Microwave Sensing

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Microwave Sensing
(www.fas.org)
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Topics

• Microwaves
• Radar
• Range and Azimuth Resolution
• Rate Determination
• Synthetic Aperture Radar

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Microwaves

• Valuable environmental and resource information
  can be acquired in the microwave portion of the
  electromagnetic spectrum, from wavelengths of 1
  mm to 1m
• These wavelengths are about 2,500,000 times
  longer than shortest light waves
• Two distinctive features characterize microwave
  energy from a remote sensing standpoint
• Microwaves penetrate atmosphere under virtually
  all conditions
• Depending on the wavelength -- haze, light rain,
  snow, clouds, and smoke can be penetrated
• Microwave reflections or emissions from Earth
  materials bear no direct relationship to
  counterparts in the visible or thermal portions
  of the spectrum
• Surfaces appearing rough in the visible spectrum
  may appear smooth in the microwave regime
• Microwaves generally give a different view than
  light or thermal spectra

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The Microwave Radio Spectrum

• Canada Center
• for Remote Sensing

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Microwave Bands

• Most remote sensing radar wavelengths are between
  .5 cm to 75 cm.
• The microwave frequencies have been arbitrarily
  assigned to bands identified by letter the most
  popular imaging radars include
• X- band from 2.4 - 3.75 cm (12.5 - 8 GHz).
• Widely used for military reconnaissance and
  commercially for terrain surveys.
• C- band from 3.75 - 7.5 cm (8 - 4 GHz).
• Used in many spaceborne SARs, such as ERS- 1 and
  RADARSAT.
• S- band from 7.5 - 15 cm (4 - 2 GHz).
• Used in Almaz.
• L- band from 15 - 30 cm (2 - 1 GHz).
• Used on SEASAT and JERS- 1.
• P- band from 30 - 100 cm (1 - 0.3 GHz).
• Used on NASA/ JPL AIRSAR.

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Microwave Bands

• Radio wave penetration through a surface layer is
  increased with longer wavelengths
• Radars operating at wavelengths greater than 2 cm
  are not significantly affected by cloud cover,
  however, rain does become a factor at wavelengths
  shorter than 4 cm.

Canada Center for Remote Sensing
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Microwaves (Concluded)

• Microwave sensing systems can be active and
  passive
• An active system supplies its source of
  illumination
• The passive system, such as a microwave
  radiometer, responds to the low levels of
  microwave energy that are naturally emitted
  and/or reflected by terrain features
• RADAR is an acronym, and now a proper noun, from
  radio detection and ranging
• Data from radar and passive microwave systems are
  relatively limited compared to photographic or
  scanning systems
• Increasing availability of spaceborne radars may
  allow the microwave database to catch up
• Like RADAR, LIDAR, light detection and ranging,
  use an active source with a sensor
• Lidars use pulses of laser light, rather than
  microwave energy, to illuminate the terrain

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Radar Configuration

• A transmitter generates radio waves radiated by
  an antenna
• The receiver, tuned to transmitter frequency,
  listens for an echo at the other antenna
• The transmitter and receiver generally share an
  antenna
• The receiver is blanked during transmission to
  avoid interference
• The antenna concentrates radiated energy into a
  narrow beam, thus allowing
• Differentiation between targets
• Detection of targets at greater range
• This is called antenna gain, or directivity

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Some Fundamental Concepts

• Resolution
• Range Resolution
• Azimuth Resolution
• Signal Processing
• Doppler Processing
• Range-Doppler Ambiguities
• Synthetic Array Weighting
• Motion Compensation

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Range Azimuth Resolution
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Microwave Resolution
Range Resolution
Azimuth Resolution
(JPL/NASA)
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Range Resolution

• For pulse duration ?, pulse width c?
• Range resolution c? / (2cos ?d), with no pulse
  compression
• c light speed, ?d angle of depression.
• Used by conventional and SAR systems

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Range Resolution with No Pulse Compression

• For no depression angle, range resolution c? /2
• Range Resolution of Two Targets
• Trailing edge of transmitted pulse must have
  passed near target before leading edge of echo
  from far target reaches near target

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A Last Look at Range Resolution
?d Depression angle
?I Look angle
Slant range resolution
Ground range resolution
Ground range resolution
?d
?
Slant Range resolution
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Azimuth Resolution

• Azimuth resolution of side-looking radar (SLR) is
  determined by angular beamwidth ? of the antenna
  and ground range GR (optimally the value should
  be small)

• Objects at points A and B would be resolved, or
  imaged separately, better at G-R1 than at G-R2

(Remote Sensing and Image Interpretation)
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Azimuth Resolution
(Remote Sensing and Image Interpretation)
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Azimuth Resolution (Concluded)

• Systems where beamwidth is controlled by physical
  antenna length are called
• Brute force radars
• Real aperture radars
• or Noncoherent radars
• Antenna must be many wavelengths long for antenna
  beamwidth to be narrow
• 2 mrad resolution requires antenna 25 m long
• Brute force systems, however, are relatively
  simple, and large
• A means to overcome these deficiencies is
  Synthetic Aperture Radar (SAR)

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Rate Determination

• There are two methods of determining changes of
  range, or rate, the variation of distance.
• Most static range determination is accomplished
  by time of flight sending and return of energy
  packets.
• The distance is from the radio transmitter is
  determined by the delay time of the echo.
• Another method, depends on the Doppler effect,
  the shifting of frequency of reflected signals
  when acted upon by objects in motion.
• The rate analysis is best done in the frequency
  domain, unlike the time domain analysis for time
  of flight signals.

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Rate Determination by Doppler

• Consider the following diagram
• A source transmitting a continuous sinusoidal
  wave
• The wave crosses a distance to a stationary object

• The object reflects, or scatters the energy
• Some of the reflected energy is sent back to the
  source
• This received wave is the same frequency as which
  left the source, fobject ftx

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Rate Determination by Doppler
Canada Centre for Remote Sensing
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Rate Determination by Doppler

• Now imagine the object moving to the right, away
  from the source, at velocity v.
• The waves will intercept the object at a lower
  frequency by

v

• where
• v velocity of moving object with respect to the
  source
• c speed of light
• If the object is moving away from the source, the
  distance is increasing, thus v gt 0, hence the
  minus sign to show that fobject lt fTX.

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Rate Determination by Doppler

• This is only half the analysis the moving object
  scatters the wave and sends part of the energy
  back in the direction of the source.
• The received wavelength at the moving object,
  ?object c/fobject, is transmitted as a longer
  wavelength, since the generation of this
  reflected electromagnetic wave is from a moving
  object away from the original source.
• This new wavelength, allowing for both the object
  moving away () and towards () the source, is

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Rate Determination by Doppler

• Combining these two equations to find the final
  reflected frequency at the source, yields
• The upper sign designates the object is moving
  away from the source, and the lower sign
  designates the object is moving toward the
  source.

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Rate Determination by Doppler

• It is assumed here that vltltc, so relativistic
  effects are ignored, and thus this equation can
  be simplified by expanding the Doppler factor
  about a Taylors series and ignoring all but the
  first two terms
• This gives

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Rate Determination by Doppler

• But how to best analyze the signal?
• Frequencies in the microwave x or k bands are
  very high compared to a Doppler shifted
  displacement frequency.
• Consider a moving automobile that is being
  tracked by radar, say moving at 100 km/hour, or
  about 27.8 m/s
• The Doppler shift factor is

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Rate Determination by Doppler

• For a nominal x-band microwave frequency of 10
  GHz, the reflected wave is thus 9.99999800000
  GHz, a difference of only 1853 Hz, or less than
  210-5
• This is difficult to sense, and then compare with
  the nominal transmitted frequency.
• A way to detect the shift easier is by
  interference, or beating the signals together.

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Rate Determination by Doppler
f0 Oscillator
Radar Antenna
Output of consisting of cos(f0)cos(fRX)
fRX Signal
Mixer

• This is a diagram of a superheterodyne mixer that
  beats an incoming signal with an oscillator wave.

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Rate Determination by Doppler

• The result of mixing two signals, here assumed to
  be sine wave signals of the nominal transmitted
  signal cos(f0), and the received Doppler shifted
  signal cos(fRX), is
• cos(f0)cos(fRX) cos(f0fRX) cos(f0fRX)/2
• Note that the beated signals produce both a sum
  and a difference frequency
• The next receiver stage is tuned to the second
  terms relatively small difference frequency
• This frequency is also proportional to the speed
  of the object

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Rate Determination by Doppler

• The difference frequency of a Doppler shifted
  wave being reflected off of a 100 km/hr
  automobile is 1.853 kHz, in the audio frequency
  (ELF) range, quite different than the SHF
  carrier.
• A means to confuse the ranging transmitter is to
  send the nominal radar frequency wave but
  modulated with an audio frequency representing a
  low automobile speed at a signal strength higher
  than the reflected Doppler shifted wave.

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Synthetic Aperture Radar
(Sandia)
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Synthetic Aperture (alias Array) Radar (SAR)
Background

• Different operations require broad area imaging
  at high resolution, such as
• Environmental monitoring
• Earth-resource managing
• Military systems
• Imagery must be acquired in bad weather and night
  as well as day
• SAR has provided this capability
• Terrain structural information for mineral
  exploration
• Oil spill boundaries
• Sea state and ice maps
• Reconnaissance and targeting information

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SAR Fundamentals

• Airborne SLR images are taken perpendicular to
  flight path direction
• Two dimensional (2-D) image typically produced
• One dimension is called range, or cross-track
  dimension
• Measure of line-of-sight distance
• Precisely measure transmission to reception time,
  or time of flight
• Resolution determined by pulse width or
  compressed pulse width
• The other dimension is called azimuth, or
  along-track dimension
• Physically large antenna focuses the transmitted
  and received energy into a sharp beam
• Optical systems--telescopes--also require large
  aperture mirrors or lenses to obtain fine imaging
  resolution
• SLR systems--much lower in frequency than optical
  systems--need a physically unrealizable antenna
  for use on-board a plane

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SAR Fundamentals (Continued)

• The bottom line is that SLR resolutions need
  antenna lengths on the order of several hundred
  meters
• The saving feature is that airborne radar can
  collect data while airplane flies the
  corresponding distance
• Data is then processed as if it came from a
  physically long antenna
• The distance the radar source flies in
  synthesizing the antenna is known as the
  synthetic aperture
• A narrow synthetic beamwidth results from the
  long synthetic aperture, thus a finer resolution
  is obtained than is possible with a smaller
  physical antenna

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SAR Fundamentals (Continued)

• The realization of SAR fine azimuth resolution
  can also be approached from a Doppler processing
  standpoint
• A targets position along the flight path
  determines the Doppler frequency of its echoes
• Targets ahead reflect a positive Doppler offset
• Targets behind reflect a negative Doppler offset
• The aircrafts flight through the synthetic
  aperture distance allows echoes to be resolved
  into a number of Doppler frequencies
• Thus the targets Doppler frequency determines
  its azimuth position

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Synthetic Array Radar (SAR)
(ASF)
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SAR Fundamentals (Continued)

• Transmitting pulses short enough to provide the
  desired resolution are generally not practical
• Longer pulses with wide bandwidth modulation are
  typically transmitted for pulse compression
  purposes
• Peak power requirements are reduced
• For even moderate azimuth resolutions, SAR
  processing is complicated, requiring digital
  signal processing (DSP)
• Energy reflected from the target must be
  mathematically manipulated to compensate for the
  range dependence across the synthetic aperture
  prior to image development
• For fine resolution imagery, range and azimuth
  channels are coupled, which greatly increases
  complexity and computational processing

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What Is Imaging Radar?

• Provides illumination of ground area
• Uses antenna and digital computer tapes to record
  image
• Brighter areas mean higher back-scatter
• Rule of thumb is that brighter backscatter on
  image means rougher surface being imaged

(Virtual Science Centre)
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Radar Shadows
(ccrs.nrcan)
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Science Objectives of Imaging Radar

• Complementary to visible, near IR, and thermal IR
• Waves penetrate clouds
• Provides own illumination and produces reliable
  multi-temporal data independent of weather or
  solar illumination
• Can penetrate--under certain circumstances--vegeta
  tion canopies and very dry sand or soil, making
  it possible to examine near-surface zones
• Side-looking geometry better for viewing of 3D
  features

(Virtual Science Centre)
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Diffuse and Specular Reflectance
Canada Center for Remote Sensing
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Specular Reflectance and Diffuse Scatter

• Microwave reflectivity is a function of surface
  roughness, and thus the brightness of features on
  radar imagery.
• Smooth surfaces, where roughness height ltlt ?,
  reflect most of the incident energy in one
  direction, are called specular reflectors, from
  the Latin word speculum, meaning mirror.
• Such specular surfaces, as calm water or playas,
  appear dark on radar imagery.

• Microwaves incident upon a rough surface are
  scattered in broader directions, including back
  at the source - this is known as diffuse scatter
  or distributed reflectance.
• Vegetation surfaces will cause diffuse scatter,
  and result in a brighter tone on the radar
  imagery.

Canada Center for Remote Sensing
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Surface Roughness
Canada Center for Remote Sensing
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Surface Roughness

• Roughness of a scattering surface is a function
  of radar wavelength and incident angle.
• A surface is considered smooth if its height
  variations are smaller than the radar wavelength,
  and rough when greater.

• A surface appears rougher as incident angle
  increases.
• Rough surfaces will usually appear brighter due
  to backscatter on radar imagery than smoother
  surfaces of like material.

Canada Center for Remote Sensing
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Supplemental References

• Lillesand, T.M., R.W. Kieffer, J.W. Chipman,
  Remote Sensing and Image Interpretation, Wiley
  publisher
• Radar Imaging, http//southport.jpl.nasa.gov
• Remote Sensing Tutorial, http//rst.gsfc.nasa.gov
• The Virtual Science Centre Project on Remote
  Sensing, http//www.sci-ctr.edu.sg/ssc/publication
  /remotesense/rms1.htm
• Microwave Remote Sensing, http//www.ccrs.nrcan.gc
  .ca/ccrs/eduref/tutorial/chap3/c3p4e.html
• IFSAR and Shuttle Radar Topography Mission
  (SRTM), http//www.fas.org/irp/program/collect/isf
  ar.htm

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Supplemental References

• What is Synthetic Aperture Radar? Sandia
  Laboratories, Brian Mileshosky,
  http//www.sandia.gov/RADAR/whatis.html
• SAR Imagery Sandia Laboratories,
  http//www.sandia.gov/images/estancia.html
• Radar Band Designations, http//geog.hkbu.edu.hk/G
  EOG3610/Lect11/sld016.htm
• Synthetic Imaging Radar, http//geog.hkbu.edu.hk/G
  EOG3610/Lect-05
• The Planet Venus, http//csep10.phys.utk.edu/astr1
  61/lect/venus/venus.html
• Canada Centre for Remote Sensing, Fundamentals of
  Remote Sensing, http//www.ccrs.nrcan.gc.ca/ccrs.