Airborne Weather Radar Concept for Measuring Water Surface Backscattering Signature and Sea Wind at Circular Flight

KLJUČNE RIJEČI vjetar na moru refleksija s morske površine meteoradar u zrakoplovu algoritam Alexey Nekrasov Saint Petersburg Electrotechnical University, Saint Petersburg, Russia Southern Federal University, Taganrog, Russia Hamburg University of Technology, Hamburg, Germany e-mail: alexei-nekrassov@mail.ru Vladimir Veremyev Saint Petersburg Electrotechnical University Saint Petersburg, Russia e-mail: ver_vi@mail.ru

To explain adequately the microwave scattering signature of water surface and to apply its features to remote sensing series of experiments are required: experimental verification of combined frequency, azimuth and incidence angles and wind speed variations of the NRCS [5].For such a study, a scatterometer, i.e. radar designed for measuring the surface scatter characteristics, is used.
Research on microwave backscatter by water surface has shown that the use of a scatterometer also allows an estimation of near-surface wind speed and direction because the NRCS of sea surface depends on wind speeds and directions.Based on experimental data and scattering theory, a significant number of empirical and theoretical backscatter models and algorithms for estimation of a near-surface wind vector from satellite and airplane has been developed [14].
To study a microwave backscattering signature of water surface from aircraft, an airborne scatterometer is used.The measurements are typically performed at either a circular track flight using fixed fan-beam antenna or a rectilinear track flight using rotating antenna [5,6,8].Unfortunately, a microwave narrow-beam antenna has considerable size at Ku-, X-and C-bands that makes its placing on aircraft difficult.Therefore, a better solution needs to be found.
At least two methods can be proposed.The first method is to apply the airborne scatterometers with wide-beam antennas as it can lead to reduction in the antenna size.The second method is to use the modified conventional navigation instruments of aircraft in a scatterometer mode that seems more preferable.
From that point of view, a promising navigation instrument is airborne weather radar (AWR).In this connection, a possibility to measure the sea surface backscattering signature and to retrieve the wind speed and direction over water by the AWR in the scatterometer mode at an aircraft circular flight, in addition to its standard navigation application, is discussed in this paper.

GENERAL DESCRIPTION OF AIRBORNE WEATHER RADAR / Opis meteoradara u zrakoplovu
AWR is radar equipment mounted on aircraft for weather observation and avoidance, aircraft position finding relative to landmarks, and drift angle measuring [15].The AWR is required equipment for any civil aircraft.All military transport aircraft are usually equipped with weather radars too.Due to specificity of airborne application, designers of avionics systems always try to use the most efficient methods and reliable engineering solutions, which provide safety and regularity of flying in harsh environment [16,17].
Development of AWR is mainly associated with growing functionalities on detection of different dangerous weather phenomena.The radar observations involved in a weather mode are magnitude detection of reflections from clouds and precipitation, and Doppler measurements of the motion of particles within weather formations.Magnitude detection allows determination of particle type (rain, snow, hail, etc.) and precipitation rate.Doppler measurements can be made to yield estimates of turbulence intensity and wind speed.Reliable determination of the presence and severity of the phenomenon known as wind shear is an important area of study too [18].
The second important assignment of the AWR is providing a pilot with navigation information using earth surface mapping.In this case, a possibility to extract some navigation information that allows determining aircraft position with respect to a geographic map is very important for air navigation.Landmark's coordinates relative to the aircraft measured by the AWR allow setting a flight computer for a more exact and more efficient enroute flying, cargo delivery, and cargo throw down to the given point.These improve tactical possibilities of transport aircraft, airplanes of search-and-rescue service, and local airways [16].
Other specific function of the AWR is interaction with ground-based responder beacons.New functions of the AWR are detection and visualization of runways at approach landing as well as visualization of taxiways and obstacles on the taxiway at taxiing.
Certainly, not all of the mentioned functions are implemented in a particular airborne radar system.Nevertheless, the AWR always is a multifunctional system that provides earth surface surveillance and weather observation.Usually, weather radar should at least enable to detect clouds and precipitation, select zones of meteorological danger, and show radar image of surface in the map mode.
AWRs or multimode radars with a weather mode are usually nose mounted.Most AWRs operate in either X-or C-band [18].The λ -4 dependence of weather formations on carrier wavelength λ favours X-band radar for their detecting.At the same time, the X-band provides the performance of the long-range weather mode better than Ku-band.The AWR antenna, in the groundmapping mode, has a large cosecant-squared elevation beam where horizontal dimension is narrow (2° to 6°) while the other is relatively broad (10° to 30°), and it sweeps in an azimuth sector (up to ± 100°) [18], [19].The scan plane is horizontal because the antenna is stabilized (roll-and-pitch-stabilized).These features allow the presumption that AWR operating in the groundmapping mode can be used as a scatterometer for measuring the water surface backscattering signature and near-surface wind over water.

MEASUREMENT OF BACKSCATTERING SIGNATURE / Mjerenje refleksije
Let an aircraft equipped with an AWR make a horizontal rectilinear flight with the speed V at some altitude H above the mean sea surface, the AWR operate in the ground-mapping mode as a scatterometer, the radar antenna have different beam width in the vertical θ a.v and horizontal θ a.h planes (θ a.v > θ a.h ), and scan periodically through an azimuth in a sector as shown in Fig. 1.Also let a delay selection be used to provide necessary resolution in the vertical plane.If the azimuth direction of the beam relative to the aircraft current course is fixed, the azimuth NRCS curve can be obtained using the circular track flight.As the scan plane is horizontal because the antenna is stabilized, the aircraft roll should not exceed the maximum one allowed for ensuring the antenna stabilization and consequently the incidence angle constancy.
Let a horizontal circular flight with the speed V and the left roll at some altitude H above the mean sea surface be completed (Fig. 2).The fixed beam should be pointed to the outer side of the aircraft turn to observe a greater area of the water surface and to obtain a greater number of independent NRCS samples.From that point of view, the best beam position is when the azimuth direction of the beam is perpendicular to the aircraft current course.

Selected cell
Figure 2 Measuring geometry: R t.fa , is the radius of aircraft turn, R g is the ground range, R t.c is the radius of turn of the selected cell middle point Slika 2. Geometrija mjerenja: R t.fa , je radijus okreta zrakoplova, R g je udaljenost od tla, R t.c je radijus okreta središne točke odabrane ćelije The azimuth size of a sector observed is 5° or 10°, respectively.A middle azimuth of the sector is the azimuth of the sector observed.The azimuth size of a sector relative to the centre point of circle of the airplane track is Δα s , and the middle azimuth of a sector is α s .The NRCS samples obtained from the sector and averaged over all measurement values in that sector give the NRCS value corresponding to the real observation azimuth angle of the sector ψ s that is where s ψ ψ is the airplane course corresponding to the real observation azimuth angle of the sector.
The 360-degree azimuth space can be divided into 72 or 36 sectors under the circle NRCS measurement.Thus, to obtain an azimuth NRCS curve of water surface in the range of moderate to high incidence angles under aircraft circular flight by the AWR operating in the ground-mapping mode as a scatterometer, the measurement should be started when a stable flight at the given altitude, speed of flight, roll and pitch has been established.Measurement should be finished when the azimuth of the measurement beginning is reached.To obtain a greater number of NRCS samples for each sector observed several consecutive full circular turns for 360° may be done.

WIND VECTOR RETRIEVAL / Izračun vektora vjetra
The wind blowing over sea modifies surface backscatter properties.These depend on wind speed and direction.Wind speed U can be measured by a scatterometer because a stronger wind produces a larger NRCS ) , , ( α θ σ U  at a medium incidence angle θ, and a smaller NRCS at the small (near nadir) incidence angle.Wind direction can also be inferred because NRCS varies as a function of azimuth illumination angle α relative to up-wind direction [7].
To retrieve the wind vector from NRCS measurements, a relationship between the NRCS and near-surface wind, called "geophysical model function", must be known.Scatterometer experiments have shown that the NRCS model function for medium incidence angles at appropriate transmit and received polarization (vertical or horizontal) is one of the widely used form [20] ) 2 cos( ) , ( Let the angle between the up-wind direction and the first NRCS azimuth ψ s.1 be α, the sector width be Δα s , and the measured NRCSs , N is the number of sectors composing the measured 360° azimuth NRCS curve, . Then, in a general case, to find the wind speed and up-wind direction from the azimuth NRCS data set obtained the following system of N equations should be solved and the navigation wind direction can be found as following To investigate capability of the proposed wind algorithm, a simulation of the wind vector retrieval based on a Wismann's geophysical model function [6] of the form (2) for the incidence angle of 45° was performed.The "measured" azimuth NRCS values were generated using Rayleigh Power (Exponential) distribution.
Fig. 3 shows "measured" NRCS after averaging of 44 samples in a one-degree azimuth sector at the "true" wind speed of 10 m/s (dot trace), and Fig. 4 demonstrates "measured" NRCS after averaging of 220 samples in a five-degree azimuth sector at the same wind speed (dot trace).Solid traces in these figures show the azimuth NRCS curves by model (1).

CONCLUSION / Zaključak
The study has shown that the AWR operating in the groundmapping mode as a scatterometer can be used for measuring the water surface backscattering signature and sea wind vector in addition to its typical navigation application.
The azimuth NRCS curve can be obtained at the circular track flight when the azimuth direction of the beam relative to the aircraft current course is fixed.The fixed beam should be pointed to the outer side of the aircraft turn to observe a greater area of sea surface and to obtain a greater number of independent NRCS samples.Azimuth position of the beam should be perpendicular to the aircraft current course, or at list tend to the perpendicular position when the scanning sector is narrower than ± 90°.Incidence angle of selected cell should tend to 45° that can be explained by better usage of the anisotropic properties of water surface scattering at medium incidence angles.
The proposed concept, considered principles and developed algorithm can be used for enhancement of AWR, and for designing an airborne radar system for operational measurement of the sea roughness characteristics and winds over water at joint and stand-alone observations.