It’s educational post o’clock! This week, we thought we’d talk about Attitude Control. People often talk about Attitude Determination and Control as one entity, but it’s a large topic, so we’ll break it down into two posts. One will talk about Attitude Determination and the other about Attitude Control.
Attitude Determination and Control systems largely have one or both of two goals. First, to keep the satellite stably pointing in a single direction without excessive tumbling. Second, to point the satellite as needed for the function of other systems (e.g. solar cells must be pointed towards sun when not in eclipse, directional antennas must be pointed towards their target during communications.)
The first goal is mainly a fight against disturbance torques. These are generally very small torques that act to give the satellite angular momentum; over time they build up and case the satellite to rotate. Accepted models generally consider four sources of disturbance torques:
1) The magnetic field of the Earth
This effect results from the tendency of satellites to build up charged particles across their surfaces in solar wind. These charges often give the satellite a net magnetic dipole, like a compass needle. As a result, the Earth’s magnetic field will exert a torque that acts to align the satellite’s dipole with the magnetic field direction. Since the magnetic field strength decreases sharply further from Earth (1/r^2), the effect is more pronounced for satellites in low earth orbit like ours. It’s also more of an issue for smaller satellites.
2) Solar radiation pressure
A satellite in orbit is subjected to a great deal of illumination from the Sun. This, of course, is crucial for generating power with solar panels, but it also creates disturbance torques.
Photons carry momentum proportional to their frequency. Therefore, when they strike the satellite they transfer some or all of that momentum to it. If they don’t hit the centre of mass it will produce some torque and thus cause some tumble. With our design, in particular the antenna, is more susceptible to solar radiation pressure.
3) Atmospheric drag
As the satellite travels through its orbit, the Earth’s atmosphere exerts a drag force on it. This drag force is very unlikely to be even across the entire satellite, so net forces will be applied to different parts of the satellite. This creates torques which attempt to line up the satellite so as to minimize the drag force applied to the satellite.
Atmospheric drag is particularly of concern if there are any deployable components, such as solar panels and in our case our payload antenna.
4) Gravity Gradient
Because the gravitiational field of the Earth, like the magnetic field, falls off as 1/r^2, the minimum energy state of a satellite is to point its long axis towards the Earth. Further, if it has a large weight concentration, that point of concentration would rotate to a position to as close to the Earth as possible.
With our deployable antenna, it will double the length the long axis of the satellite, forming a gravity boom as it will move the center of gravity.
2nd Star to the Right, and Straight on ‘Till Morning,
-The Orbit Team.