Scatterometers Flashcards
Scatterometer
- 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
Scatterometry
- Form of radar RS sending short pulses of microwave EMR to surface, measuring power/amplitude of pulses that bounce/backscatter back to sensor
What was scatterometry originally designed for?
- to measure wind over oceans
- New applications, especially in cryosphere (snow cover, Arctic sea-ice extent)
Why is scatterometry used for cryosphere
- Good for large-scale distributed phenomena that can use the wide swath, low res of scatterometers
Underlying physics of scatterometry: eqn
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
PtGt/4piR^2
When combined w/ Sigma rt?
- 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
Ar/4piR^2
- Defines the total amount of backscattered energy that is received at the radar antenna
What parts of the eqn are all known quantities associated w/ the radar system?
Pt, G, and A
What is R?
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
Which variable is of the greatest interest to scatterometry?
Sigma rt
- Function of the way the transmitted EMR interacts w/ the surface
What happens when Sigma rt, the amount of energy scattered back to antenna, is integrated over a number of pulses?
- It is referred to as the scattering coefficient or backscattering coefficient denoted as sigma naught
Backscattering coefficient, Sigma Naught
- 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
Sigma Naught equation
Sigma naught (dB) = 10log sigma naught
Very high backscatter
- Above 5dB
- Man-made objects, urban
- Terrain slopes towards radar very rough surface
- Radar looking very steep
High BS
- 10 to 0dB
- Rough surface
- Dense vegetation/forest
Moderate BS
- 20 to -10dB
- Medium level of vegetation
- Agricultural crops
- Moderately rough surfaces
Low BS
Below -20dB
- Smooth surface
- calm water, road, very dry terrain (sand)
What does backscatter coefficient change as a function of?
- System and target parameters
- System are controllable
What are the target parameters that affect sigma naught?
- Target geometry
- Surface roughness
- Electrical properties (moisture)
Target geometry
- 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)
Surface roughness: Perfectly smooth surfaces
- 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
Surface roughness
- 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
How is surface roughness expressed?
- 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
Rayleigh roughness of C-band radar
- 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
Rayleigh roughness, What happens when you increase incidence angle?
- 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)
Rayleigh roughness of L-band radar
- 23cm wavelength
- At 50 degree incidence angle would only backscatter if surface features have minimum vertical relief of 4.5cm
Rayleigh roughness of L-band vs. C-band
- 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)
Target parameters affecting sigma naught, Dielectric constant
- Dielectric constant real portion in scatterometry represents the amount of energy that the medium scatters
- Imaginary portion represents amount of energy absorbed
Dielectric constant
- Electrical property that influences the interaction between matter and EM energy
- Function of temp and wavelength
Target parameters affecting sigma naught, electrical properties
- Liquid water = increase in dielectric constant in microwave region = sigma naught sensitive to moisture
- Higher BS w/ wet surface
What does the presence of liquid water and the dielectric constant mean for scatterometry?
- Property makes scatterometry useful for detecting melting of snow and ice, as well as for measuring soil moisture and vegetation content
Simple side-looking radar viewing geometry
- 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
Range
- Cross-track dimension perpendicular to flight direction
- Direction of side-looking system
Azimuth
- along-track dimension parallel to flight direction
Fan-beam type
- Covers large area around sensor on one or both sides, but not at Nadir
- Incidence angle changes further from sensor
Pencil-beam type
- Concentrates energy in narrow beams
Relationship btwn Incidence angle and sigma naught
- For a uniform target, sigma naught decreases as incidence angle increases
- Signal decreases from near-range beam to far-range
Relationship btwn Incidence angle, surface roughness, and sigma naught
- 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
Incidence Angle Normalization
- 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
Incidence Angle Normalization: What incidence angles are data usually limited to?
Angles btwn 20-50 degrees because measurements at more extreme incidence angles are more noisy
Incidence Angle Normalization: What is the common angle incidence is normalized to?
Angle of 40 degrees b/c this is roughly mid-swath for most instruments
At what range is backscatter commonly brighter? How do rougher surfaces influence this?
- Brighter in near range, darker in far
- Rougher surfaces ‘drop-off’ intensity less, therefore less change from near to far-range
How is spatial resolution defined?
- Defined in range and azimuth directions
- Range resolution, azimuth resolution
Range resolution
- 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
Azimuth resolution
- 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
Azimuth resolution, SeaWinds
- 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
Why do spaceborne scatterometers have small antenna length?
- 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
What is scatterometry’s primary objective? What spatial res does this usually operate at?
- 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
Why are scatterometers not very good for land?
- Pixel mixing, poor spatial res
- SAR will solve problem of small antenna length to enable sigma naught to be collected at high resolution
Which frequency penetrates deeper into Earth surface materials? What does this mean for sigma naught?
- 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
Which band/wavelength would be best for measuring vegetation biomass?
- 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
How does moisture affect frequency and sigma naught?
- 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
What does sigma naught relate to on water surfaces since the penetration depth due to dielectric constant is low
- Water acts as surface scatter so sigma naught relates to roughness
What does polarization relate to?
- 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
What polarization do tall objects relate stronger to? Power lines?
- Tall objects = VV stronger than HH
- Power lines = HH stronger than VV
Influence of polarization, which is better for crops and agriculture/ land cover contrast?
- HH not responsive to vertically structured crops = poor land cover type contrast
- VV sensitive to wheat stalks = more contrast
Depolarization
- 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
False colour composites
- Put different polarization combos in different guns to discern different effects
- Put HV in green to see vegetation better
Scatterometers: Wind measurements
- Increased roughness = increased BS = increased windspeed from more energy directed back at sensor
- Sensitive to capillary waves (small surface)
At what incidence angles is Radar most sensitive to wind speed?
30 to 60 degrees
- Lower and BS decibel change is not significant enough
Scatterometers: Wind Direction - What is it dependent on, and how is it accomplished?
- 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
Why is azimuth angle important for wind direction?
- Defines radar instruments ability to estimate wind direction
Scatterometers have low spatial res due to practical limitations of antennas (approx. 1m), What is used to overcome this limitation?
- SAR
