Satellite Communication - Copy

8/4/2019 Satellite Communication - Copy

1/25


2/25

Two Stations on Earth wantto communicate throughradio broadcast but are toofar away to use conventionalmeans.

The two stations can use asatellite as a relay station fortheir communication.

One Earth Station sends atransmission to the satellite.

This is called a Uplink. The satellite Transponder

converts the signal and sendsit down to the second earthstation. This is called aDownlink.


3/25


4/25


5/25

Launching satellites into orbit is costly.

Satellite bandwidth is gradually becomingused up.

There is a larger propagation delay in satellitecommunication than in terrestrialcommunication. (about 0.3sec)


6/25

Satellite communication system is broadly divided intotwo segments : SpaceSegment

Earth segment

Space segment includes the satellites and the groundfacilities needed to keep the satellites operational.

Earth segment consists of transmit and receive earthstations.


7/25

ATTITUDE AND

ORBITALCONTROL

TT&C

SUBSYSTEM

POWERSUBSYSTEM

THERMALCONTROL

COMMUNICATIONSUBSYSTEM


8/25

Attitude of a satellite refers to its orientation in space.

Attitude control is necessary to ensure directional antennas point in proper

direction.

A number of factors known as disturbance torques can alter theattitude,such as gravitational fields of earth,sun & moon,solarradiation,meteorite impacts,etc.

Controlling torques may be generated to counter the disturbance torques :Passive attitude control (eg. spin stabilization)Active attitude control (eg. momentum wheel stabilization)


9/25


10/25

Telemetry could be interpreted as measurement at a distance.

Collects data from many sensors and sends them to the control earthstation.

Data includes: Attitude informationMagnetic field intensity & directionFrequency of meteorite impactPressure in fuel tanksCurrent drawn by each subsystemCritical voltages & currents

Certain frequencies have been allocated for telemetry transmission viainternational agreement.

During transfer & drift orbital phases of satellite launch a specialchannel is used along with omnidirectional antenna.


11/25


12/25

Tracking is referred to the determination of the currentorbit and position of the spacecraft.

Satellites transmits beacon signals which are received atTT&C earth stations.

Tracking is important during transfer & drift orbital phase.

It is necessary to track the satellite & send correctionsignals.

Satellites range is also required from time to time.


13/25

All communication satellites obtain their power from solarcells.

Solar radiation falling on a geostationary spacecraft hasintensity of 1.39 kw/m2( solar cell efficiency is 10-15%).

Efficiency of solar cells falls with time due to aging and

etching of the surface.

Space crafts carry batteries to power the subsystemsduring launch and eclipses.


14/25

Satellites are subjected to large thermal gardients.

They receive radiation from the sun,earthsalbedo,equipment in the satellites also generates heat.

These heat have to be removed to maintain a stabletemperature environment.

Thermal blankets & shields may be used to provide

insulation. Radiation mirrors are often used to remove heat from

payload.

Spinning satellite provides averaging of the temperature.


15/25

Communications subsystem is the major component of a

communication satellite (transponders & antennas).

A communications satellite exists to provide a platform inthe orbit for relaying of voice, video, and data.

Communication Satellites are designed to provide thelargest traffic capacity possible. (e.g. the INTELSAT

system) The INTELSAT example shows that successive satellites

become larger, heavier, more expensive, and handlesmore traffic. Result: lower cost per telephone circuit.


16/25

Transponder provides the connecting link

between the satellites transmit and receive

antennas. It is the series of interconnected units which

forms a single communication channel betweentransmit and receive antenna.

Antennas carried aboard a satellite provide dualfunctions of receiving the uplink andtransmitting the downlink signals.


17/25

Link between two satellites is called Inter Satellite Link(ISL) and communication through this link is called as InterSatellite Communication.

One satellite could have several link to numerous othersatellites.

The first ISL was demonstrated by Radio Amateurs in1975.

Technical feasibility of ISL was provided by Lincoln

experimental satellites (LES).


18/25

Type of traffic that ISL carries are Voice,Data,Telemetry or

even Video.

Antennas used for ISL are non-directive.

The 1st ISL was established between Oscar 7 & Oscar 6.

These satellites were in polar orbit & the ISL was availableonly when the satellites were in view of each other.


19/25

An (unverified) anecdote says that the idea for Iridium wascreated in 1985 when the wife of a Motorola engineercomplained about the lack of cellular telephone coverageduring a Caribbean vacation.

Subsequently, the Iridium system was conceived to supportglobal voice, messaging, and paging service that would enablemobile subscribers to send and receive telephone calls virtually

anywhere in the world, all with one phone, one phone number,and one customer bill.

Iridium concept was originated by engineers at Motorolas

Satellite Communication Division in 1987.


20/25

Originally envisioned as consisting of 77 satellites in LEO,the nameIridim was adopted by the analogy with element iridium which has77 orbital electrons.

Further studies led to a revised constellation plan acquiring 66satellites.

The polar constellation is formed by 6 orbital planes with 11

satellites per plane. The orbital planes are co-axial at the polar axis,separated from each other at an angle of nearly 30 degrees.

Satellites communicate with neighboring satellites via Ka band

inter-satellite links.


21/25

An international consortium of telecommunications, aerospace andconstruction companies, including Motorola, Kyocera, Lockheed Martin,Raytheon, and Bechtel developed the Iridium system.

The main components of the Iridium system are:

The SPACE SEGMENT which includes the LEO satellites and relatedcontrol facilities. The nominal number of satellites is 66 (6 satellites eachin 11 orbital planes), while the actual number of satellites launched was 79,including on-orbit spares and replacements of failed satellites.

The GROUND STATIONS(gateways) which link the satellites to terrestrialcommunications systems. During its peak the Iridium system wasoperating 12 gateways in various parts of the globe.These regional

gateways handle call setup procedures and interface Iridium with theexisting PSTN.


22/25

The Iridium SUBSCRIBER EQUIPMENT(phones and pagers) which

provide mobile access to the satellite system and terrestrial wirelesssystems. A dual mode that allows users to access either a compatiblecellular telephone network or Iridium was added after it became apparentthat Iridium could not operate in complete isolation of terrestrial cellularsystems.

The TERRESTRIAL WIRELESS INTERPROTOCOL ROAMING

INFRASTRUCTURE.Iridium is designed to provide cellular like service insituations where terrestrial cellular service is unavailable, or areas wherethe PSTN is not well developed.The interprotocol roaming infrastructureallows use of Iridium phones and pagers when the user is within terrestrialnetwork coverage.


23/25

GROUND SEGMENT Number of gateways: 12 Control station: 1 (plus 1 for backup)

SPACE SEGMENT Constellation type: Polar Number of satellites: 66 (+ 6 on-orbit spares) Number of orbital planes: 6 Orbital altitude: 780 km Inclination angle: 86.4o Minimum elevation angle: 8.2o Satellite in-orbit mass: 689 kg

Iridium technical data


24/25

Radio-frequency (RF) power: 400 W Life cycle: 5 years Launch vehicles Delta II (USA), Long March (China), and Proton

(Russia)

COMMUNICATION SEGMENT User uplink and downlink bandwidth: 1621.35-1626.5 MHz Telephony and modem data rate: 2.4 kb/s Satellite capacity: 1,100 duplex channels Total system capacity: 72,600 duplex channels Link margin: 16 dB Number of inter-satellite links (ISL) per satellite: 4 ISL frequency bandwidth: 23.18-23.38 GHz. Feeder link frequency bandwidth: 29.1-29.3 GHz (uplink), 19.4-

19.6GHz (downlink) Onboard Switching, Yes


25/25

Although utilizing new technologies, the systems communications

performance was not satisfactory to all customers in urban areas or duringperiods of high usage.

It has been reported that users of the Iridium handset must stand by

windows or step outdoors to obtain a connection.

Due to the polar formation used by the constellation, the obstruction oftransmission signals by land objects was a serious problem.

As a result of the high system complexity and the relatively harshenvironment in low Earth orbit, Iridium suffered a high failure rate.

The system was architected to operate in a partially degraded mode.Within half a year after the final deployment, 23 satellites of theconstellation exhibited various technical problems.

Satellite Communication - Copy

Documents