Passive Microwave RS Flashcards
What are the 2 broad classes of microwave sensing instruments?
- Passive MW, radiometers
- Active MW, radars
What are space borne imaging radars called?
- Spaceborne imaging radars are called Synthetic Aperture Radar (SAR)
Radiometers
- Measurement of naturally emitted microwave radiation
- Sun is original source of energy
- Most antennas contain an array of feed horns which collect H and V polarized microwave radiation at different frequencies
Radars
- Measurement of backscattered microwave radiation
- Instrument provides its own source of illumination
Basic radiometer configuration
- Antenna captures emitted microwave radiation of specific frequency and polarization
- Directs it to receiver
- Signal strength converted to digital number and output
Polarization states
- H = Horizontally polarized
- V = Vertically polarized
Swath
Width of track covered by sensing system
- Often much greater than 500km for radiometers
- Less than 500km for radars
What is the average spatial res of radiometers and why?
- ## Low energy levels of naturally emitted microwave radiation means spatial resolution of PM radiometers is very low (km-scale)
What is the average spatial res of radars and why?
- Radars transmit energy, capable of achieving spatial res that are much higher than radiometers (m-scale)
Which has the highest spatial res, radiometers or radars?
- Radars (m-scale vs. km-scale)
- b/c transmit energy, not reliant on natural emission
- Swaths less than 500km vs much greater than 500km
Orbit
- Describes path of satellite through space, relative to Earth
What are the 3 main characteristics of Orbit?
- Altitude (height above ground)
- Period (time req to complete 1 trip around Earth)
- Inclination (angle of orbit relative to equator)
Inclination
- Determines area covered by the path of the satellite
- Higher inclination = more Earth surface covered but decreased period
Geostationary orbit
- Inclination 0 degrees
- Directly over equator
- Matches Earth’s rotation
- Weather satellites (e.g., NOAA, GOES)
Polar orbit
- Sometimes near-polar
- Inclination approx. 90 degrees
- Merges swath and orbit to provide regular coverage of most of Earth’s surface
- Most radiometers and SAR’s are polar orbiting
Polar Orbit Swath
- For single swath size: shorter revisit time at high latitudes (1 day) compared to equator (2 day) (i.e. swaths spaced further apart at equator)
- Comparing 2 swaths from different instruments: shorter resist time if swath is wider (e.g. radiometers compared to radars)
AMSR-E
Advanced Microwave Radiometer for EOS
- NASA’s EOS Aqua satellite
- Mission 2002-2011
- Swath 1445km
- 12 channels
- Global coverage 1-2 days (period)
Why use PM radiometers?
- All weather imaging (except high freq - smaller wavelengths interact w/ weather more)
- Day/night imaging
- Global coverage in almost 1 day (low spatial res, but high temp res)
- Sensitivity to certain geophysical phenomena
Weather effects
- Choice of wavelength: Wavelengths greater than 2cm exhibit negligible weather effects (low freq)
- Apply atm correction: atm components removed based on known or estimated stature
Target Complexity, 3 main media
- Atmosphere - Best ‘behaved’ for our purposes
- Ocean - More complex, generally well understood due to uniformity
- Land - Most complex, physical properties of surface features like land and veg cover vary significantly over space and time
AMSR-E applications
- Sea ice: seen even in polar winter w/ no light, seen in all weather
- Sea-surface temp: warm temp = hurricanes, track storm movement based on temp, leave cool temps behind and kill storms behind
- NASA data is mostly open source
Di-urnal signal in PM data
- less energy at night, not always significant
How is sensed PM measured?
- Analagous to TIR
- Upwelling microwave energy, related to temp, is detected by radiometer and converted to brightness values, forms image
- low levels of natural PM = large FOV to build strong enough signal to form image
What are the wavelengths for cell phones and why?
- Longer so they can pass through buildings
Product Levels
- Most PM radiometer data delivered in level 2 or 3
- Level 1 = engineering values from instrument (DN to output voltages, swath format)
- Level 1B = physical values observed by instrument (brightness temps, swath format)
- Level 2 = Geophysical parameters estimated by retrieval algorithms (data location and quality, swath format)
- Level 3 = Daily and monthly global grids, generated for brightness temp and geophysical parameters (temporal and spatial avg values projected to global grid)
How is sea surface wind speed measured w/ PM?
- From roughness of sea surface
- Not direct measurement
Geophysical Products
- Algorithms used to derive geophys products are continually being created, assessed and improved, and compared to data generated from other sensors e.g. ground and airborne to validate snow products
- Sometimes Level 1B brightness temps used and simple algorithm applied to create Level 2 e.g. SCA from brightness temp
Brightness Temp
- Quantity measured by a microwave radiometer
- Apparent radiant temperature of some object in some portion of microwave region
- Can be related to physical temp if emissivity is known
What is the relation of brightness temp (Tb) to emissivity?
Tb = emissivity x Temp (K)
What can measured brightness temp be used for?
- Monitor variations in temp as well as properties related to emissivity
Emissivity
- Ratio of radiant exitance of an object (M) and that of a black-body (Mb) at the same physical temp (T):
Emissivity = M/Mb
Emissivity btwn 0 and 1
2 objects have the same physical temperature, yet 1 has much lower measured Tb, why?
- Their emissivities differ
- Object 2 has higher emissivity and is a more efficient radiator
Sea ice monitoring:
- Polar ocean features on a warm day, w/ all features at the melting point (273K)
- Emissivity of seawater, 0.4 < old ice, 0.85 < young ice, 0.95
- Tb of Sea water < old ice < young ice
- old ice has more interaction from more complex structure
- Differing Tb used to differentiate areas for monitoring even though phys temp is the same
Target parameters that affect emissivity
- Wetness/dielectric constant
- Surface roughness
System parameters that affect emissivity
- Wavelength
- Incidence angle
- Polarization
PM Limitations for snow cover on land: Vegetation
- Vegetation emits own microwave radiation
- Increases emissivity and Tb
- Masks signal of underlying snow cover
PM Limitations for snow cover on land: Terrain
- Spatial heterogeneity
- Emissivity and Tb dependent on several cover types
PM Limitations for snow cover on land: Underlying Soil
- Tb increases due to increase in physical temp instead of emissivity
- Emissivity from snow masked by temp difference
PM Limitations for snow cover on land: Melt
- Strong emissivity and Tb increase when snow melts
- Wet snow approaches black-body behaviour
- Dielectric effect where increase moisture, decreases emissivity but snow packs increase water and increase emissivity (snow is weird)
- SWE and SCA estimates not attainable
- But can use to get maps of melt for climate data
PM Limitations for snow cover on land: Snow properties
- Stratification: vertical inhomogeneity of snow pack
- Aging: density of ice grain size increase w/ time, leads to decreased emissivity despite SWE remaining the same
- Ice lenses and depth hoar formation can be factors
- i.e. Aging snow decreases emissivity w/ larger grains
Applications for soil moisture data
- Days: Daily forecasting for runoff, flooding, clouds, fog development, hydrology
- Weeks: Forcasting, monitoring, managing, crop growth, hazards from floods drought and fires, global climate
- Years: Monitoring for global climate, long-term drought prediction, Agricultural suitability, land use planning
Soil Moisture
- Satellite measurements advantageous over sparse in situ data
What frequencies are used to monitor soil moisture?
- Low frequency radiometer channels 6-10 provide greater contrast btwn wet/dry
- Sensitive to changes in surface moisture
- Not affected by cloud cover and precip
- Greater penetration depth compared to higher frequencies, still only 3cm
- Estimation of deeper soil moisture values requires a model
What is the dielectric effect with soil moisture?
- Emissivity is inversely proportional to dielectric constant and moisture content
- dry emissivity much greater than wet emissivity
Generalized process for retrieving AMSR-E soil moisture product
- AMSR-E measurements
- Normalized polarization and frequency differences
- Soil moisture Look-Up Tables
- Vegetation, soil type, surface roughness, temperature, etc.
Soil moisture equations
- Normalized polarization difference at 10.7GHz = (Tbv - Tbh)/0.5*(Tbv + Tbh)
- Normalized Frequency difference of 37 and 10.7GHz = (Tbh37 - Tbh10.7)/0.5*(Tbh37 + Tbh10.7)
- Both indices increase w/ soil moisture
- Polarization index also increases w/ increased vegetation
- Denominators help minimize the effects of physical temperature
Soil moisture limitations
- Terrain
- Vegetation
- Temperature
- Radio Frequency Interference (RFI)
Soil Moisture Limitations: Vegetation
- Veg increases emissivity, masks signal
- Bare wet soil has lower emissivity than wet soil with vegetation b/c veg signal overcomes small bare soil signal
Soil Moisture Limitations: Terrain
- Surface roughness decreases sensitivity of emissivity to soil moisture
- Corrections for rough surface necessary
- Prior knowledge of surface type can be used to correct roughness effects in retrieval algorithm
- Rougher surface overwhelms emissivity, therefore emissivity much higher than smooth soil
Soil Moisture Limitations: Temperature
- Phys temp estimated from in situ data or climatology
- Tb = emissivity sfc * Physical T sfc
- Dielectric effect from increased water content still dominates
Soil Moisture Limitations: RFI
- Hot spots near metropolitan areas and transportation corridors
- 6-8GHz freq used for transmissions
- Also effects other applications which use channels in 6-8GHz range
What are major drawbacks to PM RS?
- Detected energy levels are very low
- Data collected over large regions
- Fine-scale details not resolved
- Emissivity over land is highly variable due to spatial heterogeneity
What are some major applications of PM RS?
- Possible to measure over open ocean and sea ice
- Possible to measure distributed phenomena, including snow pack and soil moisture
- Daily measurements are possible
Incidence Angle
- Angle btwn perpendicular to the image surface and the direction of received energy
- Also called observation angle and viewing angle
What is the normal incidence angle range for microwave radiometers?
- 20 - 50 degrees
Emissivity, system and target parameters
- System and target effects are relative e.g. the effect of wetness of an object is also freq dependent
- We can control system parameters as these are part of sensor design
- Surface or in-situ studies help understand system and target effects
What are 2 main target parameters that affect emissivity?
- Wetness/dielectric constant
- Surface roughness
Dielectric constant
- Related to electrical properties of the material
- Complex number with real and imaginary components
Dielectric constant eqn
Er = E’ + iE”
- where E’ is the real part related to reflectivity, describes ability of a material to transmit or ‘permit’ an electric field across a boundary, also called permittivity
- E” is imaginary part, related to loss i.e. through absorption
- E” is negligible for most applications, therefore simplify to E’
How does the emissivity of microwaves relate to the dielectric constant, Er?
- Emissivity of microwaves is inversely proportional to Er
- object becomes less efficient radiator as Er gets larger
- Er becomes larger when material moisture content increases since Er of water»_space; than of common Earth materials
- Therefore when moisture increases, Er increases, and emissivity decreases
Dielectric constant of common Earth materials and water
- Most Earth materials Er = 1-4
- Air = 1, veg = 3, ice = 3.2, water = 80
Influence of wetness/dielectric constant
- Dielectric content increases w/ increasing moisture
- Er dry soil «_space;Er water, therefore emissivity dry soil and brightness»_space; wet soil
System parameter influence on dielectric effect
- Polarization: Brightness of H-pol is «_space;than V-pol
- Incidence angle: higher incidence = greater difference between polarizations
Influence of roughness on dielectric effect
- Specular surface = no roughness effect on emissivity
- Increase roughness = increased emissivity and less difference btwn polarizations (i.e. H-pol and V-pol begin to converge w/ increasing roughness)
Emissivity depends on what 3 major factors?
- Dielectric constant
- Incidence Angle
- Polarization
What is the emissivity of a Lambertian surface?
- Emissivity will be the same, no matter the incidence angle or polarization
How is sea surface wind speed modelled with PM radiometers?
- Take roughness value on ocean surface and invert to get wind speed
- Rougher surface = stronger wind
What are the system parameters that affect emissivity
- Wavelength
- Incidence Angle
- Polarization
Influence of wavelength on emissivity
- Most Earth surface materials are selective radiators: their emissivity varies as function of wavelength
- Black-body emissivity = 1, Grey-body < 1
What is the result of wavelength influence on emissivity when it is approximately the same size as the particle?
- Particles w/in a volume causes decrease in emissivity when wavelength is approx. = to particle size
- Scatter loss/attenuation
- eg snow: at 37GHz: snow grains approx. wavelength but at 19Ghz snow grains < the longer wavelength
What is the result of incidence angle on emissivity?
- Increase incidence angle and the difference in emissivity btwn H-pol and V-pol increases (V-pol >H-pol)
What is the wavelength influence on emissivity relating to an example using snow?
- Snow grains same size = decrease energy radiating, lost w/in own volume
- Snow grains larger = increase energy radiating
- eg snow: at 37GHz: snow grains approx. wavelength but at 19Ghz snow grains < the longer wavelength
Why are radiometers set at an angle?
- So it can better detect variations in Earth surface features
Penetration depth
- Depth of a layer to which the microwave emission is reduced to 1/e where e is Euler’s number (approx. 37%)
What does penetration depth depend on?
- Wavelength (shorter /higher freq = decreased penetration)
- Dielectric constant (increased wetness = decreased penetration depth)
Brief summary of Brightness temperature Tb
- Energy detecting by radiometer is product of true temperature of an object and its emissivity
- Changes in emissivity, as well as changes in temperature, may be measured by a radiometer and used to discriminate targets
Tb = emissivity x Temp (K) - Emissivity varies by system and target parameters
- EO applications exploit system and target parameters to enable feature detection
Notable microwave radiometer missions:Instrument
- Nimbus 7: SMMR
- DMSP: SSM/I and SSMIS
- Aqua: AMSR-E
- SMOS: MIRAS
- GCOM-W1: AMSR-2
- SMAP: SMAP
- All vary in frequencies
Hydrological Applications
soil moisture, watershed surface drainage, flood mapping, mapping of surface water, snow cover extent, SWE, snow wetness
Agriculture Applications
Soil moisture distribution for crop yield estimation and irrigation scheduling, delineation of freeze-thaw boundaries
Cryosphere Applications
Monitoring and mapping sea-ice concentration, sea-ice type, mapping glacial ice sheets, monitoring ice-sheet melt conditions, snow covered areas
Ocean Applications
Measuring surface wind speed, measuring surface temperature, measuring surface salinity, monitoring oil spills, measuring and mapping rain
Why is knowledge of snow cover on land important?
- Flood forecasting
- Weather forecasting and climate monitoring
- Water resource management
- Hydroelectric power production
- Forest fire modelling
What are the snow cover properties derived from PM radiometer data?
- Snow depth, H (cm)
- Snow Water Equivalent, SWE (mm or cm)
- Snow-Covered Area, SCA (% or fraction)
- Wet-dry state (melt onset and freeze-up dates)
SWE
- linearly dependent on H and bulk density
- SWE = bulk density x snow depth
- Bulk density is a conservative value, a single value is often used to represent large areas that may have significant density variations
Dry snow over soil
- Radiation emitted from the underlying soil is scattered by snow grains
- As scattering increases in proportion to the mass of snow, emissivity (and Tb) decreases (i.e. more snow = less emissivity)
Snow cover on land: Spectral gradient
- Difference in Tb btwn 2 channels (19 and 37GHz)
- Scattering loss by snow grains and emissivity decrease at 37GHz, while no loss and greater emissivity at 19GHz
- Allows for estimates of snow depth, SWE, and SCA
Spectral gradient approach for deriving SWE
SWE = a * (Tb19V - Tb37V)
- a is a coefficient which relates change in SWE to Tb (mm K^-1), e.g. 4.8
- a determined from models and experiments
- V-pol used
Why is V-pol used to derive SWE?
- Since it is less affected b snow pack properties
- It is more directly related to snow depth
How is SCA derived?
- From snow depth or SWE products
- Simple thresholding, pixel contains or does not contain snow
- SWE >0 = yes, SWE<0 = no
Limitations to PM for Snow cover on land (SWE, SCA, depth)
- Terrain/vegetation
- Soil properties
- Melt
- Snow properties
