satellite tracking

1. Earth Station Technology
 The earth segment of a satellite communications system
consists of the transmit and receive earth stations.
 The simplest of these are the home TV receive-only (TVRO)
systems, and the most complex are the terminal stations used for
international communications networks. Also included in the
earth segment are those stations which are on ships at sea, and
commercial and military land and aeronautical mobile stations.
As mentioned in earth stations that are used for logistic sup-
port of satellites, such as providing the telemetry, tracking, and
command (TT&C) functions, are considered as part of the space
segment.

2. Transmitter and Receiver
Any earth station consists of four major subsystems
• Transmitter
• Receiver
• Antenna
• Tracking equipment
 Two other important subsystems are
• Terrestrial interface equipment
• Power supply

3.  The earth station depends on the following
parameters
• Transmitter power
• Choice of frequency
• Gain of antenna
• Antenna efficiency
• Antenna pointing accuracy
• Noise temperature
• Local conditions such as wind, weather etc,
Polarization,Propagation losses

4. The functional elements of a basic digital earth
station are shown in the below figure

5. Digital information in the form of binary digits from terrestrial
networks enters earth station and is then processed (filtered,
multiplexed, formatted etc.) by the base band equipment.
• The encoder performs error correction coding to reduce the
error rate, by introducing extra digits into digital stream generated
by the base band equipment. The extra digits carry information.
• In satellite communication, I.F carrier frequency is chosen at
70 MHz for communication using a 36 MHz transponder
bandwidth and at 140 MHz for a transponder bandwidth of 54 or
72 MHz.
• On the receive side, the earth station antenna receives the low-
level modulated R.F carrier in the downlink frequency spectrum.

6. • The low noise amplifier (LNA) is used to amplify the
weak received signals and improve the signal to Noise
ratio (SNR). The error rate requirements can be met more
easily.
• R.F is to be reconverted to I.F at 70 or 140 MHz
because it is easier design a demodulation to work at these
frequencies than 4 or 12 GHz.
• The demodulator estimate which of the possible
symbols was transmitted based on observation of the
received if carrier.

7. • The decoder performs a function opposite that of the
encoder. Because the sequence of symbols recovered by the
demodulator may contain errors, the decoder must use the
uniqueness of the redundant digits introduced by the encoder
to correct the errors and recover information-bearing digits.
• The information stream is fed to the base-band equipment
for processing for delivery to the terrestrial network.
• The tracking equipments track the satellite and align the
beam towards it to facilitate communication.

8. EARTH STATION TRACKING SYSTEM
Tracking is essential when the satellite drift, as seen by an earth
station antenna is a significant fraction of an earth station’s
antenna beam width.
An earth station’s tracking system is required to perform some
of the functions such as
i) Satellite acquisition
ii) Automatic tracking
iii) Manual tracking
iv) iv)Program tracking

9. i) Satellite acquisition
Before communication can be established it is necessary to acquire a
satellite. One method is to program the antenna to perform a scan
around the predicted position of the satellite. The automatic tacking is
switched on when the receiver signal strength is sufficient to lock the
tracking receiver to the beacon.
ii)Automatic Tracking:
After acquisition a satellite needs to be tracked continuously. This
function is performed by the automatic tracking system .Auto-tack
systems are closed-loop control systems and
are therefore highly accurate. This tracking mode is the preferred
configuration when accuracy is the dominant criterion.

10. iii)Manual track
To avoid a total loss of communication due to a failure in the
tracking system, earth stations generally also have manual mode.
In this mode an antenna is moved through manual commands.
iv) Program Track
In this tracking mode the antenna is driven to the predicted
satellite position by a computer.
•The satellite position predictions are usually supplied by the
satellite operators. It may be noted that since a program track
system is an open-loop control system, its accuracy is mainly
governed by the accuracy of the prediction data.

11. MAIN ELEMENTS OF A SATELLITE TRACKING SYSTEM

12. •Communication satellites transmit a beacon which is used by
earth stations for tracking.
•The received beacon signal is fed into the auto-track receiver
where tracking corrections or, in some auto-track systems
estimated positions of the satellite are derived
•The outputs of the auto-track receivers are processed and used to
drive each axis of the antenna to the estimated satellite position.

13. Orbit considerations
Once in orbit, the motion of a satellite is determined by orbital
mechanics. However, while the satellite moves in such a way as to
balance centrifugal and centripetal forces, the earth is also in
motion beneath it. The following are different orbital
considerations for any satellite.
1.Equatorial Orbits
2.Inclined Orbits
3.Elliptical Orbits
4.Molniya Orbit
5.Sun Synchronous Orbit

14. Equatorial orbits lie exactly in the plane of the geographical
equator of the earth. That is, the orbital path lies directly above the
equator at all times. In order to take advantage of the 0.45 km/s
eastward rotational velocity of the earth, most satellites are launched
toward the east into a pro grade orbit.
A westerly directed orbit is called a Retrograde orbit.
A satellite in an eastwardly directed equatorial orbit will have two
periods: a real orbital period that is referenced to inertial space (the
galactic background) and an apparent orbital period that is
referenced to a stationary observer on the surface of the earth.
1.Equatorial Orbits

15. The apparent orbital period to the observer on the equator will be
P hours where P = (24T)/(24 — T) hours
The real orbital period, denoted here as T hours.
The plane of a satellite's orbit must be in the plane of the equator
for the satellite to be in equatorial orbit. This can be achieved by
launching the satellite in one of two ways. The first launch method
is to locate the launch site on the equator and to launch the
spacecraft toward the east along the equatorial plane.
The second method is to launch the satellite into an inclined orbit
and to execute a maneuver either during the launch trajectory or
when the satellite is in an inclined orbit that changes the plane of
the initial orbit so that the final orbit is in the plane of the equator.

16. 2.Inclined Orbits
There are advantages and disadvantages to inclined orbits,
depending on the mission goals and the data recovery
requirements. The greater the inclination of the orbit is the
larger surface area of the earth that the satellite will pass over
at some time in its flight.
In Coverage of an equatorial orbit LEO satellite. , the
inclined orbit will take the spacecraft, at one time or another
over the earth's entire surface that lies approximately between
the latitudes given region the orbital inclination

17. FIGURE.1 (a) Coverage of an equatorial orbit LEO satellite.
(b) Coverage of an inclined orbit LEO satellite

18. FIGURE 2 (a) Store -and -forward concept.
(b) Real-time data 1-ansfer via a GEO satellite

19. 3.Elliptical Orbits
An elliptical orbit will have a nonzero eccentricity. The orbit
centricity, e, is determined by the lengths of the semimajor axis,
a, and the semi axis, b, of the orbit ellipse
e2 = 1—(b2/a2) Eq 1
Alternatively, if Ra is the distance between the center of the
earth and the apogee point of the orbit and Rp is the distance
between the center of the earth and the perigee point.
The eccentricity is
e = (Ra — Rp)/(Ra+ Rp) Eq 2

20. FIGURE: Schematic of an elliptical orbit illustrating ellipticity
Figure illustrates the geometry of Eq. (2). In Eqs. (1) and (2), if the orbit is
exactly circular, a = b and Ra = R and the eccentricity reduces to zero.

21. The eccentricity is another way of describing the variation in
the radius of the orbit. If Ra y is the average radius of an orbit
from the center of the earth, then the variation, ∆ R , in the
orbital radius, is given by
∆R = ± eR av
For a geostationary satellite (R = 42,164.17 km) with an
eccentricity of 10-4
∆R will be ±4.2 km.
For a LEO constellation with a circular orbit of approximately
800 km above the earth, with each LEO satellite having an
eccentricity of 10-4, ∆R will be - ±0.7178 km (assuming the
earth mean radius is 6378.137 km).

22. 4.Molniya Orbit
The first Molniya satellite was launched in April 1965 and it gave
its name to both the system of satellites and to the unique orbit. The
word Molniya means flash of lightning in Russian. The apogee of
the Molniya orbit is at an altitude of 39,152 km and the perigee is at
an altitude of 500 km.
The orbital period is 11 h and 38,min and the orbital inclination is
62.9°. This combination of apogee, perigee, and inclination ensures
that the ground track of the Molniya orbit repeats every other orbit.

23. FIGURE: Schematic of a Molniya orbit
In this example, the trajectory is configured to have a large dwell time over
the northern part of the orbit so that it can serve a country that has most of its
landmass in this region.
This was the design adopted for the original Molniya system of the former
Soviet Union.

24. FIGURE: Schematic of an operational Molniya
system

25.  Satellite 1 in Molniya orbit 1 is providing service over
Russia at close to its apogee while the second satellite is also
close to its apogee in Molniya orbit 2.
 Molniya orbits I and 2 are separated by 180° in their orbital
planes. By the time satellite 2 has moved around its orbit once
and back to its apogee (a period of about 12 hours), the earth
will have rotated about 180° and the second satellite will be
over Russia.

26. 5.Sun Synchronous Orbit
A sun synchronous orbit is a special form of low earth orbit
where the plane of the orbit maintains a constant aspect angle
with the direction to the sun.
Some satellite missions require a specific orbit with such a
constant relation to the direction of the sunlight. One example is
an earth resources satellite that requires a large amount of direct
sunlight to illuminate the region below the satellite so that
photographs can be taken.

27. FIGURE: Examples of two sun synchronous
orbits. In the illustration above, the is viewed
from above the North Pole, N, with the sunlight
illuminating the left side of the earth

28. FIGURE: Illustration of alignment changes of the
orbital plane of a satellite due to the movement of
the earth around the sun

29. OPERATIONAL NGSO CONSTELLATION DESIGNS
Seven satellite constellation designs are reviewed briefly in
the following discussion, four MSS offerings with multiple
beams, one with single beam coverage providing both two-
way services and one-way store -and –forward services, and
Two Internet –multimedia satellite systems.
1. Ellipso
2. Globalstar
3. New ICO
4. Iridium
5. Orbcomm
6. Skybridge
7. Teledesic

30. 1.Ellipso
The Ellipso constellation drew from studies of the world's
population distribution and the potential market for MSS users.
Additional studies concluded that an equatorial constellation of
MEO satellites could serve the bulk of the world's population.
Ellipso therefore adopted an incremental approach to their
service offering.
The equatorial orbit groups of the Ellipso system are called
Concordia and the sun synchronous group is called Borealis

31. 2.Globalstar
In a similar manner to Ellipso. Globalstar elected to develop
a constellation that was aimed at the populous regions of the
earth.
The Globalstar orbital planes are therefore inclined at 520 to
the equator, thus ignoring the sparsely populated high -latitude
regions.

32. 3.New ICO
ICO Global is the company that was spun off from the
International Maritime Satellite Organization (Inmarsat); New
ICO is the company that emerged from bankruptcy protection in
2000. Inmarsat was initially set up solely for the purpose of
providing reliable communications to maritime traffic.
.A double -hop link involves two uplinks and two downlinks. A
double –hop configuration is not feasible for a GEO constellation
since the overall delay would be completely unacceptable at
about 1 s.

33. 4.Iridium
The genesis of Iridium was formed around the need to
communicate from anywhere to anywhere on the surface of the
world, even where no telecommunications infrastructure existed.
The system therefore must be stand-alone. From this—and the
need for a low power handset—came the concept of first 77, and
then 66, almost -polar orbiting LEO satellites linked via ISLs.

34. 5.Orbcomm
Many research organizations and businesses need to
obtain data from locations that are either inaccessible on a
regular basis or are moving within areas without good
cellular telephone coverage.
Examples are buoys measuring water characteristics in
rivers and at sea, and delivery trucks. Tracking of high
value cargo on trucks is another application that needs to
send a short message to a central station at regular
intervals

35. 6.Skybridge
Skybridge evolved a similar approach to coverage as
Globalstar, by selecting an inclined orbit that covers the
major population densities.
Like Globalstar, Skybridge satellites carry a
nonprocessing payload and do not have intersatellite links;
so all traffic is transponded down to the gateway earth
stations for processing and onward routing

36. 7.Teledesic
Teledesic started from the same precept as Iridium, but is
designed for Internet –like data traffic rather than voice
communication. Any user can access any other user or ISP
(Internet service provider) independent of location and the
existing telecommunications infrastructure.
The concept of Teledesic is to provide a complete worldwide
data communications system above the surface of the earth using
satellites, instead of on the earth's surface using fiber optic cables.