Title: KA-Band Sar Iterferometry Studies for the SWOT Mission Development
Author: Daniel Esteban-Fernandez
Organization: NASA /Jet Propulsion Laboratory
Co-Authors: Lee-Lueng Fu, Ernesto Rodriguez, Eva Peral, Shannon Brown, and Richard Hodges


The primary objective of the National Research Council (NRC) Decadal Survey recommended SWOT (Surface Water and Ocean Topography) Mission is to measure the water elevation of the global oceans, as well as terrestrial water bodies (such as rivers, lakes, reservoirs, and wetlands), to answer key scientific questions on the kinetic energy of ocean circulation, the spatial and temporal variability of the world's surface freshwater storage and discharge, and to provide societal benefits on predicting climate change, coastal zone management, flood prediction, and water resources management.

The core technology for the SWOT concept is the KaRIN instrument, a Ka-band radar interferometer, originally developed from the efforts of the Wide Swath Ocean Altimeter (WSOA). We are currently funded by the NASA Earth Science Technology Office (ESTO) Instrument Incubator Program (IIP) to reduce the risk of the main technological drivers of the proposed SWOT mission, by addressing the following technologies: the Ka-band radar interferometric antenna design, the on-board interferometric SAR processor, and the internally calibrated high-frequency radiometer. The first two technologies address the challenges of the Ka-band SAR interferometry, while the high-frequency radiometer addresses the requirement for small-scale wet tropospheric corrections for coastal zone applications.

In this paper, we present the objectives and status of the technology items currently under development for the formulation of the SWOT mission. In particular:

1. The antenna would employ printed reflectarray technology, consisting of a flat reflectarray aperture of ~5 x 0.20 m with many printed Ka-band patch elements. A prototype of the antenna has been completed, and RF performance measurements results will be presented.

2. The resulting radarís output data rate would be beyond affordable downlink capabilities for global data downlink. Instead, the raw data over land would be downlinked, while the on-board processor with interferometric SAR processing and multi-looking capabilities would decrease the data rate to reasonable limits for data over the ocean before download. A prototype of the on-board processor has been developed, and performance results will be presented.

3. In order to provide high-resolution coastal altimetry, radiometer channels operating at frequencies of 90 GHz or greater can be added. However, operation at high frequencies without external calibration (that is, without periodic views of hot and/or cold loads), has, to our knowledge, never been flown in space before. Three high-frequency radiometric channels without external calibration have been developed over the course of this development, and their radiometric performance will be presented.


The research presented in the paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautic and Space Administration.