Cuneyt Utku

Aquarius Third Stokes Parameter Measurements: Initial Results

This letter reports a first look at the polarimetric (third Stokes parameter) channel on the Aquarius L-band radiometer that was launched in June of 2011 on the Aquarius/Satélite de Aplicaciones Cientificas (SAC)-D observatory. The primary purpose of the polarimetric channel is to provide an in situ measure of Faraday rotation which can be important for remote sensing at L-band, particularly in the case of sea surface salinity. However, it also provides an additional mode of observation and a chance to look for new features of the surface. Initial results show good agreement with expectations. In particular, the values of retrieved Faraday rotation agree with predicted values, and a nonzero signal is seen to occur over mixed scenes as predicted by theory.

A Model for Prediction of the Impact of Topography on Microwave Emission

Topography can be important for future passive microwave remote sensing of soil moisture from space. One of the problems in assessing the importance of topography is that a digital elevation model (DEM) for the surface does not provide an intuitive estimation of when topography will be important. This is especially true given the large footprint of L-band radiometers on future space missions such as the Soil Moisture and Ocean Salinity (SMOS), Aquarius and the Soil Moisture Active-Passive (SMAP) missions. To address this issue, the DEM has been replaced with a probability density function (pdf) for slopes. It is shown that the slope pdf can be separated into “smooth” and “rough” distributions that provide insight into when topography will be important. The model is applied to the site of the 2004 Soil Moisture Experiment (SMEX04) in Arizona, and to the site of the Australian Airborne Cal/Val Experiment for SMOS (AACES) and shown to produce results comparable to the direct application of the DEM.

Aquarius: Status and recent results

Aquarius is a combination active/passive instrument at L band designed to map sea surface salinity globally from space. The radiometer (passive) is the primary instrument for retrieving salinity, and the scatterometer (active) provides information to correct for a major source of error, sea surface roughness (waves). In addition, the radiometer includes a number of special features designed to meet the goal for this challenging measurement, including measurement of the third Stokes parameter to help with the correction for Faraday rotation and rapid sampling to help with the mitigation of radio frequency interference. Aquarius was launched on 10 June 2011 aboard the Aquarius/SAC-D observatory and has been working well. The salinity retrieval continues to improve, and the special features suggest the potential for new applications of remote sensing from space at L band.

Accurate L-band dielectric constant measurements of seawater

A temperature controlled resonant cavity is used to measure the complex dielectric constant of seawater at 30 psu over the temperature range from 10degC to 35degC at 1.413 GHz. The measurements are conducted multiple times to obtain the mean and the standard deviation at each point. This paper describes the experimental procedure and presents recent dielectric constant results. These results are compared with the values obtained using the Klein-Swift model function.

Topographic Signatures in Aquarius Radiometer and Scatterometer Response

Theory suggests that topography (large-scale roughness) will affect thermal emission at L-band and could impact remote sensing of surface parameters, such as soil moisture from space. Evidence is presented here for the existence of effects due to topography using data from the L-band radiometers and scatterometer aboard the recently launched Aquarius. The correlation with the slope standard deviation at the topographic scale is presented for data over North Africa and Australia. In the case of the radiometer, brightness temperature is observed to increase at horizontal polarization and decrease at vertical polarization. In the case of the scatterometer, an increase with topographic roughness is observed for both polarizations. The presence of the scatterometer provides an independent verification that the behavior observed is due to topography and the observations are consistent with predictions based on Monte Carlo simulations.

A new model function for the permittivity of seawater at 1.413GHZ

Measurement data for the complex permittivity of seawater at 1.413 GHz for salinities of 30.002 psu, 34.997 psu and 38.274 psu is used to construct a polynomial model function. The coefficients of the polynomial are determined by minimizing the error between the model function and the data in the least square sense. The results are compared with the Klein Swift model function and the differences are discussed.

A physical model for microwave radiometry of forest canopies

A first order scattering model is developed and tested at 1.4 GHz by using microwave brightness temperature data acquired over deciduous tree canopies in Maryland during 2007. Microwave measurements at several incident angles and supporting ground truth data (including size/angle distributions of tree constituents) have been collected over stands of deciduous Paulownia trees under full canopy and leaf-drop conditions. Detailed ground truth data obtained during this experiment have been used to compute the additional radiation due to scattering and emission by the vegetation components. The preliminary model predictions are in good agreement with the data and they give quantitative understanding for the influence of the first order scattering within the canopy on the radiometer brightness temperature. The model results using tree ground truth show that the scattering term is significant for trees and that the tau-omega model needs modification to account for additional scattering contribution. Numerical simulations also indicate that the single scattered radiation increases the canopy brightness temperature considerably. These simulations show that the scattering term has a negligible dependence on soil moisture and is only function of angle and polarization.

Remote sensing of salinity: The dielectric constant of sea water

Global monitoring of sea surface salinity from space requires an accurate model for the dielectric constant of sea water as a function of salinity and temperature to characterize the emissivity of the surface. Measurements are being made at 1.413 GHz, the center frequency of the Aquarius radiometers, using a resonant cavity and the perturbation method. The cavity is operated in a transmission mode and immersed in a liquid bath to control temperature. Multiple measurements are made at each temperature and salinity. Error budgets indicate a relative accuracy for both real and imaginary parts of the dielectric constant of about 1%.

Precise measurement of the dielectric constant of seawater at 1.413 GHz: The capillary exit hole correction

Summary form only given. The effect of the capillary-tube exit holes in the microwave resonant cavity used to measure the dielectric constant of seawater at L-band, is investigated. The cavity technique is a perturbation method that uses the change in the resonant frequency and Q of the cavity when seawater is added to determine the complex dielectric constant of seawater. The tube, through which the seawater is introduced into the cavity, enters through two small center holes in the cavitys endplates. It has been suggested in the literature that these small exit holes could affect the change in the resonant frequency measurement, thus introducing a frequency pulling error into the measurements. Numerical modeling of similar cavities indicates that a frequency pulling effect can occur due to the center holes in the endplates. These simulations indicate that a coaxial TEM-like mode is created in the connector that holds the capillary tube to the endplates. This TEM-like mode reflects from the end of the connector and acts as a resonant circuit. Since the seawater is acting as the center conductor of the TEM guide, the effect should be more noticeable as the conductivity of the seawater increases. The conductivity increases with increasingsalinity and temperature. To see whether the frequency-pulling effect is important, a lossy graphite cylinder with a hole in the center has been inserted into the connector, to attenuate the TEM wave. Measurements for high salinity and high temperatures have been made with and without the graphite cylinder. The results of the measurements will be discussed in this study.

Seawater permittivity model function with new L-band seawater measurements at 33psu

Seawater salinity measurements are currently being made at L-band (1.413 Ghz) by NASAs Aquarius instrument (on the Aquarius/SAC-D observatory). The goal of Aquarius mission is to measure the salinity of seawater to an accuracy on the order of 0.2 psu; this requires a model function of seawater permittivity with a high accuracy. Since 2011, the George Washington University (GW) has employed a cavity technique to determine the complex permittivity of seawater at 1.413 GHz. In this paper, a new seawater dielectric model function is introduced including the latest permittivity data for seawater with salinity 33 psu. Finally, the validation of the end-effect, measurement variance and data fitting will be discussed.