What are the relative merits of three-axis body stabilisation over other types of stabilisation?
Three-axis body stabilisation is the method of satellite stabilisation preferred nowadays over older methods like spin stabilisation. In a three-axis body stabilized spacecraft, the entire structure is available for mounting antennas and other devices which need high pointing accuracy. Earth-viewing area is much larger than in a pointing accuracy. Earth-viewing area is much larger than in a spinning satellite. Solar panels can be rotated so as to keep them the north-south direction and can be rotated so as to keep them perpendicular to the Sun-satellite line. Moreover, as the spacecraft does not rotate at high rates, light weight panels initially folded and kept on board can be deployed in space. A given area of the solar panel will be three times more efficient in power generation compared with a spinning satellite. It would thus be possible to generate larger quantities of power needed by advanced communication satellites.
How does three-axis stabilisation work?
In three-axis stabilisation, the controlling mechanism is based on a, momentum wheel, essentially a fly wheel actuated by an electric motor and rotated at a predetermined speed. This system is known as zero momentum stabilisation where unwanted momentum accumulated by the wheel is reduced to zero. Correction is effected by accelerating or decelerating the momentum wheel on the basis of error signals provided by the satellite’s Earth sensors which point towards the Earth and lock on to the Earth. The accuracy of alignment to the Earth is of a high order of 0.1 to 0.2 degree. This position is required to be maintained for 5 to 7 years, which nowadays is the operational lifetime of a satellite.
What is drift control of a satellite?
In order to maintain a satellite’s position, stability and accuracy in the synchronous orbit, it’s the north-south as well as east-west directions should be constantly monitored within plus or minus 0.10. Maintenance of the satellite in its place, called station-keeping, requires more fuel for the north-south maneuvers than for controlling the east-west drift. In order to minimize the use of fuel, specific time-slots are chosen to actuate the thrusters, when the orbital inclination is suitable for it. It is usual to rely upon electro thermal thrusters which have low thrust rates and would take about four hours to correct an orbit inclination of 0.20. Due to ideal-period limitations, maneuvers may last a few days.
East-west station control has become important not only to ensure the accuracy of the satellite but also to reduce the minimum separating distance between two spacecraft in synchronous orbit to 0.20, thereby efficiently using the orbit without mutual interface between satellites.
What are the on-board systems used in a satellite for orbital corrections?
The propulsion sub-system of a three-axis satellite has several thrusters of varying capacity. The heaviest, for example, could give a thrust of 22 N used for mutation or wobble control, while for pitch and yaw corrections or for east-west maneuvers only small rockets are used. Thrusters for roll control are less powerful. At the bottom of the scale are electro thermal thrusts which being very low in capacity have to be used over a period. In order to maintain stability and arrest any drifts, the north-south station-keeping has to be constantly maneuvered. It would increase the velocity of the satellite now and then and for this purpose adequate hydrazine fuel should be stored on-board.
