电子工程师手册（英文版六）：ch102

Spitzer, C.R., Martinec, D.A., Leondes, C.T., Rana, A.H., Check, W. “Aerospace Systems” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000 102 Aerospace Systems 102.1Avionics Systems A Modern Example System?Data Buses?Displays?Power? Software in Avionics?CNS/ATM?Navigation Equipment? Emphasis on Communications?Impact of “Free Flight”? Avionics in the Cabin?Avionics Standards 102.2Communications Satellite Systems: Applications Satellite Launch?Spacecraft and Systems?Earth Stations?VSAT Communication System?Video?Audio?Second-Generation Systems 102.1 Avionics Systems Cary R. Spitzer, Daniel A. Martinec, and Cornelius T. Leondes Avionics (aviation electronics) systems perform many functions: (1) for both military and civil aircraft, avionics are used for flight controls, guidance, navigation, communications, and surveillance; and (2) for military aircraft, avionics also may be used for electronic warfare, reconnaissance, fire control, and weapons guidance and control. These functions are achieved by the application of the principles presented in other chapters of this handbook, e.g., signal processing, electromagnetic, communications, etc. The reader is directed to these chapters for additional information on these topics. This section focuses on the system concepts and issues unique to avionics that provide the traditional functions listed in (1) above. Development of an avionics system follows the traditional systems engineering flow from definition and analysis of the requirements and constraints at increasing level of detail, through detailed design, construction, validation, installation, and maintenance. Like some of the other aerospace electronic systems, avionics operate in real time and perform mission- and life-critical functions. These two aspects combine to make avionics system design and verification especially challenging. Although avionics systems perform many functions, there are three elements common to most systems: data buses, displays, and power. Data buses are the signal interfaces that lead to the high degree of integration found today in many modern avionics systems. Displays are the primary form of crew interface with the aircraft and, in an indirect sense, through the display of synoptic information also aid in the integration of systems. Power, of course, is the life blood of all electronics. The generic processes in a typical avionics system are signal detection and preprocessing, signal fusion, computation, control/display information generation and transmission, and feedback of the response to the control/display information. (Of course, not every system will perform all of these functions.) A Modern Example System The B-777 Airplane Information Management System (AIMS) is the first civil transport aircraft application of the integrated, modular avionics concept, similar to that being used in the U.S. Air Force F-22. Figure 102.1 shows the AIMS cabinet with eight modules installed and three spaces for additional modules to be added as the AIMS functions are expanded. Figure 102.2 shows the AIMS architecture. Cary R. Spitzer AvioniCon Inc. Daniel A. Martinec Aeronautical Radio, Inc. Cornelius T. Leondes University of California, San Diego Abdul Hamid Rana GE LogistiCom William Check GE Spacenet ? 2000 by CRC Press LLC AIMS functions performed in both cabinets include flight management, electronic flight instrument system (EFIS) and engine indicating and crew alerting system (EICAS) displays management, central maintenance, airplane condition monitoring, communications management, data conversion and gateway (ARINC 429 and ARINC 629), and engine data interface. AIMS does not control the engines nor flight controls, nor operate any internal or external voice or data link communications hardware but does select the data link path as part of the communications management function. Subsequent generations of AIMS may include some of these latter functions. In each cabinet the line replaceable modules (LRMs) are interconnected by dual ARINC 659 backplane data buses. The cabinets are connected to the quadraplex (not shown) or triplex redundant ARINC 629 system and fly-by-wire data buses and are also connected via the system buses to the three multifunction control display units (MCDU) used by flight crew and maintenance personnel to interact with AIMS. The cabinets transmit merged and processed data over quadruple redundant custom designed 100 Mhz buses to the EFIS and EICAS displays. In the AIMS the high degree of function integration requires levels of system availability and integrity not found in traditional distributed, federated architectures. These extraordinary levels of availability and integrity are achieved by the extensive use of fault-tolerant hardware and software maintenance diagnostics and promise to reduce the chronic problem of unconfirmed removals and low mean time between unscheduled removals (MTBUR). Figure 102.3 is a top-level view of the U.S. Air Force F-22 Advanced Tactical Fighter avionics. Like many other aircraft, the F-22 architecture is hybrid, part federated and part integrated. The left side of the figure is the highly integrated portion, dominated by the two Common Integrated Processors (CIPs) that process, fuse, and distribute signals received from the various sensors on the far left. The keys to this portion of the architecture are the Processor Interconnect (PI) buses within the CIPs and the High Speed Data Buses (HSDBs). (There are provisions for a third CIP as the F-22 avionics grow in capability.) The right side of the figure shows the federated systems including the Inertial Reference, Stores Management, Integrated Flight and Propulsion Con- trol, and Vehicle Management systems and the interface of the latter two to the Integrated Vehicle System Control. The keys to this portion of the architecture are the triple or quadruple redundant AS 15531 (formerly MIL-STD-1553) command/response two-way data buses. FIGURE 102.1 Cabinet assembly outline and installation (typical installation). (Courtesy of Honeywell, Inc.) ? 2000 by CRC Press LLC Data Buses As noted earlier, data buses are the key to the emerging integrated avionics architectures. Table 102.1 summarizes the major features of the most commonly used system buses. MIL-STD-1553 and ARINC 429 were the first data buses to be used for general aircraft data communications. These are used today widely in military and civil avionics, respectively, and have demonstrated the significant potential of data buses. The others listed in the table build on their success. Displays All modern avionics systems use electronic displays, either CRTs or flat-panel LCDs that offer exceptional flexibility in display format and significantly higher reliability than electromechanical displays. Because of the very bright ambient sunlight at flight altitudes the principal challenge for an electronic display is adequate brightness. CRTs achieve the required brightness through the use of a shadow mask design coupled with narrow bandpass optical filters. Flat-panel LCDs also use narrow bandpass optical filters and a bright backlight to achieve the necessary brightness. FIGURE 102.2 Architecture for AIMSbaseline configuration. (Courtesy of Honeywell, Inc.) ? 2000 by CRC Press LLC Because of the intrinsic flexibility of electronic displays, a major issue is the design of display formats. Care must be taken not to place too much information in the display and to ensure that the information is comprehendible in high workload (aircraft emergency or combat) situations. Power Aircraft power is generally of two types: 28 vdc, and 115 vac, 400 Hz. Some 270 vdc is also used on military aircraft. Aircraft power is of poor quality when compared to power for most other electronics. Under normal conditions, there can be transients of up to 100% of the supply voltage and power interruptions of up to 1 second. This poor quality places severe design requirements on the avionics power supply, especially where the avionics are performing a full-time, flight-critical function. Back-up power sources include ram air turbines and batteries, although batteries require very rigorous maintenance practices to guarantee long-term reliable performance. Software in Avionics Most avionics currently being delivered are microprocessor controlled and are software intensive. The “power” achieved from software programs hosted on a sophisticated processor results in very complex avionics with many functions and a wide variety of options. The combination of sophistication and flexibility has resulted FIGURE 102.3 F-22 EMD Architecture. TABLE 102.1 Characteristics of Common Avionics Buses Bus Name Word Length Bit Rate Transmission Media MIL-STD-1553 20 bits 1 Mb/s Wire DOD-STD-1773 20 bits TBS Fiber optic High-speed data bus 32 bits 50 Mb/s Wire or fiber optic ARINC 429 32 bits 14.5/100 kb/s Wire ARINC 629 20 bits 2 Mb/s Wire or fiber optic ARINC 659 32 bits 100 MB/s Wire ? 2000 by CRC Press LLC WEATHER INFORMATION SYSTEMS eather is a critical factor in aircraft operations. It is the largest single contributor to flight delays and a major cause of aircraft accidents. A study conducted for NASA by Ohio State University reported that the principal diffi- culties in making proper flight decisions are the timeliness and clarity of weather data dissemination. To advance the technology of in-flight weather reporting, Langley Research Center developed in the early 1990’s a cockpit weather information system known as CWIN (Cockpit Weather Information). The system draws on several commercial data sensors to create radar maps of storms and lightning, together with reports of surface observations. Shown above is a CWIN display in the simulation cock- pit of Langley’s Transport Sys- tems Research Vehicle, a modified jetliner used to test W ? 2000 by CRC Press LLC in lengthy procedures for validation and certification. The brickwalling of software modules in a system during the initial development process to ensure isolation between critical and noncritical modules has been helpful in easing the certification process. There are no standard software programs or standard software certification procedures. RTCA has prepared Document DO-178 to provide guidance (as opposed to strict rules) regarding development and certification of avionics civil software. The techniques for developing, categorizing, and documenting avionics civil software in DO-178 are widely used. For military avionics software, the principal document is DOD-STD-498. This standard defines a set of activities and documentation suitable for the development of both weapon systems and automated information systems. Many software languages have been used in the past in avionics applications; however, today there is a strong trend for both military and avionics civil software to use Ada wherever reasonably possible. The evolving definition of a standards for Applications Exchange (APEX) software promises to provide a common software platform whereby the specialized requirements of varying hardware (processor) requirements are minimized. APEX software is a hardware interface that provides a common link with the functional software within an avionics system. The ultimate benefit is the development of software independent of the hardware platform and the ability to reuse software in systems with advanced hardware while maintaining most, if not all, of the original software design. advanced technologies. The CWIN display is the lower right screen among the four center panel screens. By push- ing a button, the pilot may select from a menu of several displays, such as a ceiling and visibility map, radar storm map, or lightning strike map. (Courtesy of National Aeronau- tics and Space Administration.) CNS/ATM The last decade of this century has seen much attention focused on Communication/Navigation/Surveillance for Air Traffic Management (CNS/ATM), a satellite-based concept developed by the Future Air Navigation System (FANS) Committees of the International Civil Aviation Organization (ICAO), a special agency of the United Nations. Many studies have predicted enormous economic rewards of CNS/ATM for both aircraft operators and air traffic services providers. The new CNS/ATM system should provide for: ? Global communications, navigation, and surveillance coverage at all altitudes and embrace remote, off- shore, and oceanic areas. ? Digital data exchange between air-ground systems (voice backup). ? Navigation/approach service for runways and other landing areas which need not be equipped with precision landing aids. Navigation Equipment A large portion of the avionics on an aircraft are dedicated to navigation. The following types of navigation and related sensors are commonly found on aircraft: ? Flight control computer (FCC) ? Flight management computer (FMC) ? Inertial navigation system (INS) ? Attitude heading and reference system (AHRS) ? Air data computer (ADC) ? Low range radio altimeter (LRRA) ? Radar ? Distance Measuring Equipment (DME) ? Instrument Landing System (ILS) ? Microwave Landing System (MLS) ? VHF OmniRange (VOR) Receiver ? Global Navigation Satellite System (GNSS) Emphasis on Communications An ever-increasing portion of avionics is dedicated to communications. Much of the increase comes in the form of digital communications for either data transfer or digitized voice. Military aircraft typically use digital communications for security. Civil aircraft use digital communications to transfer data for improved efficiency of operations and RF spectrum utilization. Both types of aircraft are focusing more on enhanced communica- tions to fulfill the requirements for better operational capability. Various types of communications equipment are used on aircraft. The following list tabulates typical com- munications equipment: ? VHF transceiver (118–136 MHz) ? UHF transceiver (225–328 MHz/335–400 MHz for military) ? HF transceiver (2.8–24 MHz) ? Satellite (1530–1559/1626.5–1660.5 MHz, various frequencies for military) ? Aircraft Communications Addressing and Reporting (ACARS) ? Joint Tactical Information Distribution System (JTIDS) In the military environment the need for communicating aircraft status and for aircraft reception of crucial information regarding mission objectives are primary drivers behind improved avionics. In the civil environment ? 2000 by CRC Press LLC HIGH SPEED RESEARCH ? 2000 by CRC Press LLC (particularly commercial transport), the desire for improved passenger services, more efficient aircraft routing and operation, safe operations, and reduced time for aircraft maintenance are the primary drivers for improving the communications capacity of the avionics. The requirements for digital communications for civil aircraft have grown so significantly that the industry as a whole embarked on a virtually total upgrade of the communications system elements. The goal is to achieve a high level of flexibility in processing varying types of information as well as attaining compatibility between a wide variety of communication devices. The approach bases both ground system and avionics design on the ISO Open System Interconnect (OSI) model. This seven-layer model separates the various factors of commu- nications into clearly definable elements of physical media, protocols, addressing, and information identification. The implementation of the OSI model requires a much higher level of complexity in the avionics as compared to avionics designed for simple dedicated point-to-point communications. The avionics interface to the physical This McDonnell Douglas conceptual design for a Mach 2.4 supersonic trans- port is sized to carry about 300 passengers over a distance of 5,000 nautical miles. A NASA/industry high speed civil transport research effort is a first step toward determining whether such a plane can be economically viable and environmentally acceptable. (Photo Courtesy of National Aeronautics and Space Administration.) ircraft manufacturers of several nations are developing technology for the next plateau of inter- national aviation: the long-range, environmentally acceptable, second generation supersonic passenger transport, which could be flying by 2010. NASA’s High Speed Research (HSR) program is intended to demonstrate the technical feasibility of a high speed civil transport (HSCT) vehicle. The program is being conducted as a national team effort with shared government/industry funding and responsibilities. The team has established a baseline design concept that serves as a common configuration for inves- tigations. A full-scale craft of this design would have a maximum cruise speed of Mach 2.4, only marginally faster than the Anglo-French Concorde supersonic transport. However, the HSCT would have double the capacity of the Concorde, and it would operate at an affordable ticket price. Phase I of the HSR program, which began in 1990, focused on environmental challenges: engine emission effects on the atmosphere, airport noise, and sonic boom. Phase II, initiated in 1994, focuses on the technology advances needed for economic viability, principally weight reductions in every aspect of the baseline configuration. In materials, the HSR team is developing, analyzing, and verifying the technology for trimming the baseline airframe by 30 to 40%. In aerodynamics, a major goal is to minimize air drag to enable a substantial increase in range. Phase II also includes computational and wind tunnel analyses of the baseline HSCT and alternative designs. Additional research involves ground and flight simulations aimed at development of advanced control systems, flight deck instrumentation, and displays. (Courtesy of National Aeronautics and Space Administration.) A medium will generally possess a higher bandwidth. The bandwidth is required to accommodate the overhead of the additional information on the communications link for the purpose of system management. The higher bandwidths pose a special problem for aircraft designers due to weight and electromagnetic interference (EMI) considerations. Additional avionics are required to perform the buffering and distribution of the information received by the aircraft. Generally a single unit, commonly identified as the communications management unit (CMU), will perform this function. The CMU can receive information via RF transceivers operating in conjunction with terrestrial, airborne, or space-based transceivers. The capability also exists for transceiver pairs employing direct wire connections or very short-range optical links to the aircraft. The CMU also provides the routing function between the avionics, when applicable. Large on-board databases, such as an electronic library, may be accessed and provide information to other avionics via the CMU. The increasing demand on data communication system capacity and flexibility is dictating the development of a system without the numerous limitations of current systems. Current communication systems require rather rigid protocols, message formatting, and addressing. The need for a more flexible and capable system has led to the initial work to develop an Aeronautical Telecommunications Network (ATN). The characteristics envisaged for the ATN are the initiation, transport, and application of virtually any type of digital message in an apparently seamless method between virtually any two end systems. The ATN is expected to be a continually evolving system. Impact of “Free Flight” “Free Flight” is a term describing an airspace navigation system in which the “normal” air traffic controls are replaced by the regular transmission of position information from the airplane to the ground. The ground system, by projecting the aircraft position and time, can determine if the intended tracks of two aircraft would result in a cohabitation of the same point in the airspace. This is commonly called “conflict probe”. If a potential conflict occurs, then a message is transmitted to one or more of the aircraft involved to make a change to course and/or speed. “Free Flight” dictates special requirements for the avionics suite. A highly accurate navigation system with high integrity is required. The communications and surveillance functions must exhibit an extremely high level of availability. GNSS Avionics performing the position determination functions will require augmentation to achieve the necessary accuracy. The augmentation will be provided by a data communications system and will be in the form of positional information correction. A data communications system will also be required to provide the frequent broadcast of position information to the ground. A modified Mode S transponder squitter is expected to provide that function. The free flight concept will require the equipage of virtually all aircraft operating within the designated free- flight airspace with a commensurate level of avionics capability. The early stages of the concept development uncovered the need to upgrade virtually all aircraft with enhanced CNS/ATM avionics. The air transport industry resolved this problem on older airplanes by developing improved and new avionics for retrofit applications. The new avionics design addresses the issues of increased accuracy of position and enhancement of navigation management in the form of the GNSS Navigation and Landing Unit (GNLU) housed in a single unit and designed to be a physical and functional replacement for the ILS and/or MLS receivers. A built-in navigator provides enhanced navigation functionality for the airplane. The GNSS can provide ILS lookalike signals and perform landing guidance functions equivalent to Category I. Avionics in the Cabin Historically, the majority of avionics have been located in the electronics bay and the cockpit of commercial air transport airplanes. Cabin electronics had generally been limited to the cabin interphone and public address system, the sound and central video system, and the lighting control system. More recently the cabin has been updated with passenger telephones using both terrestrial and satellite systems. The terrestrial telephone system operates in the 900-MHz band in the United States and will operate near 1.6 GHz in Europe. The satellite ? 2000 by CRC Press LLC system, when completely operational, will also operate near 1.6 GHz. Additional services available to the passengers are the ability to send facsimiles (FAXes) and to view virtually real-time in-flight position reporting via connection of the video system with the flight system. Private displays at each seat will allow personal viewing of various forms of entertainment including movies, games, casual reading, news programming, etc. Avionics Standards Standards play an important role in avionics. Military avionics are controlled by the various standards (MIL- STDs, DOD-STDs, etc.) for packaging, environmental performance, operating characteristics, electrical and data interfaces, and other design-related parameters. General aviation avionics are governed by fewer and less stringent standards. Technical Standard Orders (TSOs) released by the Federal Aviation Administration (FAA) are used as guidelines to ensure airworthiness of the avionics. TSOs are derived from and, in most cases, reference RTCA documents characterized as Minimum Operational Performance Standards and Minimum Avionics System Performance Standards. EUROCAE is the European counterpart of RTCA. The commercial air transport industry adheres to multiple standards at various levels. The International Civil Aviation Organization (ICAO) is commissioned by the United Nations to govern aviation systems includ- ing but not limited to Data Communications Systems, On-Board Recorders, Instrument Landing Systems, Microwave Landing Systems, VHF OmniRange Systems, and Distance Measuring Equipment. The ICAO Stan- dards and Recommended Practices (SARPS) control system performance, availability requirements, frequency utilization, etc. at the international level. The SARPS in general maintain alignment between the national avionics standards such as those published by EUROCAE and RTCA. The commercial air transport industry also uses voluntary standards created by the Airlines Electronic Engineering Committee and published by Aeronautical Radio Inc. (ARINC). The ARINC “characteristics” define form, fit, and function of airline avionics. Defining Terms ACARS: A digital communications link using the VHF spectrum for two-way transmission of data between an aircraft and ground. It is used primarily in civil aviation applications. Brickwalling: Generally used in software design in critical applications to ensure that changes in one area of software will not impact other areas of software or alter their desired function. Distance measuring equipment: The combination of a receiver and a transponder for determining aircraft distance from a remote transmitter. The calculated distance is based on the time required for the return of an interrogating pulse set initiated by the aircraft transponder. Fault tolerance: The built-in capability of a system to provide continued correct execution in the presence of a limited number of hardware or software faults. JTIDS: Joint Tactical Information Distribution System using spread spectrum techniques for secure digital communication. It is used for military applications. Validation: The process of evaluating a product at the end of the development process to ensure compliance with requirements. Verification: (1) The process of determining whether the products of a given phase of the software development cycle fulfill the requirements established during the previous phase. (2) Formal proof of program correct- ness. (3) The act of reviewing, inspecting, testing, checking, auditing, or otherwise establishing and documenting whether items, processes, services, or documents conform to specified requirements (IEEE). Related Topic 78.1 Introduction References Airlines Electronic Engineering Committee Archives, Aeronautical Radio Inc. FANS Manual, International Air Transport Association, Montreal, Version 1.1, May 1995. ? 2000 by CRC Press LLC Federal Radionavigation Plan, DOT-VNTSC-RSPA-90-3/DOD4650.4, Departments of Transportation and Defense, 1990. M.J. Morgan, “Integrated modular avionics for next generation commercial airplanes,” IEEE/AES Systems Magazine, pp. 9–12, August 1991. C.R. Spitzer, Digital Avionics Systems, 2nd ed., New York: McGraw-Hill, 1992. Further Information K. Feher, Digital Communications, Englewood Cliffs, N.J.: Prentice Hall, 1981. J.L. Farrell, Integrated Aircraft Navigation, New York: Academic Press, 1976. L.E. Tannas, Jr., Flat Panel Displays and CRTs, New York: Van Nostrand Reinhold, 1985. M. Kayton and W.R. Fried, Avionics Navigation Systems, New York: John Wiley and Sons, 1969. 102.2 Communications Satellite Systems: Applications Abdul Hamid Rana and William Check The history of satellites began in 1957 when the Soviet Union launched Sputnik I, the world’s first satellite. In the 1960s the commercial sector became actively involved in satellite communications with the launch of Telstar I by the Bell System followed by the use of a geosynchronous orbit. With this type of an orbit, an object 22,753 miles above the earth will orbit the earth once every 24 hours above the equator, and from the earth’s surface appear to be stationary. The first geostationary orbit was achieved by NASA using a SYNCOM in 1963. The Communications Satellite Act was signed by the United States Congress in 1962 and created the Communica- tions Satellite Corporation (COMSAT). This was followed by the formation of INTELSAT, an organization that is composed of over 120 countries and provides global satellite communication services. In the 1970s, multiple companies in the private sector in the United States began to operate their own domestic satellite systems. Today there are numerous companies providing this service in the United States: e.g., GE Americom, Hughes, Loral, COMSAT, and American Mobile Satellite Corporation. Other nations such as Canada, Australia, Indo- nesia, Japan, etc. have their own satellite systems. Several international and regional satellite systems have also been formed. Examples of these are INTELSAT, EUTELSAT, Intersputnik, ARABSAT, AsiaSat, etc. [Pritchard and Sciulli, 1986]. The satellite-based communications systems have significantly evolved over a three-decade period. In the 1960s, satellite communications for commercial use became a viable alternative because of the demand for reliable communications (telephony and voice). In the 1970s, technical innovations made larger, more powerful and more versatile satellites possible. Advanced modulation and multiple-access schemes resulted in smaller, less expensive earth stations and better service offerings that were lower cost and higher quality. In the 1980s very small aperture terminals (VSATs) emerged and the Ku-band frequency spectrum became widely used. In the 1990’s satellites support data, voice, and video communications applications. The VSAT industry has given an overall boost to the entire satellite communication industry. As new satellites are launched, they will have long-term applications which have expanded opportunities. These include private long-haul networks for internal communications, cable TV, pay TV, business voice and data, satellite news gathering, direct broadcast to the home, integrated VSATs, private international satellite service, high-definition TV, mobile service, personal communications, and ISDN. Disaster recovery planning increasingly includes satellites in order to overcome the coverage limitations of existing terrestrial networks. With the allocation of frequencies for personal communications, the promise of global communications and the reality of a personal phone will soon push satellite communications to a new age. This section describes satellite communications from the application point-of-view. Since VSATs initiated the growth in satellite communication, a significant portion of the section is devoted to this topic. After a review of the satellites’ launch and their characteristics, VSAT networks are discussed in detail. Video/audio applications are described next, along with the equipment necessary for these applications. The section is concluded with a summary of next-generation trends. ? 2000 by CRC Press LLC Satellite Launch Launching a communications satellite into orbit is a complex and expensive process. This first stage in a satellite’s airborne life may cost several million dollars. The cost for launching is primarily a function of the satellite’s weight and size. Traditional geosynchronous communications satellites tend to be large and more costly to launch, although the more compact digital communications payloads and longer satellite life will reduce life cycle costs. Low earth orbit communications satellites tend to be smaller and more economical to launch, but will have shorter in orbit life. A shortage of launch vehicles influenced the economics of the launch industry following the 1986 U.S. Space Shuttle Challenger disaster. The shortage has now given way to other launch alternatives. The dominant player in the satellite launch business is the French company Arianespace. Major U.S. players in the satellite launch business are Lockheed Martin, McDonnell Douglas, and Orbital Sciences Corporation. China and Russia have also begun providing launch services. The launch of a satellite payload into the geosynchronous orbit involves many complex steps. Using the launch vehicle, the payload is first placed in a parking orbit. This is a nearly circular orbit which places the satellite approximately 300 km above the earth’s surface. After reaching this orbit, the next step is to fire a motor known as the payload assist module (PAM) to place the payload in a transfer orbit. The PAM motor is discarded afterwards. The transfer orbit is an elliptical orbit whose perigee matches the parking orbit and whose apogee matches the geostationary orbit. Perigee is defined as the point in the orbit closest to the earth, while apogee is the point in the orbit furthest from the earth. The payload itself consists of the satellite with an apogee kick motor (AKM). Once in a transfer orbit, the AKM is fired at the point when the satellite has reached apogee. This firing will place the satellite in a nearly circular orbit. Final positioning of the satellite in geosynchronous orbit can then take place [Pritchard and Sciulli, 1986]. Spacecraft and Systems A satellite spacecraft employs several major subsystems. These are propulsion, electrical, tracking, telemetry command and control, and the communications subsystem. Figure 102.4 is a diagram of a typical commercial satellite. The propulsion subsystem consists of thrusters oriented in north-south and east-west directions and is used to maintain the spacecraft in the proper orbit and orientation. An electrical subsystem is used to generate electricity in the spacecraft by means of solar cells. Backup batteries are used during periods of equinoxes. The solar cells are also used to charge the batteries. The tracking, telemetry, and command subsystem is used to receive commands from the controlling ground station, as well as to allow the ground station to monitor on-board systems. The spacecraft requires some form of stabilization to prevent it from tumbling in space. There are two types of stabilization techniques: spin stabilization and three-axis stabilization. Spin stabilization uses an outside cylinder to spin, creating the effect of a gyroscope providing spacecraft stabilization. An internal platform is decoupled from the cylinder, whose orientation is fixed towards the earth. Three-axis stabilization uses internal gyros which sense movement of the spacecraft. Any movement in the axes is detected and can be compensated by firing thruster jets. The communications subsystem consists of receiver and transmitter sections. The receiver system consists of wideband redundant units. The transmitter subsystem consists of separate amplifiers (transponders) for each channel utilized. Satellite systems make use of orthogonal polarized signals in order to transmit two signals simultaneously on the same frequency, a technique known as “frequency reuse.” Two different polarization methods for signals are used: horizontal and vertical linear polarization, or clockwise and counterclockwise circular polarization. Figure 102.5 shows a simplified block diagram of a typical satellite. A matrix-type switching arrangement is provided on the input and output of the transmitter subsystem for switching to backup transponders. This satellite is three-axis stabilized and operates at Ku-band. There are 16 operational transponders with a bandwidth of 54 MHz each. The employment of frequency reuse provides nearly 1000 MHz of usable bandwidth. Fourteen of the 16 operational transponders use 20-W traveling wave tube amplifiers (TWTA) to provide ground- commandable east or west regional coverage, for 48-state (CONUS) coverage. The remaining two transponders provide 50-state coverage using 27-W TWTAs. For the 50-state channels, one spare 27-W TWTA provides ? 2000 by CRC Press LLC FIGURE 102.4 Simplified block diagram of a communications satellite. FIGURE 102.5 Simplified block diagram of GSTAR satellite. ? 2000 by CRC Press LLC protection for the two operating TWTAs (3-for-2 redundancy). For the remaining transponder channels, 5 spare 20-W TWTAs provide protection for 14 operating TWTAs (19-for-14 redundancy). Redundant communica- tions receivers are provided on a 4-for-2 basis. The power radiated from a satellite is described as its effective radiated isotopic power (EIRP) and is the radiated power of the satellite in decibels referenced to one watt of power. The units are in dBW. The strength of the signal received on the ground is a function of the spacecraft location and that of the ground station and will vary depending upon location. A map of the signal strength contours is called the satellite’s “footprint.” Geosynchronous Satellites There are over 500 Ku-band and C-band satellites in geosynchronous orbit. These satellites are typically spaced anywhere between 1 to 3 degrees apart. Older satellites no longer in active service may be spaced less than one degree in an inclined orbit. The frequency plan for C- and Ku-band satellite services is shown in Fig. 102.6. The typical transmit frequency band used for fixed satellite services in the Ku-band is 14.0–14.5 GHz. Receive frequency is 11.7–12.2 GHz. Some satellites also use the extended band. For C-band satellites, the typical operating transmit frequency is 5.925–6.425 GHz and the receive frequency is 3.7–4.2 GHz. The operating band was extended at WARC ’79 to 7.075 GHz to be assigned to individual countries for domestic satellite systems. Ka-band satellites have down- links in the frequency range 17–23 GHz and uplinks in the range 27–31 GHz. Some European and Japanese satellites operate in this range [Long, 1991]. The satellite performance data indicate a wide range of variation in the specifications among the various satellites. Most satellites have a design lifetime of 10 years. The newer GEO satellites tend to have an extended life of 12–15 years. Most domestic U.S. satellites have 24 transponders. The older generation of Asian satellites have very few transponders per satellite. Some planned satellites will have a large number of transponders. Nominal transponder bandwidths include 36, 54, and 72 MHz. Satellite power is increasing in the newer generation of satellites. Lower-power satellites have an EIRP in the 20–35 dBW range. There are a significantly large number of medium-power satellites in the 35–45 dBW range. Newer high-power satellites tend to have power in the 50–60 dBW range. Direct broadcast satellites are planned for transponder power in the 60–120 W range. The power generally varies with polarization, frequency, and beam. Table 102.2 is a profile of typical satellite performance characteristics. Mobile Satellite Systems Mobile satellite systems encompass communications on land, in the air, or over the oceans ideally allowing a person to communicate with anyone anywhere [Long, 1991]. The Inmarsat system is a mobile communications system providing global coverage through a variety of communication paths. In the United States, the FCC has authorized American Mobile Satellite Corporation (AMSC) to provide domestic mobile satellite services. AMSC makes use of geostationary satellites to provide a domestic offering similar to the international offering of Inmarsat. FIGURE 102.6 Ku- and C-band frequency allocation chart. ? 2000 by CRC Press LLC Over the past two decades, there has been active work in the area of low earth orbit (LEO) satellite systems. In general, LEOs are designed to provide a full range of communication services, both voice and data. Proposed systems are designed to complement existing cellular communications technology. Several companies have proposed LEO systems and have made application to the FCC for a “Pioneer’s Preference” license. This license allows the use of new and innovative technology. Motorola’s Iridium system is potentially the largest, using 66 satellites to provide coverage over the entire globe. Because of the low altitude of the orbit, LEO systems use multiple satellites to provide coverage over a regional area or over the entire globe. Satellites operating at a low orbit are less costly due to the reduced launch costs and reduced weight. However, a low orbit requires the use of multiple satellites since the low altitude of the system provides smaller beam coverage. Since these satellites are not geostationary, ground stations must track an LEO satellite as it passes overhead. Due to potential growth of mobile satellite communications, several systems are proposed to be in operation in the 1990s. Examples of these systems are the Iridium, Globalstar, ICO, Orbcomm, Starsys, Odyssey, and Teledisc. Direct Broadcast Satellites The direct broadcast satellites (DBS) concept is to transmit programming directly to homes using a small receive-only antenna via high-powered satellites. Through the use of a high-powered satellite, a small receive- only satellite antenna may be used for home reception, with the ultimate goal to offer antennas less than one foot in diameter. High-powered DBS satellites use high-powered transponders, i.e., 60–120 W. To prevent interference into the small receive antennas at these high power levels, the DBS satellites will be spaced further apart in geosynchronous orbit. The first efforts in DBS began in the early 1980s when COMSAT built several DBS satellites, but did not launch them. Internationally, many countries currently have DBS services. Several European countries have high-powered DBS satellites; many others use medium-powered satellites. The DBS industry in the United States is being revitalized by advances in digital video compression technology and the announcement of new players such as Hughes, Primestar, Echostar, etc. to offer DBS services. Hughes Communications and United States Satellite Broadcasting (USSB) system using a high-powered DBS satellite is in operation. As an alternative to the launch of a high-powered satellite, medium-powered DBS systems make use of existing satellites in orbit. However, larger home antennas are required, approximately 2 feet or greater in diameter. A medium-powered DBS in the U.S. is Primestar. Digital video compression techniques using the MPEG-2 standard are used to allow multiple video channels in a transponder. DIRECTV? service, launched in the summer of 1994 by Hughes Electronics, is an example of the direct satellite system. TABLE 102.2 Typical Satellite Performance Satellite Operator System Name Configuration EIRP in dBW at Edge Comments GE Americom GSTAR series Ku-band 38–48 Domestic coverage Spacenet series C- and Ku-band C-band: 34–36 Ku-band: 39 Hughes Comm Galaxy Series C- and Ku-band C-band: 34–38 Domestic coverage Ku-band: 45–49.5 Intelsat Intelsat VA (IBS) C- and Ku-band C-band: 20–26 International service, worldwideKu-band: 38–41 Intelsat VI C- and Ku-band C-band: 20–26 Ku-band: 38–41 Intelsat VII C- and Ku-band C-band: 26–36 Ku-band: 41–46 Eutelsat Eutelsat I series Ku-band 35–43.5 Covers all of Europe Eutelsat II Ku-band 42–47 NASDA-NTT (Japan) Sakura 2 C- and Ka-band C-band: 30 `Ka-band: 37 CS-4a, CS-4b in the Sakura series is scheduled for launch during 1992–94 ? 2000 by CRC Press LLC Earth Stations Earth stations are the interface point for communications to and from the satellite [Ha, 1986]. An earth station can be divided into two subsystems, the transmit chain and the receive chain. A common element between the transmit and receive chain is the antenna. Because of the large signal attenuation at RF frequencies, the earth station antenna must have high signal gain and be highly directional to focus the power to and from the satellite. A parabolic-shaped reflector antenna is used by earth stations since it can provide these characteristics. The transmit chain consists of several major components: baseband equipment, modulators, frequency upconverters, high-power amplifiers (HPA), and combiner circuitry used to switch the output of the HPAs to the antenna. The receive chain uses a low-noise amplifier to receive the satellite signals, frequency downcon- verters, demodulator, and baseband equipment. In the transit chain the signals are modulated, combined, and frequency-shifted with an upconverter to the desired satellite transmit frequency. After upconversion, the signals are amplified by HPAs. In a large earth station, there may be many HPAs which feed to a single antenna. These signals must be switched and combined appro- priately. At microwave frequencies, waveguide combiners are used to route the output of the HPAs to the antenna. In the receive chain, the counterpart to the HPA is the low-noise amplifier (LNA), which is used to amplify the signals received from the antenna. This amplifier must be designed for maximum gain with a very small noise contribution. The noise generated in this unit contributes significantly to the overall performance of the receive side of the earth station. Gallium arsenide (GaAs) FETs are commonly used in the amplifier section of the LNA because of their low-noise characteristics. The LNA feeds the signal to the frequency downconverter, which converts it to IF frequency suitable for demodulator. A hub monitoring and control (M&C) system provides the monitoring and control of the RF equipment and baseband equipment. Redundant RF equipment is common at a hub, and the M&C system is used to monitor the components and provide automatic switchover in the event of equipment failure. Switchover between equipment can occur either by operator initiation or automatically by the M&C upon sensing an equipment failure. Technical characteristics of large earth stations have been established for use with the INTELSAT system. INTELSAT categorizes two types of earth stations: multipurpose and special purpose. A multipurpose earth station can be used with any service, while a special-purpose earth station is restricted. Multipurpose standard A, B, and C earth stations have antenna diameters from 11 to 33 meters. Special-purpose standard D, E, and F earth stations have antenna diameters between 3.5 to 11 meters. In addition to fixed earth stations, “portable” earth stations, called transportables, have been manufactured which can be taken to locations originating the programming. These transportables are usually mounted on a truck or trailer and include all the components necessary for an earth station. In the case of the transportable, the antenna size is selected to be as small as 4 meters in diameter. A transportable earth station is designed to be upgraded with “building blocks” to handle heavy, medium, and thin route traffic. Transportable earth stations are designed to meet the requirements for various applications such as temporary business communications, temporary carrier service, backup during the retrofit of an existing earth station, and disaster recovery. Another type of earth station is the flyaway. This is a small remote satellite terminal which can be packed into suitcases for shipment on an airline for delivery anywhere in the world. These systems consist of a small antenna, RF unit, and baseband equipment to provide a complete satellite communications station. An example is an L-band version which provides audio communications via the Inmarsat system. Fitting into a small suitcase, it contains a telephone handset, RF electronics, and antenna that can be assembled to provide audio commu- nications anywhere in the world. Mobile satellite terminals are even smaller, suitable to be carried as handheld or briefcase units. VSAT Communication System Advances in technology have revolutionized the satellite communications industry by deployment of very small aperture terminal (VSAT) networks for data, voice, and video communication. Since the mid-1980s, VSAT networks have become widely used in the oil, lodging, financial, auto, retail, and manufacturing industries. By the 1990s, VSATs were operating in C and Ku-bands. Also by the mid-1990s, over 70% of the VSAT market ? 2000 by CRC Press LLC was accounted for by the retail, automotive, and financial industries. VSATs are making private networks a viable alternative for many companies, for applications such as point-of-sale, reservation systems, remote monitoring and control, branch office administration, financial transactions, etc. A VSAT is a small earth station suitable for installation at a customer’s premises. A VSAT typically consists of antenna less than 2.4 m, an outdoor unit to receive and transmit signals, and an indoor unit containing the satellite and terrestrial interface units [Rana et al., 1990]. VSAT networks fall into three general categories: broadcast networks, point-to-point networks, and inter- active networks. In a broadcast network, a centralized hub station broadcasts data, audio, and/or video to a group of receive-only VSATs. Low-cost receive-only VSATs can receive news, weather services, and financial information. Music distribution and video broadcast via broadcast networks is widely used. Point-to-point networks provide direct communication between two locations without the requirement of a large hub for data, voice, and image transmission. Variations of these networks include point-to-multipoint dedicated circuits or demand-assigned mesh topologies. Interactive networks are used for two-way communications services between a central hub station and a large number of VSATs in a star topology. Table 102.3 is a summary of the salient features of VSAT networks. VSATs are available for both C- and Ku-band frequency. Most VSAT systems use BPSK modulation with Rate 1/2 FEC. For interactive networks, the inbound channel is shared on contention basis to conserve space segment. More advanced systems use concatenated codes to improve performance. Recently, hybrid VSATs have been introduced to use terrestrial networks on the return channel. An example is the Hugh’s Direct PC which uses a high speed satellite receive channel, and a low speed terrestrial return channel. A critical element of VSAT networks is the network availability. The VSAT system availability is affected by three major components: effects of rain attenuation, equipment availability, and software availability. The effects of rain attenuation for Ku-band networks are significant. While link availability is usually specified at 99.5%, link performance can be optimized to nearly any desired value through the use of energy dispersion techniques or large antenna sizes. The network hardware must be highly reliable. Hub hardware should provide for optional redundancy and the ability to achieve better than 99.9% availability. The use of hub diversity and uplink power control can also be used to improve the network availability. The VSAT hardware availability is less catastrophic; the loss of one VSAT does not constitute network failure but may require a service call to rectify the problem. Hence, it is common to use nonredundant but highly reliable VSAT units. Software availability needs to be improved since software failures dominate the overall availability of interactive networks in existing VSAT products. Interactive networks have been by far the most popular for data communication and audio/video overlays. The remaining portion of this section is devoted to these networks. An interactive VSAT system consists of a TABLE 102.3 Typical VSAT Systems Features Feature Interactive Point-to-point Broadcast Topology Star Point-to-point, mesh Point-to-multipoint Communication Between hub and VSAT to VSAT Hub to VSATs VSATs, VSAT to VSAT through hub Frequency Ku-, C-band Ku-, C-band Ku-, C-band Hub antenna 3–11 m — 3–11m VSAT antenna 0.9–2.4 m 1.8, 2.4 m 0.5–2.4 m Hub to remote access TDM, SCPC, SCPC SCPC, spread spread spectrum spectrum, FM 2 Remote to hub access ALOHA, reservation SCPC — stream, CDMA Outbound data rate (Kbps) 56–512 9.6–2048 9.6–2048 Inbound data rate (Kbps) 9.6–256 9.6–2048 — Modulation BPSK, QPSK, BPSK, QPSK BPSK, QPSK, FM 2 DPSK FEC Rate 1/2, Rate 1/2, Rate 1/2, convolutional or block convolutional convolutional or block Protocols SDLC, Bisync, Clear channel Clear channel, Async, X.25, TCP/IP synchronous, Burroughs and others HDLC format ? 2000 by CRC Press LLC hub, VSAT, network management system, and associated transmission and processing subsystems. These sub- systems along with sophisticated satellite access protocols and terrestrial protocol interfaces make interactive networks a flexible and powerful communication medium. Hub The hub performs all functions that are necessary to establish and maintain virtual connections between the central location and VSATs. In private dedicated networks, the hub is co-located with the user’s data processing facility. In shared hub networks, the hub is connected to the user equipment via terrestrial backhaul circuits. Since the hub is a single point for failure in a star network, it is typically configured with 1:1 or 1:N redundancy. The hub consists of antenna, RF, and baseband equipment (Fig. 102.7). It will handle multiple channels of inbound and outbound data and often one or more channels of audio or video broadcast. The hub antenna consists of a parabolic reflector and associated electrical and mechanical support equipment. The RF subsystem converts the modulated carrier to RF frequency, provides the necessary signal amplification, and transmits the resulting RF carrier to the antenna subsystem. It also receives RF signals from the antenna subsystem, provides low-noise amplification, RF/IF conversion, and passes the resulting IF carriers to the baseband equipment subsystem. The hub baseband equipment consists of the modem equipment and the processing equipment. The hub modems employ continuous modulators and burst demodulators. The pro- cessing equipment interfaces to the modem equipment and provides the satellite access processing and protocol processing for interface to the customer host. VSAT The VSAT consists of an antenna, outdoor unit (ODU), interfacility link (IFL), and indoor unit (IDU). The IFL connects the IDU and ODU subsystems, providing the transmit and receive lines, monitor and control signals, and dc power for the ODU electronics. A single-cable IFL, in which all signals are multiplexed on the same cable, is usually used to reduce the cost of IFL. VSATs nominally use a 1.2- or 1.8-m offset feed parabolic antenna. Smaller antenna sizes are preferable to reduce the installation cost. Options for small antennas include the use of either a submeter parabolic reflector or a flat-plate antenna. The choice of antenna is a tradeoff among performance, installation cost, and aesthetic considerations. The ODU consists of a solid-state power amplifier (SSPA), a low-noise amplifier, upconverter, and a down- converter. VSAT SSPA modules are usually between 1.0 to 3.0 W. The ODU cost can be significantly lowered by utilization of a low-power SSPA (0.1 to 0.5 W) consistent with obtaining the required output power. The VSAT receive side front end can be economically configured using an LNB. Low-cost HEMT LNBs are currently available with 50–60 dB gain and noise figures lower than 1.3 dB. FIGURE 102.7 Block diagram of a hub. ? 2000 by CRC Press LLC Direct modulation of the RF carrier may lower the cost of the VSAT IF and RF electronics while consolidating modulation and upconversion functions. Direct modulation allows the design of a VSAT with fewer parts, smaller size, and lower weight than with traditional outdoor units. Figure 102.8 is a block diagram showing a conventional VSAT and a VSAT using direct modulation. An L-band receive interface between the ODU and IDU is preferable in order to receive audio and video overlays. The IDU is located near the user terminal equipment. Major IDU functions include outbound carrier signal acquisition, tracking, demodulation, bit synchronization, burst modulation, and protocol processing. It also controls the operation of the ODU, monitors VSAT health, and responds to hub commands. The baseband processing system performs satellite channel access and protocol and customer interface processing functions. A video/audio port can be provided with an RF splitter at the IDU to separate the received audio/video signal for the optional video/ audio receiver. Network Management System The network management system (NMS) is a critical element of a VSAT network. Through the NMS, the user can have full control of his network, which is usually not possible in the case of terrestrial network facilities. The NMS generally provides a centralized management tool for hub and VSAT equipment configuration control, assignment of inbound and outbound satellite channels, network monitor and control, switchover to back-up equipment, network statistics collection, downline loading of new software, and report generation. In the shared hub environment, the hub operator controls the allocation of resources among various users and controls the RF transmission facility. The user must have the ability to manage his portion of the network transparent to other users. In the case of a dedicated hub, a single management entity can exert full control over the network, including RF transmission facilities. The network management system standards community has defined five functional areas as requirements for network management systems. These areas are fault management, accounting management, configuration man- agement, performance management, and security management. The VSAT network management system should be capable of interfacing with other network management systems by supporting a standard network management protocol. The protocol standard most widely accepted is the Simple Network Management Protocol (SNMP). Transmission System Most VSAT systems employ BPSK or QPSK modulation with rate R = 1/2, K = 7 convolutional coding and soft-decision Viterbi decoding on both the inbound and outbound channels. Differential phase shift keying (DPSK) modulation may be used to reduce the demodulator complexity and cost. DPSK is relatively insensitive FIGURE 102.8 Simplified VSAT block diagram. ? 2000 by CRC Press LLC to phase noise and frequency offset, thus allowing the use of lower-cost LNBs in the VSAT terminals. However, as compared to BPSK, convolutionally encoded DPSK requires about 2 dB greater Eb/No at a BER of 10 –5 . In addition, if operation is required below 10 dB, some form of low-level interleaving may be required. In lieu of performing the VSAT demodulation function via the traditional analog circuit techniques, an all- digital implementation using digital signal processing (DSP) techniques may be considered. The merits of DSP include the development of a more testable, producible, maintainable, configurable, and cost-effective demod- ulator. Figure 102.9 presents an illustration of the DSP demodulator functions to be implemented using the DSP processor(s). The functions of the major blocks are as follows: phase locked loop (PLL) for carrier acquisition, narrowband Costas loop for data detection, external automatic gain control (AGC), dynamically advance/retard sampling to achieve optimum data sampling, and A/D converters for signal analog-to-digital conversion. A VSAT system must employ frequency agility in the remote terminal to use an assigned block of frequencies within a transponder. Within the assigned frequency band, one or more outbound carriers and a number of inbound carriers are precisely located. On the VSAT receive or outbound side, the LNB output can be demod- ulated directly using a synthesizer-controlled local oscillator, or further downconversion can be used under synthesizer control to obtain the demodulator input signal at a standard IF frequency such as 70 or 140 MHz. In the inbound direction, channel selection can be accomplished by two methods. First, the carrier frequency of the modulator can be shifted to select the appropriate channel and a fixed upconverter may be used to obtain the RF signal. Second, the synthesizer output frequency may be multiplied up to RF to obtain the carrier, which may then be modulated directly with the data as described in Cannistraro and McCarter [1990]. Satellite Access Protocols The multiple satellite access protocol is one of the most critical elements to the performance of a VSAT network. VSAT systems tend to be used in applications where message delay is critical and this protocol is the controlling element to the delay-throughput performance of the system. During the past 15 years, there have been numerous multiple-access protocols developed and simulated in the context of satellite packet communications [Ray- chaudhuri and Joseph, 1988]. Table 102.4 provides a comparison of throughput vs. delay for various satellite access protocols. In the outbound or hub-to-VSAT direction, a TDM channel is employed. This channel may be regarded as a point-to-multipoint or broadcast channel with node selectivity being achieved by the use of addressing FIGURE 102.9 DSP demodulator functional diagram. ? 2000 by CRC Press LLC information embedded in the modulated data stream. The delay performance of this channel is essentially controlled by the queuing behavior of the hub. In the VSAT-to-hub direction, a large number of VSATs share the channel to conserve space segment. Most VSAT networks utilize a combination of slotted ALOHA protocol for the interactive component of the inbound traffic and a reservation TDMA scheme for any bulk data transfers. Most protocols are adaptive in the sense that as the channel traffic increases, they automatically evolve into reservation TDMA systems. Code division multiple access (CDMA) has been used in VSATs operating at C-band. CDMA permits more than one signal to simultaneously utilize the channel bandwidth in a noninterfering manner. This makes it possible to significantly increase the utilization and throughput of the channel. Interface Capabilities Most VSAT systems support common data communications protocols such as SDLC, X.25, Async, Bisync, TCP/IP, etc. Coexistence of different protocols is allowed in a network. A VSAT supports multiple ports with common interfaces such as RS232C, RS422, V.35, etc. VSAT networks typically must provide protocol spoofing to provide acceptable delay and throughput performance to the end-user application. To minimize the effect of satellite delay, the host computer front-end processor is emulated at the VSAT location, and multiple cluster controllers are emulated at the hub location. The polling associated with the front-end processor to cluster controller communication is not carried on the satellite link, but is instead emulated locally. Video Satellites are an excellent medium for video transmission since they can provide a broadcast capability with wide bandwidth. Video on satellites is ideal for applications such as videoconferencing, business TV, distance learning, satellite news gathering, etc. Video Teleconferencing Satellite communications provides a cost-effective and flexible means of interactive videoconferencing. Tech- nological improvement in videocompression has resulted in low-cost codecs at data rates less than T1, and good quality videoconferencing is possible at data rates as low as 56 Kbps. Low-cost satellite terminals coupled with low-cost codecs are making videoconferencing via satellite affordable and practical for many organizations. Applications include all types of business meetings and technical information exchange such as management and staff meetings, new product introductions and updates, sales meetings, training, and market presentations. Videoconferencing allows people at different locations to meet with almost as much ease as being in the same room, providing benefits of increased productivity, reduced travel time and cost, and increased management visibility. A generic videoconference system is presented in Fig 102.10. The system consists of a specially designed room, video/audio equipment, transmission equipment, monitor and control computer, and space segment. The video and audio feeds from the meeting room pass through the codec and are compressed. From the codec, the signal passes to the satellite modem for modulation. The radio frequency/terminal (RFT) upconverts the modulated carrier and amplifies it for transmission to the satellite. At the other site, the process is reversed. A videoconferencing network features point-to-point, broadcast, or point-to-multipoint architectures. In a point-to-point system, two sites are configured for interactive conference with duplex audio and full motion video transmission. Videoconferencing broadcast is appropriate for formal presentations where the presenter TABLE 102.4Random Multi-Access Protocols Comparison Throughput Comments Pure ALOHA 0.13–0.18 Low cost, good for variable-length messages Slotted ALOHA 0.25–0.37 Good for fixed-length messages Selective reject ALOHA 0.20–0.30 Variation of pure ALOHA with a modified algorithm Tree CRA 0.40–0.49 Sensing capability for collision resolution, good for fixed-length messages Announced retransmission 0.50–0.60 Uses modified algorithm of slotted ALOHA by announcement of random access (ARRA) transmission Random access with notification 0.45–0.55 Uses partition for new and retransmitted message CDMA 0.10–0.40 Used in spread spectrum systems, low delay ? 2000 by CRC Press LLC does not need to see the audience, such as a speech from a senior corporate executive. In a point-to-multipoint conference, multiple sites can receive a transmitting site. Two of the primary sites are fully interactive with each other. A feature called multipoint switching has been implemented in some commercial systems. This feature allows switching of receive and transmit sites during the conference. The multipoint switching feature can be provided using either a TDMA or SCPC system. A TDMA system allows multiple sites to transmit and receive in a mesh configuration. An economical multipoint switching system is possible with SCPC using only two transmit frequencies. In a “chair” controlled conference, the chair is assigned one of these frequencies for the duration of the conference. Dynamic allocation of the second frequency is controlled by the chair to any of the participating sites at any time during the conference. Video Broadcast Video broadcast over satellite is attractive for industry segments such as educational TV, distance learning, business television, and television receive-only (TVRO) applications. Business television allows users to transmit broadcast-quality video programming from a studio to any number of specified locations equipped with TVROs. A video broadcast capability, as an overlay to interactive data networks, is becoming increasingly popular for corporate presentations, education, and training. A video uplink consists of a video exciter, HPA, antenna, and optionally an encryption system such as B-MAC (multiplexed analog component, version B) encoder, for business video broadcasts. Each remote VSAT must be configured to receive the video transmission. This involves adding a video receiver at each VSAT location that plugs into the VSAT IDU. Audio/video signals from the video receiver can be presented directly or through a B-MAC decoder to a standard TV monitor. The digital compressed video signal can be used as a replacement for an analog video distribution. Digital coding technology can be used to compress video signals to reduce data rates to 2 Mbps or even lower and reproduce near broadcast-quality video. Distribution of digital video signals at such rates requires less tran- sponder bandwidth and a smaller antenna at remote terminals. Compression techniques used are based on one or a combination of the following: inter/intra-frame prediction, adaptive differential transform, conditional replenishment, discrete cosine transform, adaptive prediction, motion compensation, and vector quantization [Patterson and Delp, 1990]. Satellite News Gathering Satellite news gathering (SNG) is used for live, on-the-spot coverage and news exchanges with other commercial broadcast stations. This is made possible by the availability of occasional-use space segment and transportable earth stations on news trucks. An SNG systems consists of a compact earth station and video/audio transmission FIGURE 102.10 A generic videoconference system. ? 2000 by CRC Press LLC system on a truck. A duplex voice channel is used to coordinate between the space segment provider, studio, and the SNG truck. Figure 102.11 presents a block diagram of a typical SNG system. The RF subsystem has a transmit path and two independent receive paths. The transmit path consists of an HPA and a frequency agile video exciter which modulates and upconverts the video signal to the satellite’s RF frequency. A waveguide switch is used to select transmit polarization. Camera signals go simultaneously to tape for storage and for transmission over the satellite. A receive path is typically provided for both receive polarizations. Each path consists of a transmit reject filter and an LNB which downconverts to L-band. The received L-band signal passes through a video satellite receiver, from which point it can be routed to various monitor or test points or be routed to a tape device for recording and storage. Audio The use of commercial broadcast audio transmission via satellite began in the late 1970s with National Public Radio and Mutual Broadcasting using Western Union’s WESTAR I satellite. The main application was to send high-quality audio to radio broadcast stations to transmit programming information. This type of system makes use of single channel per carrier (SCPC) satellite transmission, where each satellite channel corresponds to one audio channel. The entire satellite channel is FM modulated. Pre-emphasis is used over the channel to provide additional noise reduction. A variation of this technique, called multiple channel per carrier (MCPC), can be used to transmit multiple channels over a single satellite carrier. Figure 102.12 is a block diagram of the MCPC system. As the marketplace searched for lower-cost systems, the FM 2 (or FM/FM) modulation technique evolved, allowing the use of low-cost FM receivers. Through a high-powered FM modulated carrier on the satellite, a low-cost audio and data broadcast receiver can be built. This FM/FM modulation technique is widely used to distribute audio and data on a low-cost basis. In addition to audio broadcasts, satellite-based voice applications include point-to-point voice, multinode interactive voice, and voice over data VSATs. Point-to-point voice is most prevalently used for high-volume voice trunking for long-distance connectivity or transoceanic connectivity. A multinode, interactive voice architecture is ideal in providing voice connectivity to remote locations that are not serviced by terrestrial voice facilitates. Both mesh and star configurations are used to provide multinode voice connectivity. Automated satellite access control and resource allocation techniques are used to allow for granting requested on-demand FIGURE 102.11SNG vehicle video/audio system. ? 2000 by CRC Press LLC availability of voice connectivity. To support voice over data VSATs, an audio encoder is used to accept an analog voice signal, digitize and packetize it, and format it for transmission through the VSAT data network. A voice port may either be implemented as part of a “baseline” data/voice card or as an add-on stand-alone box. The integrated data/voice system employs a TDM outbound carrier and shared inbound carriers for data and voice transmission. Two types of voice network communications alternatives may be implemented for voice channel communications: a poll/response access scheme and a reservation TDMA access scheme. With the poll/response access scheme, the hub polls the VSAT voice ports on a cyclic basis. The VSATs return their responses in the form of call requests or status updates. The number of sites in the voice network determines the rate at which VSATs are polled. Thus, this scheme is suitable for a small network. Excessive polling delays will be encountered for a network with a relatively large number of remotes. In reservation TDMA, on the other hand, voice call requests are serviced by the assignment (reservation) of a logical channel for inbound voice traffic. Although various means are implemented to avoid collisions on the satellite link, the time needed to reserve capacity on an inbound carrier may be lengthy, depending on traffic conditions. Therefore, call setup times are not as predictable as they are with the poll/response access scheme. The VSAT design is ideally suited for digital compressed voice. Coding rates of 32, 16, and 9.6 kbps and lower can presently be achieved, depending on the compression technique employed. There are two classes of digitizing voice signals: waveform coding and vocoding. In waveform coding, the analog voice curve is coded and then reproduced by modeling its physical shape. Data rates are relatively high, i.e., higher than 9.6 kbps. Vocoding attempts to reproduce the analog voice curve by abstractly “identifying” the type and shape of the curve. Only a set of parameters is transmitted, describing the nature of the curve. Achieved data rates can be as low as 1.2 kbps. Second-Generation Systems The recent wave of satellites have much higher power than their predecessors. The Intelsat K satellite, for example, is equipped with 60-W TWTAs and serves increasing worldwide traffic, video, and VSAT services. Another example is the Telstar 4 satellite which has variable power up to 120 W for Ku-band transmissions and is being promoted to be HDTV compatible in preparation for expected widespread use of HDTV. Other trends in satellite design, i.e., NASA’s advanced communication technology satellite (ACTS), include the use of multiple spot beams and onboard IF and/or baseband switching. Onboard switching coupled with electronically hopped spot beams and laser intersatellite links have been proposed. Spot beams provide higher satellite EIRP which permits small, low-cost VSATs to accommodate higher bit rate transmissions. The use of multiple-beam architectures also increases bandwidth availability through frequency reuse. Advances in multibeam satellites FIGURE 102.12 Block diagram of the MCPC system. ? 2000 by CRC Press LLC ? 2000 by CRC Press LLC INTERNATIONAL SPACE STATION he International Space Station will be a permanent laboratory for human-monitored long term research in the unique environment of Earth-orbital space, an environment that cannot be duplicated on Earth for long duration experiments. This space station program draws upon the resources and scientific and technological expertise of 13 cooperating nations. This project is being constructed in three phases. Phase I included the 1995 construction of two Boeing- built nodes (Node 1 and Node 2). The nodes will serve as connecting passageways between modules. Phase I was completed in early 1996 with the production of the U.S. laboratory module where astronauts will perform continuous scientific research. Phase II of the space station program begins in November 1997 with the launch of the FGB functional cargo block on a Russian Proton vehicle. The FGB is a 21-ton element that will provide altitude control and propulsion during the early assembly operations, plus solar power and berthing ports for additional modules. In May of 1998, the embryo space station will grow with the addition of the Proton-boosted Russian service module, which provides life support and habitation facilities, utilities, and thrusters. Then the crew transfer vehicle, a Russian Soyuz TM capsule, will be joined to the station. By June 1998, the first three-person crew will begin its orbital stay. Phase II will be completed in Spring of 1999. In Phase III, the International Space Station will progress gradually to its ultimate status as a fully operational permanent orbital research facility. Among key additions to the core configuration are the remaining modules of the U.S.-built solar array; the Japanese experiment module, to be delivered in 2000; and the U.S. habitation module which contains the galley, toilet, shower, sleep stations, and medical facilities. With the delivery of a second Russian crew transfer vehicle in June 2002, the station will be virtually complete. The completed station will measure 361 feet from tip to tip of the solar arrays. The pressurized living and working space is roughly equivalent to the passenger cabin volume of two Boeing 747 jetliners. The The interim International Space Station will look like this. In the right fore- ground is the U.S. laboratory module and the station’s airlock. In the center of the horizontal string of modules is the FGB energy block. The solar power array at the top is one of four that will provide power for the complete station. Below the tower is the Russian-built universal docking module and, at bottom, one of two crew transfer vehicles. (Photo courtesy of National Aeronautics and Space Administration.) T ? 2000 by CRC Press LLC space station will contain seven laboratories. In addition, the Japanese experiment module has an exposed “back porch” with 10 mounting spaces for experiments that require long duration with the space environment. Beginning in 1997, there will be a total of 73 assembly and service flights until the station becomes fully operational in midyear 2002. (Courtesy of National Aeronautics and Space Administration.) A concept view of the International Space Station in its final configu- ration with a space shuttle orbiter docked at the fore port. The cylinder near the orbiter’s nose is the U.S. centrifuge accommodation module. Below it, hidden by the orbiter, is the U.S. laboratory module, flanked by the European (left) and Japanese laboratories. (Photo courtesy of National Aeronautics and Space Administration.) This concept view is of the station from the opposite (aft port) end. In the foreground (lower right) is the Russian service mod- ule, with living and working room for three crew members. Next, toward the center of the photo, is the FGB energy block, then (near the orbiter) the U.S. laboratory module. The vertically mounted cylinder below it is the U.S. habitation module. (Photo courtesy of National Aeronautics and Space Administration.) with onboard baseband processing allows some of the intelligence in the central hub and VSAT equipment to be moved to the satellite. The result is expected to be improved VSAT-to-VSAT communications and a platform to provide dynamic bandwidth allocation [Naderi and Wu, 1988]. The trend in deploying higher-power satellites has an inverse effect on the size of the earth station antenna. The earth stations are becoming smaller, less complex, and more cost effective. Private hubs are now typically in the range of 3.5 to 7.6 m and are not required to be staffed. Two-way VSATs antennas originally deployed in sizes from 1.2 to 1.8 m are now using elliptical or rectangular-shaped antennas with apertures equivalent to 1.0 m or less. Two-way ultra-small aperture terminals are also emerging. These lower-cost, lower-functionality earth stations are designed for thin route, niche-type applications such as point-of-sale and credit card trans- action processing. The advances in DSP technology will continue to enhance the capabilities and performance while at the same time lowering the cost of VSATs. The advances in MMIC technology continue to miniaturize the RF components while increasing reliability. With advances in digital signal processing and compression techniques, analog video and audio transmission will increasingly be converted to digital transmissions. The advanced compression techniques reduce the bandwidth requirements and allow for smaller and lower-cost VSAT antennas to be used. The continued technological advances in satellite technology and the emerging demand for more flexible communication services will generate new satellite communications applications, such as LAN interconnections and ISDN support [Murthy and Gordon, 1989]. Satellite communications will also play an increasing role in mobile communications on land, air, and on sea. In addition to telephony services, new services such as global distress and safety applications, global positioning, navigation, voice messaging, and data transmissions are now possible. Defining Terms Earth station: The interface point for communications to and from a satellite. An earth station (also known as a hub) consists of an antenna and transmit and receive subsystems. Geosynchronous orbit: An orbit 22,753 miles above the earth in which an object will orbit the earth once every 24 hours above the equator and will appear to be stationary from the earth’s surface. Protocol spoofing: A technique used by VSAT networks to reduce the network delay. The satellite network emulates the host computer front-end processor at the VSAT location and emulates the multiple cluster controllers at the hub location. Satellite access protocol: A set of rules by which a number of distributed VSATs communicate reliably over a shared satellite channel. VSAT: Very small aperture terminal. A small earth station suitable for installation at a customer’s premises. A VSAT typically consists of an antenna less than 2.4 m, an outdoor unit to receive and transmit signals, and an indoor unit containing the satellite and terrestrial interface units. Related Topics 74.1 Introduction ? 78.1 Introduction References J.C.L. Cannistraro and S. McCarter, “Direct modulation lowers VSAT equipment costs,” Microwaves and RF, pp. 99–102, August 1990. T.T. Ha, Digital Satellite Communications, New York: MacMillan, 1986. M. Long, World Satellite Almanac, 3rd ed., Winter Beach, Fla.: MLE, Inc. 1991. K.M. Murthy and K.G. Gordon, “VSAT networking concepts and new applications development,” IEEE Com- munications Magazine, pp. 43–49, May 1989. F.M. Naderi and W.W. Wu, “Advanced satellite concepts for future generation VSAT networks,” IEEE Commu- nications Magazine, vol. 26, pp. 13–22, July 1988. H.A. Patterson and E.J. Delp, “An overview of digital image bandwidth compression,” Journal of Data and Computer Communications, pp. 39–49, Winter 1990. ? 2000 by CRC Press LLC W. Pritchard and J.A. Sciulli, Satellite Communication Systems Engineering, Englewood Cliffs, N.J.: Prentice- Hall, 1986. A.H. Rana, J. McCoskey, and W. Check, “VSAT technology, trends, and applications,” IEEE Proc., vol. 78, no. 7, pp. 1087–1095, July 1990. D. Raychaudhuri and K. Joseph, “Channel access protocols for Ku-band VSAT networks: A comparative eval- uation,” IEEE Communications Magazine, vol. 26, no. 5, pp. 34–44, May 1988. Further Information The World Satellite Almanac provides a tutorial of the satellite communications industry. It includes the technical characteristics and footprint maps for geosynchronous satellites worldwide. Contact: MLE Inc., P.O. Box 159, Winter Beach, FL 32971. World Satellite Communications and Earth Station Design is a text which provides an analytical presentation of communication satellites and their applications. Contact: CRC Press, Inc., 2000 Corporate Blvd., N.W., Boca Raton, FL 33431. The monthly IEEE Communications Magazine investigates VSAT communications in a special series spanning several issues between 1988 and 1989. Contact: IEEE Service Center, 445 Hoes Lane, Piscataway, NJ 08854-4150. ? 2000 by CRC Press LLC