What is attitude control?
Given that EQUiSat’s goal is to flash an optical beacon visible to the naked eye and only one side of the satellite has LEDs, EQUiSat needs to remain in a position where the side with LEDs is oriented towards the Earth. In order to ensure EQUiSat’s orientation, we need to employ an attitude control system (ACS).
Additionally, when the satellite is initially launched into orbit, it will tumble until it is stabilized by hysteresis rods, which are also a part of the ACS.
There are two types of ACS’s: active and passive. An active ACS uses some means of attitude determination that tells the satellite its position, and then active stabilizers return the satellite to its desired orientation. A passive ACS stabilizes the satellite utilizing a physical phenomenon in which no control is required by the computer in the satellite.
Goals for the EQUiSat ACS:
- No power requirements
- No complex algorithms to de-tumble and stabilize
- A passive ACS that is within reach for amateur satellite aficionados
What is EQUiSat’s ACS?
A permanent magnet will be placed inside EQUiSat, which will constantly align its polarization axis with the Earth’s magnetic field lines. This system will exert two types of moments on the spacecraft: restoring moments and damping moments.
Restoring moments will align the desired spacecraft axis with the Earth’s magnetic field lines. This will be implemented using the permanent magnet inside EQUiSat.
Damping moments will help de-tumble the satellite after it enters orbit by converting rotational kinetic energy into thermal energy. This will be implemented using hysteresis rods aligned with EQUiSat’s other two axes, parallel with the flash panel.
Additionally, a suite of sensors will be used to determine the satellite’s attitude and decide whether to charge or flash the LED lights.
Ultimately, one of EQUiSat’s axes will oscillate +/- 15º from the Earth’s magnetic field lines, charge when in daytime and flash when in nighttime. This system will theoretically be sufficient to ensure the visibility of the flash from the ground during nighttime passes.
In Modeling EQUiSat at a 300-km, circular orbit with a 42º inclination, we calculated two values: the angle of view (AOV) and the field of view (FOV).
The AOV is the angle between Providence’s azimuthal axis and EQUiSat’s position vector.
The FOV is the angle between the Earth’s magnetic field (the ideal vector of flash beam) and the payload panel’s azimuthal axis.
Fir the flash to be visible, the AOV can be no greater than 40º, and FOV can be no greater than 45º.
From this data, we can extrapolate that EQUiSat should be visible from Providence six times each day (assuming the merciful cooperation of Rhode Island weather), giving a total of about 10 minutes of overhead flashing during its life-time.
When EQUiSat is launched into orbit, the goal of the hysteresis rods will be to reduce the satellite’s rotational motion to an oscillation +/- 15º from the Earth’s magnetic field in under two weeks
The most critical variables of hysteresis rods are the material and volume. We are considering using HyMu80 and Permanorm5000H as materials for the hysteresis rods. And, we are considering 5 cm-long hysteresis rods (Mag big) and 3 cm-long hysteresis rods (Mag small).
In these plots, you can see how larger hysteresis rods result in a larger oscillatory angle, but reach their resolution (the final oscillatory angle) faster. Whereas, smaller hysteresis rods result in less wobble, but take longer to reach their resolution.
We are still evaluating the trade-off between high-resolution (small hysteresis rods) and fast de-tumbling (large hysteresis rods).
Using a suite of simple sensors, EQUiSat will calculate if it is in daytime, during which it can recharge, or nighttime, during which it can flash its LED panel. EQUiSat will use a 3-axis magnetometer and five solar panels during the daytime, and 6 infrared radiation (IR) sensors during the nighttime. The raw data from the sensors will then be downlinked and calculations on the ground using the TRIAD algorithm will determine the attitude.
Daytime Attitude Determination
During the the daytime, EQUiSat will use its five solar panels and a 3-axis magnetometer to determine its attitude.
The current provided by each solar panel will vary depending on how directly it faces the sun. Additioanlly, the 3-axis magnetometer tells both the strength and direction of the magnetic field relative to itself, providing another vector for the TRIAD algorithm.
Nighttime Attitude Determination
During the nighttime, EQUiSat will use its six IR sensors to determine its attitude. The IR sensors will measure the infrared radiation emitted from the Earth, and by looking at the ratio of the output signals of the sensors on adjacent sides of the satellite, we can find the angle between a certain axis of EQUiSat and its position vector.