This question appears in Mumbai University > Satellite Communication and Network subject

Marks: 10 M

Year: May 2015

A despun antenna is an antenna used in a rotating communications satellite whose main beam is continuously redirected toward desired direction, illuminating the given area on the surface of the Earth. An antenna may be despun mechanically or electrically.

Working in brief –

A despun antenna is used in spin stabilization of cylindrical shaped satellite in which this antenna is mounted on the top of the cylindrical satellite and the cylindrical body of the satellite keeps spinning around its vertical axis while the despun antenna on the body keeps following the specific area on the surface of the earth.

Types of stabilization – how despinning is achieved – In Spin Stabilization method, there are two primary ways by which despinning is achieved –

• By proper placement of antennas and solar panels
• By using multiple elements and phasing

Spin-stabilization technique

In the first method, the antennas and solar panels are spun in positive direction to that of the satellite spin. The satellite body is divided into two parts – (a) the rotor that creates gyroscopic stiffness as a result of spinning about its axis of maximum moment of inertia and (b) a despin platform over which antennas and solar panels are mounted. This method is known as Split Body Stabilization.

[2nd way (if you find the above method difficult) – Despinning is achieved by using multiple elements that are phased in such a way that only the element in proper direction is activated at any time.]

Three-axes method of satellite stabilization –

Three-axes stabilization

Also called as body stabilisation, it does not require the gyroscopic rotation of the satellite body for stability. Instead it keeps the satellite body in a fixed attitude, allowing the solar energy capture and radio transmission and reception to be optimised independently. There are two basic forms of gyroscopic three-axes stabilisation:

• Momentum wheels, similar to gyroscopes, which spin in one direction only.
• Reaction wheels which can spin in both directions.

These wheels are mounted in three orthogonal directions corresponding to the yaw, roll and pitch of the satellite body and provide a stabilised inertial platform.

Accelerating or decelerating any of the wheels by means of electric motors or gas jet thrusters increases its angular momentum in that direction by an amount which is proportional to the applied motor or jet torque and this in turn creates an equal and opposite torque on the satellite body causing it to rotate in the opposite direction about the axis of the wheel. Slowing the wheel brings the satellite body back again. Thus angular momentum can be traded back and forth between the spacecraft and the wheels. Thrusters are still required for lateral movement.

With three-axis stabilization, if satellite sensors detect that the satellite is moving away from the proper orientation, the spinning wheels speed up or slow down to return the satellite to its correct position. Some spacecraft may also use small propulsion-system thrusters to continually nudge the spacecraft back and forth to keep it within a range of allowed positions. For example, Voyagers 1 and 2 stay in position using 3-axis stabilization.

An advantage of 3-axis stabilization is that optical instruments and antennas can point at desired targets without having to perform “despin” manoeuvres.