Satellite Yeni

CONTENTS
CONTENTS................................ ................................ ................................ ................................ ........ 1 ABSTRACT ................................ ................................ ................................ ................................ ........ 2 INTRODUCTION ................................ ................................ ................................ ................................ 3 CHAPTER 1 - GPS ................................ ................................ ................................ .............................. 4 1.1 W HAT S GPS ? ................................ ................................ ................................ .......................... 4 1.2 OWERW EW OF GPS ................................ ................................ ................................ .................... 4 1.3 H STORY OF GPS ................................ ................................ ................................ ......................... 5 1.4 GPS SYSTEM SEGMENTS OVERW EW ................................ ................................ ............................... 8 1.4.1 Space Segment Overwiew ................................ ................................ ................................ 8 1.4.2 Control Segment (CS) Overwiew ................................ ................................ ....................... 8 1.4.3 User Segment Overwiew ................................ ................................ ................................ .. 9 1.5 GPS SEGMENTS ................................ ................................ ................................ ........................ 10 1.6 GPS SATELL TE GENERAT ONS ................................ ................................ ................................ ...... 11 1.7 CURRENT GPS S ATELL TE CONSTELLAT ON ................................ ................................ ....................... 12 1.8 CONTROL S TES ................................ ................................ ................................ ......................... 13 1.9 GPS: THE BAS C DEA ................................ ................................ ................................ ................. 14 1.10 GPS POS T ON NG SERV CE ................................ ................................ ................................ ........ 16 1.11 WHY USE GPS? ................................ ................................ ................................ ...................... 17 1.12 GPS ERRORS AND B ASES ................................ ................................ ................................ .......... 17 1.12.1 Ionospheric delay ................................ ................................ ................................ ......... 18 1.12.2 Tropospheric delay ................................ ................................ ................................ ....... 20 CHAPTER 2 - DIRECT BROADCAST SATELL TE SERVICES................................ ................................ ... 22 2.1 ORB TAL SPAC NG ................................ ................................ ................................ ...................... 22 2.2 POWER RAT NG AND NUMBER OF TRANSPONDERS ................................ ................................ ............ 24 2.3 FREQUENC ES AND POLAR ZAT ON................................ ................................ ................................ .. 24 2.4 TRANSPONDER CAPAC TY ................................ ................................ ................................ ............. 25 2.5 B T RATES FOR D G TAL TELEV S ON................................ ................................ ................................ 26 2.6 THE HOME RECE VER I NDOOR UN T (IDU) ................................ ................................ ...................... 27 CONCLUSION ................................ ................................ ................................ ................................ . 29 REFERENCES ................................ ................................ ................................ ................................ ... 31 REFERENCES OF FIGURES ................................ ................................ ................................ ............... 33 REFERENCES OF TABLES ................................ ................................ ................................ ................. 34

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ABSTRACT
Global Positioning System (GPS) examination 3 segments. These are Space segment, Control segment and User segment. This project include GPS satellite generation, current GPS satellite constellation, control sites, GPS positioning service and GPS errors and biases. Direct broadcast satellite (DBS) is a term used to refer to satellite television broadcasts intended for home reception. I examination Orbital Spacing, Power

Rating and Number of Transponders, Frequencies and Polarization, Transponder Capacity, Bit Rates for Digital Television and The Home Receiver Indoor Unit.

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INTRODUCTION
Global Positioning Systems can impact our world in many ways. It is important to have a sense of how data can be collected with a GPS unit and how that data can be used. We will be given a problem to solve involving locating yourself on a topographical map. We will then use a GPS unit to try to solve this problem. Once we are familiar with using a GPS and understand its function. Global Positioning Systems were developed primarily for the military to use to find the longitude, latitude and elevation of a specific location. This is called "ground truthing". Recently the public has been made aware of GPS technology used for navigation by some cars. The function of a GPS is to determine its location in terms of longitude, latitude and elevation. Depending on the instrument, a GPS can be accurate from within 30m to within 05m. "Sensing" at least 4 satellites that are orbiting the earth parallel to the equator (geosynchronous) is necessary to accomplish this. Three satellites are needed to triangulate the position and the fourth to reference time. Direct satellite broadcasting was initially seen as a technology that would, because of its broad geographic coverage, break down national borders and create transnational audiences for television programming. Its effects were portrayed as relatively similar in very different contexts (Webster, 1984). However, the trends of program fragmentation and audience segmentation that have typified the multichannel television industry have also occurred with direct broadcast satellites, and much national programming has remained distinct.

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CHAPTER 1 - GPS
1.1 What is GPS ?
The Global Positioning System (GPS) is a navigation and precise-positioning tool. Developed by the Department of Defense in 1973, GPS was originally designed to assist soldiers and military vehicles, planes, and ships in accurately determining their locations world-wide. Today, the uses of GPS have extended to include both the commercial and scientific worlds. Commercially, GPS is used as a navigation and positioning tool in airplanes, boats, cars, and for almost all outdoor recreational activities such as hiking, fishing, and kayaking. In the scientific community, GPS plays an important role in the earth sciences. Meteorologists use it for weather forecasting and global climate studies; and geologists can use it as a highly accurate method of surveying and in earthquake studies to measure tectonic motions during and in between earthquakes.

1.2 Owerwiew of GPS
GPS consists, nominally, of a constellation of 24 operational satellites. This constellation, known as the initial operational capability (IOC), was completed in July 1993. The official IOC announcement, however, was made on December 8, 1993 [1]. To ensure continuous worldwide coverage, GPS satellites are arranged so that four satellites are placed in each of six orbital planes (Figure 1.1). With this constellation geometry, four to ten GPS sat- ellites will be visible anywhere in the world, if an elevation angle of 10° is considered. As discussed later, only four satellites are needed to provide the positioning, or location, information. GPS satellite orbits are nearly circular (an elliptical shape with a maximum eccentricity is about 0.01), with an inclination of about 55° to the equator. The semimajor axis of a GPS orbit is about 26,560 km (i.e., the satellite altitude of about 20,200 km above the Earth¶s surface) [2]. The corresponding GPS orbital period is about 12 sidereal hours (~11 hours, 58 minutes). The GPS system was officially declared to have achieved full operational capability (FOC) on July 17, 1995, ensuring the availability of at least 24 operational, nonexperimental, GPS satellites.

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Figure 1.1 Gps Constellation [1]

1.3 History of GPS
The design of GPS is based partly on similar ground-based radio navigation systems, such as LORAN and the Decca Navigator developed in the early 1940s, and used during World War II. In 1956 Friedwardt Winterberg[3] proposed a test of general relativity using accurate atomic clocks placed in orbit in artificial satellites. To achieve accuracy requirements, GPS uses principles of general relativity to correct the satellites' atomic clocks. Additional inspiration for GPS came when the Soviet Union launched the first man-made satellite, Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that because they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion (see Transit (satellite)). The first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. It used a constellation of five satellites and could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the Timation satellite that proved the ability to place accurate clocks in space, a technology required by GPS. In the 1970s, the ground-based Omega Navigation System, based on phase comparison of signal transmission from pairs of stations, became the first worldwide radio navigation system. Limitations of these systems drove the need for a more universal navigation solution with greater accuracy.

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While there were wide needs for accurate navigation in military and civilian sectors, almost none of those were seen as justification for the billions of dollars it would cost in research, development, deployment, and operation for a constellation of navigation satellites. During the Cold War arms race, the nuclear threat to the existence of the United States was the one need that did justify this cost in the view of the US Congress. This deterrent effect is why GPS was funded. The nuclear triad consisted of the US Navy's submarine-launched ballistic missiles (SLBMs) along with the US Air Force's strategic bombers and intercontinental ballistic missiles (ICBMs). Considered vital to the nuclear deterrence posture, accurate determination of the SLBM launch position was a force multiplier. Precise navigation would enable US submarines to get an accurate fix of their positions prior to launching their SLBMs [4]. The US Air Force with two-thirds of the nuclear triad also had requirements for a more accurate and reliable navigation system. The Navy and Air Force were developing their own technologies in parallel to solve what was essentially the same problem. To increase the survivab ility of ICBMs, there was a proposal to use mobile launch platforms so the need to fix the launch position had similarity to the SLBM situation. In 1960, the Air Force proposed a radio-navigation system called MOSAIC (Mobile System for Accurate ICBM Control) that was essentially a 3-D LORAN. A follow-on study called Project 57 was worked in 1963 and it was "in this study that the GPS concept was born." That same year the concept was pursued as Project 621B, which had "many of the attributes that you now see in GPS" and promised increased accuracy for Air Force bombers as well as ICBMs. Updates from the Navy Transit system were too slow for the high speeds of Air Force operation. The Navy Research Laboratory continued advancements with their Timation (Time Navigation) satellites, first launched in 1967, and with the third one in 1974 carrying the first atomic clock into orbit.[5] With these parallel developments in the 1960s, it was realized that a superior system could be developed by synthesizing the best technologies from 621B, Transit, Timation, and SECOR in a multi-service program.

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During Labor Day weekend in 1973, a meeting of about 12 military officers at the Pentagon discussed the creation of a Defense Navigation Satellite System (DNSS). It was at this meeting that "the real synthesis that became GPS was created." Later that year, the DNSS program was named Navstar. With the individual satellites being associated with the name Navstar (as with the predecessors Transit and Timation), a more fully encompassing name was used to identify the constellation of Navstar satellites, Navstar-GPS, which was later shortened simply to GPS. After Korean Air Lines Flight 007, carrying 269 people, was shot down in 1983 after straying into the USSR's prohibited airspace, in the vicinity of Sakhalin and Moneron Islands, President Ronald Reagan issued a directive making GPS freely available for civilian use, once it was sufficiently developed, as a common good. The first satellite was launched in 1989, and the 24th satellite was launched in 1994. Initially, the highest quality signal was reserved for military use, and the signal available for civilian use was intentionally degraded ("Selective Availability", SA). This changed with US President Bill Clinton ordering Selective Availability turned off at midnight May 1, 2000, improving the precision of civilian GPS from 100 meters (about 300 feet) to 20 meters (about 65 feet). The US military by then had the ability to deny GPS service to potential adversaries on a regional basis.[6] GPS is owned and operated by the US Government as a national resource. Department of Defense (USDOD) is the steward of GPS. Interagency GPS Executive Board (IGEB) oversaw GPS policy matters from 1996 to 2004. After that the National Space-Based Positioning, Navigation and Timing Executive Committee was established by presidential directive in 2004 to advise and coordinate federal departments and agencies on matters concerning the GPS and related systems. The executive committee is chaired jointly by the deputy secretaries of defense and transportation. Its membership includes equivalent-level officials from the departments of state, commerce, and homeland security, the joint chiefs of staff, and NASA. Components of the executive office of the president participate as observers to the executive committee, and the FCC chairman participates as a liaison.

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1.4 GPS System Segments Overwiew
GPS is comprised of three segments: satellite constellation, ground control/monitoring network, and user receiving equipment. Formal GPS JPO programmatic terms for these components are space, control, and user equipment segments, respectively. The satellite constellation is the set of satellites in orbit that provide the ranging signals and data messages to the user equipment. The control segment (CS) tracks and maintains the satellites in space. The CS monitors satellite health and signal integrity and maintains the orbital configuration of the satellites. Furthermore, the CS updates the satellite clock corrections and ephemerides as well as numerous other parameters essential to determining user PVT. Finally, the user receiver equipment (i.e., user segment) performs the navigation, timing, or other related functions(eq. surveying) 1.4.1 Space Segment Overwiew

The space segment is the constellation of satellites from which users make ranging measurements. The SVs (i.e., satellites) transmit a PRN-coded signal from which the ranging measurements are made. This concept makes GPS a passive system for the user with signals only being transmitted and the user passively receiving the signals. Thus, an unlimited number of users can simultaneously use GPS. A satellite¶s transmitted ranging signal is modulated with data that includes information that defines the position of the satellite. An SV includes payloads and vehicle control subsystems. The primary payload is the navigation payload used to support the GPS PVT mission; the secondary payload is the nuclear detonation (NUDET) detection system, which supports detection and reporting of Earth-based radiation phenomena. The vehicle control subsystems perform such functions as maintaining the satellite pointing to Earth and the solar panels pointing to the Sun. 1.4.2 Control Segment (CS) Overwiew The CS is responsible for maintaining the satellites and their proper functioning. This includes maintaining the satellites in their proper orbital positions (called stationkeeping) and monitoring satellite subsystem health and status. The CS also monitors the satellite solar arrays, battery power levels, and propellant levels
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used for maneuvers. Furthermore, the CS activates spare satellites (if available) to maintain system availability. The CS updates each satellite¶s clock, ephemeris, and almanac and other indicators in the navigation message at least once per day. Updates are more frequently scheduled when improved navigation accuracies are required. (Frequent clock and ephemeris updates result in reducing the space and control contributions to range measurement error. Further elaboration on the effects of frequent clock and ephemeris updates. The ephemeris parameters are a precise fit to the GPS satellite orbits and are valid only for a time interval of 4 hours with the once-per-day normal upload schedule. Depending on the satellite block, the navigation message data can be stored for a minimum of 14 days to a maximum of a 210-day duration in intervals of 4 hours or 6 hours for uploads as infrequent as once per two weeks and intervals of greater than 6 hours in the event that an upload cannot be provided for over 2 weeks. The almanac is a reduced precision subset of the ephemeris parameters. The almanac consists of 7 of the 15 ephemeris orbital parameters. Almanac data is used to predict the approximate satellite position and aid in satellite signal acquisition. Furthermore, the CS resolves satellite anomalies, controls SA and AS and collects pseudorange and carrier phase measurements at the remote monitor stations to determine satellite clock corrections, almanac, and ephemeris. To accomplish these functions, the CS is comprised of three different physical components: the master control station (MCS), monitor stations, and the ground antennas 1.4.3 User Segment Overwiew

The user receiving equipment comprises the user segment. Each set of equipment is typically referred to as a GPS receiver, which processes the L-band signals transmitted from the satellites to determine user PVT. While PVT determination is the most common use, receivers are designed for other applications, such as computing user platform attitude (i.e., heading, pitch, and roll) or as a timing source.

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1.5 GPS Segments
GPS consists of three segments: the space segment, the control segment, and the user segment (Figure 1.2) . The space segment consists of the 24-satellite constellation introduced in the previous section. Each GPS satellite transmits a signal, which has a number of components: two sine waves (also known as carrier frequencies), two digital codes, and a navigation message. The codes and the navigation message are added to the carriers as binary biphase modulations . The carriers and the codes are used mainly to determine the distance from the user¶s receiver to the GPS satellites. The navigation message contains, along with other information, the coordinates (the location) of the satellites as a function of time. The transmitted signals are controlled by highly accurate atomic clocks onboard the satellites. The control segment of the GPS system consists of a worldwide network of tracking stations, with a master control station (MCS) located in the United States at Colorado Springs, Colorado. The primary task of the operational control segment is tracking the GPS satellites in order to determine and predict satellite locations, system integrity, behavior of the satellite atomic clocks, atmospheric data, the satellite almanac, and other considerations. This information is then packed and uploaded into the GPS satellites through the S-band link. The user segment includes all military and civilian users. With a GPS receiver connected to a GPS antenna, a user can receive the GPS signals, which can be used to determine his or her position anywhere in the world. GPS is currently available to all users worldwide at no direct charge.

Figure 1.2 GPS segments[2]
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1.6 GPS Satellite Generations
GPS satellite constellation buildup started with a series of 11 satellites known as Block I satellites (Figure 1.3). The first satellite in this series (and in the GPS system) was launched on February 22, 1978; the last was launched on October 9, 1985. Block I satellites were built mainly for experimental purposes. The inclination angle of the orbital planes of these satellites, with respect to the equator, was 63°, which was modified in the following satellite generations [7]. Although the design lifetime of Block I satellites was 4.5 years, some remained in service for more than 10 years. The last Block I satellite was taken out of service on November 18, 1995. The second generation of the GPS satellites is known as Block II/IIA satellites . Block IIA is an advanced version of Block II, with an increase in the navigation message data storage capability from 14 days for Block II to 180 days for Block IIA. This means that Block II and Block IIA satellites can function continuously, without ground support, for periods of 14 and 180 days, respectively. A total of 28 Block II/IIA satellites were launched during the period from February 1989 to November 1997. Of these, 23 are currently in service. Unlike Block I, the orbital plane of Block II/IIA satellites are inclined by 55° with respect to the equator. The design lifetime of a Block II/IIA satellite is 7.5 years, which was exceeded by most Block II/IIA satellites. To ensure national security, some security features, known as selective availability (SA) and antispoofing, were added to Block II/IIA satellites [8]. A new generation of GPS satellites, known as Block IIR, is currently being launched (Figure 1.3). These replenishment satellites will be backward compatible with Block II/IIA, which means that the changes are transparent to the users. Block IIR consists of 21 satellites with a design life of 10 years. In addition to the expected higher accuracy, Block IIR satellites have the capability of operating autonomously for at least 180 days without ground corrections or accuracy degradation. The autonomous navigation capability of this satellite generation is achieved in part through mutual satellite ranging capabilities. In addition, predicted ephemeris and clock data for a period of 210 days are uploaded by the ground control segment to support the autonomous navigation. More features will be added to the last 12 Block IIR satellites under the GPS modernization program, which will be launched at the beginning of 2003 [9]. As of July 2001, six Block IIR satellites have been successfully launched.
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Figure 1.3 GPS satellite generations.[3]

Block IIR will be followed by another system, called Block IIF (for ³followon´), consisting of 33 satellites. The satellite life span will be 15years. Block IIF satellites will have new capabilities under the GPS modernization program that will dramatically improve the autonomous GPS positioning accuracy.The first Block IIF satellite is scheduled to be launched in 2005 or shortly after that date.

1.7 Current GPS Satellite Constellation
The current GPS constellation (as of July 2001) contains five Block II, 18 Block IIA, and six Block IIR satellites (see Table 1.1). This makes the total number of GPS satellites in the constellation to be 29, which exceeds the nominal 24-satellite constellation by five satellites [10]. All Block I satellites are no longer operational. The GPS satellites are placed in six orbital planes, which are labeled A through F. Since more satellites are currently available than the nominal 24-satellite constellation, an orbital plane may contain four or five satellites. As shown in Table 1.1, all of the orbital planes have five satellites, except for orbital plane C, which has only four. The satellites can be identified by various systems. The most popular identification systems within the GPS user community are the space vehicle number (SVN) and the pseudorandom noise (PRN); the PRN number will be defined later. Block II/IIA satellites are equipped with four onboard atomic clocks: two cesium (Cs) and two rubidium (Rb). The cesium clock is used as the primary timing source to control the GPS signal. Block IIR satellites, however, use rubidium clocks only. It should be pointed out that two satellites, PRN05 and PRN06, are equipped with
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corner cube reflectors to be tracked by laser ranging (Table 1.1).

Table 1.1 GPS Satellite Constellation as of July 2001[1]

1.8 Control Sites
The control segment of GPS consists of a master control station (MCS), a worldwide network of monitor stations, and ground control stations (Figure 1.4). The MCS, located near Colorado Springs, Colorado, is the central processing facility of the control segment and is manned at all times [11]. There are five monitor stations, located in Colorado Springs (with the MCS), Hawaii, Kwajalein, Diego Garcia, and Ascension Island. The positions (or coordinates) of these monitor stations are known very precisely.Each monitor station is equipped with high-quality GPS receivers and a cesium oscillator for the purpose of continuous tracking of all the GPS satellites in view. Three of the monitor stations (Kwajalein, Diego Garcia, and Ascension Island) are also equipped with ground antennas for uploading the information to the GPS satellites. All of the monitor stations and the ground control stations are unmanned and operated remotely from the MCS.

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Figure 1.4 GPS control sites [4]

The GPS observations collected at the monitor stations are transmitted to the MCS for processing. The outcome of the processing is predicted satellite navigation data that includes, along with other information, the satellite positions as a function of time, the satellite clock parameters, atmospheric data, satellite almanac, and others. This fresh navigation data is sent to one of the ground control stations to upload it to the GPS satellites through the S-band link. Monitoring the GPS system integrity is also one of the tasks of the MCS. The status of a satellite is set to unhealthy condition by the MCS during satellite maintenance or outages. This satellite health condition appears as a part of the satellite navigation message on a near real-time basis. Scheduled satellite maintenance or outage is reported in a message called Notice Advisory to Navstar Users (NANU), which is available to the public through, for example, the U.S. Coast Guard Navigation Center [10].

1.9 GPS: The basic idea
The idea behind GPS is rather simple. If the distances from a point on the Earth (a GPS receiver) to three GPS satellites are known along with the satellite locations, then the location of the point (or receiver) can be determined by simply
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applying the well-known concept of resection [12]. That is all! But how can we get the distances to the satellites as well as the satellite locations? As mentioned before, each GPS satellite continuously transmits a microwave radio signal composed of two carriers, two codes, and a navigation message. When a GPS receiver is switched on, it will pick up the GPS signal through the receiver antenna. Once the receiver acquires the GPS signal, it will process it using its built-in software. The partial outcome of the signal processing consists of the distances to the GPS satellites through the digital codes (known as the pseudoranges) and the satellite coordinates through the navigation message. Theoretically, only three distances to three simultaneously tracked satellites are needed. In this case, the receiver would be located at the intersection of three spheres; each has a radius of one receiver-satellite distance and is centered on that particular satellite (Figure 1.5). From the practical point of view, however, a fourth satellite is needed to account for the receiver clock offset [7]. The accuracy obtained with the method described earlier was until recently limited to 100m for the horizontal component, 156m for the vertical component, and 340 ns for the time component, all at the 95% probability level. This low accuracy level was due to the effect of the so-called selective availability, a technique used to intentionally degrade the autonomous real-time positioning accuracy to unauthorized users. With the recent presidential decision of terminating the selective availability, the obtained horizontal accuracy is expected to improve to about 22m (95% probability level) [13]. To further improve the GPS positioning accuracy, the socalled differential method, which employs two receivers simultaneously tracking the same GPS satellites, is used. In this case, positioning accuracy level of the order of a subcentimeter to a few meters can be obtained.

Figure 1.5 Basic idea of GPS positioning. [5]
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Other uses of GPS include the determination of the user¶s velocity, which could be determined by several methods. The most widely used method is based on estimating the Doppler frequency of the received GPS signal [7]. It is known that the Doppler shift occurs as a result of the relative satellite-receiver motion. GPS may also be used in determining the attitude of a rigid body, such as an aircraft or a marine vessel. The word ³attitude´ means the orientation, or the direction, of the rigid body, which can be described by the three rotation angles of the three axes of the rigid body with respect to a reference system. Attitude is determined by equipping the body with a minimum of three GPS receivers (or one special receiver) connected to three antennas, which are arranged in a nonstraight line [14]. Data collected at the receivers are then processed to obtain the attitude of the rigid body.

1.10 GPS Positioning Service
As stated earlier, GPS was originally developed as a military system, but was later made available to civilians as well. However, to keep the military advantage, the U.S. DoD provides two levels of GPS positioning and timing services: the Precise Positioning Service (PPS) and the Standard Positioning Service (SPS) [15]. PPS is the most precise autonomous positioning and timing service. It uses one of the transmitted GPS codes, known as P(Y)-code, which is accessible by authorized users only. These users include U.S. military forces. The expected positioning accuracy provided by the PPS is 16m for the horizontal component and 23m for the vertical component (95% probability level). SPS, however, is less precise than PPS. It uses the second transmitted GPS code, known as the C/A-code, which is available free of charge to all users worldwide, authorized and unauthorized. Originally, SPS provided positioning accuracy of the order of 100m for the horizontal component and 156m for the vertical component (95% probability level). This was achieved under the effect of selective availability. With the recent presidential decision of discontinuing the SA, the SPS autonomous positioning accuracy is presently at a comparable level to that of the PPS.

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1.11 Why use GPS?
GPS has revolutionized the surveying and navigation fields since its early stages of development. Although GPS was originally designed as a military system, its civil applications have grown much faster. As for the future, it is said that the number of GPS applications will be limited only to one¶s imagination. On the surveying side, GPS has replaced the conventional methods in many applications. GPS positioning has been found to be a cost-effective process, in which at least 50% cost reduction can be obtained whenever it is possible to use the socalled real-time kinematic (RTK) GPS, as compared with conventional techniques. In terms of productivity and time saving, GPS could provide more than 75% timesaving whenever it is possible to use the RTK GPS method [18]. The fact that GPS does not require intervisibility between stations has also made it more attractive to surveyors over the conventional methods. For those situations in which the GPS signal is obstructed, such as in urban canyons, GPS has been successfully integrated with other conventional equipment. GPS has numerous applications in land, marine, and air navigation.Vehicle tracking and navigation are rapidly growing applications. It isexpected that the majority of GPS users will be in vehicle navigation. Future uses of GPS will include automatic machine guidance and control, where hazardous areas can be mapped efficiently and safely using remotely controlled vehicles. The recent U.S. decision to modernize GPS and to terminate the selective availability will undoubtedly open the door for a number of other applications yet to be developed [14].

1.12 GPS Errors and Biases
GPS pseudorange and carrier-phase measurements are both affected by several types of random errors and biases (systematic errors). These errors may be classified as those originating at the satellites, those originating at the receiver, and those that are due to signal propagation (atmospheric refraction) [19]. Figure 1.6 shows the various errors and biases. The errors originating at the satellites include ephemeris, or orbital, errors, satellite clock errors, and the effect of selective availability. The latter was intentionally implemented by the U.S. DoD to degrade the autonomous GPS
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accuracy for security reasons. It was, however, terminated at midnight (eastern daylight time) on May 1, 2000 [21]. The errors originating at the receiver include receiver clock errors, multipath error, receiver noise, and antenna phase center variations. The signal propagation errors include the delays of the GPS signal as it passes through the ionospheric and tropospheric layers of the atmosphere. In fact, it is only in a vacuum (free space) that the GPS signal travels, or propagates, at the speed of light. In addition to the effect of these errors, the accuracy of the computed GPS position is also affected by the geometric locations of the GPS satellites as seen by the receiver. The more spread out the satellites are in the sky, the better the obtained accuracy (Figure 1.6).

Figure 1.6 GPS errors and biases.[2] 1.12.1 Ionospheric delay At the uppermost part of the earth¶s atmosphere, ultraviolet and X-ray radiations coming from the sun interact with the gas molecules and atoms. These interactions result in gas ionization: a large number of free ³negatively charged´ electrons and ³positively charged´ atoms and molecules [22]. Such a region of the atmosphere where gas ionization takes place is called the ionosphere. It extends from an altitude of approximately 50 km to about 1,000 km or even more (see Figure 1.6). In fact, the upper limit of the ionospheric region is not clearly defined [23,24].
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The electron density within the ionospheric region is not constant; it changes with altitude. As such, the ionospheric region is divided into subregions, or layers, according to the electron density. These layers are named D (50±90 km), E (90±140 km), F1 (140±210 km), and F2 (210±1,000 km), respectively, with F2 usually being the layer of maximum electron density. The altitude and thickness of those layers vary with time, as a result of the changes in the sun¶s radiation and the Earth¶s magnetic field. For example, the F1 layer disappears during the night and is more pronounced in the summer than in the winter [23]. The question that may arise is: How would the ionosphere affect the GPS measurements? The ionosphere is a dispersive medium, which means it bends the GPS radio signal and changes its speed as it passes through the various ionospheric layers to reach a GPS receiver. Bending the GPS signal path causes a negligible range error, particularly if the satellite elevation angle is greater than 5°. It is the change in the propagation speed that causes a significant range error, and therefore should be accounted for. The ionosphere speeds up the propagation of the carrier phase beyond the speed of light, while it slows down the PRN code (and the navigation message) by the same amount. That is, the receiver-satellite distance will be too short if measured by the carrier phase and too long if measured by the code, compared with the actual distance [15]. The ionospheric delay is proportional to the number of free electrons along the GPS signal path, called the total electron content (TEC). TEC, however, depends on a number of factors: (1) the time of day (electron density level reaches a daily maximum in early afternoon and a minimum around midnight at local time); (2) the time of year (electron density levels are higher in winter than in summer); (3) the 11-year solar cycle (electron density levels reach a maximum value approximately every 11 years, which corresponds to a peak in the solar flare activities known as the solar cycle peak²in 2001 we are currently around the peak of solar cycle number 23); and (4) the geographic location (electron density levels are minimum in midlatitude regions and highly irregular in polar, auroral, and equatorial regions). As the ionosphere is a dispersive medium, it causes a delay that is frequency dependent. The lower the frequency, the greater the delay; that is, the L2 ionospheric delay is greater than that of L1. Generally, ionospheric delay is of the order of 5m to 15m, but can reach over 150m under extreme solar activities, at midday, and near the horizon [25]. This discussion shows that the electron density level in the ionosphere varies
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with time and location. It is, however, highly correlated over relatively short distances, and therefore differencing the GPS observations between users of short separation can remove the major part of the ionospheric delay. Taking advantage of the ionosphere¶s dispersive nature, the ionospheric delay can be determined with a high degree of accuracy by combining the P-code pseudorange measurements on both L1 and L2. Unfortunately, however, the P-code is accessible by authorized users only. With the addition of a second C/A-code on L2 as part of the modernization program, this limitation will be removed [16]. The L1 and L2 carrier-phase measurements may be combined in a similar fashion to determine the variation in the ionospheric delay, not the absolute value. Users with dualfrequency receivers can combine the L1 and L2 carrier-phase measurements to generate the ionosphere-free linear combination to remove the ionospheric delay [20]. The disadvantages of the ionosphere-free linear combination, however, are: (1) it has a relatively higher observation noise, and (2) it does not preserve the integer nature of the ambiguity parameters. As such, the ionosphere-free linear combination is not recommended for short baselines. Single-frequency users cannot take advantage of the dispersive nature of the ionosphere. They can, however, use one of the empirical ionospheric models to correct up to 60% of the delay [17]. The most widely used model is the Klobuchar model, whose coefficients are transmitted as part of the navigation message. Another solution for users with single-frequency GPS receivers is to use corrections from regional networks [19]. Such corrections can be received in real time through communication links. 1.12.2 Tropospheric delay

The troposphere is the electrically neutral atmospheric region that extends up to about 50 km from the surface of the earth . The troposphere is a nondispersive medium for radio frequencies below 15 GHz[16]. As a result, it delays the GPS carriers and codes identically. That is, the measured satellite-to-receiver range will be longer than the actual geometric range, which means that a distance between two receivers will be longer than the actual distance. Unlike the ionospheric delay, the tropospheric delay cannot be removed by combining the L1 and the L2 observations.This is mainly because the tropospheric delay is frequency independent. The tropospheric delay depends on the temperature, pressure, and humidity along the
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signal path through the troposphere. Signals from satellites at low elevation angles travel a longer path through the troposphere than those at higher elevation angles. Therefore, the tropospheric delay is minimized at the user¶s zenith and maximized near the horizon. Tropospheric delay results in values of about 2.3m at zenith (satellite directly overhead), about 9.3m for a 15°-elevation angle, and about 20±28m for a 5°-elevation angle [22, 23]. Tropospheric delay may be broken into two components, dry and wet. The dry component represents about 90% of the delay and can be predicted to a high degree of accuracy using mathematical models [23]. The wet component of the tropospheric delay depends on the water vapor along the GPS signal path. Unlike the dry component, the wet component is not easy to predict. Several mathematical models use surface meteorological measurements (atmospheric pressure, temperature, and partial water vapor pressure) to compute the wet component. Unfortunately, however, the wet component is weakly correlated with surface meteorological data, which limits its prediction accuracy. It was found that using default meteorological data (1,010 mb for atmospheric pressure, 20°C for temperature, and 50% for relative humidity) gives satisfactory results in most cases.

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CHAPTER 2 - DIRECT BROADCAST SATELL TE SERVICES
Direct broadcast satellite (DBS) is a term used to refer to satellite television broadcasts intended for home reception. A designation broader than DBS would be direct-to-home signals, or DTH. This was initially meant to distinguish the transmissions directly intended for home viewers from cable television distribution services that sometimes carried on the same satellite. The term DTH predates DBS and is often used in reference to services carried by lower power satellites which required larger dishes (1.7m diameter or greater) for reception. In Europe, prior to the launch of Astra 1A in 1988, the term DBS was commonly used to describe the nationally-commissioned satellites planned and launched to provide TV broadcasts to the home within several European countries (e.g. BSB in the UK, TV-Sat in Germany). These services were to use the D-Mac and D2-Mac format and BSS frequencies with circular polarization from orbital positions allocated to each country. Before these DBS satellites, home satellite television in Europe was limited to a few channels, really intended for cable distribution, and requiring dishes typically of 1.2m SES Astra launched the Astra 1A satellite to provide services to homes across Europe receivable o dishes of just n 60 cm-80 cm and, although these mostly used PAL video format and FSS frequencies with linear polarization, the DBS name slowly came to applied to all Astra satellites and services too.[26]

2.1 Orbital Spacing
From Table 2.1 it is seen that the orbital spacing is 9° for the high power satellites, so adjacent satellite interference is considered nonexistent. The DBS orbital positions along with the transponder allocations for the United States are shown in Figure 2.1. It should be noted that although the DBS services are spaced by 9°, clusters of satellites occupy the nominal orbital positions.

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Figure 2.1 DBS orbital positions for the United States.[6]

Table2.1 Defining Characteristics of Three Categories of United States DBS Systems [2]
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2.2 Power Rating and Number of Transponders
From Table 2.1 it will be seen that satellites primarily intended for DBS have a higher [EIRP] than for the other categories, being in the range 51 to 60 dBW. At a Regional Administrative Radio Council (RARC) meeting in 1983, the value established for DBS was 57 dBW [26]. Transponders are rated by the power output of their high-power amplifiers. Typically, a satellite may carry 32 transponders. If all 32 are in use, each will operate at the lower power rating of 120 W. By doubling up the high-power amplifiers, the number of transponders is reduced by half to 16, but each transponder operates at the higher power rating of 240 W. The power rating has a direct bearing on the bit rate that can be handled.[27]

2.3 Frequencies and Polarization
The frequencies for direct broadcast satellites vary from region to region throughout the world, although these are generally in the Ku band. For region 2 , Table 2.1 shows that for high-power satellites, the primary use of which is for DBS, the uplink frequency range is 17.3 to 17.8 GHz, and the downlink range is 12.2 to 12.7 GHz. The medium-power satellites listed in Table 2.1 also operate in the Ku band at 14 to 14.5 GHz uplink and 11.7 to 12.2 GHz downlink. The primary use of these satellites, however, is for point-to-point applications, with an allowed additional use in the DBS service. In this chapter only the high-power satellites intended primarily for DBS will be discussed.[28] The available bandwidth (uplink and downlink) is seen to be 500 MHz. A total number of 32 transponder channels, each of bandwidth 24 MHz, can be accommodated. The bandwidth is sometimes specified as 27 MHz, but this includes a 3-MHz guardband allowance. Therefore, when calculating bit-rate capacity, the 24 MHz value is used. The total of 32 transponders requires the use of both righthand circular polarization (RHCP) and left-hand circular polarization (LHCP) in order to permit frequency reuse, and guard bands are inserted between channels of a given polarization. The DBS frequency plan for Region 2 is shown in Fig. 2.2.

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Figure 2.2 The DBS frequency plan for Region 2. [7]

2.4 Transponder Capacity
The 24-MHz bandwidth of a transponder is capable of carrying one analog television channel. To be commercially viable, direct broadcast satellite (DBS) television [also known as direct-to-home (DTH) television] requires many more channels, and this requires a move from analog to digital television. Digitizing the audio and video components of a television program allows signal compression to be applied, which greatly reduces the bandwidth required. The signal compression used in DBS is a highly complex process, and only a brief overview will be given here of the process. Before doing this, an estimate of the bit rate that can be carried in a 24MHz transponder will be made. The symbol rate that can be transmitted in a given bandwidth is ;

Thus, with a bandwidth of 24 MHz and allowing for a rolloff factor of 0.2, the symbol rate is ;

Satellite digital television uses QPSK. Thus, using M = 4 , gives m = 2, and the bit rate from ;

This is the bit rate that can be carried in the 24-MHz channel using QPSK.
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2.5 Bit Rates for Digital Television
The bit rate for digital television depends very much on the picture format. One way of estimating the uncompressed bit rate is to multiply the number of pixels in a frame by the number of frames per second, and multiply this by the number of bits used to encode each pixel. The number of bits per pixel depends on the color depth per pixel, for example 16 bits per pixel gives a color depth of 216 = 65536 colors. Using the HDTV format having a pixel count per frame of 1920 x 1080 and a refresh rate of 30 frames per second as shown in Table 2.2, the estimated bit rate is 995 Mbps. (A somewhat different estimate is sometimes used, which allows for 8 bits for each of the three primary colors, and this would result in a bit rate of approximately 1.49 Gbps for this version of HDTV). From Table 2.2 it is seen that the uncompressed bit rate ranges from 118 Mb/s for standard definition television at the lowest pixel resolution to 995 Mb/s for high definition TV at the highest resolution. As a note of interest, the broadcast raster for studio-quality television, when digitized according to the international CCIR-601 television standard, requires a bit rate of 216 Mb/s (Netravali and Lippman, 1995).[29]

Table 2.2 ATSC Television Formats [3]
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A single DBS transponder has to carry somewhere between four and eight TV programs to be commercially viable [26].The programs may originate from a variety of sources, for example film, analog TV, and videocassette. Before transmission, these must all be converted to digital, compressed, and then timedivision multiplexed (TDM). This TDM baseband signal is applied as QPSK modulation to the uplink carrier reaching a given transponder. The compressed bit rate, and hence the number of channels that are carried, depends on the type of program material. Talk shows where there is little movement require the lowest bit rate, while sports channels with lots of movement require comparatively large bit rates. Typical values for SDTV are in the range of 4 Mb/s for a movie channel, 5 Mb/s for a variety channel, and 6 Mb/s for a sports channel.[30] Compression is carried out to Moving Pictures Expert Group (MPEG) standards.

2.6 The Home Receiver Indoor Unit (IDU)
The block schematic for the IDU is shown in Figure 2.3. The transponder frequency bands shown in Figure 2.2 are downconverted to be in the range 950 to 1450 MHz, but of course, each transponder retains its 24-MHz bandwidth. The IDU must be able to receive any of the 32 transponders, although only 16 of these will be available for a single polarization. The tuner selects the desired transponder. It should be recalled that the carrier at the center frequency of the transponder is QPSK modulated by the bit stream, which itself may consist of four to eight TV programs TDM. Following the tuner, the carrier is demodulated, the QPSK modulation being converted to a bit stream. Error correction is carried out in the decoder block labeled FEC-1. The demultiplexer following the FEC -1 block separates the individual programs, which are then stored in buffer memories for further processing (not shown in the diagram). This further processing would include such things as conditional access, viewing history of pay-per-view (PPV) usage, and connection through a modem to the service provider (for PPV billing purposes).[31]

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Figure 2.3 Block schematic for the indoor unit (IDU).

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CONCLUSION
GPS stands for Global Positioning System. It is a network of 24 satellites placed into orbit by the Department of Defense (DOD). It works anywhere in the world, 24 hours a day, in all weather conditions, and on land, air or sea. GPS is based on the coordinate system. The best part is that it is free to use. I learn a GPS system consists of three segments, these are: Space, Control and User. The Space segment consists of the satellite constellation. The Control segment consists of the master control station located at Schriever Air Force Base in Colorado. There are also four stations that monitor the satellites and three ground antennas. The User segment consists of the receivers and antennas receiving the signal on Earth. GPS satellites weigh approximately 2,000 lbs (1 Ton), travel 7,000 mph, last ten years and are 17 feet across when solar panels are extended. They are powered by solar energy but do have backup batteries for emergencies. Satellites are orbiting 12,500 miles above the Earth. Each satellite circles the Earth twice daily. As each GPS satellite circles the earth, it transmits a radio signal called a pseudo random code. Each signal is The signal

encoded with information used to determine a receiver¶s location.

transmission includes the time the signal was sent and the satellite¶s location in space. Receivers on earth receive this signal. All the satellites in the constellation send their information at the same time. However, they arrive at different times due to the distance the signals travel. Direct Broadcast Satellite is a digital satellite system transmitting TV programs. A number of companies provide DBS and DTH service throughout the world. DBS used geosynchronous orbit (GSO). The bandwidth allocated for DBSTV is 12.1-12.7 GHz. This band is exclusively used for DBS-TV satellite in GEO. DBS seems most appealing to persons who either are disenchanted with cable television or who live in areas that are not served by cable. DBS-TV systems operate with small antennas and low cost receiving systems,and offer a very large number of video and audio channels,making them attractive to customers. Advanteges of DBS: more choice, rural availability, reliable service, interactive channel guides etc. Disadvanteges of DBS : The receiver and satellite dish can be expensive, affected by bad weather conditions, the advancing technology is making costs are reduced.
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Telecommunications in our lives if we work in the area of engineering will surely benefit of this project. In this project we learned good information and want to do good studies in the area of telecommunications.

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REFERENCES
[1] Hoffmann-Wellenhof, B., H. Lichtenegger, and J. Collins, Global Positioning System: Theory and Practice, 3rd ed., New York: Springer-Verlag, 1994. [2] Langley, R. B., ³The Orbits of GPS Satellites,´ GPS World, Vol. 2, No. 3, March 1991 [3]Wells, D. E., et al., Guide to GPS Positioning, Fredericton, New

Brunswick:Canadian GPS Associates, 1987 [4] Dr. Dennis D. McCrady. "The GPS Burst Detector W-Sensor". Sandia National Laboratories [5] "United States Updates Global Positioning System Technology". America.gov. February 3, 2006. [6] Dana, Peter H.. "Geometric Dilution of Precision (GDOP) and Visibility". University of Colorado at Boulder [7] Hoffmann-Wellenhof, B., H. Lichtenegger, and J. Collins, Global Positioning System: Theory and Practice, 3rd ed., New York: Springer-Verlag, 1994. [8] Georg zur Bonsen, Daniel Ammann, Michael Ammann, Etienne Favey, Pascal Flammant Reckoning" [9] Shaw, M., K. Sandhoo, and D. Turner, ³Modernization of the Global Positioning System,´ GPS World, Vol. 11, No. 9, September 2000 [10] U.S. Coast Guard Navigation Center, ³GPS Status,´ September 17, 2001, http://www.navcen.uscg.gov/gps/ [11] Leick, A., GPS Satellite Surveying, 2nd ed., New York: Wiley, 1995. [12] Langley, R. B., ³The Mathematics of GPS,´ GPS World, Vol. 2, No. 7, July/August 1991 [13] Conley, R., ³Life After Selective Availability,´ U.S. Institute of Navigation Newsletter, Vol. 10, No. 1, Spring 2000. [14] Kleusberg, A., ³Mathematics of Attitude Determination with GPS,´ GPS World, Vol. 6, No. 9, September 1995 [15] FRP, U.S. Federal Radionavigation Plan, 1999 [16] Langley, R. B., ³Why Is the GPS Signal So Complex?´ GPS World, Vol. 1, No. 3, May/June 1990 [17] Berg, R. E., ³Evaluation of Real-Time Kinematic GPS Versus Total Stations for
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"Continuous Navigation Combining GPS with Sensor-Based Dead

Highway Engineering Surveys,´ 8th Intl. Conf. Geomatics: Geomatics in the Era of RADARSAT, Ottawa, Canada, May 24±30, 1996 [18] Komjathy, A., Global Ionospheric Total Electron Content Mapping Using the Global Positioning System, Ph.D. dissertation, Department of Geodesy and Geomatics Engineering, Technical Report No. 188, University of New Brunswick, Fredericton, New Brunswick, Canada, 1997. [19] Langley, R. B., ³GPS, the Ionosphere, and the Solar Maximum,´ GPS World, Vol. 11, No. 7, July 2000 [20] Hay, C., and J. Wong, ³Enhancing GPS: Tropospheric Delay Prediction at the Master Control Station,´ GPS World, Vol. 11, No. 1, January 2000,pp. 56±62. [21] Brunner, F. K., and W. M. Welsch, ³Effect of the Troposphere on GPS Measurements,´ GPS World, Vol. 4, No. 1, January 1993, pp. 42±51. [22] Leick, A., GPS Satellite Surveying, 2nd ed., New York: Wiley, 1995. [23] Langley, R. B., ³Dilution of Precision,´ GPS World, Vol. 10, No. 5, May 1999, pp. 52±59. [24]U.S. Coast Guard Navigation Center, accessed 2001,

http://www.navcen.uscg.gov/GPS/default.htm#almanacs. [25] Barry G. Haskell,Atul Puri,Arun N. Netravali, Digital video: an introduction to MPEG-2,1998 [26] Donald C. Mead Direct broadcast satellite communications: an MPEG enabled service, 2000 [27] Dement, D. K. 1984. United States Direct Broadcast Satellite System Development. IEEE Communications Magazine, Vol. 22, No. 3, March. [28] Assembly of Engineering (U.S.). Board on Telecommunications--Computer Applications,National Research Council (U.S.) Symposium on Direct Broadcast Satellite Communications [29] Prichard W. L., and M. Ogata. 1990. Satellite Direct Broadcast. Proc. IEEE, Vol. 78, No. 7 July, pp. 1116±1140. [30] Kathryn M. Queeney, Direct broadcast satellites and the United Nations [31] Fogg, Chad. 1995. MPEG and DSS Technical Notes (v0.3), IEEE

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REFERENCES OF FIGURES

[1] Ahmed El-Rabbany, Introduction to GPS: the Global Positioning System, Artech House,Boston, 2002 [2] Wells, D. E., et al., Guide to GPS Positioning, Fredericton, New Brunswick: Canadian GPS Associates, 1987. [3] http:\\www2.geod.hrcan.gc.ca/~craymer/gps.html [4] http://science.jrank.org/kids/pages/103/How-GPS-Works.html [5] http://pubs.ext.vt.edu/442/442-503/442-503.html [6] Donald C. Mead Direct broadcast satellite communications: an MPEG enabled service, 2000 [7] Assembly of Engineering (U.S.). Board on Telecommunications--Computer Applications,National Research Council (U.S.) Symposium on Direct Broadcast Satellite Communications [8] Kathryn M. Queeney, Direct broadcast satellites and the United Nations

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REFERENCES OF TABLES

[1]Ahmet El-Rabbany, Introduction to GPS: the Global Positioning System, Artech House, Boston, 2002 [2] Donald C. Mead Direct broadcast satellite communications: an MPEG enabled service, 2000 [3] www.timefordvd.com, 2004.

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