What is Remote sensing

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Remote sensing is the process of identifying objects from afar by their spectral signature. In
aerial photography, this is nothin more than taking a picture of the objects on the ground. In other words, capturing the signature in the visible light spectrum. Other remote sensing devices, such as Landsat, SPOT, and AVHRR, are satellites and can measure a number of ranges of the electromagnetic spectrum, called bands, and record them in a digital image. The digitally encoded record has one digital number to represent the spectral reflectance or emittance intensity of the band at that particular point.

Remote sensing devices are often classified according to their swath width, spatial resolution, and radiometric resolution. The swath width is the length of one scan line of the system. For example, if a satellite system has a swath width of 185 km, then that is the device can evaluate a 185 km wide path as it moves along. The spatial resolution is the smallest area on the ground that the device can distinguish. For example, a spatial resolution of 30 m means that the device can identify objects in a 30 meter square area. The radiometric resolution, or radiometric sensitivity, refers to the number of categories, or levels, that the device uses to distinguish data. This is expressed as the number of binary digits, or bits, necessary to express that number of levels. For example, if a device distinguishes between 64 different levels of data, then the device has a radiometric resolution of 6 bits. The number of levels is referred to as the digital number. In the previous example, the digital number would be 64.

Landsat is a satellite operated by the United States. The Landsat series of satellites began in 1972 with Landsat 1 (called ERTS-1), and was followed by Landsat 2 (1975), Landsat 3 (1983), Landsat 4 (1982), and Landsat 5 (1984). Landsat 6 was supposed to be launched in 1994, but failed. Landsat 1 to 5 contained an electromechanical sensor called a multispectral scanner (MSS) that recorded 4 spectral bands of the electromagnetic spectrum.

Band
Wavelength (nm)
Region

MSS1
500-600
Green

MSS2
600-700
Red

MSS3
700-800
Near Infrared

MSS4
800-1100
Near Infrared


It has a swath width of 185 km, consisting of 2,340 scan lines for each band, a spatial resolution of 80 m, and a radiometric resolution of 6 bits (64 brightness levels). Landsat 4 and 5 also carried a Thematic Mapper that recorded in 7 spectral bands of the electromagnetic spectrum.

Band
Wavelength (nm)
Region

TM1
450-520
Blue

TM2
520-600
Green

TM3
630-690
Red

TM4
750-900
Near Infrared

TM5
1550-1750
Middle Infrared

TM7
2080-2350
Middle Infrared

TM6
10400-12500
Thermal Infrared


It has a swath width of 185 km, a spatial resolution of 30 m and a radiometric resolution of 8 bits (256 brightness levels).

SPOT, which stands for Satellite Pour l'Observation de la Terre, is a satellite developed by the French National Space Centre (CNES). SPOT 1 was launched in 1986, and SPOT 2 in 1988. It uses a High Resolution Visible (HRV) sensor, which can operate in either a panchromatic, single spectrum mode or a multipectral mode. The panchromatic and multispectral modes have a spatial resolution of 10 m and 20 m respectively. Both have a swath width of 117 km and a radiometric resolution of 8 bits. In multispectral mode, SPOT records in three bands.


Band
Wavelength (nm)
Region

1
500-590
Green

2
610-680
Red

3
790-890
Near Infrared


The Advanced Very High Resolution Radiometer, AVHRR, is a National Oceanic and Atmospheric Administration, NOAA, satellite that sense in 5 spectral bands.


Bands
Wavelength (nm)
Region

1
580-680
Red

2
725-1100
Near Infrared

3
3550-3930
Middle Infrared

4
10300-11300
Thermal Infrared

5
11500-12500
Thermal Infrared


It has a swath width of 3,000 km, a spatial resolution of 1.1 km, and a radiometric resolution of 10 bits.

Radio Detecting and Ranging, RADAR, is also used for remote sensing. Radar is an imaging system using the 0.8-1.0 centimeter waveband of the microwave range, and measures surface roughness rather than the color or temperature. One big advantage of radar is that is does not rely on solar reflectance, so it can be used 24 hours per day, and it is not affected by cloud cover. [/align]

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