ACTIVE MICROWAVE REMOTE SENSING

Danny M. Vaughn, Ph.D., CMS

INTRODUCTION

 RADAR (Radio Detection & Ranging).

Microwaves range in wavelength from 1 mm to 1 m.

 Active Microwave Sensors carried by aircraft & satellites.

 Microwave signals are transmitted, reflected, & received to

create digital image.

 Ranging is achieved by measuring a timed delay of signal.

 Also detects frequency & polarization shifts as the wavelength is

known.
 Transmitter - sends microwaves.

 Receiver - accepts, filters, & amplifies a reflected signal.

 Antenna Array - transmits a narrow beam of microwave

energy.

 Recorder - to record & display the signal as an image.

 Side-Looking Airborne Radar (SLAR)

 Antenna is aimed from the side of the aircraft.

 Creates an oblique strip parallel to the flight path.

 Signal penetrates clouds & most light storm activity.

 Active energy source, used at night.

 Provides crisp imagery with good positional accuracy.

 May be registered to MSS, TM imagery.

 Good for mapping terrain features, i.e. drainage.

 Provides good synoptic view of landscapes.

ORIGIN & HISTORY

 Maxwell (1831-79) - first investigated microwave & radio

energy.

 Hertz (1857-94) - studied properties & propagation of EME in

microwave & radio portions of EMS.

 He also demonstrated reflection of radio waves from metallic

surfaces leading to radio & RADAR development.

 Marconi (1874-1937) - developed antennas for radios &

RADAR. In 1895 he demonstrated the wireless telegraph.

 Taylor & Young (1922) – Marks the beginning of RADAR by

experiments with high frequency radio transmissions.
 They noted a steamer interrupted a transmitted signal across a
river implying that radio signals could be employed to detect
large objects at a distance.

 1933-35 - transmitters & receivers were integrated enabling a

pulsed signal to be transmitted & received by the same antenna.

 Also developed in Great Britain & Germany.

 Refinements produced high-power transmissions over narrow

wavelength intervals to short time pulses, & to amplify reflected
signals (Pre-WWII).

 Post WW II - RADAR reflections from ground surface &

oceans (ground clutter) varies by terrain, season, settlement
patterns, wind, & waves (Oceans) noted.
Real Aperture (antenna) Systems (RAS) - (Brute force system)
as oldest & simplest.

 Signal is generated by a transmitter (microwave).

 Antenna directs signal.

 Antenna also receives reflected signal.

 Signal is filtered & displayed on a C.R.T.

 Recorded by a moving film line by line.

Range or look direction

 The direction of illumination (azimuth).

 Perpendicular to the aircraft fuselage & azimuth direction.
 If perpendicular to topographic alignment, optimal shadow.
 In high relief shadowing is undesirable; good for low relief.

Depression or Look Angle ()

 An angle between a horizontal plane extended from the

fuselage & to a point on the ground. (figs. 7.5 & 7.6 Campbell,
& 9.3, Jensen).
 Measured from the electromagnetic pulse of energy at the
antenna.
 Look angle is in the range or look direction.
 Top (smaller angle) is termed the far-range portion.
 Bottom (steeper angle) is the near-range portion.
 Steep terrain masks illumination as RADAR shadow.

 High topographic relief.

 Flight direction relative to topography (fig. 9.4, Jensen).

 Parallel to ridges & valleys or perpendicular to them.

 Also contributes to terrain loss w/in an image.

 Shadowing is most pronounced with low depression angle

systems & in the far-range portion.
Incident Angle ( )

 Angle between the RADAR pulse of EME & a line

perpendicular to the surface.

 In flat terrain, is the complement of the depression angle

().

In sloped terrain, no relationship between 

 Describes the relationship between the RADAR beam &

surface slope.

 Often assumed to be the complement of 90.

 Slant Range Distance (fig. 7.7) - a straight-line path between
the acft. & a point of the ground.

 Ideally, present images in a ground range format with

distances portrayed in their correct positions.

 RADAR systems collect information in the slant range, thus

geometric artifacts (errors) are created.

 Field A slant range distance is compressed at a (uncorrected).

 RADAR Layover (fig. 7.8) - at near range the top of a

symmetric mountain is closer to the antenna than its base.

 The echo from the mountaintop reaches the antenna before an

echo of that of the base.
 RADAR measures distance with respect to time.

 The top of the mountain appears closer to the antenna than the
base (slant range domain).

 In ground range domain, the top of the mountain will be

projected correct.

 In slant range domain, the top is closer to the edge of the image
than the base that is geometrically incorrect.

 Analgous to relief displacement in aerial photos.

RADAR Foreshortening - occurs in moderate to high relief
terrain.

 In the mid-far portion of an image.

 In slant range domain, images of a terrain appear to have

steeper, shorter, & brighter foreslopes than those of equal angle
imaged on the back slope.

 Back slopes would be portrayed as longer, shallower, & darker.

 Lower depression angles, > foreshortening.

 Also affected by relief.

RADAR Wavelengths

P band: 107-77 cm.

UHF: 100-30 cm.
L band: 30-15 cm.
S band: 15-7.5 cm.
C band: 7.5-3.75 cm.
X band: 3.75-2.4 cm.
Ku band: 2.4-1.67 cm.
K band: 1.67-1.18 cm.
Ka band: 1.18-0.75 cm.

 Airborne imaging RADAR frequently use C, K, & X bands.

 Real aperture imaging RADAR, spatial resolution improves
with shorter wavelengths.
 Soil penetration (skin depth) is a function of wavelength.
 Longer wavelengths & dryer the soil, the deeper (several
meters) the penetration (more absorption of the RADAR wave).

 Diaelectric constants are analogous to electrical conductivity.

 Moist soil reflects more RADAR energy (higher diaelectric

constant & shallow penetration) than dry soils.

 Deeper penetration also a function of surface roughness &

incident angle.

 Penetration decreases as incident angle increases (depression

angle decreases).

 Maximum penetration at the near range end of an image

(higher depression angle) & lower incident angle.
 X & K band signals are scattered off the first surface of contact
(foliage?).

 L band signals (Longer wavelength) reach branches, trunks,

and terrain surfaces below the canopy.

Polarization (electrical wave vibrates in one plane)

 Orientation of EME is perpendicular to the direction of travel.

 Transmit horizontally, receive horizontally (HH, like-polarized

image).

 Transmit horizontally, receive vertically (HV, cross-polarized

image).
 By comparing the two images, features & areas representing
landscape regions that tend to depolarize the signal are identified.

 These areas will reflect the incident horizontally polarized

signal back as vertically polarized energy & may be identified as
bright regions on HV images.

 Surfaces that are poor polarizers will tend to scatter energy in

the same polarization in which it was transmitted.

 Areas appear dark on HH images, bright on HV images.

 Related to physical & diaelectric properties of the ground

surface & inhomogeneous medium (Varied moisture, rock
densities, & indurated horizons).
 Steeper depression angles yield less area illuminated, thus spatial
resolution in across-tract direction varies.

 Ground moisture sensitivity increases with steeper depression

angles.

 All landscape features are viewed at an oblique angle.

 Thus images are a reflection from the sides of features.

 Resolution (Spatial) dictated by –

 Size of area illuminated.

Small areas, optimal for good spatial detail.

Large areas, reflections from diverse objects are averaged.

Loss of distinctiveness.

 Antenna length to wavelength.

Long antenna lengths – energy is focused on a small

area.

Thus better resolution.

 Long antennas are not practical on spacecraft thus resolution
is coarse for R.A.S.

 Analogy of resolution to a beam of light from a flashlight.

 Near-range portions have finer resolution.

 Antenna length (A) is related to wavelength () &

determines angular resolution (beam width, ); 

 Short pulses record distinctive identities and locations (fig.

7.16).

 Long pulses would strike two nearby features at the same

time, recording the two objects as one.
Synthetic Aperture Systems (SAR)

 Objects experience illumination over time through a flight path.

 Image is received from a mobile antenna, rather than a single

antenna.

 As the signal is recorded from a mobile antenna, the analogy is

that of a constantly lengthening antenna or Synthetic Aperture.

 Must be able to assign separate components of the reflected

signal to their proper positions

 Accomplished by Doppler Shift or frequency shifts as objects

differ in distance from the receiver.
 High frequency from objects on the leading edge of a beam,
lower frequency for objects on the trailing edge (edges are on the
X axis & in the direction of flight).

 Passing train analogy.

 As knowledge of the transmitted frequency is known, the nature

& amount of shift may be determined, thus permitting the proper
assignment of position to reflections.

 Information is recorded on Signal or Data Film in the

frequency domain.

 Forms the basis for reconstructing the signal into an image by

using the signal as a microwave hologram.

 The reflected signal differs with respect to frequency & phase

from the reference signal.
 Therefore, the image is reconstructed by directing a beam of
coherent light through the signal film using a special lens to
correct for geometric errors.

 From either optical or digital.

 Coherent energy creates noisy images due to random

irregularities of landscape surfaces w/in each resolution cell
contributing to brightness variances.

 May be removed at the expense of detail.

 SAR may create fine spatial resolution w/out the constraints of

wavelength, target range, & antenna length.
Interpreting Brightness Values

 Variations in brightness corresponds in part to place to

place changes.

 Imaging RADAR is uncalibrated in that image BV's on an

image can not be quantitatively matched to backscattering
values in the landscape.

 Range of returned signals often exceeds the capacity to

record them on emulsions (BV's are in the nonliner portion
of the characteristic curve).
Brightness of a region as a function of the RADAR equation:

Pr = G2Pt2/(4)3R4

Pr = Power returned from ground surface to antenna.

R = Range to target from antenna.
Pt = Transmitted power.
 = Wavelength of energy.
G= Antenna gain (Measure of system's ability to focus
transmitted energy).
 = Backscattering coefficient (Uncontrolled) amount of
energy scattered from a region measured as °, RADAR
cross section. ° as observed isotropic (in all directions)
scattering from a large surface area & a dimensionless
ratio.
 Backscattering is related to – wavelength, polarization,
azimuth, depression angle, and landscape orientation.

Landscape considerations

 Moisture - Changes in Dielectric Constant (ability to conduct

electricity) results.

 Skin depth is decreased w/ moisture, thus reflection is more

from the surface.

 Skin depth increases w/ decreased moisture content w/ a signal

scattered from a greater thickness of soil.
 Roughness - Incidence angle (Angle between axis of incident
RADAR signal & a perpendicular line to the surface).

 A smooth (w/respect to wavelength), homogeneous (w/

respect to electrical properties) surface will reflect a signal
equal to the incidence angle (Specular reflection or in one
direction).

 Rough surfaces will reflect as diffuse or isotropic.

 Roughness as a function of:

Wavelength.
Incident angle.
Surface configuration.

 Surfaces appear rougher w/ shorter wavelengths.

 Corner Reflectors – a RADAR signal may also be affected by
complex geometric shapes, e.g. the urban landscape.

 Complex reflection analogous to a ball bouncing from the

corner of a pool table.

 The energy forms a star-like burst of brightness proportionally

greater than the size of the object that created it. (Urban areas,
complex angular shaped objects).

Satellite Imaging RADAR

 Real Aperature RADARs – require impractically large

antennas.

 Synthetic Aperture RADAR (SAR) – good spatial resolution w/

smaller RADAR systems.
 Images can be optimized when combining RADAR with multi-
spectral imagery.

SEASAT (SAR) – used to monitor winds, waves, sea ice,

temperature, & topography (coastlines) w/ active & passive
sensors.

 Three microwave radiometers, one imaging RADAR, & a

radiometer operating in the visible & IR range.

 Could image 95% of earth's surface every 36 hours.

 L-Band (23 cm. wavelength), 100 km. swath centered 20

off nadir, ground resolution of 25 x 25 m, HH polarization.

 Turned on July 1978 for 98 days acquiring 500 passes of

data.
  Data is available as optical or digital.

 Operates in daylight or darkness (ascending & descending

paths).

SHUTTLE IMAGING RADAR (SIR-A/SAR) - first scientific

payload carried on board Columbia.

 Operated 54 hours (November 1981), alt. of 259 km.

 L-Band (23,5 cm. wavelength), 40¼ depression angle, swath

of 50 km.

 Polarization of HH, & ground resolution of 40 x 40 m.

 No ability to change depression angle.

  Data was recorded on magnetic tape & signal film.

 First generation film images R.F. at 1:5,250,000.

 Completed 5 passes over U.S. & all continents except

Antarctica (4,000,000 m2).

SHUTTLE IMAGING RADAR (SIR-B/SAR) - provided

flexible depression angles.

 Orbital alt. 225 km., orbital inclination 57, 23 cm.

Wavelength.
 Resolution @ 60 depression angle = 25 x 17 m; 25 x 58
m., @ 15depression angle, swath width of 40-50 km., HH
polarization.

 Flexible depression angle enables the orientation of the

antenna to be changed.

 Thus enabling stereoscopic overlap.

 Also provides varied look directions, illumination

directions, & angles.

 Thus a single geographic area at varied depression angles &

azimuths enable derivation of temporal information relative to
surface roughness & moisture.
SIR-C/X-SAR

SIR-C - April & August 1994, C (6 cm) & L (23 cm) bands, HH
& VV polarization (SIR-C). X-SAR – European developed
(Germany & Italy), X band (3 cm).

 Ground swath – 15 – 90 km.

 Resolution – 10 – 200 m.

 Three band composite (Plate 8) as L band (red) HH, L band

(green) HV, and C band (HV) blue.
ERS SAR (European Space Agency)

 ERS 1 (1991) & ERS 2 (1995).

 C band w/ 23 depression angle,

 Swath width. - 100 km, 30 m spatial resolution, VV

polarization.
RADARSAT SAR – Canada developed, U.S. launched &
maintains receiving station (Alaska).

 Designed to image large areas in Canada, overcome poor

weather conditions, monitor sea ice, natural resource assessment
(forests & minerals).

 C band (5.6 cm SAR) launched November 1995, in sun-

synchronous orbit, & an altitude of 793 – 821 km.

 98.6 inclination, 16 day repeat cycle, crossing the equator at

0600 & 1800 hours.

 Incident angle – 10 – 60. Swath width – 45 – 510 km. Spatial

resolution – 9 x 9 m to 100 x 100 m.

 Polarization – HH.
JERS –1 (Launched February 1992)

 C band SAR from Japan.

 Also carries a CCD sensor sensitive in 7 spectral regions (0.52

– 2.40 M).

 Sun-synchronous orbit, altitude – 568 km, also L band (23 cm).

 Incidence angle – 35, Swath width – 75 km, spatial resolution

– 30 m, HH polarization.