Satellites and sensors

Two kinds of satellite orbit:

• Sun-synchronous (near-polar) satellites orbit at 700-900 km, cover 81° N to 81° S, make about 14 orbits a day, and give fine resolution (from 182 m down to about 1 m). They are used for earth resources: the Indian Remote Sensing (IRS) series.
• Geostationary satellites orbit at about 36,000 km, moving with the earth's rotation so they stay over the same place; one orbit takes 24 hours, they see about one-third of the globe, and resolution is coarse (about 1 km × 1 km). They are used for telecommunication and weather monitoring: the INSAT series.

Sensors. A sensor gathers EMR, turns it into a signal and presents it for study. Photographic (analogue) sensors, cameras, record a scene at one instant of exposure; non-photographic (digital) sensors, scanners, build the image bit by bit.

Multispectral scanners (MSS) sweep across the field of view, building the scene from scan lines. The length of a scan line is the swath. A scanning mirror sends the energy to detectors, which turn it into electrical signals and then into Digital Numbers (DN). Two kinds:

• Whiskbroom scanners: a rotating mirror and a single detector; the detector sweeps across the field of view (between 90° and 120°), in many narrow bands from visible to middle infrared. The whole sweep is the Total Field of View (TFOV); the small patch seen at any instant is the Instantaneous Field of View (IFOV).
• Pushbroom scanners: a line of detectors, one for each ground cell across the swath. Number of detectors = swath ÷ spatial resolution. The book's example is the French SPOT satellite's HRV-1 sensor: a 60 km swath at 20 m resolution needs 60,000 ÷ 20 = 3,000 detectors.

From the video: the teacher's picture is two ways of sweeping a floor: a whiskbroom flicks from side to side across the path, while a pushbroom moves forward with its whole width at once.

Example (new): a pushbroom sensor with a 70 km swath and a resolution of 5 m would need 70,000 ÷ 5 = 14,000 detectors in its line.