chapter 9 RADAR Flashcards
active microwave (RADAR) based on transmission of …
long-wavelength microwaves (3-25 cm)
LIDAR base on…
transmission of short-wave laser light (0.9 um)
SONAR based on …
transmission of sound waves through a water column
SLAR stands for
real aperture Side-Looking Airborne Radar
SAR stand
synthetic aperture radar
sent out by the transmitter through the antenna
pulse
pulse length is measured in
microseconds (usec)
microwave energy usually measured in
cm
microwave wavelengths that can penetrate clouds
L = 15.0 - 30.0 cm
RADAR can penetrate…
vegetation, sand, snow
RADAR allows resolution to be…
independent of distance to object
RADAR can operate simultaneously in …
multiple wavelengths
azimuth _______
azimuth flight direction refers to the straight line the aircraft travels in
look direction is called..
range
terrain illuminated nearest the aircraft
near-range
terrain illuminated farthest from the aircraft
far-range
Objects that trend perpendicular to the range are
enhanced
Objects that trend parrallel to the range
less enhanced
depression angle
the angle between a horizontal plane extending out from the aircraft fuselage and the electromagnetic pulse of energy from the antenna to a specifc point on the ground
average depression angle
avergage betwen near-range and far-range depression angles
incident angle
angle between the radar pulse of EMR and a line perpendicular to the earth’s surface where it makes contact. If the terrain is flat the incident angle the complement of the deppession angle. If the terrain is sloped there is no relationship between the two. Many radar studies assume the terrain is flat.
unpolarized energy
vibrates in all possible directions perpendicular to the direction of travel
polarized energy sent out by radar antennas
pulse of energy is filtered so that its electrical wave vibrations are only in a single plane that is perpendicular to the direction of travel. the pulse can be sent out vertically or horizonally
HH and VV
like-polarized
HV and VH
cross-polarized
uncorrected radar imagery displayed in…
slant-range geometry (based on the actual distance from the radar to each of the respective features in the scene)
converted slant-range display
true ground-range display
to determine spacial resolution at any point in a radar image, you must compute two dimensions…
range and azimuth
the range resolution in the across-track direction is proportional to the length of the…
microwave pulse
the shorter the microwave pulse…
the finger the range resolution
pulse length is a function of the speed of light (c) multiplied by
the duration of the transmission (t)
range resolution (R) at any point between the near and far-range of the illuminated strip can be computed if these two are known:
depression angle of the sensor at that location and the pulse length
pulse length can be converted to distance by…
multiplying it by the speed of light
The equation for coimputing the range resolution
tau = pulse length
c = speed of light (3 x 10^8 m/s)
gamma =depression angle
Azimuth resolution
determined by computing the width of the terrain strip that is illuminated by the radar beam
Real aperture active microwave radars produce a….
lobe-shaped beam which is narrower in the near-range and spreads out in the far-range.
in real aperature active microwave, the longer the wavelength…
the wider the (lobe-shaped) beam width. and the shorter the wavelength, the narrower the beam width
In real aperture radars, a shorter wavelength pulse, the…
the better the azimuth resolution (and the poorer the atmospheric and vegitation penetration)
Beam width is ____ to antenna length
inversely proportional
the longer the radar antenna, the ____ the beam width and ____ azimuth resolution
narrower, higher
relationship between wavelength and antenna length to compute azimuth resolution (formula)
L = antenna length
S = slant-range distance to point of interest
graphed azimuth resolution
graphed range resolution
foreshortening
layover
radar relief displacement
the horizontal displacement of an object in the image caused by the objects elevation. The higher the object, the sooner it is detected on on the radar image
foreshortening infuenced by three things:
object height
depression angle
location of objects in the across track range (features in the near-range are foreshortened more than identifcal features in the far)
foreshortening causes features to appear to have …
steeper slopes than they actually have in the the near range and shallower slopes that than they actually have in the far range
image layover is an extreme case of ___________
foreshortening (occurs when incident angle is smaller than the forslope)
in synthetic aperture radar systems, engeneers have developed
a way to synthesize a very long antenna electronically. Additional beams are sent toward the object. doppler principles are then used to monitor the returns to synthesize the azimuth resolution to become one very narrow beam.
there is a relationship between the ___ of the radar, the _____, and the height of local objects
wavelength, depression angle
the _____ can be used to predict what the earth’s surface will will look like in a radar image if we know the surfice roughness, wavelenth, and depression angle
modified Rayleigh criteria
graph of surface roughness
response of pine stand to 23.5 cm, 5.8 cm, and 3 cm wavelenths
