Data were acquired by the Passive and Active L- and S-band airborne sensor (PALS) during the 1999 Southern Great Plains (SGP99) experiment in Oklahoma to study remote sensing of soil moisture in vegetated terrain using low-frequency microwave radiometer and radar measurements. The PALS instrument measures radiometric brightness temperature and radar backscatter at L- and S-band frequencies with multiple polarizations and approximately equal spatial resolutions. The data acquired during SGP99 provide information on the sensitivities of multichannel low-frequency passive and active measurements to soil moisture for vegetation conditions including bare, pasture, and crop surface cover with field-averaged vegetation water contents mainly in the 0-2.5 kg m/sup -2/ range. Precipitation occurring during the experiment provided an opportunity to observe wetting and drying surface conditions. Good correlations with soil moisture were observed in the radiometric channels. The 1.41-GHz horizontal-polarization channel showed the greatest sensitivity to soil moisture over the range of vegetation observed. For the fields sampled, a radiometric soil moisture retrieval accuracy of 2.3% volumetric was obtained. The radar channels showed significant correlation with soil moisture for some individual fields, with greatest sensitivity at 1.26-GHz vertical copolarized channel. However, variability in vegetation cover degraded the radar correlations for the combined field data. Images generated from data collected on a sequence of flight lines over the watershed region showed similar patterns of soil moisture change in the radiometer and radar responses. This indicates that under vegetated conditions for which soil moisture estimates may not be feasible using current radar algorithms, the radar measurements nevertheless show a response to soil moisture change, and they can provide useful information on the spatial and temporal variability of soil moisture. An illustration of the change detection approach is given.
Pulse compression allows a substantial reduction in the peak transmitted power of a radar and is attractive for spaceborne remote sensing applications. In the case of a downward looking rain measuring radar, however, the range sidelobes associated with surface return can mask return from rain and must be kept to a minimum. The authors describe the pulse compression system for the NASA/JPL Airborne Rain Mapping Radar. This system uses time-domain weighting of the transmitted pulse and is able to achieve a range sidelobe level of -55 dB or better in flight tests. This is significantly lower than other values reported in the open literature.< >
A spaceborne radar for atmospheric observation must be able to detect atmospheric backscatter in the presence of clutter from the surface, due to antenna sidelobes. Such clutter can come from the same pulse as that observing the atmosphere if the radar antenna is pointed off-nadir. However pulses both prior and subsequent to the pulse observing the atmosphere can also contribute to clutter, and surface clutter can be a problem even for nadir-looking radars. Here, the problem is analyzed by deriving a method for computing clutter which includes effects of all contributing transmit pulses, Doppler shifting, finite receiver bandwidth, and curved Earth's surface. The results are applied to analysis of existing radars and design of future radar systems.
The authors have utilized a set of Seasat synthetic aperture radar (SAR) data that were obtained in nearly repeat ground-track orbits to demonstrate the performance of spaceborne interferometric SAR (INSAR) systems. An assessment of the topography measurement capability is presented. A phase measurement error model is described and compared with the data obtained at various baseline separations and signal-to-noise ratios. Finally, the implications of these results on future spaceborne INSAR design are discussed.< >
Dual-frequency (19 and 37 GHz), multi-incidence measurements of the Stokes parameters of sea surface microwave emission are reported. A series of aircraft polarimetric radiometer flights were carried out over the National Data Buoy Center (NDBC) moored buoys deployed off the northern California coast in July and August 1994. Measured radiometric temperatures showed a few Kelvin azimuth modulations in all Stokes parameters with respect to the wind direction. Wind directional signals observed in the 37-GHz channel were similar to those in the 19-GHz channel. This indicates that the wind direction signals in sea surface brightness temperatures have a weak frequency dependence in the range of 19-37 GHz. Harmonic coefficients of the wind direction signals were derived from experimental data versus incidence angle. It was found that the first harmonic coefficients, which are caused by the up and downwind asymmetric surface features, had a small increasing trend with the incidence angle. In contrast, the second harmonic coefficients, caused by the up and crosswind asymmetry, showed significant variations in T/sub v/ and U data, with a sign change when the incidence angle increased from 45/spl deg/ to 65/spl deg/. Besides the first three Stokes parameters, the fourth Stokes parameter, V, which had never been measured before for sea surfaces, was measured using our 19-GHz channel. The Stokes parameter V. Has an odd symmetry just like that of the third Stokes parameter U, and increases with increasing incidence angles. In summary, sea surface features created by surface winds are anisotropic in azimuth direction and modulate all Stokes parameters of sea surface microwave brightness temperatures by as large as a few Kelvin in the range of incidence angles from 45/spl deg/ to 65/spl deg/ applicable to spaceborne observations.
A preliminary geophysical model function, relating the sea surface brightness temperatures to ocean surface wind speed and direction, was developed using the data acquired at 45/spl deg/, 55/spl deg/, and 65/spl deg/ incidence angles by Jet Propulsion Laboratory's (JPL) aircraft 19- and 37-GHz polarimetric radiometers in 1994 and 1995. Radiometric temperatures from all polarization channels under cloud-free conditions showed clear dependence on surface wind direction. When there were stratus or scattered clouds, T/sub /spl nu// and T/sub h/ were significantly influenced by the radiation from cloud water, but the polarimetric channel U was found to be insensitive to clouds. The Fourier harmonic coefficients of the wind direction signals were derived from experimental data and related to the wind speed and direction, incidence angle and frequency. In general, all harmonic coefficients increase from low to moderate wind speeds, except the sin 2/spl phi/ component of U at 65/spl deg/ incidence, which peaked at low winds with a peak-to-peak amplitude of 0.6 to 1 Kelvin at about 3 m/s winds. At moderate wind speeds, 45/spl deg/ incidence angle exhibits larger second harmonic signals, but smaller first harmonic signals, than higher incidence angles. Wind direction signals were similar in 19 and 37 GHz channels, but the 37 GHz channel showed a slightly stronger wind direction sensitivity than the 19 GHz channel. The results suggest promising applications of passive microwave radiometers to ocean wind vector measurements.
Presents the first experimental evidence that the polarimetric brightness temperatures of sea surfaces are sensitive to ocean wind direction in the incidence angle range of 30 to 50 degrees. The experimental data were collected by a K-band (19.35 GHz) polarimetric radiometer (WINDRAD) mounted on the NASA DC-8 aircraft. A set of aircraft radiometer flights was successfully completed in November 1993. The first WINDRAD flight was made on November 4, 1993. There was clear weather with a wind speed of 12 m/s at 330 degrees around the Pt. Arena buoy. The buoy was circled at three incidence angles, and all data when plotted as functions of azimuth angles show clear modulations of several degrees Kelvin. At 40 degrees incidence angle, there is a 5 degrees Kelvin peak-to-peak signal in the second Stokes parameter and and the third Stokes parameter U. The Q data maximum is in the upwind direction and U has a 45 degrees phase shift in azimuth -as predicted by theory. There is also an up/downwind asymmetry of 2 degrees Kelvin in the Q data, and 1 degree Kelvin in the U data. At 50 degrees incidence angle, the collected data show very similar wind direction signatures to the SSM/I model function. Additional flight's were made on other days under cloudy conditions. Data taken at a wind speed of 8 m/s show that at 40 degrees incidence Q and U have a smaller azimuthal modulation of 3 degrees Kelvin, probably due to the lower wind speed. Additionally, the simultaneously recorded video images of sea surfaces suggested that and and U data were less sensitive to clouds, breaking waves and whitecaps, while the T/sub /spl upsi// and T/sub h/ increased by a few degrees Kelvin when the radiometer beam crossed over clouds, or there was a sudden increase of whitecaps in the radiometer footprint. The results of the authors' aircraft flights clearly indicate that passive polarimetric radiometry is a viable option in space remote sensing of ocean surface wind direction as well as wind speed.< >
Ocean backscatter data over a wide variety of oceanic and atmospheric conditions were obtained by the Jet Propulsion Laboratory NUSCAT K/sub u/-band scatterometer during the Surface Wave Dynamics Experiment (SWADE) off the coast of Virginia. A sea surface temperature front at the Gulf Stream boundary existed in the SWADE measurement area. Swells up to 6 m in significant wave height and a large range of wind speeds were encountered. Large differences in backscatter between cold and warm sides of the Gulf Stream were observed. Relations of backscatter with friction velocity are derived from the NUSCAT-SWADE data set. The increasing trend observed in the backscatter with younger wave age agrees with a rougher surface condition for a younger wave field. The results show that backscatter is more sensitive to friction velocity with smaller deviation factors compared to the backscatter functions of wave age. NUSCAT data indicate that the azimuth direction, at which the backscatter is maximum, is in between the wind direction and the dominant wave direction at light wind conditions. Excluding cases of large swells, there is no systematic trend between backscatter and significant wave height.
Statistics on the backscatter coefficient sigma /sup 0/ from the Ku-band Seasat-A Satellite Scatterometer (SASS) collected over the world's land surfaces are presented. This spaceborne scatterometer provided data on sigma /sup 0/ between latitude 80 degrees S and 80 degrees N at incidence angles up to 70 degrees . The global statistics of vertical (V) and horizontal (H) polarization backscatter coefficients for 10 degrees bands in latitude are presented for incidence angles between 20 degrees and 70 degrees and compared with the Skylab and ground spectrometer results. Global images of the time-averaged V polarization sigma /sup 0/ at a 45 degrees incidence angle and its dependence on the incidence angle are presented and compared to a generalized map of the terrain type. Global images of the differences between the V an H polarization backscatter coefficients are presented and discussed. The most inhomogeneous region, which contains the deserts of North Africa and the Arabian Peninsula, is studied in greater detail and compared with the terrain type.< >
The sensitivities of wind direction signals in passive microwave brightness temperatures of sea surfaces to wind speed, incidence angle, polarization, and frequency are presented in this paper. The experimental data were acquired from a series of aircraft flights from 1993 through 1996 by the Jet Propulsion Laboratory (JPL) using JPL 19 and 37 GHz polarimetric radiometers (WINDRAD). Fourier analysis of the data versus mind direction was carried out and the coefficients of Fourier series are illustrated against the wind speed at 45/spl deg/, 55/spl deg/, and 65/spl deg/ incidence angles. There is a good agreement between the JPL aircraft flight data and Wentz's Special Sensor Microwave/Imager (SSM/I) geophysical model function for the vertically polarized brightness temperatures, but Wentz's SSM/I wind direction model for horizontal polarization shows a significantly stronger upwind and downwind asymmetry than the aircraft flight data. Comparison of the dual-frequency WINDRAD data show's that the wind direction signals are similar at 19 and 37 GHz, although the 37 GHz data have slightly stronger signals than the 19 GHz data. In general, the azimuthal variations of brightness temperatures increase with increasing wind speed from low to moderate winds, then level off and decrease at high minds. The only exception is the U measurements at 65/spl deg/ incidence angle, which have a stronger than expected signal at low winds. An exponential function was proposed to model the sensitivities of wind direction signals to wind speeds. The coefficients of the empirical model are provided in this paper and are useful for the simulation of ocean brightness temperatures and for the development of geophysical retrieval algorithms.
