P. Snoeij

The polarization-dependent relation between radar backscatter from the ocean surface and surface wind vector at frequencies between 1 and 18 GHz

A series of airborne scatterometer measurements carried out with the DUTSCAT multifrequency airborne scatterometer are discussed. This study deals with the first results obtained from the analysis of these measurements. The objective of this activity is to establish a multifrequency dual-polarization radar signature database, and with it a multidimensional version of the current CMOD1 model. The main features of the data set are the following. The wind exponent of the upwind normalized radar cross section (NRCS) increases with frequency and incidence angle in the case of HH polarization. The upwind/downwind ratio is mainly negative at 20 degrees of incidence angle, always at C-, X-, and Ku1-bands. >

The DUT airborne scatterometer

Abstract An airborne scatterometer system operating at six frequencies simultaneously between 1 and 18 GHz has been developed for the measurement of the microwave scattering of vegetation, forests, sea and other targets. After a description of the instrument, some C- and L-band results are presented.

The PHARUS project, results of the definition study including the SAR testbed PHARS

The PHARUS (Phased Array Universal SAR) project aims for a polarimetric C-band aircraft synthetic aperture radar (SAR) that will be finalized in 1994. The system will make use of a phased array antenna with solid-state amplifiers. The project consists of a definition phase and a realization phase. The definition phase is intended to increase the knowledge on airborne SAR systems and to develop the critical technology that will be used in the final system. Three preparatory studies were carried out before the PHARUS system was designed. The study on antenna technology and calibration focused on the design of an active phased array antenna and on the calibration problems involved in using such an antenna. The second study concentrated on motion compensation and the third study consisted of the realization of a SAR testbed. Some results of the definition phase, including the results obtained from the first test flights of the SAR testbed, are given. The PHARUS predesign is described. >

A statistical model for the error bounds of an active phased array antenna for SAR applications

A model for the theoretical error bounds for the radiometric calibration of SAR imagery is described. The model is then applied utilizing the radar system parameters that will be used in the project PHARUS (Phased Array Universal SAR), a Dutch polarimetric airborne C-band universal SAR, which is currently under construction. An error model for the phased array antenna pattern is presented. This model was applied to a 16*8 phased array antenna to determine the influence of errors in the T/R modules and angle variations of the beam direction. For a wide range of variances the model is in good agreement with the exact solution, but with far less computer effort. Other, more complicated, antennas can be analyzed in a straightforward way. The model can be used as a design tool for digital phased array radar. >

First results and status of the PHARUS phased array airborne SAR

PHARUS is an airborne fully polarimetric C-band SAR. It uses a compact phased array antenna with electronic beam steering. Its design provides for a flexible and robust system concept, suited for operational use on small aircraft. PHARUS was developed in The Netherlands to cater for both the military and civil markets. Its development was initiated in 1987 and has now resulted in completion of the system, which made its maiden flight in September 1995. In the paper, the particulars of PHARUS are discussed, with special attention to the antenna, followed by the SAR processing, use of the system, its history and status, and foreseen future developments.

Evaluation Of A Two-scale Model Using Extensive Radar Backscatter And Wave Measurements In A Large Wind-wave Flume

One of the aims of the VIERS-1 project is to develop improved models for radar backscatter from water surfaces, based on a description of the underlying physical phenomena instead of an emperical parameterization. To this end, extensive wind, wave and radar backscatter measurements have been compared with model results.

First results of the VIERS-1 experiment

In February 1988, combined measurements of microwave backscatter, wind, waves and gas exchange have been carried out in the large Delft Hydraulics wind/wave tank. This experiment was the first in a series of experiments in the frame of the VIERS-1 project. In this project a number of Dutch and German laboratories cooperate. Main objective is to come to a physical description of the processes involved in wind scatterometry and, from that point, to an improvement of the algorithms used for determination of wind speed and direction from satelliteborne microwave scatterometers. A second objective is to study the relation between the gas exchange at the water surface and the microwave backscatter. To achieve these objectives two wind/wave tank experiments and one ocean based platform experiment are scheduled. In this paper, the VIERS-1 programme will be outlined. The Delft wind/wave tank experiment will be described and some first results of a preliminary comparison of backscatter and wave slope measurements will be shown.

Uniqueness of the ERS scatterometer for nowcasting and typhoon forecasting

Since the launch of the European Remote Sensing Satellite ERS-1 in 1991, surface wind-vector observations derived from space-borne scatterometer measurements have been available over the global oceans continuously. Currently, space- borne scatterometer wind products are based on the QSCAT-1 model function for the Ku-band radar frequency (QuikSCAT) and the CMOD5 model function, recently been developed at ECMWF and KNMI, for C-band (ERS-2 and the future ASCAT series). The dynamic range of ERS derived winds now extends to at least 35 m/s. The accuracy of the wind retrieval in combination with the minimal sensitivity of the C-band frequency for rain contamination makes the ERS scatterometer an unique instrument for weather research, and the improved ambiguity removal increases the usefulness in dynamical and extreme weather conditions. The reprocessing of the entire ERS mission, combined with an anticipated overlap with the forthcoming ASCAT era will provide a 30-year long continuous and high- quality surface wind data set unique for climate research. The ERS-2 Scatterometer mission is currently operated on regional basis. This results in an unprecedented data timeliness of about 30 minutes. The short timeliness makes the current ERS-2 wind product suitable for operational weather now-casting and short-range weather forecasting. This offers advantages in analyzing and forecasting extreme weather events, which leads to improved predictions of these events using ERS scatterometer surface winds in numerical weather prediction (NWP).

Comparisons of Backs Cantering Calculations with Measurements Made in a Large Wind/wave Flume

In preparation of tlie launch of tlie ERS-1 satellite, a group of Dutch and German laboratories started a study to improve the wind extraction algorithms for the scatterometer. T h e aim of this s tudy is t o create a model for wind retrieval based on physics. In this paper results of microwave backscattering calculations with three different models will be compared with measurenients in a large wind/wave flume. T h e three models are a two-scale model and two models based on approximations of the integral equation. T h e input for the bnckscattering models are measured wave parameters so the comparison permits a direct evaluatiQn of the accuracy of the model within the experimental accuracies.

Comparison Of X-band Radar Backscatter Measurements With Area Extended Wave Slope Measurements Made In A Large Wind/wave Tank.

Combined measurements of microwave backscatter, wind, waves, and gas exchange have been carried out in the large Delft Hydraulics wind/wave tank. This experiment was the first of a series of experiments in the VIERS-1 project. In this project, a number of Dutch and one German laboratory cooperate to come to a physical description of the process involved in wind scatterometry and, from that point, to an improvement of the algorithms used for determination of wind speed and direction from satellite-borne microwave scatterometers. A second objective is to study the relation between the gas exchange at the water surface and the microwave backscatter. Two wind/wave tank experiments and the ocean-based platform experiment are scheduled. The VIERS-1 program is outlined. The Delft wind/wave tank experiment is described, and some first results of a comparison of backscatter and wave slope measurements are shown.