Global Positioning System (GPS)

1.0 INTRODUCTION

The Global Positioning System (GPS) is the only system today able to show user the exact

position on Earth at anytime, anywhere, and in any weather. GPS satellites orbit 11,000

nautical miles above Earth. They are monitored continuously at ground stations located

around the world. The satellites transmit signals that can be detected by anyone with a GPS

receiver.

The first GPS satellite was launched in 1978. The first 10 satellites launched were

developmental satellites, called Block I. From 1989 to 1997, 28 production satellites, called

Block II, were launched; the last 19 satellites in the series were updated versions, called

Block IIA. The launch of the 24th GPS satellite in 1994 completed the primary system. The

third-generation satellite, Block IIR, was first launched in 1997. These satellites are being

used to replace aging satellites in the GPS constellation. The next generation, Block IIF, is

scheduled for its first launch in late 2005[1].

GPS was developed by the United States Department of Defense as a reliable means for

accurate navigation. It is based on an intricate network of 24 satellites orbiting the earth at a

very high altitude. These satellites function 24 hours a day and are designed to be resistant

to jamming and interference. GPS allows every square meter of the earth's surface to have a

unique address, which offers limitless application possibilities when coupled with today's

advanced micro-computer systems [2].

The GPS program provides critical capabilities to military, civil and commercial users around

the world. It is an engine of economic growth and jobs, and has generated billions of dollars

of economic activity. It maintains future war fighter advantage over opponents and is one of

the four core military capabilities. In addition, GPS is the backbone for modernizing the global

air traffic system.

Advances in technology and new demands on the existing system have now led to efforts to

modernize the GPS system and implement the next generation of GPS III satellites and Next

Generation Operational Control System (OCX).[2] Announcements from the Vice President

and the White House in 1998 initiated these changes. In 2000, U.S. Congress authorized the

modernization effort, referred to as GPS III.

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2.0 DISCUSSIONS

2.1 Elements of GPS

GPS has three parts: the space segment, the user segment, and the control segment. The

space segment consists of a constellation of 24 satellites plus some spares, each in its own

orbit 11,000 nautical miles above Earth. The user segment consists of receivers, which you

can hold in your hand or mount in a vehicle, like your car. The control segment consists of

ground stations (five of them, located around the world) that make sure the satellites are

working properly. The master control station at Schriever Air Force Base, near Colorado

Springs, Colorado, runs the system [3].

Figure 1: Element of GPS comprises a control, space, and user segments.

2.1.1 The space segment (A Constellation of Satellites)

An orbit is one trip in space around Earth. GPS satellites each take 12 hours to orbit Earth.

Each satellite is equipped with an atomic clock so accurate that it keeps time to within three

nanoseconds, that’s 0.000000003, or three-billionths of a second—to let it broadcast signals

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that are synchronized with those from other satellites. The signal travels to the ground at the

speed of light. Even at this speed, the signal takes a measurable amount of time to reach the

receiver. The difference between the time when the signal is received and the time when it

was sent, multiplied by the speed of light, enables the receiver to calculate the distance to

the satellite. To calculate its precise latitude, longitude, and altitude, the receiver measures

the distance to four separate GPS satellites[3].

Figure 2: Space segment of GPS satellites

The space segment consists of a 28 satellite constellation out of which 24 satellites are

active satellites and the remaining four satellites are used as in-orbit spares. The satellites

are placed in six orbital planes, with four satellites in each plane. The satellites orbit in

circular medium Earth orbits (MEOs) at an altitude of 20 200 km, inclined at 55° to the

equator as shown in Figure 2. The orbital period of each satellite is around 12 hours (11

hours, 58 mins). The MEO was chosen as a compromise between the LEO and GEO. lf the

satellites are placed in an LEO, then a large number of satellites would be needed to obtain

adequate coverage. Placing them in a GEO would reduce the required number of satellites,

but will not provide good polar coverage. The present constellation makes it possible for four

to ten satellites to be visible to all receivers anywhere in the world and hence ensure

worldwide coverage.

These satellites transmit signals, synchronized with each other on two microwave

frequencies of 1575.42 MHz (Ll) and 1227.6 MHz (L2). These signals provide navigation and

timing information to all users worldwide. The satellites also carry nuclear blast detectors as

a secondary mission, replacing the ‘Vela’ nuclear blast surveillance satellites. The satellites

are powered by solar energy. They have back-up batteries on board to keep them running in

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the event of a solar eclipse. The satellites are kept in the correct path with the help of small

rocket boosters, a process known as ‘station keeping’.

2.1.2 The user segment (Receivers)

GPS receivers can be carried in your hand or be installed on aircraft, ships, tanks,

submarines, cars, and trucks. These receivers detect, decode, and process GPS satellite

signals. More than 100 different receiver models are already in use. The typical hand-held

receiver is about the size of a cellular telephone, and the newer models are even smaller.

The commercial hand-held units distributed to U.S. armed forces personnel during the

Persian Gulf War weighed only 28 ounces (less than two pounds). Since then, basic receiver

functions have been miniaturized onto integrated circuits that weigh about one ounce[3].

The user segment includes all military and civil GPS receivers intended to provide position,

velocity and time information. These receivers are either hand held receivers or installed on

aircraft, ships, tanks, submarines, cars and trucks. The basic function of these receivers is to

detect, decode and process the GPS satellite signals. Some of the receivers have maps of

the area stored in their memory. This makes the whole GPS system more user-friendly as it

helps the receiver to navigate its way out. Most receivers trace the path of the user as they

move. Certain advanced receivers also tell the user the distance they have travelled, their

speed and time of travel. They also tell the estimated time of arrival at the current speed

when fed with destination coordinates. Moreover, there is no limit to the number of users

using the system simultaneously. Today many companies make GPS receivers, including

Garmin, Trimble, Eagle, Lorance and Magellan.

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Figure 3: A typical GPS receiver.

GPS receivers comprise three functional blocks:

• Radio frequency front end. This comprises one or more antennas to receive

the GPS signal, filters and amplifiers to discriminate the wanted signal from noise and

a down- converter to remove the carrier signal. Simple receivers process one GPS

signal at a time using multiplexing techniques. Sophisticated receivers comprise

multiple channels for processing the signal from various satellites simultaneously.

• Digital signal processing block. This correlates the signal from satellites with

signals stored in the receiver to identify the specific GPS satellite and to calculate

pseudo ranges.

• Computing unit. This determines position, velocity and other data. The display

format is also handled by the computing unit.

2.1.3 The control segment (Ground Stations)

The GPS control segment consists of several ground stations located around the world[3]:

• A master control station at Schriever Air Force Base in Colorado

• Five unstaffed monitor stations: Hawaii and Kwajalein in the Pacific Ocean; Diego

Garcia in the Indian Ocean; Ascension Island in the Atlantic Ocean; and Colorado

Springs, Colorado.

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• Four large ground-antenna stations that send commands and data up to the

satellites and collect telemetry back from them.

Figure 4 shows the locations of the stations of the control segment.

Figure 4: The locations of the stations of the control segment.

Each of the monitor stations is provided with high fidelity GPS receivers and a caesium

oscillator to track all GPS satellites in view continuously. Data from these stations is sent to

the MCS which computes precise and updated information on satellite orbits and clock status

every 15 minutes. This tracking information is uploaded to GPS satellites through ground

antenna stations once or twice per day for each satellite using S band signals. This helps to

maintain the accuracy and proper functioning of the whole system. The ground antenna

stations are also used to transmit commands to satellites and to receive satellite telemetry

data.

2.2 Working Principle of the GPS

The basis of GPS is triangulation from satellites.

Position is calculated from distance measurements

(ranges) to satellites. Mathematically we need four

satellite ranges to determine exact position. Three

ranges are enough if we reject ridiculous answers or

use other tricks[4].

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To triangulate, a GPS receiver

measures distance using the travel

time of radio signals. Distance to a

satellite is determined by measuring

how long a radio signal takes to reach

us from that satellite. To make the

measurement we assume that both

the satellite and our receiver are

generating the same pseudo-random

codes at exactly the same time. By

comparing how late the satellite's

pseudo-random code appears

compared to our receiver's code, we

determine how long it took to reach

us. Multiply that travel time by the

speed of light and you've got distance.

To measure travel time, GPS needs very

accurate timing which it achieves with some

tricks. Accurate timing is the key to measuring

distance to satellites. Satellites are accurate

because they have atomic clocks on board.

Receiver clocks don't have to be too accurate

because an extra satellite range

measurement can remove errors.

Along with distance, you need to know exactly

where the satellites are in space. High orbits

and careful monitoring are the secret. To use

the satellites as references for range

measurements we need to know exactly where

they are. GPS satellites are so high up their

orbits are very predictable. Minor variations in

their orbits are measured by the Department of

Defense. The error information is sent to the

satellites, to be transmitted along with the timing

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signals.

Finally, correct for any delays the signal

experiences as it travels through the

atmosphere. The earth's ionosphere and

atmosphere cause delays in the GPS

signal that translate into position errors.

Some errors can be factored out using

mathematics and modeling. The

configuration of the satellites in the sky

can magnify other errors. Differential

GPS can eliminate almost all error.

2.3 GPS Services Offered / Applications

2.3.1 Personal Tracking (Child, Teens, Elderly)

Personal Tracking is a system, where a person or a commodity can be tracked using devices

that are integrated with Global positioning System (GPS) and Global Service for Mobiles

(GSM). The person can carry it in their pocket like mobile phone or can install it in car to get

the location on SMS or web based services provided by GPS Integrated. It allows tracking

GPS location of person or vehicle carrying GPS device or GPS system. All GPS devices

include some sort of transmitter that ultimately sends this data back to the end user[5].

Personal GPS Tracking is the ability for any person to locate another person, object or thing

with the use of a GPS device that receives information from the GPS satellite network in

space. Whether you are concerned about the well being of your small children, or would like

to monitor elderly relatives, would like to offer additional layers of security for your loved ones

or family members, a personal GPS tracking device has many uses in today's world. You can

use this tracking system for teen tracking, asset tracking, pet tracking, car tracking,

equipment tracking, as a spy equipment etc.

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2.3.2 Vehicle Tracking (Car, Boat, Pickup)

Vehicle tracking is one of the fastest growing satellite navigation applications today. Many

fleet vehicles, public transportation systems, delivery trucks and courier services use GPS

and GLONASS receivers to monitor their locations at all times. These systems combined

with digital maps are being used for vehicle navigation applications. These digital maps

contain information like street names and directions, business listings, airports and other

important landmarks. Such units provide useful information about the car’s position and the

best travel routes to a given destination by linking itself to a built-in digital map.

2.3.3 Police, Spy Activities and Saving Lives

Many police, fire and emergency medical service units employ GPS receivers to determine

which available police car, fire truck or ambulance is nearest to the emergency site, enabling

a quick response in these critical situations. GPS equipped aircraft monitor the location of

forest fires exactly, enabling the fire supervisors to send fire fighters to the required spot on

time.

2.3.4 Fleet management (Bus, Truck)

Fleet management is the management of a company's vehicle fleet. Fleet management

includes commercial motor vehicles such as cars, vans and trucks. Fleet (vehicle)

management can include a range of functions, such as vehicle financing, vehicle

maintenance, vehicle telematics (tracking and diagnostics), driver management, fuel

management and health & safety management. Fleet Management is a function which allows

companies which rely on transportation in their business to remove or minimize the risks

associated with vehicle investment, improving efficiency, productivity and reducing their

overall transportation costs, providing 100% compliance with government legislation (duty of

care) and many more. These functions can be dealt with by either an in-house fleet-

management department or an outsourced fleet-management provider

GPS Fleet Tracking is an All-In-One Management Solution That Lets You Monitor, Manage,

And Recover Assets Effective management can be a daunting task, especially for

businesses who deal with employees or assets in transit. Fleet tracking management uses

GPS technology to effectively track vehicles, employees, and assets. By tracking your

valuables, you can better manage and monitor their whereabouts, cutting down on wasted

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time or unnecessary fuel. With GPS Fleet Tracking information, you can evaluate

performance and cut down on extra costs. This enhances your company's value by

improving the level of service provided, leading to higher profits and customer satisfaction.

Our automatic Fleet Tracking reports are easy-to-read graphs that let you identify operational

trouble spots and improve fleet management.

Fleet Management system uses satellite technology to provide real-time vehicle location and

record historical vehicle activity. Our highly skilled team of professional technicians will install

a vehicle tracking unit into each of your vehicles. There are no exposed wires or antennas,

making the equipment virtually tamper proof, and allowing for covert installation. The vehicle

tracking units transmit data to our servers via GPRS wireless networks. This information can

then be accessed 24 hours a day through any internet connection around the world.

2.3.5 Path Navigation

Whether navigating along a river, trail, or roads, there is a similarity to the navigation problem

in that the GPS is not needed to steer along the route. Actual navigation along the path is

done by reference to the river banks, trail, or road. A GPS route can be very useful to gauge

progress along the route even if it is not needed for steering.

Figure 5 shows an example of boating along a river. The put in point to the destination is a

very short distance as the crow flies, but is a considerably longer distance along the river. If

you are paddling this in a canoe or kayak, this is a significant distance. There are times that

path navigation may be used in combination with two-dimensional navigation. An example

that comes to mind is a long canoe trip. You paddle down a river using path navigation, then

come to a lake and use two-dimensional navigation to cross the lake.

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Figure 5: GPS path navigation map

2.3.6 Mapping, Construction and Surveying.

Mapping, construction and surveying companies use satellite navigation systems extensively

as they can provide real time submetre and centimetre level positioning accuracy in a cost-

effective manner. They are mainly used in road construction, earth moving and fleet

management applications. For these applications, receivers along with wireless

communication links and computer systems are installed on board the earth moving

machines. The required surface information is fed to this machine. With the help of real time

position information, an operator obtains information as to whether the work is in accordance

with the design or not. As an example, the tunnel under the English Channel was constructed

with the help of a GPS. The tunnel was constructed from both ends. The GPS receivers were

used outside the tunnel to check their positions along the way and to make sure that they

met exactly in the centre. These systems are also used for telecom power placement, laying

of pipelines, flood plain mapping, oil, gas and mineral exploration and in glacier monitoring.

2.3.7 Environmental Monitoring.

GPS-equipped balloons monitor holes in the ozone layer across the globe. Buoys tracking

major oil spills transmit data using the GPS to guide clean-up operations. GPSs are also

used in wildlife management and insect infestation. They are also used for determination of

forest boundaries.

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2.3.8 Monitoring Structural Deformations.

Navigation systems are used for measuring deformations on the Earth’s crust. This helps in

the prediction of earthquakes and volcanic eruptions. Geophysicists have been exploiting the

GPS since the mid l980s to measure continental drift and the movement of the Earth’s

surface in geologically active regions. They are also used for monitoring the deformation of

dams, bridges and TV towers.

Figure 6: Use of Navigation Satellites in Air Traffic Control

2.3.9 Archeology and Archeologists

Biologists and explorers are using the system to locate ancient ruins, migrating animal herds

and endangered species.

2.3.10 Utility industry

These systems are of tremendous help to the utility industry companies like electric, gas,

water companies, etc. Up-to-date maps provided by the navigation systems help these

companies to plan, build and maintain their assets.

2.4 User Equipment and Hardware

2.4.1 Garmin

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Garmin designs, manufactures, and markets leading-edge Global Positioning System (GPS)

technology and other navigation and communication products. Below are a few advantages

of Garmin products[10]:

• A wide variety of products to fit your needs

• User-friendly products

• Products that have rugged exteriors built to handle tough situations

• Free product manual and latest operating software for your unit download

• A vast selection of maps and charts

• Free product support assistance, even after the product warranty period ends

Figure 7: One of Garmin product (Nuvi 1300 Series)

2.5 GPS Accuracy

Most people know that GPS is proven to be a very valuable tool for the purposes of

Surveying, Tracking and Navigation. However its users must be aware of its characteristics

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and cautious of its limitations. Before discuss the detail about the limitation in terms of

accuracy in GPS satellite, we need to know meaning of GPS accuracy.

GPS accuracy: The accuracy refers to the degree of closeness the indicated readings are

to the actual position. The accuracy of a position determined with GPS depends on the type

of receiver. Most consumer GPS units have an accuracy of about +/-10 to +/-100meter.

Actually, GPS accuracy is a complex topic involving a variety of technical factors. Among the

technical factor that influences their limitation in term of accuracy include:-

• Selective Availability (SA)

• Wide Area Augmentation System (WAAS)

• Differential GPS (DGPS).

2.5.1 Selective Availability

With Selective Availability on, the GPS receiver doesn't know what time it really is at the

satellites, because the S.A. makes the satellite send the wrong time. The time the satellite

sends is usually pretty close to the real time, but not exact. Without knowing the exact times

at the satellites when they create their time message, the receiver cannot tell you the exact

location you are trying to measure. This means the GPS receiver gives you a less accurate

position because of S.A too as show in Figure 8[10]

Figure 8: GPS Accuracy Before and After SA Removal

2.5.2 Wide Area Augmentation System (WAAS)

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The images compare the accuracy of GPS with and without selective availability (SA).

Each plot shows the positional scatter of 24 hours of data (0000 to 2359 UTC) taken at one

of the Continuously Operating Reference Stations (CORS) operated by the NCAD Corp. at

Erlanger, Kentucky. On May 2, 2000, SA was set to zero. The plots show that SA causes

95% of the points to fall within a radius of 45.0 meters. Without SA, 95% of the points fall

within a radius of 6.3 meters.

Figure 9: GPS Accuracy Before and After WAAS Removal

The above diagrams illustrate GPS accuracy with and without WAAS. The points represent

100 recordings of the exact same spot on earth over an extended time period such as the

same time of day each day for 100 days. Even the position is exact; a GPS receiver will

report slightly different positions due to factors such as atmospheric ionospheric interference

and satellite geometry. Satellite geometry difference occurs because of the angle of each

satellite in relationship to the position being recorded. Accuracy is always best when at

least one satellite is directly over head of the recorded position. Most time, the satellites

are not directly overhead & slight variations are due to the angles measured. Variations in

location reporting can also be due to such things as reflected signals such as occurs in

downtown areas of major cities where many high buildings block the direct line between

satellites & the receiver.

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Figure 10: Basic component of DGPS.

2.5.3 Differential GPS (DGPS)

DGPS(Differential GPS) uses two or more GPS receivers. With a base station whose

accurate position has already been known, we can compare the known position with the new

GPS measurement. The difference means the error in the new observed data. This error

information is then transmitted to a remote GPS receiver as calibration (RTCM) to the

observed position there. The remote receivers receive this, correct the directly observed

position, and improve accuracy. This technology is called differential GPS (DGPS).[12]

A typical DGPS architecture is shown in Figure 10. The system consists of a Reference

Receiver (RR) located at a known location that has been previously surveyed, and one or

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more DGPS User Receivers (UR). The RR antenna, differential correction processing

system, and data link equipment (if used) are collectively called the Reference Station (RS).

Both the UR and the RR data can be collected and stored for later processing, or sent to the

desired location in real time via the data link. DGPS is based on the principle that receivers in

the same vicinity will simultaneously experience common errors on a particular satellite

ranging signal. In general, the UR (mobile receivers) uses measurements from the RR to

remove the common errors. In order to accomplish this, the UR must simultaneously use a

subset or the same set of satellites as the reference station. The DGPS positioning equations

are formulated so that the common errors cancel. Reference stations with precise known

locations can calculate the errors associated with each satellite and advise the users of the

corrections necessary to reduce the errors in the location calculation. The accuracy of users'

GPS receivers equipped with differential correction receivers is improved to less than 10

meters error.[13]

The accuracy quoted by many GPS manufacturers is often done using a statistic known as

CEP (Circular Error Probable) and are usually tested under ideal conditions. The accuracy

expected to be obtained using a GPS receiver will vary according to the overall system used.

While accuracy level actually achieved will depend upon many factors, typical estimations of

the level of GPS accuracy can be given. Table 1 shows the comparisons of accuracy using

several systems of GPS

Table 1: Comparisons of accuracy using several systems of GPS

GPS system Expected GPS accuracy (metres)GPS with S/A activated ±100

GPS without S/A activated ±15Differential GPS (DGPS) ±5

GPS with WAAS ±3

Figure 11: GPS Accuracy

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Accuracy (closeness to truth) of differential systems is relative to the accuracy of the

reference points used. When used in less than ideal conditions, the accuracy and precision

of any GPS system can be degraded significantly

There are many other factors that affect the accuracy of GPS receivers. The atmosphere is

one. As the radio signal passes through the ionosphere and troposphere, the water vapour

and particles can slow a signal down, therefore affecting the time. Another error called signal

multipath is caused by the satellite signal reflecting off of buildings, rocks, water, trees, etc.

Accuracy tends to be better in open areas where the likelihood of reflection is decreased.

Common Factors affecting the accuracy of GPS are:-

Figure 12: There are many causes for position errors or low signal

• Ionosphere and troposphere delays — The satellite signal slows as it

passes through the atmosphere. The GPS system uses a built-in model that

calculates an average amount of delay to partially correct for this type of error.

• Signal multi path — This occurs when the GPS signal is reflected off objects

such as tall buildings or large rock surfaces before it reaches the receiver.

This increases the travel time of the signal, thereby causing errors.

• Receiver clock errors — A receiver's built-in clock is not as accurate as the

atomic clocks onboard the GPS satellites. Therefore, it may have very slight

timing errors.

• Orbital errors — Also known as ephemeris errors, these are inaccuracies of

the satellite's reported location.

• Number of satellites visible — The more satellites a GPS receiver can

"see," the better the accuracy.

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• Buildings, terrain, electronic interference, or sometimes even dense

foliage can block signal reception, causing position errors or possibly no

position reading at all. GPS units typically will not work indoors, underwater or

underground.

• Intentional degradation of the satellite signal — Selective Availability (SA)

is an intentional degradation of the signal once imposed by the U.S. DoD. SA

was intended to prevent military adversaries from using the highly accurate

GPS signals. The government turned off SA in May 2000, which significantly

improved the accuracy of civilian GPS receivers.

2.6 Future of Satellite Navigation Systems

Satellite-based navigation systems are being further modernized in order to provide more

accurate and reliable services. The modernization process includes the launch of more

powerful satellites, use of new codes, enhancement of ground systems, etc. In fact, satellite-

based systems will be integrated with other navigation systems to increase their application

potential. The GPS has been modernized to provide more accurate, reliable and integrated

services to the users. The first efforts in modernization began with the discontinuation of the

selective availability feature, in order to improve the accuracy of the civilian receivers. In

continuation with this step, Block-IIRM satellites will carry a new civilian code on L2

frequency. This will help in further improving the accuracy by compensating for atmospheric

delays and will ensure more navigation security. Moreover, these satellites will carry a new

military code (M code) on both the L1 and L2 frequencies. This will provide increased

resistance to jamming. This new code will be operational by the year 2010.

The satellites will also have more accurate clock systems. Block-IIF satellites (to be launched

after the Block-II satellites) are planned to be launched by the year 2011 and will have a third

carrier signal, L5, at 1176.45 MHz. They will also have a longer design life, fast processors

with more memory and a new civil signal. The GPS-III phase of satellites is in the planning

stage. These satellites will employ spot beams. Use of spot beams results in increased

signal power, enabling the system to be more reliable and accurate, with precision accuracy

approaching a metre. As far as the GLONASS is concerned, an effort is being made to make

the complete system operational in order to exploit its true application potential[13].

Another satellite navigation system being developed is the European Galileo system. The

first Galileo satellite was launched on 28 December 2005. It is planned to launch another

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satellite in the near future. These satellites will characterize the critical technologies of the

system. After the characterization, four operational satellites will be launched to complete the

validation of the basic Galileo space segment and the related ground segment.

Once this in-orbit validation (IOV) phase has been completed, the remaining operational

satellites will be placed in orbit to reach full operational capability. A fully operational Galileo

system will comprise 30 satellites (27 operational and 3 active spares), positioned in three

circular medium Earth orbit (MEO) planes at 23 222 km altitude above Earth, and with each

orbital plane inclined at 56° to the equatorial plane. All these developments will expand the

horizon of their applications to newer dimensions. In fact, the future of satellite navigation

systems is as unlimited as one’s imagination.

2.7 Alternative to GPS

There should be an alternative positioning service besides GPS. Many country concern of

what will happen in the time of war or international conflict when US suddenly stop the GPS

service. For examples, when Obama visits India, USA changed the GPS timings without

informing to other countries for security reasons. At the same time, India testing its own build

BRAHMOS missile, the missile was not correctly shoots the target due to the fake GPS

timings and the test was failed. Due to this, India had huge losses. It’s a real incident which

shows the important of owning a local GPS system for homeland defence[14].

The European Commission (EC) estimates that 6-7% of European GDP – around 800 billion

by value – is already dependent on satellite navigation. But European users have no

alternative today other than to take their positions from US GPS or Russian Glonass

satellites. Satellite positioning has already become the standard way of navigating. If the

signals were switched off tomorrow, many ship and aircraft crews would find it inconvenient

and difficult to revert to traditional navigation methods.

Many utility networks are also more and more dependent on the precise time synchronisation

provided by the satellite navigation systems. As the use of satellite navigation spreads, the

implications of a signal failure will be even greater, jeopardising not only the efficient running

of transport systems, but also human safety.

2.7.1 GLONASS(Russia)

GLONASS stands for “Globalnaya navigatsionnaya sputnikovaya sistema” or Global

Navigation Satellite System. It is a Russian equivalent of the U.S Global Positioning System

(GPS) which is designed for both military and civilian use, and allows users to identify their

positions in real time. GLONASS is intended to provide an unlimited number of nautical, air,

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space, and ground users with navigation data and precise time signals at any moment and at

any point on the Earth and the near-Earth space environment[5].

Development of GLONASS began in the Soviet Union in 1976. Beginning on 12 October

1982, numerous rocket launches added satellites to the system until the "constellation" was

completed in 1995. Following completion, the system fell into disrepair with the collapse of

the Russian economy. In the early 2000s, under Vladimir Putin's presidency, the restoration

of the system was made a top government priority and funding was substantially increased.

GLONASS is currently the most expensive program of the Russian Federal Space Agency,

consuming a third of its budget in 2010[16]. By 2010, GLONASS had achieved 100% coverage

of Russia's territory and in October 2011, the full orbital constellation of 24 satellites was

restored, enabling full global coverage. The GLONASS satellites designs have undergone

several upgrades, with the latest version being GLONASS-K.

2.7.1.1 System description

The GLONASS system is composed of four main elements (see Figure 13):

● orbital constellation of GLONASS satellites

● Ground Control Segment

● rocket-space complex

● users.

The orbital constellation of the fully deployed system is composed of 24 GLONASS satellites

orbiting in three orbital planes. The operational orbit parameters are:

● altitude - 19100 km

● inclination - 64.8 degrees

● period - 11 hours 15 minutes.

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Figure 13: GLONASS system architecture

Orbital planes are spaced at 120 degrees in longitude. There are eight satellites in each

plane, which are evenly spaced at 45 degrees in phase. Moreover, the planes themselves

are phase-shifted 15 degrees with respect to each other. Such an orbital configuration

enables continuous and global coverage of the Earth’s surface and near-Earth airspace, as

well as an optimal spatial location of the satellites that increases position determination

accuracy[17].

The Ground Control Segment provides GLONASS satellite control. It is composed of the

System Control Center (SCC), located in Moscow Territory, and several Telemetry, Tracking,

and Control stations (TT&C) distributed throughout the Russian territory. GCS performs the

following tasks:

● monitoring of the orbital constellation’s normal operation

● continuous adjustment of satellite orbit parameters

● generation and uploading of time-tagged programs, control commands, and special

information.

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For normal operation of the navigation satellite system, it is very important to synchronize all

the processes taking place during system operation. That is to say, these processes shall

take place on the single time scale. To satisfy this requirement, the Synchronization System

contains a Central Synchronizer – a stationary ultra-stable hydrogen frequency standard,

which is used as the basis for GLONASS time scale generation. All onboard time scales are

synchronized with the system time scale. The Central Synchronizer is synchronized with the

State Time and Frequency Reference, located in C. Mendeleev (Moscow Territory).

Deployment and maintenance of the orbital constellation is provided by two rocket-space

systems: one based on the “Proton” launcher and one based on the “Soyuz” launcher. Each

rocket-space system includes:

● a launcher system

● a booster system

● a satellite system.

2.7.1.2 Signals

GLONASS satellites transmit two types of signal: a standard precision (SP) signal and an

obfuscated high precision (HP) signal. The signals use similar DSSS encoding and binary

phase-shift keying (BPSK) modulation as in GPS signals. All GLONASS satellites transmit

the same code as their SP signal, however each transmits on a different frequency using a

15-channel frequency division multiple access (FDMA) technique spanning either side from

1602.0 MHz, known as the L1 band. The centre frequency is 1602 MHz + n × 0.5625 MHz,

where n is a satellite's frequency channel number (n=−7,−6,−5,...0,...,6, previously

n=0,...,13). Signals are transmitted in a 38° cone, using right-hand circular polarization, at an

EIRP between 25 to 27 dBW (316 to 500 watts). Note that the 24 satellite constellation is

accommodated with only 15 channels by using identical frequency channels to support

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antipodal (opposite side of planet in orbit) satellite pairs, as these satellites will never be in

view of an earth based user at the same time[17].

The HP signal (L2) is broadcast in phase quadrature with the SP signal, effectively sharing

the same carrier wave as the SP signal, but with a ten times higher bandwidth than the SP

signal. The L2 signals use the same FDMA as the L1 band signals, but transmit straddling

1246 MHz with the center frequency determined by the[clarification needed] equation 1246

MHz + n×0.4375 MHz, where n spans the same range as for L1.Other details of the HP

signal have not been disclosed.

A combined GLONASS/GPS Personal Radio BeaconAt peak efficiency, the SP signal offers

horizontal positioning accuracy within 5–10 meters, vertical positioning within 15 meters, a

velocity vector measuring within 10 cm/s, and timing within 200 ns, all based on

measurements from four first-generation satellites simultaneously; newer satellites such as

GLONASS-M improve on this. The more accurate HP signal is available for authorized users,

such as the Russian Military, yet unlike the US P(Y) code which is modulated by an

encrypting W code, the GLONASS P codes are broadcast in the clear using only 'security

through obscurity'. Use of this signal bears risk however as the modulation (and therefore the

tracking strategy) of the data bits on the L2P code has recently changed from unmodulated

to 250 bit/s burst at random intervals. The GLONASS L1P code is modulated at 50 bit/s

without a manchester meander code, and while it carries the same orbital elements as the

CA code, it allocates more bits to critical Luni-Solar acceleration parameters and clock

correction terms.

Currently, an additional civil reference signal is broadcast in the L2 band with an identical SP

code to the L1 band signal. This is available from all satellites in the current constellation,

except satellite number 795 which is the last of the inferior original GLONASS design, and

one partially inoperable GLONASS-M satellite which is broadcasting only in the L1 band.

(See www.glonass-ianc.rsa.ru for daily updates on constellation status.)

GLONASS uses a coordinate datum named "PZ-90" (Earth Parameters 1990 – Parametry

Zemli 1990), in which the precise location of the North Pole is given as an average of its

position from 1900 to 1905. This is in contrast to the GPS's coordinate datum, WGS 84,

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which uses the location of the North Pole in 1984. As of September 17, 2007 the PZ-90

datum has been updated to differ from WGS 84 by less than 40 cm (16 in) in any given

direction.

2.7.2 GALILEO(Europe)

Galileo is Europe’s own global navigation satellite system,

providing a highly accurate, guaranteed global positioning

service under civilian control. It is inter-operable with GPS and

Glonass, the two other global satellite navigation systems. By

offering dual frequencies as standard, Galileo will deliver real-

time positioning accuracy down to the metre range. It will

guarantee availability of the service under all but the most

extreme circumstances and will inform users within seconds of any satellite failure, making it

suitable for safety-critical applications such as guiding cars, running trains and landing

aircraft[18].

ESA’s first two navigation satellites, GIOVE-A and –B, were launched in 2005 and 2008

respectively, reserving radio frequencies set aside for Galileo by the International

Telecommunications Union and testing key Galileo technologies. Then on 21 October 2011

came the first two of four operational satellites designed to validate the Galileo concept in

both space and on Earth. Two more will follow in 2012. Once this In-Orbit Validation (IOV)

phase has been completed, additional satellites will be launched to reach Initial Operational

Capability (IOC) around mid-decade.

Galileo services will come with quality and integrity guarantees which mark the key difference

of this first complete civil positioning system from the military systems that have come before.

A range of services will be extended as the system is built up from IOC to reach the Full

Operational Capability (FOC) by this decade’s end.

The fully deployed Galileo system consists of 30 satellites (27 operational + 3 active spares),

positioned in three circular Medium Earth Orbit (MEO) planes at 23 222 km altitude above

the Earth, and at an inclination of the orbital planes of 56 degrees to the equator[19].

Thereafter, four operational satellites - the basic minimum for satellite navigation in principle -

are being launched in 2011 to validate the Galileo concept with both segments: space and

related ground infrastructure. Once this In-Orbit Validation (IOV) phase has been completed,

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additional satellites will be launched to to reach the Initial Operational Capability around mid-

decade.

At this stage, The Open Service, Search and Rescue and Public Regulated Service will be

available with initial performances. Then along the build-up of the constellation, new services

will be tested and made available to reach the Full Operational Capability (FOC).

The fully deployed Galileo system consists of 30 satellites (27 operational + 3 active spares),

positioned in three circular Medium Earth Orbit (MEO) planes at 23 222 km altitude above

the Earth, and at an inclination of the orbital planes of 56 degrees with reference to the

equatorial plane.

Once this is achieved, the Galileo navigation signals will provide good coverage even at

latitudes up to 75 degrees north, which corresponds to the North Cape, and beyond. The

large number of satellites together with the optimisation of the constellation, and the

availability of the three active spare satellites, will ensure that the loss of one satellite has no

discernible effect on the user.

Two Galileo Control Centres (GCCs) have been implemented on European ground to

provide for the control of the satellites and to perform the navigation mission management.

The data provided by a global network of Galileo Sensor Stations (GSSs) will be sent to the

Galileo Control Centres through a redundant communications network. The GCCs will use

the data from the Sensor Stations to compute the integrity information and to synchronise the

time signal of all satellites with the ground station clocks. The exchange of the data between

the Control Centres and the satellites will be performed through up-link stations. Five S-band

up-link stations and 10 C-band up-link stations have been installed around the globe for this

purpose.

As a further feature, Galileo is providing a global Search and Rescue (SAR) function, based

on the operational Cospas-Sarsat system. To do so, each satellite will be equipped with a

transponder, which is able to transfer the distress signals from the user transmitters to the

Rescue Co-ordination Centre, which will then initiate the rescue operation[20].

At the same time, the system will provide a signal to the user, informing him that his situation

has been detected and that help is under way. This latter feature is new and is considered a

major upgrade compared to the existing system, which does not provide feedback to the

user.

Altogether Galileo will provide five levels of services with guaranteed quality which marks the

difference from this first complete civil positioning system.

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2.7.2.1 Galileo satellites system

• 30 in-orbit spacecraft (including 3 spares)

• Orbital altitude: 23,222 km (MEO)

• 3 orbital planes, 56° inclination, ascending nodes separated by 120° longitude (9

operational satellites and one active spare per orbital plane)

• Satellite lifetime: >12 years

• Satellite mass: 675 kg

• Satellite body dimensions: 2.7 m × 1.2 m × 1.1 m

• Span of solar arrays: 18.7 m

• Power of solar arrays: 1.5 kW (end of life)

2.7.2.2 Services

The Galileo system will have five main services:

• Open access navigation

This will be 'free to air' and for use by the mass market; Simple timing and positioning

down to 1 metre.

• Commercial navigation (encrypted)

High precision to the centimetre; guaranteed service for which service providers will

charge fees.

• Safety Of life navigation

Open service; For applications where guaranteed precision is essential; Integrity

messages will warn of errors.

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• Public regulated navigation (encrypted)

Continuous availability even in time of crisis; Government agencies will be main

users.

• Search and rescue

System will pick up distress beacon locations; Feasible to send feedback, confirming

help is on its way.

2.7.2.3 The concept

Each satellite will have 4 atomic clocks of 2 types (2 rubidium frequency standards and 2

passive hydrogen masers); critical to any satellite-navigation system and a number of other

components. These clocks will provide an accurate timing signal for a receiver to calculate

the time that it takes the signal to reach the target. This information is used to calculate the

position of the receiver by trilaterating the difference in received signals from multiple

satellites[21].

Figure 14: Galileo launch on a Soyuz rocket, 21 Oct 2011

2.7.2.4 International involvement

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In September 2003, China joined the Galileo project. China was to invest €230 million (USD

302 million, GBP 155 million, CNY 2.34 billion) in the project over the following years. In July

2004, Israel signed an agreement with the EU to become a partner in the Galileo project. On

3 June 2005 the EU and Ukraine signed an agreement for Ukraine to join the project, as

noted in a press release. As of November 2005, Morocco also joined the programme. On 12

January 2006, South Korea joined the programme. In November 2006, China opted instead

to independently develop the Beidou navigation system satellite navigation system.[31]

When Galileo was viewed as a private-sector development with public-sector financial

participation, European Commission program managers sought Chinese participation in

pursuit of Chinese cash in the short term and privileged access to China’s market for

positioning and timing applications in the longer term. However, due to security and

technology-independence policy from European Commission, China was, in effect, dis-

invited from Galileo and without a return of its monetary investment, a decision that was

reinforced by China’s move to build its own global system, called Beidou/Compass. At the

Munich Satellite Navigation Summit on March 10, a Chinese government official bluntly

asked the European Commission why it no longer wanted to work with China, and when

China’s cash investment in Galileo would be returned [22].

On 30 November 2007, the 27 member states of the European Union unanimously agreed to

move forward with the project, with plans for bases in Germany and Italy. Spain did not

approve during the initial vote, but approved it later that day. This greatly improves the

viability of the Galileo project: "The EU's executive had previously said that if agreement was

not reached by January 2008, the long-troubled project would essentially be dead."

On 3 April 2009, Norway too joined the programme pledging €68.9 million toward

development costs and allowing its companies to bid for the construction contracts. Norway,

while not a member of the EU, is a member of the ESA.

2.7.3 COMPASS (China)

The COMPASS system, which is also known as Beidou-2, BD2 is a project by China to

develop an independent global satellite navigation system. COMPASS is a new GNSS

similar that posses similar principle with GPS, GLONASS, and Galileo.

2.7.3.1 General system

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The new system will be a constellation of 35 satellites, which include 5 geostationary

orbit (GEO) satellites and 30 medium Earth orbit(MEO) satellites, that will offer complete

coverage of the globe. The ranging signals are based on the CDMA principle and have

complex structure typical to Galileo or modernized GPS. Similarly to the other GNSS, there

will be two levels of positioning service: open and restricted (military). The public service

shall be available globally to general users. When all the currently planned GNSS systems

are deployed, the users will benefit from the use of a total constellation of 75+ satellites,

which will significantly improve all the aspects of positioning, especially availability of the

signals in so-called urban canyons. The general designer of Compass navigation system

is Sun Jiadong, who is also the general designer of its predecessor, Beidou navigation

system[23].

Frequencies for Compass are allocated in four bands: E1, E2, E5B, and E6 and overlap with

Galileo. The fact of overlapping could be convenient from the point of view of the receiver

design, but on the other hand raises the issues of inter-system interference, especially within

E1 and E2 bands, which are allocated for Galileo's publicly-regulated service.[4] However,

under International Telecommunications Union (ITU) policies, the first nation to start

broadcasting in a specific frequency will have priority to that frequency, and any subsequent

users will be required to obtain permission prior to using that frequency, and otherwise

ensure that their broadcasts do not interfere with the original nation's broadcasts. It now

appears that Chinese Compass satellites will start transmitting in the E1, E2, E5B, and E6

bands before Europe's Galileo satellites and thus have primary rights to these frequency

ranges. Although almost nothing has yet been officially announced by Chinese authorities

about the signals of the new system, the launch of the first Compass satellite permitted

independent researchers not only to study general characteristics of the signals but even to

build a Compass receiver.

Compass-M1 is an experimental satellite launched for signal testing and validation and for

the frequency filing on April 14, 2007. The role of Compass-M1 for Compass is similar to the

role of GIOVE satellites for Galileo. The signals of Compass-M1 are to a great extent

unraveled by independent research. The orbit of Compass-M1 is nearly circular, has an

altitude of 21,150 km and an inclination of 55.5 degrees.

Compass-M1 is transmitting in 3 bands: E2, E5B, and E6. In each frequency band two

coherent sub-signals have been detected with a phase shift of 90 degrees. These signal

components are further referred to as "I" and "Q". The "I" components have shorter codes

and are likely to be intended for the open service. The "Q" components have much longer

codes, are more interference resistive, and are probably intended for the restricted service.

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The investigation of the transmitted signals started immediately after the launch of

COMPASS-M1 on April 14, 2007. Already in June engineers at CNES reported the spectrum

and structure of the signals. Next month researchers from the Stanford University reported

complete decoding of the “I” signal scomponents. The knowledge of the codes allowed a

group of engineers at Septentrio to build the COMPASS receiver and report tracking and

multipath characteristics of the “I” signals on E2 and E5B.

2.7.4 IRNSS (India)

India is developing its own version of the Global Positioning System. IRNSS, stands for The

Indian Regional Navigation Satellite System is expected to be fully functional by 2012 and

will be used for surveillance, telecommunications, transport, identifying disaster areas and

public safety. The IRNSS is an autonomous regional satellite navigational system that is

developed by the ISRO, which would be controlled directly by the Indian government.The

satellites will be placed at a higher geostationary orbit to have a larger signal footprint and

lower number of satellites to map the region, he said. The first satellite of the proposed

constellation, developed at a cost of Rs 1,600 core, is expected to be launched in 2009.

The Indian regional Navigational Satellite System (IRNSS) is a constellation comprising of

seven other satellites with an objective of providing access to the Global Navigation Satellite

System at the most hostile situations too. The project had been developed with an intention

to enhance the quality of Indian security system, as it can track the accuracy of the position

within a region of 2000 km. The ISRO had decided to launch in all the seven satellites by the

year 2014 with a gap of 6 months between each launching activities, the first one of the

satellites has yet to be launched in the year 2011 and it is being developed with a cost

estimation of around $342 million. The significant features of the IRNSS have been portrayed

as follows:

• On the completion status, the IRNSS would be covering all weather conditions over

the Indian landmass, round the clock and further up to extended distance coverage of

about 1500 km.

• If compared with the existing constellations, the IRNSS would be superior in the

sense of its independent functioning features and thus it would be under the direct

control of Indian central government.

• The IRNSS on its accomplishment would enter into the competitive ground with the

two space navigation system in the world, i.e. the U.S. Global Positioning System and

the Russian Global Navigation Satellite System, which would be another successful

milestone in the Indian space history.

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• The system can operate independently without any necessary influences from other

satellite systems, besides transmitting continuous navigation signals, which are

powered with powerful electronic equipment and atomic clocks.

• It could be of much help in acting as the best surveillance system, tracking in the

border activities that are happening through the mountains, terrains, and deep under

the sea… Thus, it can be equivalent in its functions to the security personnel,

especially in tracing down the infiltration activities across the boundaries.

• Further, the ground center of the IRNSS consists of Master Control Center to

estimate the satellites’ orbits, their quality, in addition to following up the health of the

satellites.

The IRNSS signals are said to consists of precision strength and special positioning service

attributes and both the signals are said to be on the ‘L’ and ‘S’ bands. Further the

navigational signals are said to be transmitted in the ‘S’ band frequency, which are then

broadcasted through the array type antenna to achieve the maximum strength and signal

coverage. The ground functionality of the IRNSS comprises of the user segments with dual

frequency receiver, which helps in receiving the signals from the other constellations of the

system and will be having a minimum of G/T of -25 dB/k which would be similar to that of the

GPS.

2.7.5 Quasi-Zenith Satellite System (Japan)

Japan has started a new project of Quasi-Zenith Satellite System (QZSS) in 2003. QZSS

consists of three satellites and will provide a regional satellite positioning service as well as

communication and broadcasting services. Each satellite is in three different orbit planes,

which are obtained by inclining the geostationary orbit (GEO) by about 45 degree. In this

system, at least one satellite stays around the zenith for about eight hours and is visible with

a higher elevation angle in mid-latitude area (e.g. at least 80 deg. in Tokyo) than in case of

using a satellite in GEO. This characteristic is very beneficial in large cities where there are

many tall buildings which block the signal from satellites in GEO. Thus, satellite availability

for satellite positioning and mobile communication services is expected to be greatly

improved. Several organizations from government and private sectors cooperate to develop

QZSS. The private sector has established a joint company, the Advanced Satellite Business

Corporation (ASBC), which will develop and operate the communications and broadcasting

system as well as satellite bus system. From the government sectors, four Ministries and

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their relating research institutions participate in the project and develop the satellite

positioning system.

2.7.5.1 QZSS and positioning augmentation

QZSS can enhance GPS services in two ways: first, availability enhancement, whereby the

availability of GPS signals is improved, second, performance enhancement whereby the

accuracy and reliability of GPS derived navigation solutions is increased.

Because the GPS availability enhancement signals transmitted from Quasi-Zenith Satellites

are compatible with modernized GPS signals, and hence interoperability is ensured, the

QZSs will transmit the L1C/A signal, L1C signal, L2C signal and L5 signal. This minimizes

changes to specifications and receiver designs.

Compared to standalone GPS, the combined system GPS plus QZSS delivers improved

positioning performance via ranging correction data provided through the transmission of

submeter-class performance enhancement signals L1-SAIF and LEX from QZS. It also

improves reliability by means of failure monitoring and system health data notifications.

QZSS also provides other support data to users to improve GPS satellite acquisition.

According to its original plan, QZSs was to carry two types of space-borne atomic clocks; a

hydrogen maser and a Rb atomic clock. The development of a passive hydrogen maser for

QZSs was abandoned in 2006. The positioning signal will be generated by a Rb clock and an

architecture similar to the GPS timekeeping system will be employed. QZSS will also be able

to use a Two-Way Satellite Time and Frequency Transfer (TWSTFT) scheme, which will be

employed to gain some fundamental knowledge of satellite atomic standard behavior in

space as well as for other research purposes.

2.7.5.2 QZSS timekeeping and remote synchronization

Although the first generation QZSS timekeeping system (TKS) will be based on the Rb clock,

the first QZS, will carry a basic prototype of an experimental crystal clock synchronization

system. During the first half of the two year in-orbit test phase, preliminary tests will

investigate the feasibility of the atomic clock-less technology which might be employed in the

second generation QZSS.

The mentioned QZSS TKS technology is a novel satellite timekeeping system which does

not require on-board atomic clocks as used by existing navigation satellite systems such as

GPS, GLONASS or the planned Galileo system. This concept is differentiated by the

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employment of a synchronization framework combined with lightweight steerable on-board

clocks which act as transponders re-broadcasting the precise time remotely provided by the

time synchronization network located on the ground. This allows the system to operate

optimally when satellites are in direct contact with the ground station, making it suitable for a

system like the Japanese QZSS. Low satellite mass and low satellite manufacturing and

launch cost are significant advantages of this novel system. An outline of this concept as well

as two possible implementations of the time synchronization network for QZSS were studied

and published in Fabrizio Tappero's PhD work.

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3.0 CONCLUSION

GPS has become a widely deployed and useful tool for commerce, scientific uses, tracking,

and surveillance. GPS's accurate time facilitates everyday activities such as banking, mobile

phone operations, and even the control of power grids by allowing well synchronized hand-

off switching. While originally a military project, GPS is considered a dual-use technology,

meaning it has significant military and civilian applications.

All GPS navigation and surveying techniques have limitations that may not permit desired

accuracies in a given environment. The cause for poor accuracy is not always obvious but is

usually attributable to one of the following source of error is multipath / signal corruption, Low

number of satellites / poor satellite geometry and erratic Ionospheric activity. These errors

can lead to position errors as large as several of meters or more.

Comparison between WAAS, SA and DGPS that attributable to accuracy of GPS satellite

system; conclude that the best technique or factor that the best reducing the accuracy is

WAAS is +/- 3m.

GPS modernization has now become an ongoing initiative to upgrade the Global Positioning

System with new capabilities to meet growing military, civil, and commercial needs. The

program is being implemented through a series of satellite acquisitions, including GPS Block

III and the Next Generation Operational Control System (OCX). The U.S. Government

continues to improve the GPS space and ground segments to increase performance and

accuracy.

In addition to GPS, other systems are in use or under development. The Russian GLObal

NAvigation Satellite System (GLONASS) was in use by only the Russian military, until it was

made fully available to civilians in 2007. There are also the planned European Union Galileo

positioning system, Chinese Compass navigation system, and Indian Regional Navigational

Satellite System.

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4.0 REFERENCES

[1] http://www.gpsintegrated.com/

[2] http://www.gpsintegrated.com/Services/GPSPassengerInformationSystem.aspx

[3] http://www.aero.org/education/primers/gps/howgpsworks.html

[4] http://www.gpsintegrated.com/Services/PersonalGPSTracking.aspx

[5] http://www.gpstrackingservice.net/

[6] Satellite Technology – Principles and Applications (Anil K. Maini, Varsha Agrawal)

[7] Satellite System Engineering in IPV6 Environment (Daniel Minoli)

[8] John Bell, “Basic GPS Navigation: A Practical Guide to GPS Navigation”, January

2008.

[9] “Global System for Mobile Communication (GSM)”, Online-Education Tutorial,

International Engineering Consortium, 9th September 2005.

[10] http://www.unavco.org/edu_outreach/tutorial/sa.html

[11] http://proceedings.esri.com/library/userconf/proc97/proc97/to450/pap428/p428.htm

[12] Cell-ID location technique, limits and benefits: an experimental study. Emiliano

Trevisani Dipartimento di Informatica e Sistemistica Universit`a di Roma “La

Sapienza” via Salaria 113, 00198 Rome, Italy trevisani@dis.uniroma1.it

[13] http://www.gps.gov/technical/

[14] http://www.beacon-egypt.com/dgps.htm

[15] "QZSS in 2010". Magazine article. Asian Surveying and Mapping. 2009-05-07.

[16] “The System”. GPS World Online, November 2007.

[17] "India to develop its own version of GPS". www.Rediff.com.

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[18] S. Anandan (2010-04-10). "Launch of first satellite for Indian Regional Navigation

Satellite system next year". Beta.thehindu.com. Retrieved 2010-12-30.

[19] "Compass due Next year". Magazine article. Asian Surveying and Mapping.

[20] Galileo, Compass on collision course, GPS World, April 2008, p. 27

[21] T. Grelier, J. Dantepal, A. Delatour, A. Ghion, L. Ries, Initial observation and analysis

of Compass MEO satellite signals, Inside GNSS, May/June 2007

[22] www.insidegnss.com

[23] www.irnssindia.com

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