Introduction
Satellite Orbits
Satellites have unique orbit altitudes and speeds that affect the area they can image, and the time they take to revisit a location.
Spatial Resolution and Extent
There is a fundamental trade-off between the area that is imaged by a sensor, and the spatial detail or resolution that it records.
Concept Check
Can you explain the relationship between image detail and image extent?
Image detail refers to the size of features that can be seen within an image, and is inversely related to the spatial resolution or pixel size of the sensor. As such, a large pixel size reveals low detail and vice versa. The number of pixels in rows and columns that the sensor can capture controls the image extent. Thus, as the pixel size becomes larger (and therefore the detail reduces), the image extent increases.
What are the factors that affect the frequency with which satellites and sensors can revisit a location?
Temporal resolution or revisit frequency is controlled by both platform and sensor characteristics. For example, an increase in platform altitude results in an increased sensor ‘footprint’, allowing it to view large areas at any one time therefore taking fewer orbits to return to its origin.
Platform orbital speed is also important in determining the time taken to revisit a location. Sensor design is also important. The sensor footprint is directly related to the angular field of view. Broad angles therefore facilitate image capture over large areas with a consequent rapid revisit time.
The exception to this simple relationship are sensors that have ‘agile’ optics, meaning that their viewing angle is not fixed but rather they have the ability to point towards locations of interest. In this way, a sensor does not have to be directly above (nadir) a target to capture an image, but can do so in subsequent orbits. This also results in an increased revisit frequency.
Key Terms
- Geostationary
- Spatial extent
- Spatial resolution
- Sun synchronous