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(Prof.S.N.Mitra Memorial Award-2007 Lecture)Globa l Naviga tio n Satel li te Syste m (GNSS)(A vast system of systems providing global positioning, navigation & timing

information to scores of user community)

Conventional Satellite technology has got three applications : Communication, Remote

sensing and, Scientific studies. The latest one to add to this list is Satellite Based

Navigation also referred as Satellite Navigation/Global Positioning System and lately

termed as Global Navigation Satellite System (GNSS). With the technological

advancement taking place in mobile communications, controls, automobiles, aviation,

geodesy, geological survey, military operations, precision farming, town planning,

banking, weather predictions, power grid synchronization etc., in spite of each one

having separate domain, there is one thing common in all of them for their future; that

is the Precise-position, Timing and Velocity (PVT) information, which can only be

provided by Global Navigation Satellite System (GNSS).

Global Navigation Satellite System (GNSS) is a vast system of systems, providing global

positioning, navigation and timing information to scores of users in oceans, land, air

and even in space. The paper/talk traces the history of navigation, evolution of

navigation satellites, the three present constellations and a world scenario in this

direction. India has taken a significant step in this direction, with its SBAS system

GAGAN and deployment of its own Regional Navigation Satellite Constellation (IRNSS).

The paper will touch upon the various GNSS connected aspects, their applications and

the Indian perspective.

________________________________________________________________________Dr. S Pal DFIETE, FIEEE, FNAE, FNAScDistinguished ScientistProgramme Director, Satellite Navigation Program / Chairman, GAGAN PMBDeputy Director, Digital & Communication AreaISRO Satellite Centre, Bangalore-17pal_surendra@hotmail.com

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Dr. P S Goel, President INAE, Members of the INAE Council, INAE Fellows,

distinguished guests, ladies and gentleman,

First of all I wish to thank the INAE President Dr. Goel and Council Members for

conferring Prof. S. N. Mitra Memosrial Award on me. It is indeed a great honour toreceive an award in the name of an eminent electronics and RF engineer who worked in

the field of electronics & RF communication when the technologies were not so

advanced, even in advanced countries, what to talk about India. Prof. Mitra was also a

founder member of INAE. I feel highly honoured with the conferment of this award on

me.

As an award lecture I am going to talk to your on Global Navigation Satellite System

GNSS and efforts being made by India in this direction.

Indian Space Programme as Envisaged by Prof. Vikram Sarabhai to start with had four

components:

Rockets (Launchers like SLV,ASLV,PSLV,GSLV)

Satellites (Communication Satellites, Remote Sensing & Scientific Satellites)

Space Sciences and Space Technology Application

In the last decade the planners of Indian Space Programme (Present Chairman Dr. G

Madhavan Nair , Former Chairman, Dr. Rangan, and Ex Director, ISAC presently

Secretary, Ministry of Earth Sciences, Dr. Goel) have added Satellite Based Navigation

System to the Indian Space Activities. Satellite Based Navigation System is generally

termed as Global Navigation Satellite System GNSS. India although a late entrant in

this arena is going to play a major role.

ISRO has taken up GPS Aided GEO Augmented Navigation System (GAGAN) the

Indian Satellite Based Augmentation System for GPS (S-BAS) along with Airport

Authority of India, Indian Regional Navigation Satellite System IRNSS Indias ownindependent constellation, participation & Co-operation with GALILEO, GLONASS &

thus India is becoming an important member of world GNSS fraternity. Dr. P S Goel

then INAE President then Director of ISRO Satellite Centre did play an important role

in defining GAGAN & IRNSS.

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As a part of this lecture, I am going to talk to you and tell you about the history of

navigation, global satellite based navigation scenario, Indian participation &

contribution to this newer phenomena of space technology. The talk will end with

GNSS applications & issues.

It is really a great honour for me to deliver the Prof. S N Mitra Memorial (2007) Award

Lecture. It is my humble tribute to the great electronics & RF communication engineer

& one of the founder member of INAE.

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History of Navigation:

Navigation is the science of charting ones own route from Point `A to Point `B

with respect to known references both in spatial as well as temporal domain.

Navigation is also the process of planning, recording and controlling the movements of a

craft from one place to another.

Navigation is the determination of the position & velocity of a moving vehicle or a craft.

Apparently the word `NAVIGATE is derived from the latin word NAVIS meaning ship

and agere to move, steeror to direct. There is another thinking about the word

`Navigation having its origin in Sanskrit where `Nav or `Nau means `Nauka boat

and `gati means `velocity. Some scholars feel that art of navigation was born in the

Sindhu Valley 6000 years ago.

The early man wondered away from

his hut or cave, he asked himself,

where am I? That became the need

and may also be the origin of

navigation. He also perhaps wanted

to know as how to go back to his

place, that is which way is his

destination? The tips to returnbecame tools ofguidance. He had no

concept of position on earth, whose

size and shape could not be conceived

even for centuries to come.

Identifying and remembering objects and land marks like rocks, trees, rivers, marking

on trees or leaving stones/flags, looking at Sun and Moon as points of reference were

the techniques and navigational aids that the early man used to find his way in jungles,

desserts, mountains etc. Perhaps the time reference was day/night or to start with

even could be seasons.

The situation changed drastically when man started long voyages on oceans. During the

sea voyages to start with the boats were kept near the contours of the shores.

Vegetation, water currents, land contours, birds, water, temperature, wind speed &

4

HISTORY OF NAVIGATION

Navigation is the science of charting ones own

route from point A to point B with respect toknown references both in spatial as well as in

temporal domain

Identifying and remembering objects and land marks

like rocks, trees, rivers, marking on trees or leavingstones/flags and looking at Sun and Moon, as pointsof reference were the techniques and navigational

aids that the early man used to find his way in jungles, deserts, mountains etc. Perhaps the time

reference was day/night or even could be seasons.

The situation changed drastically when man startedlong voyages on oceans.

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direction and even water smell were used for navigation. With slight passing of time

celestial objects like Moon, Sun, stars and various constellations were used, based on

the fact that the relative position of stars and their geometrical arrangements look

different from different places on earth. By observing this phenomenon one was able to

compute his position on Earth and the direction that he should take. Great Bear andSmall Bear along with other constellations & planets like Venus, Mars & Jupiter were

extensively used. Slowly Pole Star also came in the arena.

Perhaps the first person to think about navigation from air or space/above land, was

the great Sanskrit scholar `Kalidas. In his famous literary creation Meghdoot, the

Yaksha instructs/navigates the journey of the Megh above the land, and tells him all

the ground location and ground control points like rivers, mountains, forests, cities and

even flowers and vegetation so that the Megh can reach to Alkapuri and deliver

Yakshas message to his beloved. In Meghdoot the way `Kalidas uses biosphere,

animals, birds, vegetation, fragerances and even emotions of the human beings for

navigation, perhaps with most advanced navigational skills of the present technological

paradigm, one will not be able to do.

History of NavigationThe great sanskrit scholar Kalidas was the first one to imagine aboveland navigation. In his famous Sanskrit Kavya `Meghdoot KalidassYaksha instructs `Megha ,how to navigate from Ramagiri to Alkapuri.

Soar up high and head North.Lift yourselves a little higher westward and keep moving. Relax for a while on

the top ofMount Amrakuta, whose burning woods you will have helpedsoak

As you lighten you will pick up speed and reach the rocky VindhyaRange.

.The wind there will be too weak to hoist you.

.The chataka birds will follow as you travel shedding rain catching the headyscent of flowers and charred wood charred summer fires

..when you reach Dashran, you will see garden hedges white with Ketakiflowers

..In the royal city of Vidisha you will be able to sip the sweet waters of theVetravati River. Go ahead and rest for a white on the low peaks ofNichais.

.Dont forget to detour a little and checkout the view ofUjjayinis whitemansions and savor..

Along the way fill yourselves up at the nirvindhya River.

When you reach Awanti look forVishala, a city made inheaven.

There the cool morning breeze, fragrant from lotus blossoms on theShipra River,,,,,,,,,,,,

.Nurse the lotus flowers in the Manasa Lake with your water

There to the north ofKuberas estate is our house with a large rainbowlike gate and a Mandartree which is just like a.

HISTORY OF NAVIGATION Contd

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Phoenicians, Vickings,

Irish Monks and Greeks

were undertaking sea

voyages and had great

navigation skills even 3000 years back. Phoenicians

claimed to have

circumnavigated Africa

from Red Sea, sailing via

the Cape of Good Hope.

During those days perhaps

burning fire on mountain

tops were used as light houses. The Legendary Light house of Alexandria was an

example.

Although `Nav word has its origin in `Naoka - `Nav in Sanskrit gati means velocity,

not much is known about Indians capabilities in navigation, though great work was

reported in the area of astronomy, time measurement, to certain extent terrain

mapping, village and city planning. In Mohanjadaro ruins ,one clay tablet was found

which depicted a boat. Sindhu or Indus valley civilization which was spread in parts of

Pakistan & Gujarat, do show that India had ports and perhaps a successful business

with Romans, Babylonian and Sumerian civilizations. Bate Dwarka marine

archeological findings indicate the existence of a well developed marine navigation in

1000 BC.

The archeological site at Lothal

(near Ahmedabad, Gujarat,

India) has got remains of a port

which indicates that more than

4500 years back India also had

an advanced sea-transportation

system. The dock is almost ofthe same size as that of

modern Vishakapatanam dock.

However not much is known

about the navigation aids used

by the sea travelers. Cholas in the past and Marathas in the recent past had large

navy. As per the classic Tamil literature out of the 18 Tamil Sidhas, Sidha

6

HISTORY OF NAVIGATION

Phoenicians , Vikings and Greek were undertaking sea voyages andhad navigation skills even 3000 years back. Phoenicians claimed tohave circumnavigated Africa from Red sea, sailing via the Cape ofGood Hope.

Burning fire on mountain tops were used as light houses. The

legendary Light House of Alexandria was an example. Navigation word has perhaps its origin in Naoka- Nav boat + Gati-velocity , in Sanskrit.

Not much is written in the modern history about Navigation activitiesin Asia-Pacific region. Chinese, Arabs etc., had under taken lot of seavoyages.

In Mohanjadaro ruins (Indian sub continent ) one clay tablet was foundwhich depicted a boat.

Sindhu or Indus valley civilization ruins ( parts of Pakistan, Gujarat ,Harayana ) do show that perhaps a successful business with Romans,Babylonians and Sumerian civilizations.

Out of 18 Tamil Sidhas, Sidha Bhoganathar went to China via searoute (even he is supposed to have designed an aeroplane) and livedin China as Lao-tzu, spread Taosim. He is attributed to have greatnavigational skills.

Lothal

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Bhoganathar went to China via sea route (even he is supposed to have designed an

aeroplace) & lived in China as Lao-tzu, spread Taosim, also had great navigational

skills. Sumerians, Arabs & Chinese also undertook long sea voyages earlier to 3rd BC.

In the recorded history Megallan (1550 AD) went on his voyage to circumnavigateearth equipped with primitive sea charts, an earth globe, cross staff, dead reckoning

tools wooden and metal theodolites and quadrants, hour glasses, a log and knotted

rope. Christopher Columbus started his journey on 6 th September 1492. Columbus

had very primitive navigation tools and his place in navigation history is recorded

owing to his courage, resolution and audacity rather than to his insight, intellect

or erudition.

In 324 BC Alexander the great supposed to have expressed his desire to his admiral to

sail to Africa. On Eastern side Arabs had sailed to Malabar Cost and Mallaca strait,

Sumatra and even reached China in 800 AD. Chinese had some fragmented maps

which talked about geographical features of sea, rivers even in 2000 B.C. During Tang

dynasty (700 AD ) such navigational directions extended from Korea around to East,

Africa and the Persian Gulf in the West. Intercontinental trade was pioneered by

Persian Jews, through the `Silk route even in 5th century BCE. Jews traveled West to

East and spoke many languages. Even Vasco Da Gama met on Indian shores a Jew ,

whom he supposed to have baptized. Earlier cross-staff astrolable, traverse board and

dead reckoning tools were used. With these tools & great personal skills the sailors

could estimate the ships speed direction and approximate latitude but not longitude.

Earlier navigator some

how could determine

their latitude but not

longitude and speed.

They had hour glass,

pendulum clocks and all

primitive time measuringequipments. Around

1500 AD, Chinese

invented compass for

direction finding, but

earlier to them even

Europeans, particularly, Italian used compass needles named as lodestone needles for

7

Cross-staff

Astrolable

Traverse board

Position

ingdur

ing14-16

century

Sextant

Compass Rose

Radio Communication ( I & II World War)

Radar (Robert Watson Watt -1935)

Chronometer

George Harrison, 1764 A.D

Positionin

gdurin

g17-20c

entury

Dead reckoning tools

Compass

Latitu

de

Speed

Directio

n

Longitu

de

RadioR

anging

VOR

LORAN

LandBa

sedRad

io

Position

ing

TRANSIT

GPS

SpaceBa

sed

Position

ing...

..

Egyptian Groma

20th Century

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direction finding. During long voyages a great need was felt for a chronometer which

did not get affect by gravity, temperature, humidity and loses less than a few seconds in

24 hours. In seventeenth century AD, Queen Anne of England announced 20000

reward for a certified chronometer. In 1764 George Harrison, a carpenter claimed the

award.

It is interesting to note that although most of the navigators could determine latitude

with very good accuracy, but determination of longitude of a place always remained a

problematic exercise, in want of an accurate clock. However by 1700 AD longitude of

many places were determined with respect to Paris Observatory. The methodology

depended on pendulum clocks and telescopes. Pendulum clocks could not be used in

sea due to errors on account of gravity variations, humidity, temperature effect and

added instability of the boat made it difficult to determine the latitude. With the advent

of chronometer life became much simpler to determine both longitude and latitudes in

sea.

It took another 220 years after the invention of chronometer to estimate very accurately

the position, velocity and time instantaneously independent of location and weather.

With the advent of Marine Chronometer by George Harison, the British Parliament

declared Greenwich laboratory longitude to be 0 longitude reference and

Greenwich time to be the Mean time reference for all purposes. Later on in the

year 1884at Washington this was ratified by an international agreement and the

Greenwich meridian was adopted as the prime meridian. French for long time to

come were still using Paris observatory meridian as 0 meridian.

Till the end of the nineteenth century for positioning, timing and navigation

chronometers, sextants and various types of compasses were the main tool. The

beginning of twentieth century brought the use of radio-telegraph, HF radio while first

and second world wars brought the use of radars for navigation. Over the years some of

the land based radio positioning equipments and systems like LORAN, OMEGA, DECCA& VOR. ALPHA & CHAKYA came into use. Quite a few of them are still in use and may

continue to be in use in spite of satellite based navigation.

With the advent of space technology Transit, GPS & GLONASS positioning and

navigation systems were evolved, which could provide position and guidance accuracies

in meters and submeters.

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Besides the above, for guidance purposes for aircrafts, instrument landing system and

microwave landing system are used in poor visibility. IR & laser sensors are used for

missile guidance systems.

Basic Principle of Radio Navigation

Radio Navigation technique involves positioning and accurate determination of time in

some standard reference frame. There are three basic principles of positioning, based

on the fact that radio waves travel with the speed of light, a well known parameter:

1) Trilateration

2) Hyperbolic positioning

3) Doppler positioning

Trilateration: If the distance from three known location transmitters is known then the

observer can compute its position unambiguously. Estimation of a position based on

measurement of distance is referred as trilateration. An rf navigation system working

on this principle is referred as a time of arrival system (TOA system).

Hyperbolic positioning : is a

system where distance from a

master station and twosynchronized slave stations are

plotted and the point of intersection

is the position of the observer.

Loran-C is an example of this

system.

Doppler positioning system : is based on continuous monitoring of the frequency shift

of a stable transmitter by observer & then based on number of observations, position of

the observer is determined.

Satellite based navigation like GPS, GLONASS are based on TOA/Trilateration while

earlier system like TRANSIT, TSIKADA etc. were using Doppler Positioning and TOA.

9

FIRST RADIO-POSITIONG SYSTEM FORMARITIME APPLICATIONS

SERVICE FROM 30 CHAINS FOR WIDECOVERAGE

PRINCIPLE OF RANGING FOR POSITION-FIX:

RADIO-PULSE TRANSMISSION FROM MASTER AND

SECONDARY STATIONS (OVER A GLOBAL NETWORK)

RECIEVER GETS BOTH PULSES AND TIMEDIFFERENCES (TD) FOR EACH PAIR OF MASTER-

SECONDARY STATIONS IS COMPUTED

LOCUS OF POINTS HAVING THE SAME TD FROM A

SPECIFIC MASTER-SECONDARY PAIR IS A CURVED

LINE OF POSITION (LOP).

POSITION DETERMINED BY INTERSECTION OF 2 LOPs

TD IS USED WITH MAPS TO ESTIMATE LAT/LONG

PHASE MEASUREMENTS IMPROVES PRECISION

LORAN OPERATING RANGE : 90-110 KHZ

LIMITED COVERAGE: ~1000km RANGE

OBSTRUCTION/INTERFERENCE FROM GROUND

FEATURES

REFLECTION BY IONOSPHERE

POSITION ACCURACY: ~460m (AT BEST)

LAND BASED LORAN -C (LONG RANGE NAVIGATION)

x2,y2x1,y1

x3,y3x,y

21

2

1

2

1

2

2

2

2 )()()()( dyyxxyyxx =++

31

2

1

2

1

2

3

2

3 )()()()( dyyxxyyxx =++

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Space Based Navigation

After the launch of Sputnik-I on 4th October 1957 by the erstwhile Soviet Union, two

scientists (Dr. William H Guier & Dr. George C Wieffenbach) at the Applied Physics

Laboratory (APL) of John Hopkins University were carefully studying the rf transmission

and observed certain regularities, the most important was the prominent changes in

doppler shift produced by an over flight, caused by accelerations along the line of sight,

which were enhanced by the spacecraft high speed and low orbital latitude. They

determined the orbit of Sputnik very accurately from the doppler frequency data,

observed from one location in a single pass, since the satellite orbit obeyed Keplers

Laws. This lead to the idea that If the satellites orbit were already known, a radio

receiver (observer) unknown position could be determined accurately from the doppler

measurements.

This idea gave birth

to TRANSIT system

navigation concept.

The Transit

spacecraft provided

inputs for analyzing

Earths gravity,ionospheric

refraction

correction,

development of

reliable mechanical

and electronic

satellite

construction techniques. TRANSIT could give best position accuracy (approx.) 25M.

TIMATION a programme of US Naval Research Laboratory used the concept of

synchronized tone transmission. It also had onboard stable atomic frequency standards

(Rubidium and Cesium). TIMATION provided precise time & accurate position to

passive terrestrial observers using range better than Doppler measurement. Meanwhile

U.S Airforce under a programme termed as 621B launched satellites where ranges

were measured to four satellites simultaneously in view by matching (correlating) the

10

SATELLITE

NAVIGATION &

POSITION SYSTEMS

SECOR (Sequential Collation of Range)

SECOR was a U.S army satellite navigation and positioning system. Thirteen satellites

were launched between 1964 and 1969.

TIMATION

Developed in 1972 by the Naval Research Laboratory (NRL),TIMATION satellites were intended to provide time and frequencytransfer. The third satellite acted as a GPS technologydemonstrator.

TSIKADA

Russian four satellite civil navigation system

TSYKLON

First navigation satellite launched by soviet union in late 1967.The satellite is based on doppler technique similar to TRANSIT

system.

TRANSIT

Operated in 100 MHz and 400 MHz frequency bands and allowed the user to determine

their position by measuring the Doppler shift of a radio signal transmitted bythe satellite.When man moves from one place to another 3D positioning (latitude,

longitude & height) are required.

GPS (1978) &

GLONASS

SPUTNIK

First artificial Satellite launched from Russia. Operated using

Doppler frequency shift to obtain position.

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incoming PRN signal with a user-generated replica signal and measuring the received

phase against the users (receiver) crystal clock. With this concept the users latitude,

longitude, altitude and a correction to the users clock could be determined. In 1978 all

the programmes were merged by US government in to a single entity and a Joint

Program Office for Global Positioning System (GPS) NAVSATARwas created . Themoto of GPS JPO was:

o Drop 5 bombs simultaneously in the same hole

o Build a cheap set that navigation (

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GPS, GLONASS & GALILEO - Configuration

Constellation GPS GLONASS GALILEOTotal Satellites 24+3 24 (4 Opr) 27+3

Orbital Period 12 hrs 11hrs 15min 14Hrs 22min

Orbital planes 6 3 3

Orbital height (km) 20200 19100 23616

Sat. In each plane 4 8 10

Inclination 55 deg 64.8 deg 56 deg

Plane Separation 60 deg 120 deg 120 degFrequency 1575.42MHz

1227.6MHz

1246 - 1257 MHz

1602 - 1616 MHz

1164 - 1300 MHz

1559 - 1591 MHz

Modulation CDMA FDMA CDMA

Indian Scenario

India is developing GAGAN its Satellite Based Augmented System, IRNSS Indian

Regional Navigation Satellite System and also participating in GALILEO & GLONASS

Programme.

Working of Navigation Satellites

As explained earlier, navigation satellites were operating using either Doppler Effect or

TOA of signal (Time of Arrival) principle. For NAVSTAR GPS or GLONASS the basic

position determination methodology attempts to determine the least directed line

tangent to four spherical shells centered on four spacecraft. The radius of each shell is

determined by the Time of Arrival (TOA) of the radio signal. The straight line need to be

defined due to the fact that observations are made over a large period of time and the

observer may not be stationary. The RF signals when pass through ionosphere and

troposphere gets slowed down, some times even angle of arrival also changes. Hence

the observer receivers are equipped with algorithams and look up data to correct for

these. The receivers reduce the error so caused using signals from multiple satellites,

multiple correlators. Kalman filter techniques are used for estimation of position, time

& velocity.

Both GPS & GLONASS constellations were aimed and designed primarily for military

uses. In 1996 first time GPS was opened for civilian uses, however the signal for

civilian use (Standard Positioning System SPS) had a feature called selective

availability (SA) where clock and ephemeris were intentionally tampered to give position

accuracies of the order of 100 meter while the Precision Positioning System (PPS) could

give even sub meters accuracy. On 1st May 2000 U.S President Bill Clinton removed the

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SA feature there by even SPS accuracy has come to be ~25 meters. PPS services are

reserved for military applications. There is an underlying warning that ability to

supply satellite navigation signals is also the ability to deny their availability. The

administration who controls a particular navigation satellite constellation, potentially

has the ability to degrade or eliminate satellite navigation services over any territory itdesires. This makes most of the users vulnerable to this veiled threat. Due to this

reason only new constellations are in offing.

GLONASS the Russian constellation was complete till 1998, later on with the fall of

mighty Soviet Union lots of satellites became unuseable. As of now, there are around

11 spacecraft. India will be helping in launching GLONASS-M spacecraft and also

manufacturing GLONASS-K bus. It is hoped that by 2010-2011 both the constellation

(GPS & GLONASS) will be in operation and overall position determination accuracies

will get enhanced.

Satellite Constellation Design Criterion

The basic criterion for selection of constellation for global coverage are:

The orbital height should be above 18500 KMS, to be above the Van Allen

Radiation belts The orbital period should be almost approx. 12 hrs for greater visibility

To give a coverage at high latitudes that is near or on the poles the orbitalinclination should be >50 deg.

The spacecraft design should be suchthat it should be autonomous to the

maximum extent and orbitalcorrection are rarely done, since ittakes almost 24 hours for spacecraftto get stabilized for navigationpurposes

The spacecraft dimensions should be

such that solar radiation pressure(Impacts eccentricity) and Luni/solarforces (Impacts inclination) effectsare minimum

For efficient launch considerations

and optimum in orbit spare policy, thetotal constellation should haveatleast three planes

(GPS-6, GLONASS-3, GALILEO-3)

The spacecraft payload should be

based on atomic clocks (min. of

13

SATELLITE CONSTELLATION

DESIGN PARAMETER

ORBIT CHARACTERISTICS

ORBITAL HEIGHT >= 20,000

KM

LONGER VISIBILITY

ORBITAL PERIOD

PERTURBATIONS(MINIMUM)

SOLAR RADIATION PRESSURE

(IMPACTS ECCENTRICITY)

LUNI SOLAR FORCES (IMPACTS

INCLINATION)

COMMUNICATION

ANTENNA

ISO FLUX (MORE THAN EARTH DISC)

FREQUENCY - L BAND

MINIMUM BACKGROUND THERMAL

NOISE

MINIMUM PATHLOSS

MINIMAL IONOSPHERIC GROUP DELAY

MINIMAL ATTENUATION

3,

2

an

nT

==

)Re

Re(

1cos

alt+

=

SATELLITE CONSTELLATION DESIGN PARAMETER

1. ORBIT CHARACTERISTICS

ORBITAL HEIGHT >= 20,000 KM

LONGER VISIBILITY

ORBITAL PERIOD

PERTURBATIONS(MINIMUM)

SOLAR RADIATION PRESSURE(IMPACTS ECCENTRICITY)

LUNI SOLAR FORCES (IMPACTSINCLINATION)

PLANES

LAUNCH CONSIDERATIONS

SPARE REQUIREMENT

INCLINATION

GLOBAL/HIGH LATITUDE COVERAGE

2. COMMUNICATION

ANTENNA

ISO FLUX (MORE THAN EARTH DISC)

FREQUENCY - L BAND

MINIMUM BACKGROUND THERMAL NOISE

MINIMUM PATHLOSS

MINIMAL IONOSPHERIC GROUP DELAY

MINIMAL ATTENUATION

MODULATION - CDMA/FDMA

MODULATION OF BPSK & SPREADSPECTRUM

CDMA- SINGLE FREQUENCY FOR

MULTIPLE SATELLITE DOWNLINK

FDMA- MULTIPLE FREQUENCY JAMMINGDIFFICULT

3,

2

an

nT

==

)Re

Re(1

cosalt+

=

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three with two in hot redundancy) & antenna should be iso- flux (slightly morethan earths disc coverage)

Frequency is L-band to take advantage of minimum background thermal noiseand lesser path loss.

The present day navigation

satellites work on Time of

Arrival parameter from a

number of navigation

satellites whose position

and orbital parameters are

known to a great accuracy

and equipped with onboard

atomic clocks. The receiver

correlates the information,

uses Kalman filters and estimates the ranges (position, velocity & time). To estimate

only position, data from three satellites are enough, but to remove clock bias the fourth

satellite is needed for time parameter. The range estimated this way is only `PSUEDO

RANGE and has got errors contributed by various sources like system noise,

ionosphere, clock etc. To get the true range one has to apply correction for all the

errors. Besides the error correction to estimate better accuracy even the satellites used

for observations should be geographically widely separated to give minimumgeometrical dilution of precision (G-DOP) which is inversely proportional to the volume

enclosing visible satellites. For satellite position fixing accurately one has to apply

geopotential, atmospheric drag, solar radiation pressure and luni-solar effects.

14

REALTIME POSITION FIXING USING SATELLITES

1

2

3

4

(X,Y,Z, b)(Un Known)

(XI,YI,ZI) REAL-TIME 3D POSITION FIXING:

1-WAY RANGING

ATOMIC CLOCK FOR PRECISE

RANGING

MIN OF 4 SATELLITES VISIBLE

ANYTIME

WORLD-WIDE TIME SYNCHRONISATION

2-FREQUENCY FOR IONOSPHERIC

CORRECTIONS

SIMPLE USER-END EQUIPMENT

ACCURACY: FEW METRES

bZZYYXXP

bZZYYXXP

bZZYYXXP

bZZYYXXP

+++=

+++=

+++=

+++=

2

4

2

4

2

44

2

3

2

3

2

33

2

2

2

2

2

22

2

1

2

1

2

11

)()()(

)()()(

)()()(

)()()(

SOURCES OF ERRORSystem Noise ~ 2m

Ephemeris ~ 5m

Satellite clock ~ 1m

Receiver clock ~ 2m

Multi-path ~ 0.5m

Troposphere delay ~ 1m

Ionosphere delay ~10m

Ionospheric delay

Tropospheric delay

ELEMENTS OF A SATELLITE POSITION FIXING

)]nm

((sinsin)mn

P

n

re

R

2n

n

1mnm

J(sinn

P

n

re

R

2nn

J[1r

V

==

+

== )

MEASUREMENT (UHF, S-BAND, LASER)

MODELLING (Geo-Potential, Drag, SRP,Luni-Solar)

ESTIMATION (Least-Square, Kalman filter)

rrd

dragvv

m

ACP )(

2

1=

uvm

APP

drag ))1( +=

)](2sinsinsin)sin(2sin[cos8

3 22

1jjjjj

j

iiiiydt

di +==

LEO

(m/s2)

Atm drag 6*10-5

SRP 4.7*10-6

Sun 5.6*10-7

Moon 1.2*10-6

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DILUTION OF PRECISION AND IMPACT ON

POSITION ACCURACY

Range

1,4i

1iuziz

iuyiy

iuxixAwhere,

1ATATraceDOP

==

=

=

1 2

2

1

DOP 1/volume

POSITION ERROR IS A FUNCTION

OF:

DILUTION OF PRECISION

MEASUREMENT ACCURACY

MEASUREMENT ERROR

Satellite Based Positioning System

Satellite Positioning System mainly consists of three segments:

1. Space Segment A constellation oforbiting or

Geostationary/Geosynchronous

satellites whose orbital parameters

are accurately known and are

equipped with atomic clocks.

2. Ground Control Systems - Ground

Control Segment consists of a

number of monitoring and message

uplinking stations. The ground segment maintains the constellation,

monitors satellite health, finds out accurate orbital parameter (using CDMA,

laser or one way ranging across the globe), maintains the network time and

uplinks the various parameters to satellites for transmitting to users.

3. User Segment consists of a multichannel receivers with high sensitivity

(-160 dBW) and fast processors to give the position to the user. The user

segment is only one way receiving system which does not have any linkage

with the constellation except receiving signals. The constellation & ground

segment are blind to the user.

Global Scenario of GNSS

GPS and GLONASS were the two constellations which were completely operational from

years 1995 to 1998. GLONASS service was badly affected due to the fall of Soviet

Union. However GPS programme continued. In the year 2000 AD, European Union

announced its ambitious plan of a parallel constellation of 30 satellites in three planes

15

SATELLITE POSITIONING SYSTEM SEGMENT

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with a large number of free and paid services. The constellation is called GALILEO.

They have put two

spacecraft in orbit for

experiments. The whole

constellation is likely tobe completed by 2012-

2013. Meanwhile

Russians are making

efforts to make the

GLONASS constellation

complete by 2010.

Since satellite life is limited and based on experience, US has planned modernization of

the GPS signals by increasing BW and power and additing extra signal L5, with ME

code on L1/L2, but still providing the existing services.

As stated earlier the GLONASS constellation dwindled post soviet era. However

Russians have planned modernization & revival of the full constellation using M & K

series of spacecraft.

GALILEO constellation envisage a large number of services. GALILEO has discussed

interoperability issues with GPS & GLONASS. Galileo will also be having a service

termed as PRS, (Public Regulated Services) available on the lines of PPS & M code of

GPS to selected users of the European Union.

The space segment of any GNSS constellation provides one way ranging where the user

will never have communication with spacecraft and the space segment is always blind

of the users. All constellation except GLONAS transmit their signals using CDMA, to

have resistance to jamming & interference. The same thing is achieved in GLONAS by

FDMA. Russians have 14 sets of frequencies. They repeat the frequency for satellites

on antipodal mode. However for interoperability considerations under GLONASmodernization. GLONAS will also be transmitting one signal using CDMA.

L-band frequency spectra from 1164 to 1620 is completely occupied by GPS, GLONASS

& GALILEO transmitting frequencies.

16

GPS, GLONASS & GALILEO - Configuration

Constellation GPS GLONASS GALILEOTotal Satellites 24+3 24 (4 Opr) 27+3

Orbital Period 12 hrs 11hrs 15min 14Hrs 22min

Orbital planes 6 3 3

Orbital height (km) 20200 19100 23616

Sat. In each plane 4 8 10

Inclination 55 deg 64.8 deg 56 deg

Plane Separation 60 deg 120 deg 120 deg

Frequency 1575.42MHz1227.6MHz

1246 - 1257 MHz1602 - 1616 MHz

1164 - 1300 MHz1559 - 1591 MHz

Modulation CDMA FDMA CDMA

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GPS GPSGPSGALILEO GLONASS GLONASS

1164.000MHz

1188.000MHz

1212.000MHz

1215.000MHz

1215.600MHz

1260.000MHz

1237.827MHz

1239.600MHz

1261.610MHz

GALILEO

1300.000MHz

1559.000MHz

1592.952MHz

1610.000MHz

1620.610MHz

1626.500MHz

1587.420MHz

1563.420MHz GALILEO

5010.000MHz

5030.000MHz

Radioastronomy

1610.6 1613.6 MHz

GPS, GLONASS & GALILEO

Frequency Plans

All the three constellations have carried out extensive coordination with each other.

Needless to add that GPS & GLONASS were the earlier players. However new services

are also being planned in the overlapping frequencies but using orthogonal & BOC

modulations techniques. L-band is the most crowded band in the available space to

17

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earth links. India for this very reason has gone to L& S-bands of frequencies for its

IRNSS Constellation.

Thankfully borrowed from ICG Bangalore Meet

GPS, GLONASS stand alone, cannot satisfy the integrity, accuracy and availability

requirements for all phases of flight, particularly for the more stringent precision

approaches. Integrity is

not guaranteed, since

all satellites may not be

satisfactorily working

all the times. Time to

alarm could be from

minutes to hours and

there is no indication of

quality of service.

Accuracy is not

sufficient even withS/A off in GPS. The

vertical accuracy for 95% of the time is >10m. For GPS & GLONASS stand alone

systems availability and continuity are not assured (while for GALILEO for certain

services integrity, accuracy and availability are assured). All these calls for a special

18

LIMITATIONS OF GPS AND GLONASS GPS stand alone, cannot satisfy the integrity,

accuracy & availability requirements for allphases of flight, particularly for the morestringent precision approaches.

Integrity is not guaranteed, since all satellitesmay not be satisfactorily working all times.

Time to alarm could be from minutes to hoursand there is no indication of quality of service.

Accuracy is not sufficient even with S/A off, thevertical accuracy for 95% of the time is >10m.

For GPS & GLONASS stand alone systemsavailability & continuity are not assured.

All these calls for a special system addressing allthe above, which could be done by augmenting

the GNSS systems.

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system addressing all the above, which could be done by augmenting the GNSS

systems.

For the safety-critical applications like civil aviation sector, it is essential that a user be

assured that the system is operating within design tolerances and the positionestimates derived from

it can be trusted to be

within specifications

This is the so called

integrity requirement.

Timely warning of a

system anomaly (which

may be hazardous is

called time to alarm.

The Space Based

Augmentation termed as S-BAS is the most popular system for augmenting the existing

constellation. In space Based Augmentation system a GPS like signal is transmitted by

a geostationary satellite (there are no S-BAS system as of now for GLONASS &

GALILEO) which ensures the integrity parameter of the constellation & besides that

transmits correction related to ionosphere, troposphere, ephemeris and timings.

Presently American WAAS (Wide Area Augmentation System) is the only certified S-BAS

system. EGNOS (European Geo Navigation Overlay System) is under certifications,

while GAGAN of India and M-SAS of Japan are under deployment. Brazil, Nigeria,

Russia and China have also expressed their intention of having their own S-BAS

systems.

19

REQUIREMENT OF ENHANCEMENT OF

ACCURACY, AVAILABILITY AND INTEGRITY

For the safety-critical applications it is essential

that a user be assured that the system is

operating within design tolerances and the

position estimates derived from it can be trusted

to be within specifications This is the so

called integrity requirement.

Timely warning of a system anomaly (which may

be hazardous is called time to alarm.

30Sec En-route

6 Sec APV II (Approach with Vertical Guidance)

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GPS Wide Area Augmentation SystemsGPS Wide Area Augmentation Systems

GAGAN

MSASEGNOS

C-WAAS

WAAS

*

*

**

(* INTENDED SYSTEMS)

The US & European Systems are the forerunners

consisting of a large number of reference, uplink

and Master Control Centres with each one

having 2 GEO spacecraft.

20

AUGMENTATION OF GPS / GLONASSLIMITATIONS OF GPS:

SIGNAL NOT AVAILABLE INSIDE

TUNNEL & WATER

NO ASSURANCE OF AVAILABILITY

AND INTEGRITY OF DATA

CRITICAL FOR AVIATIONAPPLICATIONS

ACCURACY REQUIREMENTS

STRINGENT

SPACE BASED AUGMENTATION (SBAS)

WAAS, EGNOS, MSAS & GAGAN

GROUND BASED AUGMENTATION (GBAS)

LAAS, PSUEDOLITE, DGPS

AIRCRAFT BASED AUGMENTATION

(ABAS)

RAIM (RECEIVER AUTONOMOUS

INTEGRITY MONITORING TECHNIQUE)

US WIDE AREA AUGMENTATION SYSTEM OF GPS

WAAS

24 Wide Area

Reference Stations

2 Wide Area MasterStations

2 Navigation Land

Uplink Stations

2 GEOs AOR &

POR

FAA presentation to ISRO

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EUROPEAN GEOSTATIONARY NAVIGATIONOVERLAY SERVICE - EGNOS

34 Range IntegrityMonitoring Stations Rims

4 Master Control Stations

2 Navigation Land UplinkStations

2 GEOs INMARSATAOR E & IOR andpresently working onARTEMIS

EGNOSS presentation to ISRO

Japan has planned M-SAS along with its own novel system QZSS (a tear drop shape

constellation) to avoid problems of low look angles.

21

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Function distributed in each institute

Timing management, WDGPScorrection, etc.

QZSS

SLR Site

Geonet GSI

Monitor Station NW

User Receiver

TT&CNAV

Message UplinkStation

WDGPS CorrectionMessage, LEX NAV

L1-SAIF: 1575.42 MHzLEX: 1278.75 MHz

Laser Ranging

Navigation SignalsL1: 1575.42 MHzL2: 1227.60 MHzL5: 1176.45 MHzLEX: 1278.75 MHz

TWSTFTUp: 14.43453GHz

Down: 12.30669GHz

Time ManagementStation

SLR: Satellite Laser Ranging, TWSTFT: Two Way Satellite Time and Frequency Transfer

Master Control Station MCS)

TT&C, NAVMessage Upload**

**: Under trade-off study between S (Up: 2025-2110, Down: 2200-2290MHz)and C (Up:5000-5010, Down:5010-5030MHz) band

QZSS Navigation System

Navigation System Architecture

China had its Beidou system. China is going in a big way with its COMASS (~ 35

satellites) system which will have GEO, MEO Components. Their plans are to have a

global system.

Indian Scenario in GNSS

India has entered in the arena of GNSS for the last seven years. We have used Satellite

Positioning System (SPS) in IRS & Scientific satellites and have completed GAGAN-TDS

the technology demonstration phase of Indian S-BAS system along with Airports

Authority of India. India plans to participate in GALILEO & GLONASS. We have also

22

Japanese S-BAS System

(MSAS)

IbarakiIbaraki

MCSMCS

Sapporo GMSSapporo GMS

Fukuoka GMS

Naha GMSNaha GMS

UserUser

Australia MRSAustralia MRS

Hawaii MRSHawaii MRS

Kobe MCSKobe MCS

TokyoTokyo

GMSGMS

GPS ConstellationGPS ConstellationMTSATMTSAT

MCS Master Control StationMCS Master Control Station

MRS Monitor and Ranging StationMRS Monitor and Ranging Station

GMS Ground Monitor StationGMS Ground Monitor Station

KDD 128KbsKDD 128Kbs

NTT 64KbsNTT 64Kbs

MSAS is the Wide area Augmentation System of Japan like WAAS and is based on MTSAT.

ICG 2007, Bangalore meet

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planned to have our own regional constellation (IRNSS) which will provide accuracies

over the land mass comparable or better than GPS. We have also taken up in a big way

the Ionospheric & Tropospheric studies and their modeling. India may becomea

biggest user of GNSS for GIS, mobile, survey, mining, fishing industry, aviation, road,

rail transport etc.

India has presently six components of

its space programme.

GNSS is the latest one to enter in to

Indian Space Arena and will augment

the existing worldwide satellite based

Navigation Systems.

To start with India has undertaken (ISRO & Airports Authority of India joint

venture) to establish its own Satellite Based Augmentation System for the GPS

constellation named GAGAN (GPS Augmented GEO Aided Navigation System) on the

lines with WAAS of US. In the Technology Demonstration (TDS) phase the system

consists of eight reference stations to measure ionospheric, ephemeris and time

corrections, one master control centre at Bangalore along with one uplink station for

GEO. GSAT-4 spacecraft will carry the

required L-band transponder, transmitting

corrections for L1 & L5 (GRS) frequencies.

As of today for the TDS phase we are using

INMARSAT 4F1 spacecraft. GAGAN Final

Operational Phase (FOP) has begun. In the

FOP phase we shall have minimum of two

Master Control Centres, 16 reference

23

INSAT IRSLAUNCH

VEHICLES

NATIONAL SPACE SYSTEMS

GNSS

GAGAN

IRNSSApplications Science

40 50 60 70 80 90 100 110-10

-5

0

5

10

15

20

25

30

35

40

INDIAN AIRSPACE TO BE SERVICEDINDIAN AIRSPACE TO BE SERVICED

INRESINRES

INMCC

BANGALOREINLUS

BANGALORE

GEO

C BAND

L1 & L5

GAGAN USER

GPS L1 & L2GPS L1 & L2

GPS Nav Data GPS Nav Data

GEO D/L

in L1

GPS & GEO data

Correction

Messages

GPS & GEO data

D/L in C and L

U/L in C

GEO D/L

in L1

GPS and GEO

Broadcast Messages

INSAT Coverage83 Deg.GAGANCOVERAGETHROUGHINSAT

INDIAN S-BAS PROGRAM GAGAN

GPS AIDED GEO AUGMENTED

NAVIGATION

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stations with (triple receiver redundancy) & redundant communication links. The

whole system will be certified system for safety of life services. The GAGAN FOP is likely

to be completed by 2010.

The GAGAN is required to service the Indian Airspace (As defined by AAI). Howeverthe GEO foot prints cover a larger area, provides opportunity to serve the neighbouring

countries by establishing suitable reference station.

[GAGAN is a fine example of great cooperation and understanding between an R&D

organization (ISRO) and a large Public Sector Service Provider (AAI) for executing a

complex technological project]

GAGAN-TDS Phase position accuracy results are very encouraging, clearly indicating

the improvements shown by the augmentations achieved over the GPS alone system.

The position accuracies from GAGAN-TDS results show very encouraging results. The

red and blue shaded areas show with and without S_BAS correction. With corrections

accuracies are approx. 3 meter.

An SBAS receiver was flown on the NRSA aircraft. The SIS (Signal in Space) from

GAGAN was verified and the performances were compared with the INMCC (Indian

Master Control Centre) generated HPL/VPL (Horizontal and Vertical Protection Limits)

contours and were observed to be in perfect agreement.

24

Preliminary System Acceptance Test Results

Achieved

position

accuracy in

North, East

and Up

directions is

better than

the Exit

Criteria

PSAT Exit Criteria

Position

Accuracy betterthan 7.5 Meters

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25

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GGTAGGTA

26

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India falls on the magnetic equator & under the equatorial ionosphere whose behaviouris quite unpredicted. Ionosphere correction and modeling are two important tasks for

any satellite based augmentation system. To study this phenomena ISRO has

established almost 28 Total

Electron Content (TEC)

Monitoring Stations, around the

country and taken up iono

studies in a big way. The

models & results will be used for

GAGAN & future navigation

projects.

Indian Regional Navigation Satellite System (IRNSS)

India has planned its own Regional Navigation Satellite System consisting of seven

satellites in GEO orbit.

27

POSITION OF MAGNETIC EQUATOR AND

SCINTILLATION REGIONS

INDIAN REGION EXPERIENCES UNPREDICTABLE

IONOSPHERIC DISTURBANCES

SUCCESS OF GAGAN IS DEPENDANT ON THE STUDY AND

MODEL THE IONOSPHERE OVER THE REGION.

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340

550

830

1110

1320

IRNSS

INDIAN REGIONAL NAVIGATIONAL SATELLITE SYSTEM

28

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The space segment will consist of seven

spacecraft (3 in GEO and 4 in Geo Geo

Synchronous Orbit (29 deg inclination)

covering the GEO arc from 34 deg to 111 deg.

The IRNSS system will be transmitting siz

signals in L1, L5 & S-band frequencies, for

standard positioning and precision

positioning applications. All the satellites will

be providing position accuracies, over the

Indian Geo political Boundary + 1500 KMS

areas, equivalent or better than GPS/GLONASS or GALILEO constellations.

Under IRNSS besides the Satellite Control Centre, Navigation Control Centre and IRNSS

Network timing centres

are planned to be

established.

29

IRNSS Architecture

Space Segment Seven satellite configuration, 3 SVs in Geo-Stationary orbit ( 34,

83 and 132 East), 4 SVs are in GEO Synchronous orbit placed atinclination of 29 (with Longitude crossing at 55 and 111 East)

The configuration takes care of continuity of service with a failureof one satellite.

The satellites are of 1 ton class with navigation payload of 102Kgs and power consumption of 676 Watts .

There will be two downlinks (L and S bands) providing dualfrequency operation with EIRP of 31.5 dBW at EOC.

The payload will have 3 Rubidium clocks. Ground Segment

Master Control Center

IRNSS Ranging & Integrity Monitoring stations (IRIM)

IRNSS Telemetry and Command stations

Navigation Control Centre

IRNSS Network Timing Centre

User SegmentPlanned operationalization by 2011-2012

HDOP & VDOP (99%) for the

Proposed Constellation

GEO 34,83,132

GSO 55(55,235), 111(111,291)

User Mask Angle 5deg

IRNSS Coverage Area

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IRNSS constellation will be operational by the year 2011 AD.

GNSS Applications and Related Issues:

Perhaps next to INTERNET if any single technological phenomena which is going

to influence many walks of human life will be GNSS. GNSS applications besides

navigation and timing informations are numerous. The most common applications are

mapping, surveying, natural resources and land management (town planning, forest

mapping, epidemic mapping and management, precision farming etc.) scientific studies

(Iono, Tropo and atmospheric studies), health monitoring of tall buildings, long bridges,

search and rescue, powergrid synchronization, banking and mobile services time

control etc., are a few important applications.

In the near future mobile, satellite based navigation services, internet and other telecom

services will get merged in to one service giving birth to many newer applications which

will be termed as GNSS Assisted Applications.

Satellite positioning

systems (GPS, Galileo,

GLONASS)

Satellite positioning

augmentation systems

(EGNOS, WAAS)

Mobile

communications

signaling network(s)

Location server(s)Fixed

telecommunications

network nodes with

short-range wireless

data communications

equipment (Bluetooth,

WLAN)

Terrestrial positioning

systems (LORAN C)

Inertial navigation

sensors (implemented

into the rover itself:

accelerometer,

barometer)

ASSISTED-GNSS

POSITIONING

ALGORITHM

Assisted GNSS Applications

30

NAVIGATION

SPACECRAFT

AIRCRAFT

SHIP

VEHICLE

GEOGRAPHIC DATA

COLLECTION

MAPPING SURVEYING

ENGINEERING

SCIENTIFIC RESEARCH

ATMOSPHERIC STUDIES

GEODYNAMICS

CRUSTAL MOVEMENTS

CRUSTAL DEFORMATIONS

MILITARY

NATURAL RESOURCE AND LAND

MANAGEMENT

GIS INGEST

FOREST MENSURATION

TOWN PLANNING

FLEET MOVEMENT

ROUTING/ALIGNMENT

MONITORING THE HEALTH OF TALL

BUILDINGS/TOWERS, LONG BRIDGES

Power grid synchronization

AGRICULTURE

PRECISION FARMING

EMERGENCY RESPONSE

SEARCH AND RESCUE

BUSINESS SOLUTIONS

LOCATION BASED SERVICES

MOBILE

TOURISM

RETAILING/Banking

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AREAS OF RESEARCH & DEVELOPMENT IN

POSITIONING AND TIMING SYSTEM

(GNSS)

SCIENCE

IONO-TROPO MODELLING IN

THE EQUATORIAL REGION IN L-

BAND

RADIO OCCULTATION STUDIES

FOR NEAR EARTH

ATMOSPHERIC TEMPERATURE

PROFILE

REAL-TIME WEATHER

FORECASTING

TECHNOLOGY

PRECISION ORBITS

TIME SYNCHRONISATION

DEVELOPMENT OF NAVIGATION

SOFTWARE

ATOMIC CLOCK RUBIDIUM, CESIUM,

HYDROGEN MASERS

ISOFLUX ANTENNAS FOR SPACECRAFT

DUAL RECEIVERS (GPS+GLONAAS,

GPS+GALILEO)

ACCURATE ESTIMATE OF PHASE DELAYS

ONBOARD SATELLITE

Although GNSS is going to be a big phenomena but interoperability between various

constellations, interferences, standardization of signals and receivers and use of

precision measurement equipments by terrorist/anti-social elements are some of the

issues which need to be tackled.

31

Issues related with GNSS

Interoperabilityrefers to the ability of open globaland regional satellite navigation and timing servicesto be used together to provide better capabilities atthe user level than would be achieved by relyingsolely on one service or signal.

Compatibility refers to the ability of space-basedpositioning, navigation, and timing services to beused separately or together without interfering witheach individual service or signal.

Issues related with GNSS

Intentional and Unintentional Interferences

Multipath, Indoor and Urban Environment

Over crowding of Frequency Spectra

Need for higher anti-jamming margins

Protection of RNSS and Radio Astronomy bands

Continuity of existing and planned constellations

Ionospheric and Solar weather impact on GNSS signals

Standardization of Civilian Signals and Receivers

Universal Time and Reference Frames (EachConstellation as of today has adopted different time andgeodetic reference frames)

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CONCLUSION

After all, we need measurements of space and time for

almost all our activities and GNSS provides these.

Hence, GNSS will influence our life more than any othertechnological advent.

Acknowledgements:

The Author wishes to express sincere thanks and gratitude to ISRO Management; Dr.

G. Madhavan Nair, Chairman-ISRO/Secretary DOS, Dr. K Kasturirangan, Former

Chairman, ISRO and Dr. P S Goel, Former Director, ISAC Dr. K N Shankara, Director-

ISAC, for giving opportunities to work and lead the Indian Satellite Based Navigation

Program. The author also puts on record his gratitude towards Dr. Ramalingam,

Chairman, Airports Authority of India, AAI/GAGAN colleagues for their wholehearted

support to the GAGAN Project. Dr. Ramalingam played the most important role in

bringing AAI & ISRO together to realize GAGAN, whose fruits will be reaped by many

GNSS users in the years to come not only by India but, the entire world civil aviation

community.

References:

i) Lecture on Global Navigation Satellite Systems- A Vast System of

Systems. Houston System of Systems Seminar, IEEE-AESS,

NASAS/JSC/Gilruth Centre, Houston, Texas, USA, 11-12th October

2007.

ii) Global Navigation Satellite System (GNSS) An Indian Scenario, Vikram

Sarabhai Memorial Lecture, IETE Mid Term Symposium-2007, Vadodara,

India (April 2007).

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