Satellite Tracking System

Satellite Tracking System

ABSTRACT Tracking

of

satellite provides the

most

vital

information about its position and its orbit in space. Tracking is very much essential not only during its launch for initial critical maneuvers but even after launch to maintain it assigned slot in space. An effort has been made in

this

paper

to

present

various

basic

techniques

of satellite

tracking and their merits. A brief description of precision tracking system has been also presented.

V.Y.W.S. Polytechnic, Badnera.

Satellite Tracking System

1.0 INTRODUCTION When satellite is launched from the launch pad, the most important requirement is to find out the exact position of the satellite in its orbit. For that it is essential that it is tracked. Tracking is the process of determining the location, radial velocity, direction of motion of satellite. Also the tracking is not only important during

launching

but even after

launch to maintain it is assigned slot in space. Actually tracking involve the collection of following data about the satellite i)

Range of a satellite

ii)

Azimuth angle

iii)

Elevation angle

iv)

Velocity of satellite This data is collected continuously

the processed data, the position of satellite

can

& processed from

be determined

and

its

future position can be predicted with some accuracy. Satellite Tracking helps in i)

Pin pointing the position of satellite in space.

ii)

Pointing the telecommand (TC) and Telemetry (TM) antenna in the direction of satellite for commanding the space-craft or receiving the signal from it.

iii)

Determining the instants for orbit corrections where thrusters can be fired for orbit corrections.

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Satellite Tracking System 1.1 NECESSITY OF TRACKING :IMPORTANCE OF SATELLITE ORBITS :The orbit of a satellite used for communication purpose has special significance. Such satellite are called gee-stationary and these have to maintained

geo-stationary at

all

the

costs.

The

two

main

problems with the communication satellite regarding their orbit are as -

i)

Launching and putting the satellite in desired geo-stationary orbit.

ii)

To maintain it in desired slot in that orbit which is known as ‘Station Keeping’. Once the satellite is launched in the geostationary orbit,

it has to be parked there till its whole life and to maintain it in assigned clot, the tracking of satellite is necessary. There are several standard techniques for keeping the satellite at exact location in its orbit. This avoids the unnecessary losses of information transmitted by satellite. That is why the tracking of a satellite is very much important. Actually tracking of satellite starts much before it is launched into space. At launch pad the rocket is first tracked by the radar. When the rocket is fired, radar keeps it tracking and once the satellite is separated from the rocket, the key systems on the satellite is turns on. Then radio-tracking of a satellite starts using its own downlink signals. All the decisions such as, to open the solar panels towards the sun, pointing the V.Y.W.S. Polytechnic, Badnera.

Satellite Tracking System satellite antenna towards the earth, even of firing the thrusters for orbit adjustment, these all decisions depends upon available tracking of satellite is required to maintain it in desired orbit. Because there may change in the orbit due to disbalancing of a satellite which can be caused by several factors such as gravitation force from the sun, moon and planets, solar pressure acting as antennas and earth's magnetic field etc. Due to this reason while working with the communication satellite earth station antenna, must be pointed toward satellite within same accuracy. Otherwise it would have result in the antenna pointing loss and we will not be able to get maximum signal from the satellite. For that some antenna tracking systems are used. For tracking of geostationary satellite within accuracy of ±0.1 0 , in its longitudinal slot certain corrections in it location against the different variations in the orbit are carried out. These corrections causes slight change in look angles of the ground station antennas. Hence antenna has to be properly monitored for the maximum signal. These are several methods, available for tracking the antenna of ground station. e.g.

i) Step-tracking ii) Monopulse tracking

But for satellite tracking, there are two basic techniques These are

i) Radio tracking system ii) Optical tracking system

V.Y.W.S. Polytechnic, Badnera.

Satellite Tracking System

2.0 RADIO TRACKING SYSTEM The

radio-tracking

system

uses

the

in-formation

contained in the radio signal transmitted by satellite itself to ground station. Basically radio-tracking system is used to measure the range, range-rate angular measurements of the satellite. This system basically consists of a transmitter system, a receiving system and a tracking signal processor, 2.1 RANGE MEASUREMENT :The principle of range measurement depends on the measurement of delay of time between the transmitted wave and its reception at particular instant. There are two ways of Range measurement. i) One Way Ranging :In one way ranging system, the time delay of a radio signal transmitted from space-craft is measured at ground

station. The

time of transmission should be accurately known. ii) Two Way Ranging :The round

trip

time

delay

of

transmitted from the ground station to a satellite and station.

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e

radio

wave

back to the ground

Satellite Tracking System 2.1.1 One Way Ranging :In one way ranging a satellite carries a radio source or a beacon, through which ranging signal is transmitted to the ground station. For one way ranging the down link frequency and the time at

which

the signal

is

transmitted

from

the satellite

must be

accurately known. Then the time required to reach this signal at ground station is measured. From this the range of a satellite can be calculated as follows R=C\ Where

R = Range in Km C = Velocity of radiowave in free space \ = Time delay suffered by radio signal

2.1.2 Two Way Ranging :In this system, the ranging signal transmitted by the ground station to the satellite. The signal may be processed or may not be processed on the satellite. After that it is transmitted by the satellite to earth station. The round time trip time delay of ranging signal is measured. The angle of satellite, then given asR = C \ /2 Where C, \ & R have the usual notations as before.

V.Y.W.S. Polytechnic, Badnera.

Satellite Tracking System 2.2

RANGE RATE MEASUREMENT :For measuring the range rate of a satellite radio-tracking

system

utilizes

the

significance

of

‘Doppler Effect’. When a radio

source moves with respect to observer its frequency appears to change as it approaches or moves away from it. This is trailed as ‘Doppler effect’ and this apparent

change in frequency is called

as

‘Doppler frequency’.

The doppler or range rate depends upon radial velocity and orbit of the satellite with respect to ground station. The doppler frequency f d is given asfd = fR – fT = V r f T /C Where

f d = Doppler frequency

f R = Frequency of

received signal

f T = Frequency of

radio source

V r = Radial velocity of the satellite with respect to ground station. The doppler frequency can be found out by counting the frequency of satellite signal between the two instants in the desired interval of the orbit. Thus the radial velocity of satellite is computed by using the doppler frequency and position of satellite is fixed.

V.Y.W.S. Polytechnic, Badnera.

Satellite Tracking System 2.3 ANGLE MEASUREMENT :For angle measurement two receiving system. A and B are used which are separated by some known distance which is multiple of signal wavelength. At both the -signal from the

satellite

system the phase difference at a radio

is measured. Due to different path lengths

travelled by radio wave the phase measured at antenna A would be different with respect

to

reference antenna B. This phase

difference

is used to

compute the angle α as follows α = cos -1 [φ \ / 2π] Where φ = Phase difference due to extra path travelled by signal with respect to station B. \ = Signal wavelength

Signal Amplitude t

t

This measurement provide the information about the angle of arrival of the signal i.e. elevation angle but they don't tell in which direction to look for the rising satellite. To obtain this direction another V.Y.W.S. Polytechnic, Badnera.

Satellite Tracking System set of two receiving systems are used at angles to the line joining the A and B. Thus two direction cosine will be known which would provide third direction cosine. Using them angular

position of the satellite can be

determined. 2.3.1 MONOPULSE RADAR :The monopulse radar provides azimuth and elevation position of the satellite in the space. These angular values are fed to the ground station antenna servomotor so that it tracks closely the movement of satellite in space. The system consists of two pairs of feed points located on the antenna. One pair is in the horizontal plane and other pair is in vertical plane placed symmetrically from the central axis of antenna. The

horizontal

and vertical

feeds provides azimuth and

elevation

information respectively. If the target is exactly located on the beam axis then the signal received at all the four feed is came. But target located to one side of beam axis produced strong signal in one feed than other. The difference in the signal strength measured in horizontal feeds provides azimuth information while that measured between vertical feeds provides the

elevation information. The some of

the

two signals provides net

strength of the target and is used as a reference. These error voltage direct the movement of ground station antenna in the direction of satellite signal.

V.Y.W.S. Polytechnic, Badnera.

Satellite Tracking System

3.0 STEP TRACKING SYSTEM As the earth station antenna is subjected to wind loading effects and there is a drift in the orbit of the satellite as mentioned before, an antenna tracking system is necessary for large diameter antenna to minimize

the pointing error which is also called as antenna pointing

loss. As shown in Fig. the maximum gain of a antenna can be achieved if the antenna beam is pointed accurately towards the satellite. A loss in gain can occur, if the antenna pointing vector is not in line with satellite position vector. To avoid such a loss antenna tracking system is used. Basically antenna tracking system is close loop pointing system i.e. antenna pointing vector which is the function of azimuth and elevation angles is

derived

from received

signal.

In step tracking

the

antenna pointing vector is derived from the signal strength of a satellite beacon signal.

3.1 PRINCIPLE OF STEP TRACKING :Generally all the tracking system works in two modes i) Search mode ii) Track mode

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Satellite Tracking System i ) Search Mode :At first the approximate satellite

position

is fixed

with the azimuth, elevation data and antenna is move to four positions of the satellite as shown in Fig.

elevation SA

B SA

SB ∆EL(offset in elevation)

0

SD D

SC C

∆AZ

offset in AZimuth Let these positions are A, B, C and D reap. The average signal strength is measured at these four points by taking several samples. For scanning, the antenna may rotated in helical -fashion, that is antenna must be rotated simultaneously in horizontal and vertical plane. This way antenna

covers

a whole vertical range once

in

every

rotations. Thus the system determines the average signal points. Let these be S A , S B , S C , S D respectively.

V.Y.W.S. Polytechnic, Badnera.

'n' horizontal at

these -four

Satellite Tracking System Thus

in

search mode small range of

azimuth

and

elevation angle is determined in which the target can be found out. The exact position is not determined. ii) Track Mode :In

the

above

Fig. the

centre

of

square

ABCD

corresponds to assumed satellite position. Thus if the satellite position vector is aligned with antenna signal received at each

of four

pointing vector, points A,

the amplitude of beacon

B, C and D will

be

equal.

Therefore by comparing the average amplitude of A and B with that of C and

D, the difference in elevation error can be found

out. Similarly by

comparing the average amplitude of A and D with that of B and C, the azimuth angle error can be determined. If the value of

( S B + S C ) > (S A + S D ) and

( S A + S B ) > (S C + S D ) then system commands the antenna to move in one direction. But if the value of ( S B + S C ) > (S A + S D ) and ( S A + S B ) > (S C + S D ) then system commands the antenna to move in another direction and accordingly origin

is selected. Before each shift in

origin a check is made to see whether the real origin is approached or not. This check can be made by comparing ( S B + S C ) -  (S A + S D ) and  ( S A + S B )  -  (S C + S D )  with some threshold signal level called as s. This threshold is selected as low as possible. When the above difference in V.Y.W.S. Polytechnic, Badnera.

Satellite Tracking System some average levels of signals becomes less than s, no further correction is done

on the origin and precision step track is said

Then antenna is commanded to move to

be completed.

the corrected origin and it is

locked to that position till next correction is initiated.

V.Y.W.S. Polytechnic, Badnera.

to

Satellite Tracking System

4.0 OTHER TRACKING SYSTEMS In many studies or application, the position of satellite is required to the known very accurately. In such cases other tracking systems such as i)

Optical tracking system

ii)

Laser tracking system

are used. 4.1 OPTICAL TRACKING SYSTEM :The position of a satellite can also be determined by tracking it optically. In this tracking system special type of cameras are used to take the photograph spread

all over

the

of satellite. There are several optical stations

world. These stations are used with Baker-Nunn-

Cameras. These cameras have field view of 30 0 along the track of satellite and 5 0 in the direction at right angle to it.

It has special mirror 30 inches in

diameter with some special mounting so that camera can be pointed in any desired location. Approximate position of satellite must be known so that camera may be pointed in that direction.

The satellite track is

photographed in the different segments against the background of fixed stars using a rotating shutter which allows the light from the space-craft at regular intervals. A series of exposures of the moving target is taken on the same plate. From the photograph taken at different stations a satellite path and its

angle

are computed

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with

respect to known stars

in

Satellite Tracking System background.

The photographs are taken at dawn and dusk, when the

satellite is in illuminated condition with sun’s ray just below the horizon and the background stars are visible. 4.2 LASER TRACKING :Laser can also be used for tracking of satellite. The laser beam is highly directional and narrow. The spreading of beam is very less as compare to radio-wave. The laser beam is flashed to satellite at certain fixed

rate with

reflector

the

help

of telescope, A number

are attached to the body of satellite. The

of

small

angular

laser beam projected

towards satellite will be reflected back from these reflector to the ground station while some gets scattered in other directions. At ground station the

time between transmission of signal and upto that it is

received is

measured and from this the range of satellite can be .determined. As most of the incident energy is reflected back from the satellite, a transponder or repeater is not required. Optical tracking with laser beam is much more accurate

than radio-tracking system. The major drawback

system is that they depends on light and weather conditions.

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of this

Satellite Tracking System

5.0 CONCLUSION Tracking of satellite is very essential requirement, during the launching of satellite as well as during its working period. For satellite tracking mainly the radio-tracking systems are used. But for specific missions where more accurate orbit determination is required, a laser or optical tracking systems are used. For such a missions precision tracking system can also be used.

V.Y.W.S. Polytechnic, Badnera.

Satellite Tracking System

6.0 REFERENCES

1] Ha T. T.

"Digital Satellite Communication", Mac-millan Publishing Corporation, New York.

2] Dr. D. C. Agrawal,

"Satellite Communication",

Khanna Publishers. 3] S. N. Prasad S. Pal

"Satellite Tracking System", IETE Students Journal, Vol.-36.

4] K. A. Krishnamurthy, "A Microprocessor Based Precision Step Tracking System", IETE Technical Review, Vol.-II

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