Daniel Esteban-Fernandez

The Surface Water and Ocean Topography Mission: Observing Terrestrial Surface Water and Oceanic Submesoscale Eddies

The elevation of the ocean surface has been measured for over two decades from spaceborne altimeters. However, existing altimeter measurements are not adequate to characterize the dynamic variations of most inland water bodies, nor of ocean eddies at scales of less than about 100 km, notwithstanding that such eddies play a key role in ocean circulation and climate change. For terrestrial hydrology, in situ and spaceborne measurements of water surface elevation form the basis for estimates of water storage change in lakes, reservoirs, and wetlands, and of river discharge. However, storage in most inland water bodies, e.g., millions of Arctic lakes, is not readily measured using existing technologies. A solution to the needs of both surface water hydrology and physical oceanography communities is the measurement of water elevations along rivers, lakes, streams, and wetlands and over the ocean surface using swath altimetry. The proposed surface water and ocean topography (SWOT) mission will make such measurements. The core technology for SWOT is the Ka-band radar interferometer (KaRIN), which would achieve spatial resolution on the order of tens of meters and centimetric vertical precision when averaged over targets of interest. Average revisit times will depend upon latitude, with two to four revisits at low to mid latitudes and up to ten revisits at high latitudes per ~20-day orbit repeat period.

Obtaining Accurate Ocean Surface Winds in Hurricane Conditions: A Dual-Frequency Scatterometry Approach

We describe a method for retrieving winds from colocated Ku- and C-band ocean wind scatterometers. The method utilizes an artificial neural network technique to optimize the weighting of the information from the two frequencies and to use the extra degrees of freedom to account for rain contamination in the measurements. A high-fidelity scatterometer simulation is used to evaluate the efficacy of the technique for retrieving hurricane force winds in the presence of heavy precipitation. Realistic hurricane wind and precipitation fields were simulated for three Atlantic hurricanes, Katrina and Rita in 2005 and Helene in 2006, using the Weather Research and Forecasting model. These fields were then input into a radar simulation previously used to evaluate the Extreme Ocean Vector Wind Mission dual-frequency scatterometer mission concept. The simulation produced high-resolution dual-frequency normalized radar cross-section (NRCS) measurements. The simulated NRCS measurements were binned into 5 x 5 km wind cells. Wind speeds in each cell were estimated using an artificial neural network technique. The method was shown to retrieve accurate winds up to 50 m/s even in intense rain.

Impact of Surface Waves on SWOT’s Projected Ocean Accuracy

The Surface Water and Ocean Topography (SWOT) mission being considered by NASA has, as one of its main objectives, to measure ocean topography with centimeter scale accuracy over kilometer scale spatial resolution. This paper investigates the impact of ocean waves on SWOT’s projected performance. Several effects will be examined: volumetric decorrelation, aliasing of ocean waves, backscattering modulation, and the so-called surfboard sampling.

Ka-band SAR interferometry studies for the SWOT mission

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 worlds surface freshwater storage and discharge, and to provide societal benefits on predicting climate change, coastal zone management, flood prediction, and water resources management. In this paper, we present the overall concept of the SWOT mission, as well as the scientific rational, objectives and development status of the technology items currently under development.

Design considerations for a dual-frequency radar for sea spray measurement in hurricanes

Over the last few years, researchers have determined that sea spray from breaking waves can have a large effect on the magnitude and distribution of the air-sea energy flux at hurricane-force wind speeds. Characterizing the fluxes requires estimates of the height-dependent droplet size distribution (DSD). Currently, the few available measurements have been acquired with spectrometer probes, which can provide only flight-level measurements. As such, in-situ measurement of near-surface droplet fluxes in hurricanes with these instruments is, at best, extremely challenging, if at all possible. This paper describes an airborne dual-wavelength radar profiler concept to retrieve the DSD of sea spray.

The Effect of Atmospheric Water Vapor Content on the Performance of Future Wide-Swath Ocean Altimetry Measurement

AbstractMeasurement of sea surface height (SSH) over a finite swath along satellite tracks has been planned for future space missions. The effect of water vapor in the troposphere on the delay of radar signal must be corrected for in the SSH measurement. The efficacy of a nadir-looking radiometer that has been the approach for conventional altimetry is examined in the study. The focus is placed on the cross-track variability of water vapor that is not measured by the nadir-looking radiometer. Simulations of the 2D field of water vapor were performed by spectral analysis of existing radiometer data. The residual error from the application of the correction made by a nadir-looking radiometer was computed over the global ocean and compared to the SSH signal estimated from satellite altimeter data. Global maps of the signal-to-error ratio (the square root of spectral variance at wavelengths shorter than 500 km) were created, showing values of 20–50 in the regions of high SSH variability of the boundary curren...

Development path of a Ka-Band Extended Interaction Klystron for space-borne interferometer

An Extended Interaction Klystron (EIK) design has been developed to the support extreme signal stability requirements of space-borne interferometric applications. This is an unprecedented set of requirements for an EIK. In conjunction with the Canadian Space Agency (ASC/CSA)and Jet Propulsion Laboratory, Communications and Power Industries, Canada (CPI) advanced this development through testing of a heritage design EIK, testing a dedicated Demonstration Model EIK and an Engineering Model EIK. Each step of testing provided guidance in the development of the EIK Design, finally providing confidence in the EIKs ability to support the needs of the interferometric radar.

Assessment of near-nadir correlation characteristics over water bodies using interferometric SAR: Implications for the swot mission

This paper introduces the use of an airborne interferometric synthetic aperture radar (InSAR) to estimate water surface decorrelation times at Ka-Band. Such an assessment is directly relevant to the upcoming Surface Water and Ocean Topography mission, especially for surface water bodies such as lakes and rivers since the surface decorrelation may limit the spatial resolution achievable by the mission to delineate water spatial boundaries. Initial assessments indicate decorrelation times consistent with limited published observations for the ocean and fresh water bodies (several milliseconds). However, there are challenges both in terms of the phenomenology and in the instrument sensitivity to longer decorrelations.