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Other Remote Sensing Systems - SPOT and Charge-Coupled Devices (CCDs)

Scanners, such as those on the Landsats (MSS and TM) were the prime Earth-observing sensors during the 1970s into the 1980s. But these instruments contained moving parts, such as oscillating mirrors that were subject to wear and failure (although remarkably, the MSS on Landsat 5 continues to operate into 1999 after launch in March of 1984). Another approach to sensing EM radiation was developed in the interim, namely the Pushbroom Scanner, which uses charge-coupled devices (CCDs) as the detector. A CCD is an extremely small, silicon chip, which is light-sensitive. When photons strike a CCD, electronic charges develop whose magnitudes are proportional to the intensity of the impinging radiation during a short time interval (exposure time). From 3,000 to more than 10,000 detector elements (the CCDs) can occupy a linear space less than 15 cm in length. The number of elements per unit length, along with the optics, determine the spatial resolution of the instrument. Using integrated circuits each linear array is sampled very rapidly in sequence, producing an electrical signal that varies with the radiation striking the array. This changing signal recording goes through a processor to a recorder, and finally, is used to drive an electro-optical device to make a black and white image, similar to MSS or TM signals. After the instrument samples, the array discharges electronically fast enough to allow the next incoming radiation to be detected independently. A linear (one-dimensional) array acting as the detecting sensor advances with the spacecraft's orbital motion, producing successive lines of image data (analogous to the forward sweep of a pushbroom). Using filters to select wavelength intervals, each associated with a CCD array, we get multiband sensing. The one disadvantage of current CCD systems is their limitation to visible and near IR (VNIR) intervals of the EM spectrum.

CCD detectors are now in common use on air- and space-borne sensors (including the Hubble Space Telescope which captures astronomical scenes on a two-dimensional array, i.e., many parallel rows of detectors). Their first use on an Earth-observing spacecraft was on the French SPOT-1 launched in 1986. (Section 3 describes the SPOT system.) An example of a SPOT image, from its high-resolution video (HRV) camera, covering a 60 km section (at 20 m. spatial resolution) of the coastal region in southwest Oregon, is the next image we show. Note that scan lines are absent, because each CCD element is, in effect, a tiny area analogous to a pixel. The second image is a panchromatic image (with 10 meters resolution) showing the edge of Orlando, Florida, including its airport.

SPOT-1 image of southwest Oregon.

Southwest Oregon

SPOT-1 image of Orlando, Florida.

Orlando, Florida

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Primary Author: Nicholas M. Short, Sr. email:

Collaborators: Code 935 NASA GSFC, GST, USAF Academy
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