Some missions have been carried out to measure wave directional spectrum based on airborne real
aperture radar in a low incidence. For this study, the authors simulate the satellite-based real aperture
radar image spectrum to validate the radar performance of measuring the directional wave spectrum
based on the same principal as airborne one. In the processes of the simulation, the authors don't take
into account the rotation of antenna and the disturbing of the noise in order that the simulation can
exclude other factors to purely verify the measuring theory feasibility in essence. The authors use
Wen's spectrum as the input spectrum of the simulation, which is a kind of wind-generated wave
spectrum that can better describe the state of developing wave and the statuses of the Chinese sea fields.
Following that, the simulated sea surface, backscattered radar signal, modulation spectrum and the one
dimension wave spectrum (two dimension one can be obtained by synthesizing every directional one
dimension spectrum) are obtained, which is compared with the input spectrum. To have a
comprehensive realization to the performance of the radar, different cases including different
developing degree, wind speed and angle between dominant waves propagation direction and radar
look direction are considered. Presented results show that Satellite-based real aperture radar can mostly
measure the wave spectrum, but the capacity is limited on the cases of low wind speed, developing
waves and the big angles.
Modern microwave satellite altimeters can measure the instantaneous sea surface height to a precision of approximately
4.1 cm in the open ocean. One limiting factor is that if land appears within the altimeter footprint the data is flagged as
useless due to land contamination of the altimeter return waveforms and inappropriate geophysical corrections. This
means that many valuable coastal altimetry data values are simply edited out. In order to make improved use of the
altimetry waveform data near the coastal area, we derived the altimeter ranges from one year (March, 2006 to February,
2007; cycle 155 to cycle 188) of Jason1 waveform off the China coast (14-45°N, 105-130°E) by using four specialized
retrackers: Ocean, Ice-2, OCOG (Offset Centre of Gravity), and Threshold retracking algorithms, which are employed by
Envisat RA2 altimeter standard processing. In order to compare the four retracking algorithms quantitatively, we
calculated the bias, root mean square, and standard deviation of the sea level anomaly difference between the ascending
and descending tracks at crossovers. In addition, we compared the sea surface height derived from the four retracking
algorithms and in-situ tide gauge station measurements. The comparisons showed that the OCOG algorithm provides
more accurate results than the other three in coastal waters.
Altimetry data under extreme weather events such as tropical cyclones, typhoons and hurricanes are valuable although
the measurements are affected by various factors associated with high sea states, such as big winds, high waves,
especially atmospheric rains and ocean surface foams. Both rains and foams can strongly affect the propagation and
reflection of microwave signal, but the effects of sea foam on altimeter measurements have not been adequately
addressed. Although the sea foam only appears when the wind speed is high and ocean wave is breaking, the attenuation
effect of foam in typhoons should not be neglected. The major challenge is the effects of rain and foam on altimeter
received backscatter are similar and are hard to be separated. In this paper, we proposed a iterative method to correct
both rain and sea foam effects using a simplified stratified foam model and the liquid water content measurements from
nadir viewing Jason1 Microwave Radiometer. Finally a case study to maintain accurate wave height, wind speed, and
rain rate measurements and to retrieve the additional results of foam coverage considering both the effects of rain and sea
foam in the typhoon Shanshan is presented.
By analyzing several collocated QuikSCAT measurements and Special Sensor Microwave Imager (SSM/I) rain rate in
typhoons, the evidence of rain influence to wind retrieval is presented. The results show that the present of heavy rain in
typhoons will cause large bias in both wind speed and wind direction for QuikSCAT wind retrieval. So that the
Holland's model is applied in the ambiguity clear process to correct the errors in wind direction induced by rain. Also, a
radiate transfer function (RTF) and collocated SSM/I rain rate is employed to correct the attenuation and scattering
effects of rain. The performance of RTF method is limited due to lack of information of the collocated rain rate and the
un-precise of the RTF. As an alternative way to RTF, an interpolation method which just uses the data from QuikSCAT
itself is introduced to correct the attenuation and scattering effects of rain. This method is applied to retrieve the wind
vector in typhoon Ioke, the result shows that the bias in the wind direction and wind speed induced by rain has both been
well corrected, indicating that our correction to the impacts of rain on QuikSCAT wind retrieval in typhoons is effective.
With the availability of scatterometer data, surface wind vectors can be estimated from the backscatter measurement over
oceans, guarantee global, long-term monitoring of the winds on the oceans, which make them very valuable for climate
studies and other applications. At moderate wind speeds, the wind speed derived by scatterometer is considered reliable.
But at higher wind speeds, scatterometers appear to underestimate the wind speed, especially in tropical cyclones,
because of deficiencies of the geophysical model function for high winds, attenuation caused by rain, influence of wind
gradient, and the saturation of the backscattering under high wind.
As a passive microwave sensor, radiometer does not show obvious saturation phenomena under high wind, therefore it is
an appropriate candidate to be used to retrieve high wind speed. In this paper, combined scatterometer and radiometer
data is used to retrieve wind field under high wind condition. Using in situ data and meteorological data as a criterion,
we compared the wind retrieval performances of scatterometer and radiometer. Results show that it is better to use
radiometer data as a replacement of scatterometer while observing high wind speed.
Time phase shift is an effective, reliable, and commonly used technique to measure the phase of fringe patterns. The major disadvantage of this technique is that it involves phase-shift devices, which introduce errors during the process of phase shift and require high compatibility of the photoelectrical performance of different detectors used in the experiment. More importantly, since at least three fringe patterns are needed to reconstruct the 3-D surface of the object, this technique is inappropriate for use under dynamic conditions. We propose and demonstrate a novel technique, i.e., grating projection tricolor fringe profilometry, for rapid determination of the profiles of objects. The technique, which is based on only one fringe pattern, obviates the need of phase-shift devices. Thus, errors resulting from the process of phase shift can be eliminated and rapid measurement can be achieved.