Characteristics of remote sensing satellites

Satellites generally vary in their architecture Usually remote sensing satellites are having two plateforms These are main plateform (MPL) and payload plateform (PPL) MPL is having power systems, telemetry, tracking and command systems, attitude and orbit control sensors and communication subsystems

MPL is also known as module or satellite bus PPL is having the imaging sensors, star sensors Satellite housing: all the subsystems are mounted on it and it provides the overall mechanical integrity to the satellite Power system: for every satellite the main source of power comes from solar cells Solar cells convert light energy into electricity

Solar cells are arranged and attached on a surface called solar panel or solar arrays Panels are folded while launching To provide peak power requirements as well as during satellite eclipse, batteries are used Batteries are recharged during the sunlit portion of the orbit Star sensors is a devise which measures the position of the stars using the camera

Attitude and orbit control system A satellite must maintain a certain attitude (orientation) to allow pointing the camera to the earth Factors such as magnetic field, earth’s non uniform gravitational field, solar wind etc disturb the satellite orbit and attitude that needs continuous adjustment Attitude control of a satellite is done by two methods Spin control and three axis control

Spin control is used for geosynchronous meteorological satellites Three axis control is mainly for remote sensing satellites Orbital changes are corrected by the propulsion system carried in the satellite Propulsion systems are small rocket boosters that give a thrust on the various faces of the satellite to bring it back into the correct orbit and attitude

Propulsion system is operated by giving command from the ground

The entire satellite including its antenna(s) spins Spinning can be achieved using rods with coils of wire around them When current is passed through the rods, magnetic field is generated around the wires When this rod's magnetic field interacts with Earth's magnetic field, the rod begins to spin If we have rods in three opposing directions we can have our satellite spin stably in all three axes (up, down, and out) Spin control

Three axis control Three-axis stabilization is done by using electrically- powered reaction wheels, also called as momentum wheelsreaction wheels These wheels are mounted in three orthogonal axes on the spacecraft To rotate the vehicle in one direction, we spin up the proper wheel in the opposite direction.

Telemetry tracking and command Telemetry system collects data on the health of the satellite and communicate it with the satellite control station Command system receives and process the command from the ground station Commands are sent from the ground such as to correct the orbit or to turn on/off the remote sensing camera

Communication subsystems Responsible for maintaining communication links between satellite and ground stations It is used to send the information about the satellite health or to send a data recorded by the remote sensor to ground