Active Microwave Physics and Basics

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Simon YuehJPL, Pasadena, CA

August 14, 2014

How Deep Can the Radio Waves Penetrate• 10 to17 GHz microwave can penetrate dry

snowpack with a broad range of depth (1 to 5 m)

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• Experiment, Radio Laboratory, Helsinki University of Technology in 1987• Theoretical simulations from bicontinuous medium/NMM3D, Xu et al, 2012

Frequency Penetration Depth

10 GHz (X) ~5 m14 GHz (Ku)

~1 m

18 GHz (K) ~0.5 m37 GHz (Ka)

~0.1 m

0.01m

0.1m

1m

10m

Radar Sounding of SnowSurface Scattering

• Surface scattering dominates at near nadir looking• Early demonstration by late Prof. Hal Boyne (CSU)

• Current Status – A well-developed tool for probing the snow stratigraphy– Marsahll et al., ground-based FMCW Radar – Gogineni et al., aircraft-based Snow Radar

Courtesy of Boyne

• What is the resolution?– ΔR=Range resolution=C/2B– ΔH=H(1/cosθ-1) for rough interface

• Beamwidth (2θ) and height (H)– Horizontal resolution=2Hθ – limited by

beamwidth ΔRΔH

B ΔR

1 GHz 15 cm

5 GHz 3 cm

H ΔH

1000 m, 10deg 3.8m

10 m, 5 deg 1 cm

Off-nadir Looking RadarVolume Scattering

SAR processing can achieve horizontal resolution of a few meters from space

Backscatter contributions:Volume, surface, and interaction terms.Observed backscatter coefficient σ° :

asgvvg '0

At off-nadir angles (30-50 degrees incidence angles)Volume scattering starts to dominateSurface scattering diminishes

Main parameters for snow backscatter:Dry snow• Snow water equivalent • Grain size (d)• Density (ρ)• Soil background signalWet snow• Liquid water content (radar signal does not

penetrate)

One example of data and theoryMore data acquired through CLPX2, SnowScat and SnowSAR campaigns

• Snow

50 60 70 80 90 100 110 120-20

-18

-16

-14

-12

-10

-8

-6

SWE (mm)

VV

(dB

)

SnowSCAT backscatter against SWE, 40 , 16.7GHz

10.2GHz13.3GHz16.7GHz

03/01/201112/28/2010

SnowSCAT backscatter time series σvv with 40∘ incidence angle against SWE. Data taken from at Sodankylä between 12/28 /2010 and 03/01/2011.

Simulated radar backscatter using the DMRT/QCA for snow volume scattering at three frequencies. All three frequencies show response to snow water equivalent for moderate and large grain size.

SAR Snow TomographySide-looking radar with multiple baselines

• Snow stratigraphy - Metamorphism and environmental factors create complex layering structures in the snow pack

• SAR Tomography will provide insight into snow and ice – Lack of comprehensive theoretical

development and experimental testing for snow

• SAR Tomography – Tested for 3-D forest canopy mapping– Coherence and multiple baselines– Demosntrated by GB-SAR, K Morrison of Cranfield U.

Measurements at Reynolds Creek study site, 200 meters from tower - 116 manual probe depth measurements. (Marshall et al. of BSU)

Leln

r

dr

Hei

ght (

m)

Sla

nt R

ange

(m)

Polarimetric tomographic profile over a forested area using DLR’s E-SAR system at L-band [Moreira et al., IEEE GRS magazine, 2013].

Recent campaigns covering main snow regimes

Churchill, Canada, Tundra

(Near-)Coincident Ku-band and X-band scatterometers and SAR used

Sodankylä, Finland, Taiga

Innsbruck, Austria, Alpine

Colorado, USA Alpine/Tundra/ Taiga/Prairie

Inuvik, Canada, Tundra

Kuparuk, Alaska, Tundra

Radar backscatter versus SWE – from Sodankylä, Finland, Taiga

Backscatter versus observed SWE, Sodankylä, Finland , SnowScat measurements for winter I , for winter II radiative transfer model calculation for 3 different values of grain size

SnowScat measurements at 40° for two winters

Radar backscatter versus SWE – from Rocky Mountain, Colorado

Backscatter for VV, HH, and VH polarizations shows sensitivity to SWE for three sampling sitesYueh et al., Airborne Ku-band Polarimetric Radar Remote Sensing of Terrestrial Snow Cover, IEEE TGRS, Vol. 47, No. 10, 3347-3364, 2009.

NASA/JPL POLSCAT measurements