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Über dieses Buch

This book discusses methods for measuring the water surface backscattering signature and estimating the near-surface wind vector over water using airborne radars, in addition to their standard application. Airborne FMCW demonstrator system, Doppler navigation system, airborne weather radar, airborne radar altimeter, and airborne precipitation radar are analyzed in order to be used for that purpose. The radars functionality is enhanced for their operation in a scatterometer mode. A circle flight and/or a rectilinear flight of an aircraft over the water surface is considered depending on the radar design features to perform measurements of the azimuth normalized radar cross section curve of the water surface and/or the near-surface wind speed and direction. Flight recommendations to perform measurements along with algorithms for measuring the water surface backscattering signature and for retrieval of the wind speed and direction over water are presented.

Inhaltsverzeichnis

Frontmatter

Chapter 1. Introduction

Abstract
Satellite remote sensing has demonstrated its potential to provide measurements of weather conditions on a global scale as well as airborne remote sensing on a local scale. Near-surface wind measurements over the sea are very important for operational oceanography, as well as for meteorology and navigation. Wind and wave measurements by those remote sensing instruments are based on features of microwave backscattering from the water surface. To study the microwave backscattering signature of the water surface from aircraft, an airborne scatterometer is used. The measurements are typically performed at either a circle track flight using a fixed fan-beam antenna or a rectilinear track flight using a rotating antenna. For such an airborne measurement of winds, antennas with comparatively narrow beams are commonly used. Unfortunately, a microwave narrow-beam antenna has considerable size at Ku-, X-, and C-bands that makes its placing on an aircraft difficult, especially on a seaplane or an amphibious aircraft. Therefore, a better way needs to be found. At least two options can be proposed. The first option is to apply airborne scatterometers with wide-beam antennas as it can lead to reduction of the antenna size. The second option is to use modified conventional navigation instruments of the aircraft in a scatterometer mode, which seems more preferable.
Alexey Nekrasov

Chapter 2. Water-Surface Backscattering and Wind Retrieval

Abstract
Research on microwave backscatter by the water surface has shown that the use of a scatterometer also allows an estimation of the near-surface wind vector because the Normalized radar cross section (NRCS) of water surface depends on wind speed and direction. Based on experimental data and scattering theory, a significant number of empirical and theoretical backscatter models and algorithms for estimation of near-surface wind vector over water from satellite and airplane have been developed. To retrieve the wind vector from NRCS measurements, a relationship between NRCS and near-surface wind, called the “geophysical model function” is used. A scatterometer having an antenna with an inclined beam allows measuring NRCS azimuth curve of a water surface and provides retrieval of both wind speed and wind direction over water. A scatterometer equipped with a nadir-looking antenna allows measuring nadir NRCS and estimating sea surface wind speed but provides no information on the wind direction.
Alexey Nekrasov

Chapter 3. FM-CW Demonstrator System as an Instrument for Measuring Sea-Surface Backscattering Signature and Wind

Abstract
In the field of airborne earth observation, there is growing interest in small, cost-effective radar systems. Such radar systems should consume little power and be small enough to be mounted on light, possibly even unmanned, aircraft. FM-CW radar systems are generally compact, relatively cheap, and they consume little power. Consequently, FM-CW radar technology seems to be of interest to civil airborne earth observation, particularly in combination with high resolution SAR techniques.
Alexey Nekrasov

Chapter 4. Doppler Navigation System Application for Measuring Backscattering Signature and Wind Over Water

Abstract
DNS is a self-contained radar system that uses the Doppler effect (Doppler radar) for measuring ground speed and drift angle of aircraft and performs its dead-reckoning navigation (Sosnovskiy and Khaymovich 1987).
Alexey Nekrasov

Chapter 5. Measuring Water-Surface Backscattering Signature and Wind by Means of Airborne Weather Radar

Abstract
Measuring Water Surface Backscattering Signature and Wind by Means of Airborne Weather Radar: Airborne weather radar (AWR) is the type of radar equipment mounted on an aircraft for purposes of weather observation and avoidance, aircraft position finding relative to landmarks, and drift angle measurement. AWRs or multimode radars with a weather mode are usually nose mounted. The AWR antenna, in the ground-mapping mode, has a large cosecant-squared elevation beam where the horizontal dimension is narrow while the other is relatively broad, and it sweeps in an azimuth sector of up to ± 100°. Therefore, those features can be used for AWR enhancement to measure the water-surface backscattering signature and wind vector over water when it operates in a scatterometer mode. The analysis of AWR has shown that the radar employed in the ground-mapping mode as a scatterometer can be used for remote measuring of the sea surface backscattering signature at a circular flight along with recovering the wind speed and direction over the water surface from NRCS azimuth curves obtained as well as for measuring wind vector during a rectilinear flight in addition to its typical meteorological and navigation application.
Alexey Nekrasov

Chapter 6. Water-Surface Wind Retrieval Using Airborne Radar Altimeter

Abstract
Radar altimeters are frequently used by aircraft. The primary function of the Airborne radar altimeter (ARA) is to provide terrain clearance or altitude with respect to the ground level directly beneath the airplane or helicopter. The ARA may also provide a vertical rate of climb or descent and selectable low altitude warning. Typical scatterometer wind measurements are commonly performed using antennas with comparatively narrow beams. As the ARA has a wide-beam antenna, the beam sharpening technologies should be used to apply the ARA for wind vector measurement. For that prepuce, Doppler discrimination along with range discrimination have been employed. The wind retrieval algorithms for several particular beam geometries are presented. The study has shown that wind vector over sea can be measured by means of the ARA employed as a nadir-looking wide-beam short-pulse scatterometer in conjunction with Doppler filtering. The measuring instrument should be equipped with two additional Doppler filters (a fore-Doppler filter and an aft-Doppler filter) to provide for spatial selection under the wind measurements.
Alexey Nekrasov

Chapter 7. Near-Nadir Wind Estimation Over Water with Airborne Precipitation Radar

Abstract
APR-2 is an airborne, dual-frequency (Ku- and Ka-band), dual-polarization Doppler rain profiling radar. Its antenna scans in the ±25° cross-track elevation range allowing the radar to measure atmospheric precipitation and sea surface NRCS. The wind measuring algorithm has been proposed and investigated with the help of simulations. The study has shown that airborne radar instrument for measurement geometry similar to the APR-2 geometry and operating in a scatterometer mode can be applied to remote measurement of sea surface wind speed and direction at near-nadir incidence angles based on the measuring algorithm developed in case of precipitation absence. As azimuth water surface slope variance has been assumed to be elliptical, wind direction can be estimated, unfortunately, only with an ambiguity of 180°. It means that in principle the APR-2 also can be applied to wind retrieval over water surface during a rectilinear flight in addition to its typical meteorological application.
Alexey Nekrasov

Backmatter

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