Radar
Acronym for radio detection and ranging. Radar is an active form of remote sensing that operates in the microwave and radio wavelength regions.

Radar altimeter
A non-imaging device that records the time of radar returns from vertically beneath a platform to estimate the distance to and hence the elevation of the surface; carried by Seasat and the EAS-ERS-1 platforms.

Radar cross section
A measure of the intensity of backscattered radar energy from a point target. Expressed as the area of a hypothetica surface which scatters radar equally in all directions and which would return the same energy to the antenna.

Radar scattering coefficient
A measure of the back-scattered energy from a target with a large area. Expressed as the average radar cross section per unit area in decibels (dB). It is the fundamental measure of the radar properties of a surface.

Radar scatterometer
A non-imaging device that records radar energy backscattered from terrain
as a function of depression angle.

Radar shadow
Dark signature on a radar image representing no signal return. A shadow extends in the far-range direction form an object that intercepts the radar beam.

Radial relief displacement
The tendency of vertical objects to appear to learn radially away from the center of a vertical aerial photograph. Caused by the conical field of view of the camera lens.

Radian
Angle subtended by an arc of a circle equal in length to the radius of the circle 1 rad = 57.3¡.

Radiant energy peak
Wavelength at which the maximum electromagnetic energy is radiated at a particular temperature.

Radiant flux
Rate of flow of electromagnetic radiation measured in watts per square centimeter.

Radiant temperature
Concentration of the radiant flux from a material. Radiant temperature is the kinetic temperature multiplied by the emissivity to the one-fourth power.

Radiation
Propagation of energy in the form of electromagnetic waves.

Radiometer
Device for quantitatively measuring radiant energy, especially thermal radiation.

Radiometry
Measure of the variations in light intensity.

Random line dropout
In scanner images, the loss of data from individual scan lines in a nonsystematic fashion.

Range
In radar usage this is the distance in the direction of radar propagation, usually to the side of the platform in an imaging radar system. The slant range is the direct distance from the antenna to the object, whereas the distance from the ground track of the platform to the object is termed the ground range.

Range direction
See look direction.

Range resolution
In radar images, the spatial resolution in the range direction, which is determined by the pulse length of the transmitted microwave energy.

Raster
The scanned and illuminated area of a video display, produced by a modulated beam of electrons sweeping the phosphorescent screen line by line from top to bottom at a regular rate of repetition.

Raster format
A means of representing spatial data in the from of a grid of DN, each line of which can be used to modulate the lines of a video raster.

Raster pattern
Pattern of horizontal lines swept by an electron beam across the face of a CRT that constitute the image display.

Ratio image
An image prepared by processing digital multi-spectral data as follows: for each pixel, the value for one band is divided by that of another. The resulting digital values are displayed as an image.

Rayleigh criterion
In radar, the relationship between surface roughness, depression angle, and wavelength that determines whether a surface will respond in a rough or smooth fashion to the radar pulse.

Rayleigh scattering
Selective scattering of light in the atmosphere by particle that are small compared
with the wavelength of light.

Real-Aperture radar
Radar system in which azimuth resolution is determined by the transmitted beam width, which is in turn determined by the physical length of the antenna and by the wavelength.

Real time
Refers to images or data made available for inspection simultaneously with their acquisition.

Recognizability
Ability to identify an object on an image.

Rectilinear
Refers to images with no geometric distortion in which the scales in the horizontal and vertical directions are identical.

Redundancy
Information on an image which is either not required for interpretation or cannot be seen. Redundancy may be spatial or spectral. The term also refers to multispectral data where the degree of correlation between bands is so high that one band contains virtually the same information as all the bands.

Reflectance
Ratio of the radiant energy reflected by a body to the energy incident on it. Spectral reflectance is the reflectance measured within a specific wavelength interval.

Reflected Energy Peak
Wavelength (0.5 µm) at which maximum amount of energy is reflected from the earth's surface.

Reflected IR
Electromagnetic energy of wavelengths from 0.7 µm to about 3 µm that consists
primarily of reflected solar radiation.

Reflectivity
Ability of a surface to reflect incident energy.

Refraction
Bending of electromagnetic rays as they pass from one medium into another when each medium
has a different index of refraction.

Registration
Process of superposing two or more images or photographs so that equivalent
geographic points coincide.

Relief
Vertical irregularities of a surface.

Relief displacement
Geometric distortion on vertical aerial photographs. The tops of objects appear in the photograph to be radially displaced from their bases outward from the photograph's centerpoint.

Remote sensing
Collection and interpretation of information about an object without being in physical contact with the object.

Resampling
The calculation of new DN for pixels created during geometric correction of a digital scene, based on the values in the local area around the uncorrected pixels.

Reseau marks
Pattern of small crosses added to photographs.

Resolution
Ability to separate closely spaced objects on an image or photograph. Resolution is commonly expressed as the most closely spaced line-pairs per unit distance that can be distinguished. Also called spatial resolution.

Resolution target
Series of regularly spaced alternating light and dark bars used to evaluate the resolution of images or photographs.

Resolving power
A measure of the ability of individual components. and of remote sensing systems, to separate closely spaced targets.

Restitution
Process which converts disparity information into elevation information in a cartographic reference once viewing parameters are known.

Return
In radar, a pulse of microwave energy reflected by the terrain and received at the radar antenna. The strength of a return is referred to as return intensity.

RMS (Root Mean Square) Error
All AIRDATA products are characterized by their root-mean-square (or RMS) error to within one (1) standard deviation. This statistic describes the divergence of a random variable (e.g., pixel position) from its mean. The interval within one standard deviation of the mean contains approximately 68% of the sample. RMS error must not be confused with the maximum error (that may be associated with a given value).

Rods
The receptors in the retina that are sensitive to brightness variations.

Roll
Rotation of an aircraft that causes a wing-up or wing-down attitude.

Roll compensation system
Component of an airborne scanner system that measures and records the roll of the aircraft. This information is used to correct the imagery for distortion due to roll.

Rough criterion
In radar, the relationship between surface roughness, depression angle, and wavelength that determines whether a surface will scatter the incident radar pulse in a rough or intermediate fashion.

Roughness
In radar, the average vertical relief of a small-scale irregularities of the terrain surface. Also called surface roughness.