Sensors
A satellite is composed of modular units, each of which is equipped with a set of sensors.
Another possibility to classify earth observation satellites is according to the sensors used:
- passive sensors which measure the reflected sunlight or thermal radiation(Optical)
- active sensors which make use of their own source of radiation (Radar)
Most of the remote sensing satellite platforms today are in near-polar orbits. The satellite travels northwards on one side of the Earth and then toward the southern pole on the second half of its orbit.
These are called ascending (ANX) and descending (DNX) passes, respectively.
Passive sensors recording reflected solar energy only image the surface on a descending pass, when solar illumination is available. Active sensors which provide their own illumination can also image the surface on ascending passes.
Swaths
Swath of a satellite is the width of the area on the surface of the Earth, which is imaged by the sensor during a single pass.
An orbital cycle is completed when the satellite retraces its path, i.e., when the nadir point (point on the Earth’s surface directly below the satellite) of the satellite passes over the same point on the Earth’s surface for a second time. Orbital cycle is also known as repeat cycle of the satellite.
- The field of regard (FOR) is the total area that can be captured by a movable sensor.
- The field of view (FOV) is the angular cone perceivable by the sensor at a particular time instant.
The field of regard is the total area that a sensing system can perceive by pointing the sensor, which is typically much larger than the sensor’s FOV. For a stationary sensor, the FOR and FOV coincide.
