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Geog319RmSns > Passive Microwave RS > Flashcards

Passive Microwave RS Flashcards

1
Q

What are the 2 broad classes of microwave sensing instruments?

A
  • Passive MW, radiometers

- Active MW, radars

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2
Q

What are space borne imaging radars called?

A
  • Spaceborne imaging radars are called Synthetic Aperture Radar (SAR)
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3
Q

Radiometers

A
  • 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
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4
Q

Radars

A
  • Measurement of backscattered microwave radiation

- Instrument provides its own source of illumination

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5
Q

Basic radiometer configuration

A
  • Antenna captures emitted microwave radiation of specific frequency and polarization
  • Directs it to receiver
  • Signal strength converted to digital number and output
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6
Q

Polarization states

A
  • H = Horizontally polarized

- V = Vertically polarized

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7
Q

Swath

A

Width of track covered by sensing system

  • Often much greater than 500km for radiometers
  • Less than 500km for radars
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8
Q

What is the average spatial res of radiometers and why?

A
  • ## Low energy levels of naturally emitted microwave radiation means spatial resolution of PM radiometers is very low (km-scale)
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9
Q

What is the average spatial res of radars and why?

A
  • Radars transmit energy, capable of achieving spatial res that are much higher than radiometers (m-scale)
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10
Q

Which has the highest spatial res, radiometers or radars?

A
  • Radars (m-scale vs. km-scale)
  • b/c transmit energy, not reliant on natural emission
  • Swaths less than 500km vs much greater than 500km
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11
Q

Orbit

A
  • Describes path of satellite through space, relative to Earth
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12
Q

What are the 3 main characteristics of Orbit?

A
  • Altitude (height above ground)
  • Period (time req to complete 1 trip around Earth)
  • Inclination (angle of orbit relative to equator)
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13
Q

Inclination

A
  • Determines area covered by the path of the satellite

- Higher inclination = more Earth surface covered but decreased period

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14
Q

Geostationary orbit

A
  • Inclination 0 degrees
  • Directly over equator
  • Matches Earth’s rotation
  • Weather satellites (e.g., NOAA, GOES)
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15
Q

Polar orbit

A
  • 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
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16
Q

Polar Orbit Swath

A
  • 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)
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17
Q

AMSR-E

A

Advanced Microwave Radiometer for EOS

  • NASA’s EOS Aqua satellite
  • Mission 2002-2011
  • Swath 1445km
  • 12 channels
  • Global coverage 1-2 days (period)
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18
Q

Why use PM radiometers?

A
  • 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
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19
Q

Weather effects

A
  • 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
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20
Q

Target Complexity, 3 main media

A
  • 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
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21
Q

AMSR-E applications

A
  • 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
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22
Q

Di-urnal signal in PM data

A
  • less energy at night, not always significant
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23
Q

How is sensed PM measured?

A
  • 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
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24
Q

What are the wavelengths for cell phones and why?

A
  • Longer so they can pass through buildings
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25
Q

Product Levels

A
  • 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)
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26
Q

How is sea surface wind speed measured w/ PM?

A
  • From roughness of sea surface

- Not direct measurement

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27
Q

Geophysical Products

A
  • 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
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28
Q

Brightness Temp

A
  • 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
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29
Q

What is the relation of brightness temp (Tb) to emissivity?

A

Tb = emissivity x Temp (K)

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30
Q

What can measured brightness temp be used for?

A
  • Monitor variations in temp as well as properties related to emissivity
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31
Q

Emissivity

A
  • 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
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32
Q

2 objects have the same physical temperature, yet 1 has much lower measured Tb, why?

A
  • Their emissivities differ

- Object 2 has higher emissivity and is a more efficient radiator

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33
Q

Sea ice monitoring:

- Polar ocean features on a warm day, w/ all features at the melting point (273K)

A
  • 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
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34
Q

Target parameters that affect emissivity

A
  • Wetness/dielectric constant

- Surface roughness

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35
Q

System parameters that affect emissivity

A
  • Wavelength
  • Incidence angle
  • Polarization
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36
Q

PM Limitations for snow cover on land: Vegetation

A
  • Vegetation emits own microwave radiation
  • Increases emissivity and Tb
  • Masks signal of underlying snow cover
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37
Q

PM Limitations for snow cover on land: Terrain

A
  • Spatial heterogeneity

- Emissivity and Tb dependent on several cover types

38
Q

PM Limitations for snow cover on land: Underlying Soil

A
  • Tb increases due to increase in physical temp instead of emissivity
  • Emissivity from snow masked by temp difference
39
Q

PM Limitations for snow cover on land: Melt

A
  • 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
40
Q

PM Limitations for snow cover on land: Snow properties

A
  • 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
41
Q

Applications for soil moisture data

A
  • 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
42
Q

Soil Moisture

A
  • Satellite measurements advantageous over sparse in situ data
43
Q

What frequencies are used to monitor soil moisture?

A
  • 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
44
Q

What is the dielectric effect with soil moisture?

A
  • Emissivity is inversely proportional to dielectric constant and moisture content
  • dry emissivity much greater than wet emissivity
45
Q

Generalized process for retrieving AMSR-E soil moisture product

A
  • AMSR-E measurements
  • Normalized polarization and frequency differences
  • Soil moisture Look-Up Tables
  • Vegetation, soil type, surface roughness, temperature, etc.
46
Q

Soil moisture equations

A
  • 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
47
Q

Soil moisture limitations

A
  • Terrain
  • Vegetation
  • Temperature
  • Radio Frequency Interference (RFI)
48
Q

Soil Moisture Limitations: Vegetation

A
  • 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

49
Q

Soil Moisture Limitations: Terrain

A
  • 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
50
Q

Soil Moisture Limitations: Temperature

A
  • Phys temp estimated from in situ data or climatology
  • Tb = emissivity sfc * Physical T sfc
  • Dielectric effect from increased water content still dominates
51
Q

Soil Moisture Limitations: RFI

A
  • 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
52
Q

What are major drawbacks to PM RS?

A
  • 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
53
Q

What are some major applications of PM RS?

A
  • Possible to measure over open ocean and sea ice
  • Possible to measure distributed phenomena, including snow pack and soil moisture
  • Daily measurements are possible
54
Q

Incidence Angle

A
  • Angle btwn perpendicular to the image surface and the direction of received energy
  • Also called observation angle and viewing angle
55
Q

What is the normal incidence angle range for microwave radiometers?

A
  • 20 - 50 degrees
56
Q

Emissivity, system and target parameters

A
  • 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
57
Q

What are 2 main target parameters that affect emissivity?

A
  • Wetness/dielectric constant

- Surface roughness

58
Q

Dielectric constant

A
  • Related to electrical properties of the material

- Complex number with real and imaginary components

59
Q

Dielectric constant eqn

A

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’
60
Q

How does the emissivity of microwaves relate to the dielectric constant, Er?

A
  • 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&raquo_space; than of common Earth materials
  • Therefore when moisture increases, Er increases, and emissivity decreases
61
Q

Dielectric constant of common Earth materials and water

A
  • Most Earth materials Er = 1-4

- Air = 1, veg = 3, ice = 3.2, water = 80

62
Q

Influence of wetness/dielectric constant

A
  • Dielectric content increases w/ increasing moisture

- Er dry soil &laquo_space;Er water, therefore emissivity dry soil and brightness&raquo_space; wet soil

63
Q

System parameter influence on dielectric effect

A
  • Polarization: Brightness of H-pol is &laquo_space;than V-pol

- Incidence angle: higher incidence = greater difference between polarizations

64
Q

Influence of roughness on dielectric effect

A
  • 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)
65
Q

Emissivity depends on what 3 major factors?

A
  • Dielectric constant
  • Incidence Angle
  • Polarization
66
Q

What is the emissivity of a Lambertian surface?

A
  • Emissivity will be the same, no matter the incidence angle or polarization
67
Q

How is sea surface wind speed modelled with PM radiometers?

A
  • Take roughness value on ocean surface and invert to get wind speed
  • Rougher surface = stronger wind
68
Q

What are the system parameters that affect emissivity

A
  • Wavelength
  • Incidence Angle
  • Polarization
69
Q

Influence of wavelength on emissivity

A
  • Most Earth surface materials are selective radiators: their emissivity varies as function of wavelength
  • Black-body emissivity = 1, Grey-body < 1
70
Q

What is the result of wavelength influence on emissivity when it is approximately the same size as the particle?

A
  • 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
71
Q

What is the result of incidence angle on emissivity?

A
  • Increase incidence angle and the difference in emissivity btwn H-pol and V-pol increases (V-pol >H-pol)
72
Q

What is the wavelength influence on emissivity relating to an example using snow?

A
  • 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
73
Q

Why are radiometers set at an angle?

A
  • So it can better detect variations in Earth surface features
74
Q

Penetration depth

A
  • Depth of a layer to which the microwave emission is reduced to 1/e where e is Euler’s number (approx. 37%)
75
Q

What does penetration depth depend on?

A
  • Wavelength (shorter /higher freq = decreased penetration)

- Dielectric constant (increased wetness = decreased penetration depth)

76
Q

Brief summary of Brightness temperature Tb

A
  • 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
77
Q

Notable microwave radiometer missions:Instrument

A
  • Nimbus 7: SMMR
  • DMSP: SSM/I and SSMIS
  • Aqua: AMSR-E
  • SMOS: MIRAS
  • GCOM-W1: AMSR-2
  • SMAP: SMAP
  • All vary in frequencies
78
Q

Hydrological Applications

A

soil moisture, watershed surface drainage, flood mapping, mapping of surface water, snow cover extent, SWE, snow wetness

79
Q

Agriculture Applications

A

Soil moisture distribution for crop yield estimation and irrigation scheduling, delineation of freeze-thaw boundaries

80
Q

Cryosphere Applications

A

Monitoring and mapping sea-ice concentration, sea-ice type, mapping glacial ice sheets, monitoring ice-sheet melt conditions, snow covered areas

81
Q

Ocean Applications

A

Measuring surface wind speed, measuring surface temperature, measuring surface salinity, monitoring oil spills, measuring and mapping rain

82
Q

Why is knowledge of snow cover on land important?

A
  • Flood forecasting
  • Weather forecasting and climate monitoring
  • Water resource management
  • Hydroelectric power production
  • Forest fire modelling
83
Q

What are the snow cover properties derived from PM radiometer data?

A
  • 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)
84
Q

SWE

A
  • 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
85
Q

Dry snow over soil

A
  • 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)
86
Q

Snow cover on land: Spectral gradient

A
  • 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
87
Q

Spectral gradient approach for deriving SWE

A

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
88
Q

Why is V-pol used to derive SWE?

A
  • Since it is less affected b snow pack properties

- It is more directly related to snow depth

89
Q

How is SCA derived?

A
  • From snow depth or SWE products
  • Simple thresholding, pixel contains or does not contain snow
  • SWE >0 = yes, SWE<0 = no
90
Q

Limitations to PM for Snow cover on land (SWE, SCA, depth)

A
  • Terrain/vegetation
  • Soil properties
  • Melt
  • Snow properties

Geog319RmSns (15 decks)

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