To calculate the attitude and position, sensors such as Sun Sensors, GPS, Magnetometers, Horizon Sensors and Gyroscopes are used together to create an array of processable data. By using the controller algorithm, designed in advance, the position and orientation may be calculated.
This page gives a brief overview of each sensor with more details and calculations on the individual pages of the sensors.
Sun sensors are essentially photodiodes, which convert light into an electrical current. By reading the strength of the current you can measure the angle at which the sunlight is interacting with the sun sensor. When the sensor reaches its saturated current, the light is orthogonal to the sun and the face of the sensor is facing directly at the sun. Alternatively, when the current is zero, the sensor is not facing towards the sun. By measuring the strength of the current from multiple sun sensors placed strategically around the satellite we can calculate an approximate angle location of the sun relative to the satellite. Depending on the accuracy of the individual sensor, an accuracy of 0.01°-3° can be reached.
Horizon sensors are a type of infrared sensor which detects the difference between the electromagnetic thermal radiation on earth and in the atmosphere with the frigid openness of space. There are two types of horizon sensors commonly in use on spacecraft: static and scanning.
Static horizon sensors are used on satellites which spin in space as they will "scan" their field of view to detect the edges of the earth and space. Using this information, it can then calculate an approximate estimation of the satellites nadir pointing angle.
Scanning horizon sensors are typically used on satellites which do not rotate in space. To simulate the the change in field of view for the infrared sensor, a beam of light is shone on a series of rotating mirrors which detects when the infrared signal is received and then lost. Just like the static horizon sensor, it can use this information to calculate an approximate estimation of the statellites nadir pointing angle.
Depending on the individual sensor, typical accuracies range between 0.1°-0.25°. Although our satellite should not be rotating all the time, a static horizon sensor is still the better option for our CubeSat as it requires less power and we can use our actuators to rotate the satellite to increase the field of view of the sensor if necessary.
Magnetometers measure the strength and direction of earth's magnetic field. Since we will be using a set of magnetorquers, this sensor will be especially important. The accuracy of the sensor is typically in the range of 0.5°-3°.
Gyro sensors have a material embedded in them, such as a crystal, which measure the Coriolis force applied from the vibrations in the sensor as it moves. By measuring this force, the angular velocity is calculated and transmitted as an electrical signal. The accuracy of the sensor is highly dependent on the type of material used to sense the vibrations, with crystals such as quartz glass or single-crystalline sapphire being a good balance between speed and precision.
A single component that combines multiple sensors is a good way to save on save on space, power and computing. Wherever possible, we'd like to select sensors that combine functionality without compromising accuracy.
A comparison of different sensors was conducted for the CSDC5 and is summarized in this spreadsheet.
Obsolete Sensor Selection
The following are sensors selected for a previous CSDC and are kept for reference purposes only.
- CSDC 4spreadsheet.
Satellite Attitude Determination with Low-Cost Sensors Attitude Determination for Small Satellites with Infrared Earth Horizon Sensors Scanning Earth horizon sensor Infrared Horizon Sensors for Attitude Determination