Radar: RAdio Detection And Ranging. A method, system or technique, including
equipment components, for using beamed, reflected, and timed electromagnetic
radiation to detect, locate and (or) track objects, to measure altitude
and to acquire a terrain image. In remote sensing of the Earth's or a planetary
surface, it is used for measuring and, often, mapping the scattering properties
of the surface.
Radar beam: The vertical fan-shaped beam of electromagnetic energy
produced by the radar transmitter.
Radar reflectivity: The measure of the efficiency of a radar target
in intercepting and returning a radar signal. This depends upon the size,
shape, aspect, and the dielectric properties at the surface of the target
and includes the effects of not only reflection but also scattering and
diffraction.
Radarsat: Polar-orbiting, Canadian active microwave satellite launched
in 1995.
Radar scatterometer: A non-imaging device that records radar energy
backscattered from terrain as a function of the depression angle.
Radar shadow: A dark area of no return on a radar image that extends
in the far-range direction from an object on the terrain that intercepts
the radar beam.
Radar, synthetic aperture (SAR): A radar in which a synthetically
long apparent or effective aperture is constructed by integrating multiple
returns from the same ground cell, taking advantage of the Doppler effect
to produce a phase history film or tape that may be optically or digitally
processed to reproduce an image.
Radiance: Total energy radiated by an object of unit area per solid
angle of measurement. Standard measurement unit W m^-2. This geometric radiation
quantity is what is measured by remote sensing devices.
Radiant density (W): Total energy radiated by an object of unit volume
in all directions. Standard measurement units J m^-3.
Radiant energy (Q): Total energy radiated in all directions. Standard
measurement unit J.
Radiant exitance (M): Total energy radiated in all directions from
an object of a unit area per unit time. Standard measurement unit W m^-2
Radiant flux (F): Total energy radiated in all directions from an
object per unit time; sometimes referred to as radiant power. Standard measurement
unit W.
Radiant intensity (I): Total energy radiated from an object per solid
angle of measurement. Standard measurement unit W sr-1.
Radiant power: Rate of change of radiant energy with time. May be
further qualified as spectral radiant power, at a given wavelength. Also
called radiant flux.
Radiation: The emission and propagation of energy through space or
through a material medium in the form of waves; eg, the emission and propagation
of electromagnetic waves, or of sound and elastic waves. The process of
emitting radiant energy.
Radiation temperature: Temperature of a blackbody that gives the
same radiant exitance as the object being studied.
Radiometer: An instrument for quantitatively measuring the intensity
of electromagnetic radiation in some band of wavelengths in any part of
the electromagnetic spectrum. Usually used with a modifier, such as an IR
radiometer or a microwave radiometer.
Radiometric correction: Correction of gain and offset variations
in satellite data.
Radiometric degradation: The effects of atmosphere and imaging systems
that result in a blurred image. Degradation resulting from factors such
as non- linear amplitude response, vignetting, shading, transmission noise,
atmospheric interference or variable surface illumination.
Radiometric resolution: Amount of energy required to increase a remotely
sensed pixel value by one quantisation level (or DN).
Radiometric transformations: Adjustments made in data to convert
the raw multi-spectral data to a radiometrically consistent set of measurements;
may be used to compensate for sensor system irregularities or environmental
variations.
Radiometry: Measurement of radiant energy based on fundamental physical
units of energy and flux. Remotely sensed measurements are usually (attempted
to be) related back to this measurement system.
Range, dynamic: The ratio of maximum measurable signal to minimum
detectable signal. The upper limit usually is set by saturation and the
lower limit by noise.
RAR: Real-Aperture Radar.
Raster: Grid. A raster display device stores and displays data as
horizontal rows of uniform grid or picture cells (pixels). Contrasts with
vector.
Ratio image: An image in which one of the original data channels
is replaced by the ratio of two or more of the raw data or transformed data
channels. The 'greenness ratio' (NIR/red) is an example of a ratio channel.
Raw data image: The image of radiance values as received by the sensor
device and processed into a basic CCT.
Rayleigh scattering: The wavelength-dependent scattering of electromagnetic
radiation by particles in the atmosphere much smaller than the wavelengths
scattered.
RBV: Return Beam Vidicon.
RCP: Relative Control Point.
Real-aperture radar (RAR): SLAR system in which azimuth resolution
is determined by the physical length of the antenna and by the wavelength.
The radar returns are recorded directly to produce images; also called brute-force
radar.
Real-time: Time in which reporting on events or recording of events
is simultaneous with the events. For example, the real time of a satellite
is the time in which it simultaneously reports its environment as it encounters
it; the real time of a computer is the time during which it is accepting
data and performing operations on it.
Rectification: The process of correcting distortions in remotely
sensed imagery so that its geometry accurately represents the geometric
features of the Earth's surface.
Redundancy: Information in an image which is either not required
for interpretation or cannot be seen. Redundancy may be spatial or spectral.
It also refers to multi-spectral data where the degree of correlation between
bands is so high than one band contains virtually the same information as
all of the bands.
Reflectance (r): The ratio of the radiant flux reflected by an object
to that incident upon it.
Reflection: Radiation neither absorbed nor transmitted is reflected.
Reflection may be diffuse when the incident radiation is scattered upon
being reflected from the surface, or specular, when all or most angles of
reflection are equal to the angle of incidence.
Reflective infrared: Portion of the electromagnetic spectrum, approximately
0.72-3.0 µm; often subdivided into near infrared or middle infrared.
Reflectivity: A fundamental property of a material that has a reflective
surface and is sufficiently thick to be opaque. One may further qualify
it as spectral reflectivity.
Refraction: The bending of electromagnetic radiation waves when they
pass from one medium into another having a different index of refraction
or dielectric coefficient. Electromagnetic radiation rays also bend in media
that have continuous variations in their indices of refraction or dielectric
coefficients.
Registration: The process of geometrically matching different spatial
data sets, such as imagery and/or maps, so that positions in one data set
may be accurately located in others.
Relative Control Points (RCPs): Points which are used to establish
the relative geometry between two or more images so that these images can
be registered to each other. These may be any features that can be detected
and precisely located on both images.
Relief: The vertical changes of an elevation surface.
Relief displacement: A shift in position of the optical image of
an object caused by height of the object above or depth below a datum plane.
Remote sensing: In the broadest sense, the measurement or acquisition
of information about some property of an object or phenomenon, by a recording
device that is not in physical or intimate contact with the object or phenomenon
under study.
Resampling: Modifying the geometry of an image (which may be from
either a remotely sensed or map data source). This process usually involve
rectification and/or registration.
Resolution: Intensity or rate of data sampling. In remotely sensed
imagery, resolution is significant in four measurement dimensions: spectral,
spatial, radiometric and temporal.
Resolution cell: The smallest area in a scene considered as a unit
of data. For Landsat MSS, the resolution cell approximates a rectangular
ground area of 0.44 ha or 1.1 acres (see pixel,
instantaneous field of view).
Resolving power: An instrument's ability to resolve (see resolution).
Resource matrix: A cross tabulation between two sets of resource
information. For example, in image classification this may be between land
cover classes labelled by an interpreter and spectral classes generated
by the computer.
Return beam vidicon (RBV): A modified vidicon television camera tube,
in which the output signal is derived from the depleted electron beam reflected
from the tube target. The RBV can be considered as a cross between a vidicon
and an orthicon. RBVs provide highest resolution TV imagery, and were used
in the Landsat 1, 2 and 3 satellites.
RGB: Red, Green, Blue; commonly used to refer to the colour space,
mixing system or monitor in colour computer graphics. In RGB, a colour is
defined as proportions of red, green and blue (the additive primaries).
Rods: The receptors in the retina which are sensitive to variations
in achromatic brightness.
Roll: Rotation of an aircraft about the longitudinal axis to cause
a wing-up or wing-down attitude.
Roughness: For radar images this term describes the average vertical
relief of small-scale irregularities of the terrain surface.
Row: (1) The latitudinal (nominal) centre line of a Landsat scene.
(2) The scan lines that constitute an image.