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.
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.
cross-track scanner :
- “back and forth” motion of the fore-optics scans each ground resolution cell one-by-one
- Instantaneous Field of View (IFOV) of instrument determines pixel size
- Image is built up by movement of satellite along the orbital track and scanning acrosstrack
along-track scanner (“Pushbroom"):
- Linear array of detectors (aligned cross-track) – reflected radiance passes through a lens and onto a line of detectors
- Image is built up by movement of the satellite along its orbital track.
- Area array can also be used for multi-spectral remote sensing.
Sensors can be further divided in:
- Fixed sensors- instruments with a fixed field of view and orientation which provides always the same footprint on ground
- Steerable sensors- instruments with a field of view geometry that can be steered within certain limits, either via mechanical steering of the sensor or the full satellite body, or electronically like for Synthetic-aperture radar (SAR) instruments. The sensor geometry is characterized by a Field of Regard virtual swath that represents the access boundaries within which the sensor can operate.
Getting the Data to the Ground
- On-board recording and pre-processing
- Direct telemetry to ground stations – receive data transmissions from satellites – transmit commands to satellites (pointing, turning maneuvers, software updating)
- Indirect transmission through Tracking and Data Relay Satellites (TDRS).