Prof. Samirsinh P ParmarAsst. Prof. Dharmasinh Desai University, Nadiad, Gujarat, INDIAMail: samirddu@gmail.com

PROVISION OF HORIZONTAL CONTROL IN DIGITAL ERA

2REFERENCE ELLIPSOID

All the measurements are done on the surface of the Earth. Various computations are required to determine the coordinates, distances, area etc.

The surface of the earth is irregular and therefore unsuitable for such computations. We need a smooth mathematical surface for these computations.

The best mathematical figure that can represent the earth is an ellipsoid. Ellipsoid is used as reference surface.

3TYPES OF ELLIPSOIDS

1. Best-Fit Ellipsoid/Local Ellipsoid: based on the measurements within a region, so it best fits that region only. The centre of such reference ellipsoid does not coincide with the centre of gravity of the Earth.

Example : Everest Ellipsoid

2. Geocentric Ellipsoid: The centre of geocentric ellipsoid coincides with the centre of the Earth. This type of the ellipsoid can be used worldwide.

Example : WGS 84 ellipsoid

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Best Fit and Geocentric Ellipsoids

Centre of Best Fit Ellipsoid

Centre of Geocentric Ellipsoid

Best Fit Ellipsoid

Geocentric Ellipsoid

Earth Surface

Best Fit Ellipsoid

5 EVEREST ELLIPSOID

Everest Ellipsoid evolved from the measurement of the ‘Great Arc’ from Cape-Comorin to foothills of Himalaya at Mussoorie.

Geometrical parameters: - Semi major axis a = 6377301.243 m Semi minor axis b = 6356100.231 m Flattening (a-b)/a = f = 1/300.8017 Eccentricity (a2-b2)/a2 = 0.00663784607

6GEODETIC REFERENCE SYSTEM 1980(GRS 80)

The IUGG in December 1980 adopted a new model of the Earth called the Geodetic Reference System (GRS) 80. It replaced the GRS 67.

The ellipsoid defined by GRS 80 is geocentric.

The rotation axis of reference ellipsoid has the direction of Conventional International Origin for polar motion.

Geometrical parameters of GRS 80 : -• Semi major axis a = 6378137 m• Semi minor axis b = 6356752.314 m

7 WORLD GEODETIC SYSTEM 84(WGS 84)

U.S. military efforts produced a new System called World Geodetic System 84 or simply WGS 84.

It is the reference system used by the Global Positioning System.

8 WGS 84 contd.

The geometrical parameters of WGS84 Ellipsoid are: - Semi major axis a = 6378137 m Semi minor axis b = 6356752.3142 m Flattening (a-b)/a = f = 1/298.2572 Eccentricity (a2-b2)/a2 = 0.00669437999013

9IGS (International GNSS Service)

In 1991, International Association of Geodesy (IAG) decided to establish International GPS Service for Geodynamics (IGS) to promote and support activities such as maintenance of permanent network of GPS tracking station and continuous computational satellite ephemeredes and ground station coordinates.

The name was changed to International GPS Service. Now it is called the International GNSS Service (IGS). It is a voluntary federation of more than 200 worldwide

agencies that pool resources and permanent GPS & GLONASS station data to generate precise GPS & GLONASS products.

10 IGS contd.

IGS provides highest quality data in support of Earth science research, multidisciplinary applications, and education.

Currently the IGS includes GPS and the Russian GLONASS, and intends to incorporate future GNSS like Galilio.

It can be thought of as the highest-precision international civilian GPS community.

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12International Terrestrial Reference Frame( ITRF )

The coordinates of IGS tracking stations are expressed and periodically re-determined in the reference system known as International Terrestrial Reference System (ITRS).

The realization of ITRS is International Terrestrial Reference Frame (ITRF).

The ITRF is produced by the International Earth Rotation Service (IERS) based in Paris.

It includes more than 500 stations at 290 sites all over the world.

13 ITRF contd. Four different space positioning methods contribute to the ITRF:

Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), GPS Doppler Ranging Integrated on Satellite (DORIS).

Each has strengths and weaknesses - their combination produces a strong multi-purpose TRF.

ITRF was created by the civil GPS community, quite independently of the US military organizations.

14 ITRF contd.

Each version of the ITRF is given a year code to identify it like ITRFyy. The current version is ITRF2000.

It is simply a list of coordinates (X Y and Z in meters) and velocities (dX, dY and dZ in meters per year) of each station in the TRF.

The coordinates relate to the time yy. To obtain the coordinates of a station at any other time, the station velocity is applied appropriately. This is to cope with the effects of tectonic plate motion.

15ITRF contd.

The ITRF and WGS84 are very similar and the difference between them is within centimeter level worldwide.

ITRF is the most accurate global TRF ever constructed.

We can use IGS stations as reference points of known coordinates to precisely coordinate our own GPS stations, using GPS data downloaded from the Internet. This procedure is known as ‘fiducial GPS analysis’.

We can also obtain precise satellite positions (known as ephemeredes) in the ITRF which are not subject to the deliberate degradation of selective availability.

Both these vital geodetic services are provided free by the International GPS Service on the Internet.

16 Switching Over to Geocentric Coordinate System

Transformation Parameters Accuracy Expected

GCP Library

17Ionosphere

18VLBI

VLBI – Very Long Baseline Interferometry A geodetic space technique used to determine

very precise distances between radio telescopes Initially developed to perform the detailed study

of compact radio sources Used to study the geodetic, astronomic and

geodynamic parameters of Earth It can determine distance between radio

telescopes to within a millimeter across an entire continent.

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NON REAL-TIMECORRELATER

TAPES

D = τ c

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2

ψ B

RADIO WAVES FROM DISTANT QUASAR

Basic Geometry of VLBI

TIMING &RECORDING TIMING &

RECORDING

TAPES

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VLBI ANTENNAS OF THE WORLD

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