main applications of SCS Allocations of frequency for satellite International organizations for SCS We just snapshot the satellite communication systems and their main applications. Compared to other wireless communication systems, the key features of the SCS are their radio link and operation frequency. Therefore, the system design strongly depends on the link design. In the following lectures, we are going to discuss the issues related to the link design. EE5404 Introduction 20

21 GPS Global Positioning System! What is GPS? Why GPS? How does GPS work?. EE5404 Introduction 21

22 What is GPS? The GPS is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. GPS uses these "man-made stars" as reference points to calculate positions accurate to a matter of meters. In fact, with advanced forms of GPS you can make measurements to better than a centimeter! In a sense it's like giving every square meter on the planet a unique address. GPS receivers have been miniaturized to just a few integrated circuits and so are becoming very economical. And that makes the technology accessible to virtually everyone. These days GPS is finding its way into cars, boats, planes, construction equipment, movie making gear, farm machinery, even laptop computers. Soon GPS will become almost as basic as the telephone. Indeed, it just may become a universal utility. EE5404 Introduction 22

23 Why GPS? Trying to figure out where you are and where you're going is probably one of man's oldest pastimes. Navigation and positioning are crucial to so many activities and yet the process has always been quite cumbersome. Over the years all kinds of technologies have tried to simplify the task but every one has had some disadvantage. The result is the Global Positioning System, a system that's changed navigation forever EE5404 Introduction 23

24 How does GPS work? Here's how GPS works in five logical steps: 1.The basis of GPS is "triangulation" from satellites. 2.To "triangulate," a GPS receiver measures distance using the travel time of radio signals. 3.To measure travel time, GPS needs very accurate timing which it achieves with some tricks. 4.Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret. 5.Finally you must correct for any delays the signal experiences as it travels through the atmosphere. EE5404 Introduction 24

25 How does GPS work? Improbable as it may seem, the whole idea behind GPS is to use satellites in space as reference points for locations here on earth. That's right, by very, very accurately measuring our distance from three satellites we can " triangulate " our position anywhere on earth. Forget for a moment how our receiver measures this distance. We'll get to that later. First consider how distance measurements from three satellites can pinpoint you in space. Suppose we measure our distance from a satellite and find it to be 11,000 miles. Knowing that we're 11,000 miles from a particular satellite narrows down all the possible locations we could be in the whole universe to the surface of a sphere that is centered on this satellite and has a radius of 11,000 miles. Next, say we measure our distance to a second satellite and find out that it's 12,000 miles away. That tells us that we're not only on the first sphere but we're also on a sphere that's 12,000 miles from the second satellite. Or in other words, we're somewhere on the circle where these two spheres intersect. If we then make a measurement from a third satellite and find that we're 13,000 miles from that one, that narrows our position down even further, to the two points where the 13,000 mile sphere cuts through the circle that's the intersection of the first two spheres. So by ranging from three satellites we can narrow our position to just two points in space. EE5404 Introduction 25

26 How does GPS work? To decide which one is our true location we could make a fourth measurement. But usually one of the two points is a ridiculous answer (either too far from Earth or moving at an impossible velocity) and can be rejected without a measurement. A fourth measurement does come in very handy for another reason however, but we'll tell you about that later. Next we'll see how the system measures distances to satellites. 1.Position is calculated from distance measurements (ranges) to satellites. 2.Mathematically we need four satellite ranges to determine exact position. 3.Three ranges are enough if we reject ridiculous answers or use other tricks. 4.Another range is required for technical reasons to be discussed later. EE5404 Introduction 26

27 Web sites related to GPS: EE5404 Introduction 27

28 Problems: Describe basic elements and functions of a satellite communications system, such as fixed, mobile and direct broadcast system. What are the important considerations in the design of satellite communication systems? Describe the GPS. EE5404 Introduction 28