Inae 7 Dec 07

(Prof.S.N.Mitra Memorial Award-2007 Lecture)

Globa l Na viga tio n Sa tel li te Syste m (G NSS) (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, FNASc Distinguished Scientist Programme Director, Satellite Navigation Program / Chairman, GAGAN PMB Deputy Director, Digital & Communication Area ISRO Satellite Centre, Bangalore-17 [email protected]

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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 to receive 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 – India’s own independent 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,

contribution to this newer phenomena of space technology.

Indian

participation

&

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 one’s 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, steer’or ‘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.

HISTORY OF NAVIGATION

The early man wondered away from his hut or cave, he asked himself,

• Navigation is the science of charting one’s own route from point ‘A’ to point ‘B’ with respect to known 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 leaving stones/flags and 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, deserts, mountains etc. Perhaps the time reference was day/night or even could be seasons. • The situation changed drastically when man started long voyages on oceans.

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

destination?

is

which

The

way

tips

to

is

his

return

became tools of guidance. 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 &

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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 and Small 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 Yaksha’s

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 OFNavigation NAVIGATION Contd History of The great sanskrit scholar Kalidas was the first one to imagine above land navigation. In his famous Sanskrit Kavya `Meghdoot’ Kalidas’s Yaksha 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 of Mount Amrakuta, whose burning woods you will have helped soak……… ………As you lighten you will pick up speed and reach the rocky Vindhya Range……. ……….The wind there will be too weak to hoist you……. ……….The chataka birds will follow as you travel shedding rain catching the heady scent of flowers and charred wood charred summer fires…… ………..when you reach Dashran, you will see garden hedges white with Ketaki flowers…… ………..In the royal city of Vidisha you will be able to sip the sweet waters of the Vetravati River…. …………Go ahead and rest for a white on the low peaks of Nichais……. ………….Don’t forget to detour a little and checkout the view of Ujjayini’s white mansions and savor………………….. ………Along the way fill yourselves up at the nirvindhya River………. ……………When you reach Awanti look for Vishala, a city made in heaven……………. ……………There the cool morning breeze, fragrant from lotus blossoms on the Shipra River,,,,,,,,,,,, …………….Nurse the lotus flowers in the Manasa Lake with your water……………… ……………There to the north of Kubera’s estate is our house with a large rainbow like gate and a Mandar tree which is just like a……….

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Phoenicians,

HISTORY OF NAVIGATION •

• • • • • •

Vickings,

Irish Monks and Greeks

Phoenicians , Vikings and Greek were undertaking sea voyages and had navigation skills even 3000 years back. Phoenicians claimed to have circumnavigated Africa from Red sea, sailing via the Cape of Good 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 activities in Asia-Pacific region. Chinese, Arabs etc., had under taken lot of sea voyages. In Mohanjadaro ruins (Indian sub continent ) one clay tablet was found which 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 sea route (even he is supposed to have designed an aeroplane) and lived in China as Lao-tzu, spread Taosim. He is attributed to have great navigational skills.

were voyages

undertaking and

had

sea great

navigation skills even 3000 years back. claimed

Phoenicians 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 Indian’s 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

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 of the

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

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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 3 rd BC. In the recorded history – Megallan (1550 AD) went on his voyage to circumnavigate earth 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 Columbus

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

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 ship’s speed direction and approximate latitude but not longitude.

Compass

turyDead reckoning tools

nin itio s o P

cen 14 -16 g Traverse board rin

u g d

Astrolable

ed Spe

Cross-staff

Egyptian Groma

ing

n itio Pos

n ctio Dire

tury ude 0 cen t i t La g 17-2 in dur

Radar (Robert Watson – Watt -1935) 20th Century Radio Communication ( I & II World War)

g ngin o Ra

i Rad

Compass Rose

Sextant Chronometer George Harrison, 1764 A.D

...

GPS

TRANSIT

. VOR . de gitu adio Lon LORAN Based R ng d Lan ositioni P

ased ce B Spa tioning Posi

Earlier

navigator

how

could

their

latitude

determine but

longitude

and

They

hour

had

some not

speed. glass,

pendulum clocks and all primitive time measuring equipments.

Around

1500

Chinese

AD,

invented

compass

for

direction

finding,

but

them

even

earlier

to

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

7

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 0º longitude reference and Greenwich time to be the Mean time reference for all purposes. Later on in the year 1884 at 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 0º 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). LAND BASED LORAN -C (LONG RANGE NAVIGATION) • FIRST RADIO-POSITIONG MARITIME APPLICATIONS • SERVICE FROM COVERAGE

30

SYSTEM

CHAINS

FOR

FOR

system

WIDE

• RADIO-PULSE TRANSMISSION FROM MASTER AND SECONDARY STATIONS (OVER A GLOBAL NETWORK)

• TD IS USED WITH MAPS TO ESTIMATE LAT/LONG

Loran-C

• PHASE MEASUREMENTS IMPROVES PRECISION

x1,y1

• LORAN OPERATING RANGE : 90-110 KHZ

• REFLECTION BY IONOSPHERE

is

a

from

a

and

two

stations

are

is the position of the observer.

• POSITION DETERMINED BY INTERSECTION OF 2 LOPs

GROUND

slave

:

plotted and the point of intersection

• LOCUS OF POINTS HAVING THE SAME TD FROM A SPECIFIC MASTER-SECONDARY PAIR IS A CURVED LINE OF POSITION (LOP).

FROM

distance

station

synchronized

• RECIEVER GETS BOTH PULSES AND TIME DIFFERENCES (TD) FOR EACH PAIR OF MASTERSECONDARY STATIONS IS COMPUTED

• OBSTRUCTION/INTERFERENCE FEATURES

positioning

where

master

• PRINCIPLE OF RANGING FOR POSITION-FIX:

• LIMITED COVERAGE: ~1000km RANGE

Hyperbolic

x3,y3

x2,y2

x,y

is

an

example

of

this

system.

( x2 − x ) 2 + ( y2 − y ) 2 − ( x1 − x ) 2 + ( y1 − y ) 2 = d 21 ( x3 − x ) 2 + ( y3 − y ) 2 − ( x1 − x ) 2 + ( y1 − y ) 2 = d 31

• POSITION ACCURACY: ~460m (AT BEST)

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.

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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 Kepler’s 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.

SATELLITE NAVIGATION & POSITION SYSTEMS

GPS (1978) & GLONASS TIMATION Developed in 1972 by the Naval Research Laboratory (NRL), TIMATION satellites were intended to provide time and frequency transfer. The third satellite acted as a GPS technology demonstrator. 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.

SECOR (Sequential Collation of Range) SECOR was a U.S army satellite navigation and positioning system. Thirteen satellites were launched between 1964 and 1969. 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 by

the satellite. When man moves from one place to another 3D positioning (latitude, longitude & height) are required. SPUTNIK First artificial Satellite launched from Russia. Operated using Doppler frequency shift to obtain position.

This idea gave birth to TRANSIT system navigation concept. The

Transit

spacecraft provided inputs for analyzing Earth’s

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

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incoming PRN signal with a user-generated replica signal and measuring the received phase against the user’s (receiver) crystal clock.

With this concept the user’s latitude,

longitude, altitude and a correction to the user’s 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) – NAVSATAR was created . The moto of GPS – JPO was:

o Drop 5 bombs simultaneously in the same hole o

Build a cheap set that navigation (<$10000)

[Both the targets have since been met more than adequately] Parallely Soviet Union was also working on satellite based navigation and they launched TSYKLON & TSIKADA series of navigation satellites which finally culminated into GLONASS constellation. Both GPS & GLONASS constellations used onboard atomic clocks and PRN signals. GPS had two frequencies but different codes for all satellites (CDMA) while GLONASS

Navigation Satellites • •

• •

doppler

•

TRANSIT (US Navy satellite developed by John Hokins, Applied Physics Laboratory , 1960-1996). Based on doppler shift measurements of a 400 MHz tone. TIMATION ( TIMe/navigATION) Programme. - 2 satellites (1967 & 1969) also called NCST ( Navy Centre for Space Technology) satellites carried quartz oscillator which were regularly updated by master clocks. NAVSTAR ( NAVigation Satellite Timing and Ranging) of US Air force PROJECT621. used pseudorandom noise ranging signals. Under TIMATION Program two more satellites viz., NTS-I ( Navigation Technology Satellite) and NTS-II were launched in 1974 and 1977 and carried Rubidium and cesium Atomic clocks. In 1978, US Govt. Decided to bring all the above technologies under one head and made a joint program office under which umbrella, GPS satellites were developed and first block 1 satellites were launched during 1978-1985 and second block during 1989 to 1990. GPS constellation was completed by 1995.

&

codes

 ∂t +ρ , ρ = (x s −xu )2 +(y s −yu )2 +(z s −zu )2 ρ = ∫ρ 0

for

satellites

(FDMA). Both

the

constellation

worked well and were in full operational mode in 1995-98. European

Zenith Elevation Horizon

time

had separate frequencies

Meanwhile Union

also

announced its intention

of putting their own navigation satellite constellation GALILEO by 2012. Many other nations have also notified their intentions of having their own constellation or participating in GNSS.

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GPS, GLONASS & GALILEO - Configuration Constellation

GPS

GLONASS

GALILEO

Total Satellites Orbital Period Orbital planes Orbital height (km) Sat. In each plane Inclination Plane Separation Frequency

24+3 12 hrs 6 20200 4 55 deg 60 deg 1575.42MHz 1227.6MHz CDMA

24 (4 Opr) 11hrs 15min 3 19100 8 64.8 deg 120 deg 1246 - 1257 MHz 1602 - 1616 MHz FDMA

27+3 14Hrs 22min 3 23616 10 56 deg 120 deg 1164 - 1300 MHz 1559 - 1591 MHz CDMA

Modulation

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 it desires. 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 orbital inclination should be >50 deg. • The spacecraft design should be such SATELLITE CONSTELLATION that it should be autonomous to the DESIGN PARAMETER maximum extent and orbital ORBIT CHARACTERISTICS COMMUNICATION correction are rarely done, since it • ANTENNA takes almost 24 hours for spacecraft •ORBITAL HEIGHT >= 20,000 – ISO FLUX (MORE THAN EARTH DISC) KM to get stabilized for navigation • FREQUENCY - L BAND –LONGER VISIBILITY θ – MINIMUM BACKGROUND THERMAL purposes π µ NOISE –ORBITAL PERIOD – MINIMUM PATHLOSS • The spacecraft dimensions should be •PERTURBATIONS(MINIMUM) – MINIMAL IONOSPHERIC GROUP DELAY such that solar radiation pressure – MINIMAL ATTENUATION –SOLAR RADIATION PRESSURE (Impacts eccentricity) and Luni/solar (IMPACTS ECCENTRICITY) forces (Impacts – inclination) effects –LUNI SOLAR FORCES (IMPACTS are minimum INCLINATION) • For efficient launch considerations and optimum in orbit spare policy, the total constellation should have atleast three planes SATELLITE CONSTELLATION DESIGN PARAMETER 1. ORBIT CHARACTERISTICS • (GPS-6, GLONASS-3, GALILEO-3) 2. COMMUNICATION Re ) Re + alt 2 , n= 3 n a

= cos −1(

T=

•ORBITAL HEIGHT >= 20,000 KM –LONGER VISIBILITY –ORBITAL PERIOD

θ = cos −1(

• ANTENNA

Re ) Re + alt

2π µ T= , n= n a3

– MINIMUM BACKGROUND THERMAL NOISE

•PERTURBATIONS(MINIMUM)

– MINIMUM PATHLOSS

–SOLAR RADIATION PRESSURE (IMPACTS ECCENTRICITY) –LUNI SOLAR FORCES (IMPACTS INCLINATION)

– ISO FLUX (MORE THAN EARTH DISC)

• FREQUENCY - L BAND

•

The spacecraft payload should be based on atomic clocks (min. of

– MINIMAL IONOSPHERIC GROUP DELAY – MINIMAL ATTENUATION

• MODULATION - CDMA/FDMA

•PLANES –LAUNCH CONSIDERATIONS –SPARE REQUIREMENT

•INCLINATION –GLOBAL/HIGH LATITUDE COVERAGE

– MODULATION OF BPSK & SPREAD SPECTRUM – CDMA- SINGLE FREQUENCY FOR MULTIPLE SATELLITE DOWNLINK – FDMA- MULTIPLE FREQUENCY JAMMING DIFFICULT

13

•

three with two in hot redundancy) & antenna should be iso- flux (slightly more than earth’s disc coverage) Frequency is L-band to take advantage of minimum background thermal noise and lesser path loss.

The present day navigation satellites work on “Time of

REALTIME POSITION FIXING USING SATELLITES •

of

satellites

whose

navigation position

and orbital parameters are known to a great accuracy and equipped with onboard atomic clocks. The receiver

ρ4

• 1-WAY RANGING

Arrival” parameter from a number

(XI,YI,ZI)

REAL-TIME 3D POSITION FIXING: • ATOMIC CLOCK FOR PRECISE RANGING

Ionospheric delay

•

WORLD-WIDE TIME SYNCHRONISATION

•

2-FREQUENCY FOR IONOSPHERIC CORRECTIONS

•

SIMPLE USER-END EQUIPMENT

•

ACCURACY: FEW METRES

SOURCES OF ERROR System Noise ~ 2m Ephemeris ~ 5m Satellite clock ~ 1m Receiver clock ~ 2m Multi-path ~ 0.5m Troposphere delay ~ 1m Ionosphere delay ~10m

ρ3

ρ2

Tropospheric delay

• MIN OF 4 SATELLITES VISIBLE ANYTIME

ρ1

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

P1 =

( X − X 1 ) 2 + (Y −Y1 ) 2 + ( Z − Z1 ) 2 + b

P2 =

( X − X 2 ) 2 + (Y −Y2 ) 2 + ( Z − Z 2 ) 2 + b

P3 =

( X − X 3 ) 2 + (Y − Y3 ) 2 + ( Z − Z 3 ) 2 + b

P4 =

( X − X 4 ) 2 + (Y −Y4 ) 2 + ( Z − Z 4 ) 2 + b

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 minimum geometrical 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.

ELEMENTS OF A SATELLITE POSITION FIXING • MEASUREMENT (UHF, S-BAND, LASER) • MODELLING (Geo-Potential, Drag, SRP, Luni-Solar) • ESTIMATION (Least-Square, Kalman filter)

n n ∞ ∞ n R   Re  μ   P V = − [1− ∑ J  e  P (sin φ) + ∑ (sinφsinφ) (λ − λ )] ∑ J  n n nm  r  nm nm r n =2  r  n = 2m = 1  

1 C A Pdrag = − ρ ( d )vr vr 2 m

Pdrag = −P (1 +η)

A ) vu m LEO

di 2 3 = ∑ y j [cos i sin 2i j sin(Ω − Ω j ) + sin i sin 2 i j sin 2(Ω − Ω j )] dt j =1 8

(m/s2) Atm drag SRP Sun Moon

6*10-5 4.7*10-6 5.6*10-7 1.2*10-6

14

DILUTION OF PRECISION AND IMPACT ON POSITION ACCURACY • POSITION ERROR IS A FUNCTION OF: • DILUTION OF PRECISION

ρ2

ρ1

• MEASUREMENT ACCURACY

ρ1

DOP =

D O P w here,

α 1/volume

ρ2

MEASUREMENT ERROR

−1  AT A       xi − xu yi − yu zi − zu  A =  1 ρi ρi ρi  i =1,4 Trace

ρ =R ange

Satellite Based Positioning System Satellite Positioning System mainly consists of three segments:

1.

Space Segment – A constellation of orbiting

SATELLITE POSITIONING SYSTEM SEGMENTS

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

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

GPS, GLONASS & GALILEO - Configuration Constellation

GPS

GLONASS

GALILEO

Total Satellites Orbital Period Orbital planes Orbital height (km) Sat. In each plane Inclination Plane Separation Frequency

24+3 12 hrs 6 20200 4 55 deg 60 deg 1575.42MHz 1227.6MHz

24 (4 Opr) 11hrs 15min 3 19100 8 64.8 deg 120 deg 1246 - 1257 MHz 1602 - 1616 MHz

27+3 14Hrs 22min 3 23616 10 56 deg 120 deg 1164 - 1300 MHz 1559 - 1591 MHz

Modulation

CDMA

FDMA

CDMA

They

have

spacecraft

put

in

experiments.

orbit

two for

The whole

constellation is likely to be completed by 20122013.

Meanwhile

Russians efforts

are to

GLONASS

making

make

the

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 GLONAS modernization. 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

5030.000 MHz

GALILEO

5010.000 MHz

1626.500 MHz

1620.610 MHz

1610.000 MHz

GLONASS

1592.952 MHz

1587.420 MHz

1563.420 MHz

1300.000 MHz

GPS 1559.000 MHz

GALILEO

1261.610 MHz

GLONASS

1239.600 MHz

1237.827 MHz

1215.600 MHz

GPS 1215.000 MHz

1212.000 MHz

GALILEO

1188.000 MHz

1164.000 MHz

GPS

1260.000 MHz

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

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

with

LIMITATIONS OF GPS AND GLONASS • GPS stand alone, cannot satisfy the integrity, accuracy & 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 times. • Time to alarm could be from minutes to hours and there is no indication of quality of service. • Accuracy is not sufficient even with S/A off, the vertical accuracy for 95% of the time is >10m. • For GPS & GLONASS stand alone systems availability & continuity are not assured. • All these calls for a special system addressing all the above, which could be done by augmenting the GNSS systems.

S/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

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 position REQUIREMENT OF ENHANCEMENT OF ACCURACY, AVAILABILITY AND INTEGRITY

estimates derived from

• 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”.

within specifications –

– 30Sec – 6 Sec

En-route APV II (Approach with Vertical Guidance)

it can be trusted to be This is the so called integrity Timely

requirement. 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

AUGMENTATION OF GPS / GLONASS LIMITATIONS OF GPS: • SIGNAL NOT AVAILABLE INSIDE TUNNEL & WATER • NO ASSURANCE OF AVAILABILITY AND INTEGRITY OF DATA – CRITICAL FOR AVIATION APPLICATIONS – 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)

GPS Wide Area Augmentation Systems C-WAAS WAAS

*

EGNOS

*

* *

MSAS

GAGAN

(* INTENDED SYSTEMS)

US WIDE AREA AUGMENTATION SYSTEM OF GPS –

The US & European Systems are the forerunners

WAAS

consisting of a large number of reference, uplink and Master Control Centres

• 24 Wide Area Reference Stations • 2 Wide Area Master Stations • 2 Navigation Land Uplink Stations • 2 GEOs –AOR & POR

with each one

having 2 GEO spacecraft.

FAA presentation to ISRO

20

EUROPEAN GEOSTATIONARY NAVIGATION OVERLAY SERVICE - EGNOS • 34 Range Integrity Monitoring Stations – Rims • 4 Master Control Stations • 2 Navigation Land Uplink Stations • 2 GEOs – INMARSAT AOR E & IOR and presently working on ARTEMIS 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

Japanese S-BAS System (MSAS) GPS Constellation ellation

MTSAT

Sapporo GMS

NTT 64Kbs User Kobe MCS Tokyo GMS

Fukuoka GMS

Ibaraki MCS

KDD 128Kbs Hawaii MRS MCS Master Control Station MRS Monitor and Ranging Station Australia MRS

Naha GMS

GMS Ground Monitor Station

MSAS is the Wide area Augmentation System of Japan like WAAS and is based on MTSAT. ICG 2007, Bangalore meet

QZSS Navigation System Navigation System Architecture Navigation Signals L1: 1575.42 MHz L2: 1227.60 MHz L5: 1176.45 MHz LEX: 1278.75 MHz

QZSS

WDGPS Correction Message, LEX NAV

TWSTFT Up: 14.43453GHz Down: 12.30669GHz

L1-SAIF: 1575.42 MHz LEX: 1278.75 MHz

Laser Ranging

TT&C, NAV Message Upload**

SLR Site

Monitor Station NW

Time Management Station

TT&C・NAV Message Uplink Station

User Receiver Master Control Station MCS)

Geonet GSI

Function distributed in each institute Timing management, WDGPS correction, etc.

**: Under trade-off study between S (Up: 2025-2110, Down: 2200-2290MHz) and C (Up:5000-5010, Down:5010-5030MHz) band SLR: Satellite Laser Ranging, TWSTFT: Two Way Satellite Time and Frequency Transfer

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

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. NATIONAL SPACE SYSTEMS

India has presently six components of

GNSS LAUNCH VEHICLES

GAGAN

INSAT IRS

IRNSS

Applications

its space programme.

Science

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

INDIAN AIRSPACE TO BE SERVICED C BAND L1 & L5

GPS L1 & L2

40 GEO

GPS L1 & L2

35 30

GPS Nav Data

GEO D/L in L1

GPS Nav Data

25

GEO D/L in L1

20 15

GAGAN USER

GPS and GEO Broadcast Messages

INRES

INRES GPS & GEO data D/L in C and L

GPS & GEO data

U/L in C

INMCC INSAT Coverage 83 Deg.

GAGAN COVERAGE THROUGH INSAT

BANGALORE

Correction Messages

INLUS

BANGALORE

INDIAN S-BAS PROGRAM – GAGAN GPS AIDED GEO AUGMENTED NAVIGATION

10 5 0

23

­5 ­10 40

50

60

70

80

90

100

110

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). However the 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.

Preliminary System Acceptance Test Results PSAT Exit Criteria – Position Accuracy better than 7.5 Meters

Achieved position accuracy in North, East and Up directions is better than the Exit Criteria

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

25

GGTA

26

India falls on the magnetic equator & under the equatorial ionosphere whose behaviour is quite unpredicted. Ionosphere correction and modeling are two important tasks for any satellite based augmentation system. POSITION OF MAGNETIC EQUATOR AND SCINTILLATION REGIONS

To study this phenomena ISRO has established

almost

Electron

Content

28

Total (TEC)

Monitoring Stations, around the country

and

taken

up

studies in a big way.

iono The

models & results will be used for GAGAN • INDIAN REGION EXPERIENCES UNPREDICTABLE IONOSPHERIC DISTURBANCES • SUCCESS OF GAGAN IS DEPENDANT ON THE STUDY AND MODEL THE IONOSPHERE OVER THE REGION.

&

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

340

1320 1110

550 830

IRNSS INDIAN REGIONAL NAVIGATIONAL SATELLITE SYSTEM

28

IRNSS Architecture •

Space Segment – – – – –

•

Ground Segment – – – – –

•

Seven satellite configuration, 3 SVs in Geo-Stationary orbit ( 34°, 83° and 132° East), 4 SVs are in GEO Synchronous orbit placed at inclination of 29 ° (with Longitude crossing at 55° and 111° East) The configuration takes care of continuity of service with a failure of one satellite. The satellites are of 1 ton class with navigation payload of 102 Kgs and power consumption of 676 Watts . There will be two downlinks (L and S bands) providing dual frequency operation with EIRP of 31.5 dBW at EOC. The payload will have 3 Rubidium clocks. Master Control Center IRNSS Ranging & Integrity Monitoring stations (IRIM) IRNSS Telemetry and Command stations Navigation Control Centre IRNSS Network Timing Centre

User Segment Planned operationalization by 2011-2012

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.

IRNSS Coverage Area HDOP & VDOP (99%) for the Proposed Constellation GEO 34,83,132 GSO 55(55,235), 111(111,291) User Mask Angle 5deg

29

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.

Assisted GNSS Applications •

NAVIGATION – – – –

•

SPACECRAFT AIRCRAFT SHIP VEHICLE

GEOGRAPHIC DATA COLLECTION – MAPPING – SURVEYING – ENGINEERING

•

•

SCIENTIFIC RESEARCH – ATMOSPHERIC STUDIES

•

GEODYNAMICS

•

MILITARY

– CRUSTAL MOVEMENTS – CRUSTAL DEFORMATIONS

– – – – –

GIS INGEST FOREST MENSURATION TOWN PLANNING FLEET MOVEMENT ROUTING/ALIGNMENT

•

MONITORING THE HEALTH OF TALL BUILDINGS/TOWERS, LONG BRIDGES

• •

Power grid s ynchronization AGRICULTURE

•

EMERGENCY RESPONSE

– PRECISION FARMING – SEARCH AND RESCUE

•

Satellite positioning systems (GPS, Galileo, GLONASS)

NATURAL RESOURCE AND LAND MANAGEMENT

BUSINESS SOLUTIONS – LOCATION BASED SERVICES • MOBILE • TOURISM • RETAILING/Banking

Mobile communications signaling network(s)

Location server(s)

ASSISTED-GNSS POSITIONING ALGORITHM

Fixed telecommunications network nodes with short-range wireless data communications equipment (Bluetooth, WLAN)

Satellite positioning augmentation systems (EGNOS, WAAS)

Inertial navigation sensors (implemented into the rover itself: accelerometer, barometer)

Terrestrial positioning systems (LORAN C)

30

AREAS OF RESEARCH & DEVELOPMENT IN POSITIONING AND TIMING SYSTEM (GNSS) TECHNOLOGY SCIENCE • IONO-TROPO MODELLING IN THE EQUATORIAL REGION IN LBAND • RADIO OCCULTATION STUDIES FOR NEAR EARTH ATMOSPHERIC TEMPERATURE PROFILE • REAL-TIME WEATHER FORECASTING

• 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.

Issues related with GNSS • Interoperability refers to the ability of open global and regional satellite navigation and timing services to be used together to provide better capabilities at the user level than would be achieved by relying solely on one service or signal. • Compatibility refers to the ability of space-based positioning, navigation, and timing services to be used separately or together without interfering with each 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 (Each Constellation as of today has adopted different time and geodetic reference frames)

31

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 other technological 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, DirectorISAC, 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).

32

33