IV/IV B.Tech Regular Degree Examination, November 2017

Scheme of Evaluation

Subject code:- 14EC705D

Subject Name:- Satellite Communications

Faculty: Head of Department

Sk. Idrish Dr. B. Chandra Mohan

Assistant Professor Professor & Head

9866600354

BAPATLA ENGINEERING COLLEGE:: BAPATLA

Department of Electronics and Communication Engineering

IV/IV B.Tech (Regular) Degree Examination

Scheme of Evaluation

Satellite Communications (14EC705D)

November 2017 Seventh Semester Max Marks:60 Marks ----------------------------------------------------------------------------------------------------------------

Answer Question No.1 Compulsorily. (1X12=12M)

Answer one Question from each unit. (4X12=48M)

1. Answer all Questions 1X12=12M

a) I-Law: The orbit is an ellipse with larger body (earth) at one focus.

II-Law: The satellite sweeps out equal areas in equal time.

III-Law:The Square of period of revolution is proportional to cube of semi major

axis.

b) At higher frequency attenuation is more hence more power will be required for signal

transmission to ensure that it reaches the destination with the required minimum

power. Higher power requirements involve the use of high power amplifiers with high

ratings and heat sinks. This will increase the weight and power supply ratings will not

make any difference. However for the satellite this will result in higher power

consumption, which results in avoidable inefficiency.

c) i)Satellite always in same position relative to earth.

ii)Antennas do not need orientation

d) The coordinates to which an earth station antenna must be pointed to communicate

with the satellite are called look angles. These are expressed as Azimuth and elevation

angle.Azimuth is measured East word from geographical North to the projection of

satellite path on horizontal plane at the earth station. Elevation is angle measured

upward from local horizontal plane at earth station to satellite Path.

e) Horn antennas are used for relatively wide beams like global beam at microwave

frequencies. A horn has an aperture of several wavelengths having good match between

the waveguide and free space. Horns are also used as feeds for reflectors either singly or

in clusters.

f) C/N=(PtGt/KBn)(λ/4πR)(Gr/Ts)

g) A wireless communications device usually attached to a satellite. A transponder receives

and transmits radio signals at a prescribed frequency range. After receiving the signal a

transponder will amplifies and broadcast the signal at a different frequency. The term is a

combination of the words transmitter and responder.

h) In FDMA all users share the satellite at the same time but each user transmits at a

unique allocated frequency. In TDMA each user is allocated a unique time slot at the

satellite so that signals pass through the transponder sequential.

i) If the proportion allocated to each earth station fixed in advance the system is call Fixed

Access.The resource is allocated as needed depending on changing traffic conditions the

multiple access technique is called Demand access.

J) The term VSATs stand for Very Small Aperture Terminal, these are fixed satellite

terminals that are used to provide interactive or receive -only communications.

K) The major sources of errors are:- Satellite clock and ephemeris errors.

Selective availability.

Ionospheric delay

Tropospheric delay.

Receiver noise.

Multipath.

l) The term VSATs stand for Very Small Aperture Terminal, these are fixed satellite

terminals that are used to provide interactive or receive -only communications.

UNIT I

2)a) List and explanation of orbit parameters and elements 3M+3M=6M

2)b) Giving brief history of satellite (6M)

History of satellite communications:

(OR)

3)a) Each bit calculation gives 2M+2M+2M=6M

3)b) Explanation of solar eclipse and suntransit outage 3M+3M=6M

UNIT –II

4)a) Diagram of TTC&M and explanation of each block 3M+3M=6M

The telemetry system at the satellite collects data from many sensors within satellite and

sends them to the controlling earth station. As many as 100 or more sensors monitor pressure

on fuel tanks, voltage and current in the power conditioning unit, critical voltage and current

in communication electronics, temperature of subsystems, status of subsystems, positions of

switches, and sighting device for altitude control. Telemetry data is usually transmitted as

FSK or PSK of low-power telemetry carrier using time division techniques. Low data rate is

normally used to allow the earth station receiver to have a narrow bandwidth and thus

maintain high carrier-to-noise ratio (C/N). An entire TDM frame may contain thousands of

bits of data and take several seconds to transmit.

Tracking:Tracking system at the control earth station provides information about the

range, elevation, and azimuth for a satellite. Data from velocity and acceleration sensors on

the satellite can be used to establish the change in the orbit from the last known position by

integrating data. Doppler shift observation at the control earth station provides the rate of

range change. Active determination of range can be achieved by transmitting pulses to the

satellite from the control earth station and measuring its round-trip delay. The position of a

satellite can be measured by triangulation from multiple earth stations. The position of a

satellite can be determined within 100 m.

Command: A secure and effective command structure is vital to the successful

launch and operation of any communication satellite. The command system makes changes

the position and attitude of the satellite, controls antenna positioning and communication

system configuration, and operates switches at the satellite. During launch it is used to control

the firing of the apogee kick motor (AKM) and to spin up a spinner or extend the solar sails

of a three-axis stabilization satellite. The command structure must have safeguards against

inadvertent operation of a control due to error. The control code is converted into a command

word which is sent in a TDM frame. After checking for validity in the satellite, the command

word is sent back to the control earth station via the telemetry link where it is checked again.

If it is received correctly, then an execute instruction is sent to the satellite so that the

command is executed. The entire process may take 5 -10 seconds, but minimizes the risk of

erroneous commands causing a satellite malfunction. The command and telemetry links are

usually separated from the communication system, although they may operate in the same

frequency band (for example, 6 and 4 GHz). Two levels of command system are used in the

Intelsat satellite: the main system operates in the 6-GHz band, in a gap between the

communication channel frequencies; the main telemetry system uses a similar gap in the 4 -

GHz band.

4)b) Explanation of different antennas using in satellite 6M

Satellite Antennas

Four main types of antennas used on satellites:

1. Wire antennas: monopoles and dipoles 2. Horn antennas. 3. Reflector antennas. 4. Array

antennas. Wire antennas are used to provide omni-directional coverage mainly at VHF and

UHF for the TT&C systems. As most satellites are only a few wavelength long at VHF

frequencies, it is difficult to get the required antenna patterns and there tend to be some

orientations of satellite in which the gain for the TT&C system is reduced by nulls in the

antenna pattern. Typical satellite antenna coverage zones are shown in figure.

The antenna for the global beam is usually a waveguide horn. Scanning beams and

shaped beams require phased array antennas or reflector antennas with phased array feeds.

The pattern is specified by its 3-dB beamwidth. Figure shows the contours of satellite

transmit antenna gain expressed by the EIRP of the satellite antenna and transmitter. In

Figure contours are in 1 dB steps, normalized to 0 dB at the center of the beam.To calculate

the exact EIRP it is needed to know the location of the earth station relative to the satellite

transmit antenna contour. When the satellite transmit antenna contour is not known, the

antenna gain in a given direction may be estimated if the antenna boresight (or beam axis

direction) and its beamwidth are known. Horn antennas are used for relatively wide beams

like global beam at microwave frequencies. A horn has an aperture of several wavelengths

having good match between the waveguide and free space. Horns are also used as feeds for

reflectors either singly or in clusters. It is difficult to obtain gains much greater than 23 dB or

beamwidths narrower than o 10 with horn antennas. Reflector antennas have one or more

horns as feeds and provide a larger aperture than that can be achieved with a horn alone. To

obtain maximum gain it is designed to generate a plane wave in the aperture of the reflector.

To calculate the exact EIRP it is needed to know the location of the earth station

relative to the satellite transmit antenna contour. When the satellite transmit antenna contour

is not known, the antenna gain in a given direction may be estimated if the antenna boresight

(or beam axis direction) and its beamwidth are known. Horn antennas are used for relatively

wide beams like global beam at microwave frequencies. A horn has an aperture of several

wavelengths having good match between the waveguide and free space. Horns are also used

as feeds for reflectors either singly or in clusters. It is difficult to obtain gains much greater

than 23 dB or beamwidths narrower than o 10 with horn antennas. Reflector antennas have

one or more horns as feeds and provide a larger aperture than that can be achieved with a

horn alone. To obtain maximum gain it is designed to generate a plane wave in the aperture

of the reflector. This is achieved by choosing a reflector profile that has equal path lengths

from the feed to the aperture such as a paraboloid with a feed at its focus. In many satellite

antennas, modified papaboloidal reflector profiles are used to tailor the beam pattern to a

particular coverage zone.

(OR)

5)a) Diagram and explanation of satellite uplink model 3M+3M=6M

The primary component within the uplink section of a satellite system is the earth

station transmitter. A typical earth station transmitter consists of an IF modulator. an IF-to-

RF microwave up-converter, a high-power amplifier (HPA). and some means of band

limiting the final output spectrum (i.e., an output bandpass filter). The IF modulator converts

the input baseband signals to either an FM, a PSK, or a QAM modulated intermediate

frequency. The up-converter (mixer and bandpass filter) converts the IF to an appropriate RF

carrier frequency. The HPA provides adequate input sensitivity and output power to

propagate the signal to the satellite transponder. HPAs commonly used are klystons and

traveling-wave tubes.

5)b) Calculating G/T for a satellite transponder 6M

Receiver antenna gain Ar=22db

Gain of LNA A(LNA)=10db

Equivalent noise temperature Te=30dbK

G/T is given by

G/Te= 22+10-30

G/Te=2dBk

UNIT-III

6)a) Giving similarities and differences 3M+3M=6M

Sr.No

.

Multiplexing

Multiple Access

1

“In telecommunications and computer

networks,multiplexingis a process where

multiple analog message signals or digital

data streams are combined into

one signal over a shared medium.”

“In telecommunications and computer

networks,a channel access method or

multiple access method allows several

terminals connected to thesame multipoint

physical medium to transmit over it and to

share its capacity.”

2

The multiplexed signal is transmitted

over a communication channel, which

may be a physical transmission medium.

A channel-access scheme is based on a

multiplex method that allows several data

streams or signals to share the same

communication channel or physical media

3

A device that performs the multiplexing

is called a multiplexer (MUX), and a

device that performs the reverse process

is called a demultiplexer (DEMUX).

A channel-access scheme is also based on a

Multiple access protocol and control

mechanism, also known as media access

control (MAC). This protocol deals with

issues such as addressing, assigning

multiplex channels to different users, and

avoiding collisions.

4

It works on the physical layer

(L1) of OSI model.

It works on the Data Link layer

(L2) of OSI model.

5

Classification of multiplexing:

(w.r.t. channelization methods)

a.Time-division multiplexing (TDM)

b.Frequency-division multiplexing(FDM)

c.Code-division multiplexing (CDM)

Classification of Multiple Access:(w.r.t.

channelizationmethods)

a.TDMA, MF- TDMA

b.FDMA

c.Code division multiple access (CDMA).

6)b) Explanation of VSAT overview 6M

VSAT (Very Small Aperture Terminal) describes a small terminal that can be used for

two-way communications via satellite. VSAT networks offer value-added satellite-based

services capable of supporting the Internet, data, video, LAN, voice/fax communications, and

can provide powerful private and public network communication solutions. They are

becoming increasingly popular, as VSATs are a single, flexible communications platform

that can be installed quickly and cost efficiently to provide telecoms solutions for consumers,

governments and corporations.

The benefits of VSAT technology are being realized in many sectors, both private and

public. From banks to administrations, schools, hospitals and rural telecommunications,

VSATs are being seized upon to elevate economic, educational, and health standards. VSATs

have been in use for more than 20 years and, with already millions installed all over the

world, VSATs are a mature and proven technology. VSAT comprises of two modules viz. an

outdoor unit and an indoor unit. Outdoor unit mainly houses Antenna, feed

horn,RFTransceiver,LNA,Power amplifier. The antenna size is typically 1.8 or 2.4 meter in

diameter, although smaller antennas are also in use. The indoor unit functions as mux-demux,

modem and interfaces with the end user equipments like PCs, LANs, Telephones or an

EPABX. Following diagram describes typical schematic consisting various VSAT

subsystems. A VSAT network consists of three components: A central hub A satelliteA

virtually unlimited number of VSAT user terminals. How vsat network work : The user’s

devie is connected to the satellite dish . This transmitter mounted on the satellite dish sends

and receives digital signals to a satellite at the same time. When a client requests data from

the Internet, the client’s VSAT dish transmits a signal to the satellite, which in turn

retransmits the signal to a central hub earth station, which in turn is connected, to the Internet.

Data from the Internet to the VSAT dish follows the reverse path. The hub or earth station

manages, controls and monitors the network.VSAT is capable of providing Broadband

services.

The satellite is located in a fixed-point 36,000kms above the surface of the earth. This

position is known as the geostationary orbit. Satellites located at this height orbit the earth at

the same speed as the earth rotates on its own axis. This allows the VSAT earth station

equipment to be positioned at the satellite and avoids the need for the VSAT earth satellite

dish to be repositioned once the initial installation has been completed. The VSAT earth

stations are usually made up of an outside mounted satellite dish(approximately 1m in

diameter) and a coaxial wire connection to specially installed electronic boards mounted

inside the master computer.

These boards allow signals to be transmitted and received from the satellite dish.

Transmissions between the earth station and the satellite station generally operate in the 12

and 14 Ghz frequency bands. Because of the high frequencies used high levels of rainfall can

attenuate the signals. The design of the installation is generally engineered to allow for the

reductions in the radio signal paths during periods of intense rainfall. The design of the

satellite link is generally to guarantee communications for 99.999% time, which is equivalent

to a loss of signal for about 4 minutes per year. During this trial no outages were recorded as

a result of rainfall. The central satellite hub station allows communications to and from the

satellite to connect to the Internet. The central hub station can be located anywhere within the

satellite footprint area. For some of the larger satellite operators a number of central hub

stations are operated at different locations to ensure alternative telecommunications paths in

the event of a failure of one of the hubs.

(OR)

7)a) Block diagram and explanation of STAR and MESH 3M+3M=6M

Vsat network Architectures any telecommunication services there are three basic

implementations services: one-way, split-two-way (referred to as split-IP sometimes, when

referring to internet traffic) and two-way implementation. Further division of two-way

implementation are star and mesh network architectures.

There are two Architectures; Star: In Star network architecture, all traffic is routed via

the main hub station. If a VSAT want to communicate with another VSAT, they have to go

through the hub station. This makes double hop link via the satellite. Star is The most

common VSAT configuration os the TDM/TDMA. These have a high bit rate outbound

carrier (Time Division Multiplexed) from the hub to the remote earth stations, and one or

more low or medium bit rate (Time Division Multiple Access) inbound carriers. In a typical

VSAT network, remote users have a number of personal computers or dumb terminals

that are connected to the VSAT terminal that in turn is connected to a centralised host

computer either at individual sites or at a data processing centre. Data sent to the VSAT

terminal from the data terminal equipment (DTEs) is buffered and transmitted to the hub in

packets.Mesh: Meshed VSAT networks provide a way to set up a switched point to point data

network that can have the capability for high data rates of up to 2Mb/s. Links are set up

directly between remote terminals usually on a call by call basis. These networks are usually

configured to operate without a large central earth station and carry a mix of data traffic and

telephony traffic or only data traffic. These networks generally will have a network control

station, which controls the allocation of resources across the network. This control centre is

only involved in the signalling for the call setup/teardown and in monitoring the operation of

the network.

Vsat star topology Vsat mesh topology

7)b) Explanation of basic techniques in VSAT( Protocol also) 6M

The selecting an appropriate multiple access scheme, evaluating signal formats, and

establishing effective coding and interference practices.

There are three fundamental multiple access schemes- FDMA, TDMA and CDMA. Within

TDMA there are two broad subdivisions of access-those that are closely controlled in time

and access ability and those that are loosely controlled in time and access ability.

The intended application and the potential interference environment often determine the

choice between FDMA, TDMA, CDMA for VSAT networks with economics also playing a

major part.

More explanation

UNIT IV

8(a) Explanation of each segment 2M+2M+2M=6M

GPS systems are made up of 3 segments

Space Segment (SS)

Control Segment (CS)

User Segment (US)

Control Segment

Space Segment

User Segment

Three Segments of the GPS

Monitor Stations

GroundAntennas

Master Station

8)b) Calculating Doppler shift and velocity of GPS satellite 3M+3M=6M

(OR)

9)a) Explanation of GPS navigation message and functions of GPS receiver3M+3M=6M

GPS navigation message subframe details

Basic GPS receiver functions

9)b) Explaining different errors of C/A code accuracy 6M

Prepared by

Sk.Idrish

Assistant Professor

Department of ECE

9866600354