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Geog319RmSns > Scatterometers > Flashcards

Scatterometers Flashcards

1
Q

Scatterometer

A
  • Low resolution and large swaths enable regular global coverage at low cost from space borne scatterometers
  • Comparable to radiometers (low res, large swath)
  • Data can be mapped to a grid and visualized as an image
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2
Q

Scatterometry

A
  • Form of radar RS sending short pulses of microwave EMR to surface, measuring power/amplitude of pulses that bounce/backscatter back to sensor
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3
Q

What was scatterometry originally designed for?

A
  • to measure wind over oceans

- New applications, especially in cryosphere (snow cover, Arctic sea-ice extent)

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4
Q

Why is scatterometry used for cryosphere

A
  • Good for large-scale distributed phenomena that can use the wide swath, low res of scatterometers
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5
Q

Underlying physics of scatterometry: eqn

A

Pr = (PtGt/4piR^2)Sigma rt(Ar/4piR^2)

  • Pr = received power
  • Pt = transmitted power
  • Gt = gain of transmitting antenna in direction of target
  • R = distance btwn target and antenna
  • Sigma rt = Radar cross-section, the area of the tartest intercepting the transmitted pulse that produces a return pulse equal to the received power
  • Ar = effective receiving area of the receiving antenna aperture
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6
Q

PtGt/4piR^2

When combined w/ Sigma rt?

A
  • Defines the total amount of transmitted power reaching a given target
  • When amount of energy reaching target is multiplied by radar cross-section Sigma rt, this determines the amount of energy that the target scatters back to antenna
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7
Q

Ar/4piR^2

A
  • Defines the total amount of backscattered energy that is received at the radar antenna
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8
Q

What parts of the eqn are all known quantities associated w/ the radar system?

A

Pt, G, and A

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9
Q

What is R?

A

Related to the location of the target and can be determined from the duration it takes for the transmitted pulse to return to the antenna

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10
Q

Which variable is of the greatest interest to scatterometry?

A

Sigma rt

- Function of the way the transmitted EMR interacts w/ the surface

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11
Q

What happens when Sigma rt, the amount of energy scattered back to antenna, is integrated over a number of pulses?

A
  • It is referred to as the scattering coefficient or backscattering coefficient denoted as sigma naught
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12
Q

Backscattering coefficient, Sigma Naught

A
  • Dimensionless number, describes backscatter level (tone or brightness) when visualized as an image
  • Analogous to the reflectance of Earth surface materials at visible and infrared wavelengths used in optical remote sensing
  • High dynamic range (order of 10^5) expressed in decibels (dB)
  • Used to derive geophysical parameters and is primary variable used for scatterometer data
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13
Q

Sigma Naught equation

A

Sigma naught (dB) = 10log sigma naught

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14
Q

Very high backscatter

A
  • Above 5dB
  • Man-made objects, urban
  • Terrain slopes towards radar very rough surface
  • Radar looking very steep
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15
Q

High BS

A
  • 10 to 0dB
  • Rough surface
  • Dense vegetation/forest
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16
Q

Moderate BS

A
  • 20 to -10dB
  • Medium level of vegetation
  • Agricultural crops
  • Moderately rough surfaces
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17
Q

Low BS

A

Below -20dB

  • Smooth surface
  • calm water, road, very dry terrain (sand)
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18
Q

What does backscatter coefficient change as a function of?

A
  • System and target parameters

- System are controllable

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19
Q

What are the target parameters that affect sigma naught?

A
  • Target geometry
  • Surface roughness
  • Electrical properties (moisture)
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20
Q

Target geometry

A
  • General term describing the effect of structures on sigma naught
  • Related to scattering mechanism
  • Rougher structures give more intensity (hills more than trees more than rivers)
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21
Q

Surface roughness: Perfectly smooth surfaces

A
  • Reflect an incident radar pulse like a mirror, 90 degrees in the opposite direction from which it arrived
  • No energy scattered back to sensor/direction pulse arrived from
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22
Q

Surface roughness

A
  • Surface must be rough enough that energy is backscattered to antenna
  • Result is rougher surfaces have higher values of sigma naught
  • Surface is rough from perspective of radar pulse depending on the height of the roughness features on the surface relative to the radar’s wavelength
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23
Q

How is surface roughness expressed?

A
  • Rayleigh roughness criterion
  • Considers a surface to be rough if:
    Vertical relief of the surface roughness features (h) > (radar wavelength/(8cos theta)
  • Where theta is incidence angel of the radar pulse, in radians
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24
Q

Rayleigh roughness of C-band radar

A
  • 5cm wavelength/5GHz
  • At 50 degree incidence angle backscattering would only occur if surface had features w/ minimum roughness of 1 cm
  • At 20 degrees the surface must have minimum vertical relief of only 0.7cm
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25
Q

Rayleigh roughness, What happens when you increase incidence angle?

A
  • Roughness minimum relief of surface decreases
  • Smaller relief features can be scattered back
  • Therefore incidence angle of 20 is stronger than 50 degrees (20 closer to nadir, therefore steeper)
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26
Q

Rayleigh roughness of L-band radar

A
  • 23cm wavelength

- At 50 degree incidence angle would only backscatter if surface features have minimum vertical relief of 4.5cm

27
Q

Rayleigh roughness of L-band vs. C-band

A
  • C-band (shorter wavelength/higher freq) commonly used b/c it can detect finer scale roughness (C-band 1cm vs. 4.5cm at 50 degree incidence for L-band)
28
Q

Target parameters affecting sigma naught, Dielectric constant

A
  • Dielectric constant real portion in scatterometry represents the amount of energy that the medium scatters
  • Imaginary portion represents amount of energy absorbed
29
Q

Dielectric constant

A
  • Electrical property that influences the interaction between matter and EM energy
  • Function of temp and wavelength
30
Q

Target parameters affecting sigma naught, electrical properties

A
  • Liquid water = increase in dielectric constant in microwave region = sigma naught sensitive to moisture
  • Higher BS w/ wet surface
31
Q

What does the presence of liquid water and the dielectric constant mean for scatterometry?

A
  • Property makes scatterometry useful for detecting melting of snow and ice, as well as for measuring soil moisture and vegetation content
32
Q

Simple side-looking radar viewing geometry

A
  • Microwave beam is transmitted obliquely at right angles to the direction of flight
  • Illuminates a swath, offset from nadir (directly below platform)
  • Range and azimuth of swath
33
Q

Range

A
  • Cross-track dimension perpendicular to flight direction

- Direction of side-looking system

34
Q

Azimuth

A
  • along-track dimension parallel to flight direction
35
Q

Fan-beam type

A
  • Covers large area around sensor on one or both sides, but not at Nadir
  • Incidence angle changes further from sensor
36
Q

Pencil-beam type

A
  • Concentrates energy in narrow beams
37
Q

Relationship btwn Incidence angle and sigma naught

A
  • For a uniform target, sigma naught decreases as incidence angle increases
  • Signal decreases from near-range beam to far-range
38
Q

Relationship btwn Incidence angle, surface roughness, and sigma naught

A
  • Rate of sigma naught by incidence angle decreases more rapidly with smoother surfaces
  • Rough surfaces stronger than smooth
  • Rough surface sigma naught decrease at slower rate than smooth w/ increasing incidence angle
39
Q

Incidence Angle Normalization

A
  • To compare measurements taken at different incidence angles so that measurements btwn different areas, times, or instruments can be more readily compared
  • Data usually limited to incidence angles btwn 20-50 degrees because measurements at more extreme incidence angles are more noisy
  • Commonly normalized to incidence angle of 40 degrees b/c this is roughly mid-swath for most instruments
40
Q

Incidence Angle Normalization: What incidence angles are data usually limited to?

A

Angles btwn 20-50 degrees because measurements at more extreme incidence angles are more noisy

41
Q

Incidence Angle Normalization: What is the common angle incidence is normalized to?

A

Angle of 40 degrees b/c this is roughly mid-swath for most instruments

42
Q

At what range is backscatter commonly brighter? How do rougher surfaces influence this?

A
  • Brighter in near range, darker in far

- Rougher surfaces ‘drop-off’ intensity less, therefore less change from near to far-range

43
Q

How is spatial resolution defined?

A
  • Defined in range and azimuth directions

- Range resolution, azimuth resolution

44
Q

Range resolution

A
  • 2 ground targets resolved in range if their separation is greater than half the pulse length, if not echoes will overlap
  • Near range can’t resolve objects that are too close together
  • Short pulse length = fine range resolution
45
Q

Azimuth resolution

A
  • Dependent on beam width and range (distance)
  • Range gets wider further from sensor
  • Beam width, beta, is inversely proportional to antenna length (aperture), i.e a larger antenna = tighter beam
  • Features in far range may not be as separable as beam widens in azimuth direction
46
Q

Azimuth resolution, SeaWinds

A
  • Ku-band, wavelength, 2.2cm
  • Aperture, dH, 1m
  • Altitude, R, 822km
  • Bandwidth = wavelength/aperture = 0.022m/1m = 0.022
  • Azimuth res = altitudeBeam width = 822km0.022=18km
47
Q

Why do spaceborne scatterometers have small antenna length?

A
  • Practical reasons
  • For fine spatial res the antenna would need to be huge
  • Therefore spatial res is usually coarse for scatterometers
  • SAR will solve problem of small antenna length to enable sigma naught to be collected at high resolution
48
Q

What is scatterometry’s primary objective? What spatial res does this usually operate at?

A
  • Objective for sigma naught to map ocean at large scales and large-scale cryospheric parameters
  • 25-50km spatial res sufficient for this
  • Good b/c scatterometers do not have good res
49
Q

Why are scatterometers not very good for land?

A
  • Pixel mixing, poor spatial res

- SAR will solve problem of small antenna length to enable sigma naught to be collected at high resolution

50
Q

Which frequency penetrates deeper into Earth surface materials? What does this mean for sigma naught?

A
  • Lower freq, longer wavelength
  • Means sigma naught can be from surface, volume or surface and volume of a material like veg, soil, or ice depending on radar freq
51
Q

Which band/wavelength would be best for measuring vegetation biomass?

A
  • X band (3cm) would only give surface scattering from top layer, not enough penetration for veg biomass
  • L band (23cm) would give surface and volume and double-bounce scattering so sigma naught would be from top layer and branches and tree trunks
  • C band (5cm) would give surface and volume, sigma naught from top layer and branches, therefore best for veg biomass
52
Q

How does moisture affect frequency and sigma naught?

A
  • Presence of moisture will significantly reduce penetration depth of microwave energy at all freq
  • Large dielectric constant of water (80) compared to Earth materials causes this
  • Penetration depth into water is very small at all freq
53
Q

What does sigma naught relate to on water surfaces since the penetration depth due to dielectric constant is low

A
  • Water acts as surface scatter so sigma naught relates to roughness
54
Q

What does polarization relate to?

A
  • Related to physical structure and orientation of backscattering objects
  • Waves of pol interact w/ surface features in similar orientation
  • BS recorded at that pol combination (HH or VV) is greater
55
Q

What polarization do tall objects relate stronger to? Power lines?

A
  • Tall objects = VV stronger than HH

- Power lines = HH stronger than VV

56
Q

Influence of polarization, which is better for crops and agriculture/ land cover contrast?

A
  • HH not responsive to vertically structured crops = poor land cover type contrast
  • VV sensitive to wheat stalks = more contrast
57
Q

Depolarization

A
  • Multiple bounces of microwave energy tend to cause depolarization (H no longer H, V no longer V)
  • Effect is associated w/ volume scattering mechanisms like trees/veg
  • Detect effect in cross-polar measured in HV or VH to fix
58
Q

False colour composites

A
  • Put different polarization combos in different guns to discern different effects
  • Put HV in green to see vegetation better
59
Q

Scatterometers: Wind measurements

A
  • Increased roughness = increased BS = increased windspeed from more energy directed back at sensor
  • Sensitive to capillary waves (small surface)
60
Q

At what incidence angles is Radar most sensitive to wind speed?

A

30 to 60 degrees

- Lower and BS decibel change is not significant enough

61
Q

Scatterometers: Wind Direction - What is it dependent on, and how is it accomplished?

A
  • Dependence on Azimuth angle i.e. directionality of sensor
  • Sequential observations of BS on ocean surface from 3 directions (azimuth angles)
  • Different angles needed to resolve direction
  • Peak-to-Valley differences in BS reveal wind direction
  • Compare intensity of BS btwn angles to find angle to get orientation of waves towards sensor
62
Q

Why is azimuth angle important for wind direction?

A
  • Defines radar instruments ability to estimate wind direction
63
Q

Scatterometers have low spatial res due to practical limitations of antennas (approx. 1m), What is used to overcome this limitation?

A
  • SAR

Geog319RmSns (15 decks)

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