X. Bosch-Lluis 1 , H. Park 2 , A. Camps 2 , S.C. Reising 1 , S. Sahoo 1 ,

1. A RAdiometEr Concept to Retrieve 3-D Radiometric Emission from Atmospheric Temperature and water vapor DENSITY X. Bosch-Lluis1, H. Park2, A. Camps2, S.C. Reising1, S. Sahoo1, S. Padmanabhan3, N. Rodriguez-Alvarez2, I. Ramos-Perez2, and E. Valencia2 1. Microwave Systems Laboratory - ECE, Colorado State University, Fort Collins, CO, USA. 2. Remote Sensing Lab, Dept. Teoria del SenyaliComunicacions, UniversitatPolitècnica de Catalunya and IEEC CRAE/UPC, Barcelona, Spain.3. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA. E-mail: xbosch@mail.colostate.edu IGARSS’11 – Vancouver, Canada, 29thJuly 2011 FR3.T03: Microwave Radiometry Missions and Instrument Performance III

2. Presentation Outline • Motivation • Introduction to Atmospheric Sounding • New Concept Proposal • Theoretical Development • Simulation Results • Future Lines and Conclusions

3. Motivation Radiometric measurements of the atmosphere provide brightness temperatures according to the radiative transfer equation. Retrieval algorithms are used to obtain information on profiles of atmospheric parameters such as water vapor content (WVC). Weighting functions and in-situ measurements from radiosondes (RAOB) are required to perform such retrievals. Here we propose a new approach to this problem which may enable the development of new solutions to the atmospheric profile retrieval problem. Specifically, the goal of this work is to measure the structure of the radiometric emission from the atmosphere using two antennas separated by a certain distance and pointing to the same point in the atmosphere.

4. Atmospheric Sounding I – Radiative Transfer Equation (RTE) Basis Assuming a stratified atmosphere and a pencil beam antenna TC Layer N (10 km) dz ds Atmosphere ds Layer 1 Atmospheric attenuation (from the layer to the ground) Ground level Atmospheric attenuation Physical Temperature RTE DiscreteRTE Absorption coefficient Cosmic Background

5. Atmospheric Sounding II – Retrieval Algorithm Linearization Linearizing the discrete problem for retrieving Weighting Function (MxN) (Jacobian or Kernel) Linearization point (Mx1) Linearization error Linearization point (measured using RAOBmeasurements) (Nx1) M radiometric measurements (Mx1) @ several frequency channels WVC profile to retrieve (Nx1) The linearization approximation applies only for a certain period of time. It requires the launch of ROAB periodically.

6. Atmospheric Sounding II – Retrieval Algorithm Linearization • N is the number of atmospheric layers to retrieve • M is the number of uncorrelated channels that the radiometer measures • Usually N >M → ill posed problem • An information content analysis of the measurement determines the quality of the retrieval , i.e. a trade-off between accuracy and spatial resolution Various inversion methods can be used for retrieving WVC: Newtonian Iteration retrieval Regression retrieval Neural Network Bayesian Maximum likelihood

7. New Concept proposal Brightness temperature from different atmospheric volumes could be measured independently, without the cumulative effect of the RTE Measure Brightness temperature using a CROSS-BEAM Interferometer TCB Layer N (10 km) (x0, y0, z0) Atmosphere z y x Baseline Layer 1 Ground level Antenna #2 (x2, y2, 0) Antenna #1 (x1, y1, 0) LPF True time delay for measuring at points which have different distance with respect Ant1 and Ant2, and sub-overlapping measurements

8. Theoretical development • Visibility sample: Main differences between this concept and interferometric synthetic aperture radiometer: Narrower antenna beamwidth Only one visibility sample, not a set of visibility samples Adjustable true time delay Visibility sample written in Cartesian coordinates: Integration variable, spans the whole atmosphere • . Distance from antenna 1 and 2 to the integration variable Brightness temperature of the point Fringe-washing function

9. Antenna beamwidth and overlapping volume effect Main challenge posed by this technique To retrieve the brightness temperature the cross-beam interferometric measurement must be multiplied by the inverse of If the overlapping solid angle decreases in comparison to the solid angles of the beams , the radiometric resolution (standard deviation) of the measurement increases. Very narrow beams mitigate this effect, then same order of magnitude between the overlapping solid angle and the beams’ solid angles

10. Spatial Resolution 1/2 • Horizontal Spatial Resolution (determined by hardware decorrelation time) • The spatial resolution is determined by the bandwidth of both receivers (FWF) If with rectangular shapes and

11. Spatial Resolution 2/2, Horizontal Spatial Resolution Moreover, it allows several measurements in the same beam-overlapping volume by changing the delay between both receivers 3 sub beam-overlapping volume measurements obtained changing the relative delay Outside overlapping volume (This volume does not contribute to the measured visibility function.) Beam-overlapping volume (This volume contributes to the measured visibility function.) • The overlapped volume depends on the antennas beamwidth () size of both antennas and on their spacing

12. Simulation Assumptions and Considerations Assumptions and considerations for the simulation 2D atmosphere for simplicity Stratified atmosphere with dx=dz=33 meters Atmosphere dimensions 10x66 Km (303x2000 voxels) Van Vleck model for absorption coefficients, using RAOB measurements for the water vapor, pressure and temperature profile. F=22.12 and 24.50 GHz the same channels as the CMR-H radiometers CSU, channels suitable for WVC retrieval. Gaussian antenna patterns. Identical and perfectly rectangular response of both systems WVC profile used for the synthetic atmosphere, obtained using a RAOB WVC [gr/m3]

13. Simulation Results, Vertical Scans 1/3 Measured temperatures using the cross-beam interferometer: D=600 m Centers of the overlapping area, scanned sequentially #2 #1 =0.5 degrees =100 MHz, #2 #1 D=600 m

14. Simulation Results, Vertical Scans 3/3 Antenna spacing change Measured temperatures using the cross-beam interferometer: #2 #1 D=600 m #2 #1 D=6600 m

15. Simulation Results, Horizontal Scans 1/2 =0.5 degrees =100 MHz, Measured temperatures using the cross-beam interferometer: Height #1 #2 D=600 m X axis [m] Atmosphere attenuation effect Height #1 #2 D=600 m X axis [m]

16. Simulation Results, Horizontal Scans 2/2 Measured temperatures using the cross-beam interferometer: Height #1 #2 D=600 m X axis [m] Height #1 #2 D=6600 m X axis [m]

17. Conclusions And Open Issues • A new radiometric for retrieving VWC proposed using interferometric cross-beam techniques. • The system can measure brightness temperatures independent of the RTE. • Spatial resolution depends on: • The horizontal spatial resolution depends on the BW. • The vertical spatial resolution depends on the antenna spacing and beamwidth • Ongoing: • Keep on studying this technique to better understand its limitations and constraints. • Estimate the radiometric resolution for ±10% error WVC retrieval depending on the altitude. • Determinate methods for calibrate different parts of the system • Phase • Amplitude • Offset • Radiometric calibration (hot and cold load) • Perform retrievals from simulated atmospheres (applying techniques such as “onion peeling” instead of a Weighting function approximation). • Retrieval error compressive study

18. THANK YOU !