1. Spaceborne Polarimetric Microwave Radiometer Brandon Ravenscroft
WindSat
Spaceborne Polarimetric Microwave Radiometer
Brandon Ravenscroft
EECS 823 – 12/06/2016

2. WindSat Radiometer
Multi-frequency, polarimetric, space borne microwave radiometer (first of its kind)
Primary payload of DoD Coriolis Satellite (other is SMEI)
Developed by NRL for U.S. Navy and National Polar-orbiting Operational Environment Satellite System Integrated Program Office (NPOESS-IPO)
Launched January 2003 and still operating
Objective: show capability of polarimetric microwave radiometry to measure global ocean surface wind direction/velocity
Provide: information for weather forecasting and warnings, naval missions, ocean circulation and oceanography studies [a]

3. Operation 840 km helio-synchronous near-polar orbit
Also Provides: column integrated cloud liquid water, column integrated precipitable water and sea surface temperature
22 discrete channels operating at five different frequency bands
Measures near-surface ocean wind speed and direction (ocean wind vector) by measuring orthogonal brightness temperatures at three frequency bands and forming third and fourth Stokes parameters
Stokes parameters compared to well known models of ocean surface roughness, temperature and salinity to infer wind speed and direction [a]

4. Relating Surface Roughness to Temperature
Measurement of apparent, sky and water temperatures provides relationship to surface emissivity (reflectivity)
Source: [g]
Comparison of derived emissivity is compared to complex models [e.g. h] relating wind surface temperature and direction to emissivity as a function of incidence angle, azimuth angle and polarization

5. Stokes Parameters Vector Specific Intensity s = scattered v = vertical
h = horizontal
η = free-space intrinsic impedance
Source: [c, pp. 169]
Stoke’s Parameters

6. Stokes Applied to Polarimetric Radiometry
Polarimetric Brightness Temperature Vector
h = horizontal
v = vertical
η0 = free-space intrinsic impedance
λ0 = center frequency wavelength
K = Boltzmann’s constant
B = Radiometer bandwidth
Brightness Temperatures
Source: [c, pp. 257]

7. Incoherent Detection
WindSat utilizes Incoherent Detection: requires direct measurement at 6 principal polarization states: V/H, LHC/RHC, ±45
Coherent Detection requires only one orthogonal pair of antennas (lower SWaP)
Source: [c, pp. 257] & [a, (1)]

8. Incoherent Detection
WindSat utilizes incoherent detection and measures the brightness temperatures in previous equations directly using conventional total-power radiometer and square law detection (very well known).
All brightness temperatures calibrated with a blackbody load and cold sky reflector before and after earth measurements
Common mode biases in calibration between V/H, ±45 and L/R are cancelled in equations for and
Result is more accurate brightness temperature measurements
Antenna design provides mechanical and electrical symmetries, further reducing common mode bias [c, pp ]

9. Radiometer Channel Configuration [b]
Band (GHz)
Polarization
Bandwidth (MHz)
EIA (deg.)
Horizontal Spatial Resolution (km)
6.8
V/H
125
53.5
40 x 60
10.7
V/H, ±45, L/R
300
49.9
25 x 38
18.7
750
55.3
16 x 27
23.8
500
53.0
12 x 20
37.0
2000
8 x 13
10.7, 18.7 and 37.0 GHz bands measure wind speed and direction
6.8 GHz is more sensitive to Sea Surface Temperature (SST) and removes measurement noise due to SST variations
23.8 GHz is sensitive to water vapor and is used to remove attenuation effects of water vapor

10. Radiometer Channel Configuration
18.7 GHz
6.8 GHz
37.0 GHz
10.7 GHz
23.8 GHz
Source: [a, fig. 3]

11. Processed Results Ascending Ocean Surface Wind Speed Map – 08/28/2005
Source: [d]

12. Processed Results Hurricane Katrina
Ascending Ocean Surface Wind Speed Map – 08/28/2005
Source: [d]

13. Processed Results Hurricane Katrina
Descending Ocean Surface Wind Speed Map – 08/28/2005
Source: [d]

14. Processed Results
Source: [e]
Source: [f]

15. Processed Results
Source: [e]
Source: [f]

16. Analysis Brightness temperature is a function of many parameters
Sensor set: frequency, incidence angle, antenna polarization
Geophysical set: physical temperature, wind speed, salinity, precipitable water vapor, cloud-liquid water path
Why is each frequency band used?
Sensitivity analysis: water surface temperature sensitivity peaks around 5 GHz, water vapor sensitivity peaks at 22 GHz, wind speed sensitivity decreases linearly below 10 GHz, liquid cloud sensitivity increases exponentially with frequency

17. Analysis Relation of antenna main beam efficiency and 3db BW
Noise equivalent differential temperature
Relating mission requirement of 2 m/s wind velocity accuracy to brightness temperature measurement accuracy: does not appear to be enough publicly available information.

18. References
- Cover Photo: Gaiser, P.W. "WindSat - Remote Sensing of Ocean Surface Winds." WindSat - Remote Sensing of Ocean Surface Winds - U.S. Naval Research Laboratory. Web. 01 Dec [a] P. W. Gaiser et al., "The WindSat spaceborne polarimetric microwave radiometer: sensor description and early orbit performance," in IEEE Transactions on Geoscience and Remote Sensing, vol. 42, no. 11, pp , Nov [b] Gaiser, P.W. "WindSat - Remote Sensing of Ocean Surface Winds." WindSat - Remote Sensing of Ocean Surface Winds - U.S. Naval Research Laboratory. Web. 01 Dec [c] Ulaby, Fawwaz T., David G. Long, William J. Blackwell, Charles Elachi, Adrian K. Fung, Chris Ruf, Kamal Sarabandi, Howard A. Zebker, and Jakob Van Zyl. Microwave Radar and Radiometric Remote Sensing. Ann Arbor: U of Michigan, Print.

19. References
-[d] “August 28, 2015”. Accessed 04 Dec [e] Wentz, F.J., L.Ricciardulli, C.Gentemann, T. Meissner, K.A. Hilburn, J. Scott, 2013: Remote Sensing Systems Coriolis WindSat [indicate whether you used Daily, 3-Day, Weekly, or Monthly] Environmental Suite on 0.25 deg grid, Version 7.0.1, [wsat_ rt_d3d]. Remote Sensing Systems, Santa Rosa, CA. Available online at [Accessed 05 Dec 2016]. -[f] Wentz, F.J., L.Ricciardulli, C.Gentemann, T. Meissner, K.A. Hilburn, J. Scott, 2013: Remote Sensing Systems Coriolis WindSat [indicate whether you used Daily, 3-Day, Weekly, or Monthly] Environmental Suite on 0.25 deg grid, Version 7.0.1, [wsat_ rt]. Remote Sensing Systems, Santa Rosa, CA. Available online at [Accessed 05 Dec 2016]. -[g] Allen, Chris(1998). Radiometry [PowerPoint slides]. Retrieved from -[h] S. T. Brown, C. S. Ruf and D. R. Lyzenga, "An emissivity-based wind vector retrieval algorithm for the WindSat polarimetric radiometer," in IEEE Transactions on Geoscience and Remote Sensing, vol. 44, no. 3, pp , March 2006.