absorbed radiation  Electromagnetic energy that is not reflected by a surface. It raises the internal energy of the material.

active sensors  Sensors that send out electromagnetic energy and derive information from the reflection of this energy.

analog  Quantities that can be represented by a continuous sequence of numbers.

AVHRR  Advanced Very High Resolution Radiometer. This sensor, located on a NOAA satellite detected radiation in Infrared wavelengths.

classification  The grouping of patterns of reflectance according to some criterion, such as surface composition, temperature, land use, etc.

color-coded image   An image depicting a property derived by applying an algorithm to the spectral bands. A color lookup table is stored with the imagewhich corresponds to the property values in the image.   Such colors need not be "true-to-life."

color guns  In a color TV or monitor, the three electron beams that produce the colored image on the screen. Color guns produce red, green and blue, hence the "RGB" designation on monitors.

detector  A device designed to respond to (sense) one or more parts of the electromagnetic spectrum (the human eye is a detector). See also sensor.

digital  Quantities represented by a very limited set of numbers; 0 and 1 in binary notation, and 0 to 255 in "8 bit" computers.

electromagnetic radiation  The complete spread of waves, formed by traveling electric and magnetic fields. Called EMR, these waves span the range from radio to television, microwaves, heat (Infrared), visible light, ultraviolet, x-rays, gamma rays, and cosmic radiation.

emitted radiation  Radiation actually given off by a source (e.g., the Sun) as opposed to radiation reflected by a surface.

enhancement  Altering an image to highlight some feature. This may include sharpening, outlining, reducing noise, etc.

false-color image  When the reds, greens, and blues (RGB) of the computer screen do not correspond to the red, green and blue visible bands measured by the satellite sensor, a false-color image is displayed.  This image is not true to what your eye would expect to see.

geostationary orbits  Satellites placed in orbit at an altitude of approx. 22,000 miles take 24 hrs. to revolve about the Earth. Since this is the same speed the Earth rotates, such satellites appear to be stationary above the surface.

infrared radiation (IR) (far infrared)   Heat radiation whose wavelength is much longer than that of red visible light. This is the heat energy that the Earth emits, as opposed to near infrared.

image  A digital representation of a surface or object, composed of pixels. They are not called photographs, since no film is used in their production.

Landsat  Originally called the Earth Resources Technology Satellite (ERTS), a series of satellites which observe the Earth in seven spectral bands. Landsat 1 was launched in 1972, and the current unit, Landsat 5, in 1984. A replacement for this aging unit, launched in 1993, crashed before reaching orbit.

micron  A unit of measure equal to 1/1 000 000 (one-millionth) of a meter. Represented by the symbol "µm."

multispectral scanner  A device capable of looking at an area (scanning) in more than one wavelength at one time.

nadir  The point in the sky (on the Celestial Sphere) directly below an observer.

nanometer  A unit of measure equal to 1/1 000 000 000 (one-billionth) of a meter.

near infrared radiation (NIR)  Heat radiation of just slightly longer wavelength than visible red light. This part of solar radiation, reflected by the Earth, is one of the bands scanned by both Landsat and SPOT satellites.

passive sensors  Sensors that derive information from radiation emitted from or reflected by a surface. These sensors do not send out any radiation.

pixel  A "picture element," the smallest part of a digital image.

polar orbits  Satellite orbits in which the satellite travels North and South, passing over each pole once each orbit. 

radar  "Radio Detection and Ranging." Active sensors that use microwave radiation.

reflected radiation  Electromagnetic radiation that "bounces off" a surface without interacting with it.

remote sensing  Acquiring data, usually by radio/t.v. waves, from a distant location. Historically kites, balloons, aircraft and now Earth-orbiting satellites have been used.

sensor  A device designed to respond to (sense) one or more parts of the electromagnetic spectrum (the human eye is a sensor).

scan line  (see SWATH)

spatial resolution  Measures the smallest object/area that can be seen by a satellite. It is the size of a single pixel in the satellite's image, and ranges from kilometers down to meters.

spectral resolution  The range of wavelengths seen by a particular sensor. The smaller this range, the more specific the information the sensor can provide. 

spectral signature  The specific manner in which a particular surface reflects electromagnetic radiation. Usually given as the percentage reflected in each spectral band.

spectral band  A narrow set of wavelengths scanned by a sensor. The "blue band" represents a group of wavelengths that all appear "blue."

SPOT  Acronym for "Satellite (or Systeme) Probatoire l'Observation de la Terre," a French satellite launched 2/21/86.

sun-synchronous orbits  Orbits designed so that the satellite always passes over a location at the same solar time.

swath  The ground to either side of its track that a satellite scans as it passes over an area.

thematic map  A map that provides information about a certain theme or topic. Geographic, topographic, political, population, and land use maps are all thematic maps.

thermal infrared radiation  Heat radiated by all bodies, including the earth. This radiation is related to the temperature of the object. Sometimes called far infrared or infrared (IR).

track  The path on the ground directly below a satellite as it travels.

true-color image  When the reds, greens and blues (RGB) of the computer screen correspond to the same visible bands measured by the satellite sensor, a true-color image is displayed.  This image is true to what your eye would expect to see.

visible light  The portion of the electromagnetic spectrum from about 0.4 µm - 0.8 µm (or 400 - 800 nanometers) This, the portion that we see, is the familiar "spectrum," or rainbow. Violet light represents the shortest wavelengths, and red light the longest.

wavelength  In any wave (think about water waves) the distance between consecutive Òcrests.Ó The wavelength determines the characteristics of the wave. For Example, in visible light, blue light has a shorter wavelength than red light.