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8/8/2019 Satellite Yeni

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CONTENTS

CONTENTS................................ ................................ ................................ ................................ ........ 1

ABSTRACT ................................ ................................ ................................ ................................ ........ 2

INTRODUCTION ................................ ................................ ................................ ................................ 3

CHAPTER 1 - GPS ................................ ................................ ................................ .............................. 4

1.1WHAT S GPS ? ................................ ................................ ................................ .......................... 4

1.2OWERWEW OF GPS ................................ ................................ ................................ .................... 4

1.3HSTORY OF GPS ................................ ................................ ................................ ......................... 5

1.4GPSSYSTEM SEGMENTS OVERWEW ................................ ................................ ............................... 8

1.4.1 Space Segment Overwiew ................................ ................................ ................................ 8

1.4.2 Control Segment (CS) Overwiew ................................ ................................ ....................... 8

1.4.3 User Segment Overwiew ................................ ................................ ................................ .. 91.5GPSSEGMENTS ................................ ................................ ................................ ........................ 10

1.6GPSSATELLTE GENERATONS ................................ ................................ ................................ ...... 11

1.7CURRENT GPSSATELLTE CONSTELLATON ................................ ................................ ....................... 12

1.8CONTROL STES ................................ ................................ ................................ ......................... 13

1.9GPS:THE BASC DEA ................................ ................................ ................................ ................. 14

1.10GPSPOSTONNG SERVCE ................................ ................................ ................................ ........ 16

1.11WHY USE GPS? ................................ ................................ ................................ ...................... 17

1.12GPSERRORS AND BASES ................................ ................................ ................................ .......... 17

1.12.1 Ionospheric delay ................................ ................................ ................................ ......... 181.12.2 Tropospheric delay ................................ ................................ ................................ ....... 20

CHAPTER 2 - DIRECT BROADCAST SATELLTE SERVICES................................ ................................ ... 22

2.1ORBTAL SPACNG................................ ................................ ................................ ...................... 22

2.2POWER RATNG AND NUMBER OF TRANSPONDERS ................................ ................................ ............ 24

2.3FREQUENCES AND POLARZATON................................ ................................ ................................ .. 24

2.4TRANSPONDER CAPACTY................................ ................................ ................................ ............. 25

2.5BT RATES FOR DGTAL TELEVSON................................ ................................ ................................ 26

2.6THE HOME RECEVER INDOOR UNT (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 serviceand 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 ona 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,

andmuch 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 sothat four satellites are placed in each of six orbital planes (Figure 1.1). With this

constellation geometry, four to ten GPS satellites 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 Earths 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 ofthe 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 survivability 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. Thefirst 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 satellites 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 satellites 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 users

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 follow-

on), 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 so-

called 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 users 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 attitude 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 ones 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 so-

called 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 attractiveto 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 earths 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 negatively charged

electrons and positively 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 (5090 km), E (90140

km), F1 (140210 km), and F2 (2101,000 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 suns radiation and the Earths

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 peakin 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 ionospheres 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 users 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 2028m

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, 20C for temperature, and 50% for relative

humidity) gives satisfactory results in most cases.


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CHAPTER 2 - DIRECT BROADCAST SATELLTE

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 on dishes of just

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 thatalthough the DBS services are spaced by

9, clusters of satellites occupy the nominal orbital positions.


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Figure 2.1DBS 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 forDBS have

a higher [EIRP] than for the other categories, being in therange 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-poweramplifiers. Typically, a satellite may carry 32 transponders. If all

32 arein use, each will operate at the lower power rating of 120 W. By doublingup

the high-power amplifiers, the number of transponders is reduced byhalf to 16, but

each transponder operates at the higher power rating of240 W. The power rating has

a direct bearing on the bit rate that canbe 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 to14.5 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 as27 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 forRegion 2 is shown in Fig. 2.2.


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Figure 2.2The 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 compressionto 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 24-

MHz 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, usingM= 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, andmultiply 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 HDTVformat having a pixel count per frame of 1920 x 1080 and a

refresh rateof 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/sfor 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 time-

division 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 asconditional access, viewing history ofpay-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 theworld, 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

encoded with information used to determine a receivers location. The signal

transmission includes the time the signal was sent and the satellites 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 DBS-

TV 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 operatewith 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., The 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 "Continuous Navigation Combining GPS with Sensor-Based Dead

Reckoning"

[9] Shaw, M., K. Sandhoo, and D. Turner, Modernization of the Global Positioning

System, GPS World, Vol. 11, No. 9, September 2000

[10] U.S. Coast Guard Navigation Center, GPS 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., The Mathematics of GPS, GPS World, Vol. 2, No. 7,

July/August 1991

[13] Conley, R., Life After Selective Availability, U.S. Institute of Navigation

Newsletter, Vol. 10, No. 1, Spring 2000.

[14] Kleusberg, A., Mathematics 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., Why Is the GPS Signal So Complex? GPS World, Vol. 1, No.

3, May/June 1990

[17] Berg, R. E., Evaluation of Real-Time Kinematic GPS Versus Total Stations for


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Highway Engineering Surveys, 8th Intl. Conf. Geomatics: Geomatics in the Era of

RADARSAT, Ottawa, Canada, May 2430, 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., GPS, the Ionosphere, and the Solar Maximum, GPS World,

Vol. 11, No. 7, July 2000

[20] Hay, C., and J. Wong, Enhancing GPS: Tropospheric Delay Prediction at the

Master Control Station, GPS World, Vol. 11, No. 1, January 2000,pp. 5662.

[21] Brunner, F. K., and W. M. Welsch, Effect of the Troposphere on GPS

Measurements, GPS World, Vol. 4, No. 1, January 1993, pp. 4251.

[22] Leick, A., GPS Satellite Surveying, 2nd ed., New York: Wiley, 1995.

[23] Langley, R. B., Dilution of Precision, GPS World, Vol. 10, No. 5, May 1999,

pp. 5259.

[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. 11161140.

[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.

satellite yeni

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