What are Earth/Horizon Sensors
Earth/Horizon Sensor is used to observe the earth and collect the data to determine the nadir vector relative to the sensor. The nadir vector is the vector that points directly below a particular location, that is, it is one of the two vertical directions or it points in the direction of the force of gravity at that location. It is also commonly used as a reference vector in autonomous navigation, especially in the satellites' orbit determination.
The calculated nadir vector is passed to the spacecraft. The spacecraft then uses this information to determine its orientation and make necessary attitude adjustments.
Earth Horizon Sensors can further be classified as follows:
- Static Earth Horizon Sensor – It contains several sensors and senses infrared radiation from the Earth’s surface with a field of view slightly larger than the Earth with its use being restricted to spacecraft with circular orbits.
- Scanning Earth Horizon Sensor – They primarily use a spinning mirror or prism that focuses a narrow beam of light onto a sensing element called the bolometer. The spinning of the mirror or prism causes the device to sweep out the areas of a cone and electronics inside the sensor which then detects when the infrared signal from Earth is first received and when it is lost. The time elapsed is used to determine Earth’s width. From this, the roll angle can also be determined.
- GPS - This sensor uses one signal to determine many characteristics such as satellite identification, position, the duration of the propagated signal, and clock information. Using a constellation of GPS satellites, navigation, positioning, precise time, orbit, and attitude can be determined.
