Methods of Developing Radio Frequency Interference Mitigation for Microwave Radiometry

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Methods of Developing Radio Frequency
Interference Mitigation for Microwave Radiometry

• Roger De Roo

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Outline

• Monitoring the soil moisture with microwave
  radiometry
• Motivation
• Physics of passive remote sensing
• Radio Frequency Interference a major problem
• Summary of RFI detection approaches
• Methodologies of the kurtosis algorithm
  development
• Conclusions

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Soil Moisture Active and Passive (SMAP)

• NASA environmental satellite
• Currently in planning stages
• Launch 2010-2013

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Soil Moisture who cares?

• Soil Moisture regulates plant transpiration
• Transpiration determines humidity
• Humidity gives rise to clouds
• No widespread measurements of soil moisture
  currently

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Whats so great about Microwave Remote Sensing?
Long wavelengths (3mm to 30cm) dont scatter off
of objects the size of cloud droplets --
microwaves see through clouds
Radar Radiometry

• Very high spatial resolution
• Power hungry expensive
• Sensitive to geometry of water eg. Movement of
  trees causes big signal changes

• Poor spatial resolution
• Low power requirements
• Insensitive to geometry of water

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Planck Blackbody Radiation
6000K white hot the Sun
3000K red hot
300K room temp
30K
3K outer space
1 GHz
1 THz
1 PHz frequency wavelength
0.3 m
0.3 mm 0.3 um
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Microwave Brightness and Moisture

• Water molecules have large electric dipole,
  unlike rest of nature
• An interface w/ high contrast of index of
  refraction leads to reflection
• Dry soils appear warm, while wet soils appear
  cold, at the same temp.

H - O H
Liquid water molecules will orient itself with
passing electromagnetic waves,slowing the wave
down The molecule can keep up with the wave until
9 GHz (index of refraction n 9 at 1GHz, but n
2 at 100 GHz)
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Example Brightness Image from Space
NASDA
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Sensitivity of Radiobrightness to Soil Moisture
Under a Vegetation Canopy
19 GHz
6.9 GHz
1.4 GHz
Courtesy of P. ONeill
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University of Michigan Radiometers
L-band 1.4 GHz l 21 cm satellites none
yet! SMOS 08 Aquarius 10 SMAP 14
C-band 6.7 GHz l 4.5 cm satellites AMSR-E 02
19 GHz 37 GHz l 1.6 cm l 0.8 cm
satellites SSM/I etc. 87 to present
Antenna size is proportional to wavelength
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Diurnal Brightness Measurements
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Brightness of Tundra and Shrubs
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Trouble with the 1.4GHz Radiometer
Both of these ranges appear plausible
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Potentially Interfering RadarsCobra Dane
Peak Transmit Power 16.8 MW Transmit Frequency
1.215-1.375GHz
Raytheon
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Surrounded by Interfering Radars?
FPS-124
FPS-108 Cobra Dane
Observation site Toolik Lake
FPS-117
ITT, 05
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AMSR-E Interference at 6.9GHz
If it is not purple, we cannot use the data from
that location If it is purple, the data from that
location might be OKor not
Li et al., 04
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Approaches to Detecting RFI

1. Time domain look for pulses
2. Frequency domain look for carrier frequencies
3. Amplitude domain look for non-thermal
   distribution

Gaussian pdf
Non-Gaussian pdf
Sinusoidal waveform
Thermal waveform
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Digital Radiometry
Digital radiometers use fast analog-to-digital
converters to measure the voltage waveform
Power is determined by finding the variance
(2nd moment) of the quantized data Processing
capability allows for implementation of one or
more RFI mitigation strategies
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Methods

• Lit search!
• Theory development (analysis in the math sense
  of the word)
• Simulation
• Highly controlled experiments in the laboratory
• Less controlled experiments in the field

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Literature Search

• What has already been done on this problem, or
  related problems?
• For RFI mitigation, nothing in the amplitude
  domain. Some in time-domain and some in
  frequency domain.
• However, testing for normality of a distribution
  does have a rich literature. Lotsa ways to do
  it, and it is known how well they work.

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Is it Normal? The kurtosis

• Statistical moments
• 0th event count
• 1st Mean
• 2nd Variance
• 3rd Skew
• 4th Kurtosis

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Skew

• Measures how asymmetric a distribution is
• Normal distribution has zero skew
• So does RFI ?

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Kurtosis

• Kurtosis measure peakedness of a distribution
• Normal distribution has kurtosis 3
• RFI can have any kurtosis

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Theory Development

• Paper and pencil derivations of what to expect
• Requires assumptions
• Results are general if the assumptions hold true

Geothermal Signal
Atmosphere
Antenna
Receiver
Data Processor
Radio Frequency Interference
Atmosphere
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Theory Development some assumptions
All curves have the same variance A radiometer
will report all of these signals as the same
brightness
Pulsed sinusoid to noise ratio S dA2/2s2
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Simulation

• Mimic the expected process in a computer
• Use to verify/ validate the analysis
• Not general Can only conclude about the cases
  simulated.
• Can also be used to extend the analysis to cases
  that violate the assumptions

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Simulation

• One example
• Spread of kurtosis due to finite no. of samples
• Curves are a prediction
• Dots are calculated by computer program

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Laboratory Experiments

• Check assumptions about radiometer operation
• RFI prescribed to conform to theory assumptions

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Laboratory Experiment Results

• Curves are theory
• Marks are data
• They match!!!

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Field Experiments

• Check assumptions about RFI very realistic
  environment
• No control of when RFI comes, or what kind it is,
  or how strong.

RFI flags
TSYS (counts2)
sky
sky
absorber
seconds
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Field Experiments

• A lot of fun to do!
• Takes lots of people () to do.

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Field Campaign Results
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Conclusions

• Different research methods have different
  strengths and weaknesses
• For RFI research, the methods can be considered
  to lie on a spectrum
• Theory Simulation Laboratory Experiment
  Field Experiment
• Most general
  Very Specific
• Many assumptions
  Very Realistic
• Relatively cheap
  Quite Expensive
• Combinations of methods draws on strengths of each

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Thank You!
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Backup Slides
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Tanana River Breakup at Nenana
Guess the moment of breakup! Tickets cost 2.50
each Typical Jackpot 300,000 www.nenanaakicec
lassic.com
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Observed Global Temperature Trends
IPCC 01
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Projected Global Temperature Trends
2071-2100 temperatures relative to
1961-1990. Special Report on Emissions Scenarios
Storyline B2 (middle of the road warming).
IPCC 01
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Carbon Stocks by Biome
Atmospheric stock is about 750PgC
IPCC 01
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Permafrost extent
Global Terrestrial Network for Permafrost
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20m Borehole Temperature Trends in AK
Hinzman et al 2005
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Permafrost structure
NSIDC
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Active Layer Depth Trends
Circumpolar Active Layer Monitoring Network
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The Tundra Landscape
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Strategy for Estimating Temperature and Moisture
Profiles
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Calibrated LSP/R model of Prairie Grassland
Judge et al. 1999
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Correlated Noise Calibration System
To Radiometer
From AWG
Low Noise Amplifier (LNA) input is a matched
source of sub-ambient noise it is an electronic
device which, at RF, looks like it is at LN2
temperatures CNCS concept Onto this very low
noise background, couple in some much stronger
noise. This much stronger noise can be
generated in a COTS Arbitrary Waveform
Generator CNCS extension This same concept
can be used to create known weak RFI
Ruf and Li, 03
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Detection and Mitigation Testbed
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Conclusions
Microwave Radiometry has been demonstrated to
have high sensitivity to surface soil
moisture. Hydrologic models can use this
measurement to constrain the evolution of
profiles of temperature and moisture. This
technique should work well for the low
vegetation content of the Arctic. Understanding
the evolution of the active layer will help us
understand the threat of carbon release from
Arctic soils in response to climate
change. Microwave observations are very
susceptible to interference. RFI mitigation for
microwave radiometry is an emerging research area
at Michigan