Subsystems(1)

8/4/2019 Subsystems(1)

http://slidepdf.com/reader/full/subsystems1 1/6

EE 418 Comm. Satellite sub-systems (131) Dr . M. M. Daw oud

Communications Satellite Sub-systems

The major subsystems required on the communications satellite are:

• Attitude & orbit control system (AOCS), consists of rocketmotors that are used to move the satellite back to the correct orbitwhen external forces causes it to dri ft.

• Telemetry, Tracking & Command (TT&C), this is partly on thesatellite and partly on the controlling earth station. A dedicatedearth station is used for this purpose.

• Power system (mainly solar cells)

• Communications subsystem, these are the major components of acommunications satellite (transponders & antennas)

Attitude & Orbital Control

• AOC system is necessary to ensure that the narrow beam

antennas are pointing correctly to the earth, (within ±±±± 0.1o)

• Several factors make the space craft tend to rotate and wobbleand change orbit (e.g. gravitational forces from sun, moon, andother planets.)

• Different forms of stabilization are used e.g. entire craft is rotatedat 30-100 RPM to provide gyroscopic action by using spinners. 3-axis stabilization using 3 momentum wheels mounted on 3orthogonal axes. Also c losed loop control of the attitude.)

Telemetry System

• Collects data from many sensors and sends them to the controlearth station.

• Pressure in fuel tanks




• Current drawn by each subsystem

• Critical voltages & currents

• Temperatures.

• Status & positions of switches

• sighting devices used to maintain attitude

• Low data rate is used to allow the receiver at the earth station tohave narrow band-width and maintain high C/N ratio.

Tracking• The determination of the current orbit and position of the

spacecraft.

• Velocity & acceleration sensors are employed.

• The control earth station can observe the doppler shift of thetelemetry carrier to determine the rate of change of the range.

• Triangulation can be used from measurements from several earthstations observing the satellite.

Command System

• Secure & effective command structure is vital for the successfullaunch and operation of a communication satellite.

• The command system is used for:

• Making changes in attitude & orbit correction

• Controlling the communications system

• Controlling the firing of the apogee boost motor

• Spinning up a spinner spacecraft




• Extending the solar sails of a 3-axis stabilizedspacecraft

• Safeguards against errors in received commands are built incommand structure.

– Command originates at the control terminal by converting acontrol code into a command word which is sent in a TDMframe to the satellite.

– Validity is checked and sent back via the telemetry linkwhere it is checked again in the computer.

– If the command word is received correctly, an executeinstruction wil l be sent to the satellite.

– The entire process takes 5-10 sec. And minimizes the risk of malfunctioning.

Typical Tracking, Telemetery & Command System




Power System

➸ All communication satellites obtain their power from solar cells.➸ Solar radiation falling on a geostationary spacecraft has intensity

of 1.39 kw/m2( solar cell effic iency 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 subsystems duringlaunch and eclipses.

Communications System

➸ A communications satellite exists to provide a platform in the orbitfor relaying of vo ice, video, and data.

➸ Comm. Satellites are designed to provide the largest trafficcapacity possible. (e.g. the INTELSAT system)

➸ The INTELSAT example shows that successive satellites becomelarger, heavier, more expensive, and handles more traffic. Result:lower cost per telephone circui t.




Growth of INTELSAT Satellites Over Two Decades

➸ Down link design is the critical part due to the limitedtransmitter power and antenna size and gain.

➸ Received power levels ~ 10-10

W. Satisfactory performance (S/N ~5 - 25 dB) depending on the bandwidth of tr. Signal &modulation used.

➸ Low power transmitters leads to narrow rec. bandwidth tomaintain the required S/N.

➸ High power transmitters & more directional antennas enablelarge bandwidths and increased sat. capacity.




➸ The trend in high capacity satellites is to reuse the availablebands by employing different techniques:

➸ Multiple directional antenna beams at the samefrequency (spatial frequency reuse).

➸ Orthogonal polarizations at the same frequency(polarization reuse).

➸ Example: INTELSAT V achieves and effective bandwidth of 2250MHz in its communication system within 00 MHz band at 6/4 GHzand 250 MHz at the 14/11 GHz by acombination of spatial &

polarization resue.

Documents

Subsystems(1)