Study On The Detection Technology Of Taihu Lake Bloom By Spaceborne GNSS-R

For a long time,the outbreak of cyanobacteria blooms in lakes has caused immeasurable economic losses and environmental hazards to local residents,seriously affecting the quality of life of local people.The 2007 blackwater mass crisis caused by an outbreak of cyanobacterial blooms in Taihu Lake left millions of residents in Wuxi without water for a week.After the outbreak of this crisis,the state has stepped up efforts to control the water body of Taihu Lake.With the continuous advancement of the comprehensive management of cyanobacteria blooms in Taihu Lake,the overall water quality of Taihu Lake has been significantly improved.However,the number of cyanobacteria blooms and the area of blooms did not decrease significantly.At present,cyanobacteria blooms still threaten the ecological environment of Taihu Lake.Therefore,when cyanobacteria blooms break out,how to quickly and accurately detect water blooms is particularly important.Spaceborne GNSS-R(Global Navigation Satellite System–Reflectometry)is a passive remote sensing technology,which has the advantages of high data revisit cycle,all-day,all-weather,rich signal sources,etc.,and GNSS-R has low cost,wide coverage,and is not subject to The influence of weather and environment such as clouds.The principle of GNSS-R technology is to receive the direct and reflected signals through the GNSS receiver,and analyze the code delay and correlation function waveform and its trailing edge characteristics to obtain the specific parameter information of the target reflective surface.The waveform,time delay,and Doppler frequency shift of the reflected signal can reflect the characteristic information of the reflecting surface.Based on this scattering characteristic,the roughness of the reflective surface,the dielectric constant of the reflective surface,etc.can be obtained by inversion,so as to determine the nature and state of the reflective surface.At present,spaceborne GNSS-R technology has been applied in the fields of sea surface altimetry,sea surface wind field detection,sea ice detection,soil moisture detection and other fields.This paper studies the detection of Taihu Lake algae bloom by spaceborne GNSS-R for the first time.Algal blooms are produced by sudden blooms of plankton,forming clearly visible algae aggregates and causing a reactive change in the color of the water.There are many algae in Taihu Lake,and the dominant algae is Microcystis,which has a certain thickness and viscosity.When a cyanobacteria bloom occurs in the Taihu Lake area,the cyanobacteria floating on the lake surface will reduce the tension of the water surface and make the reflective surface smooth,thereby enhancing the front of the radar wave.to scatter.Spaceborne GNSS-R can effectively detect the roughness of the reflective surface.When the reflective surface is smooth,the Delay Doppler Maps(DDM)of the spaceborne GNSS-R shows coherent reflection.On the contrary,when the reflective surface is rough Incoherent reflections occur.The main factors affecting the reflecting surface in the Taihu Lake area are wind speed and cyanobacteria bloom.In this paper,by using coherent reflection to characterize the roughness of the reflective surface,the relationship between coherent reflection and cyanobacterial blooms in different wind speed ranges,as well as the relationship between power ratio and chlorophyll a concentration,is studied using the "Sentinel-3" satellite water color remote sensing instrument(OLCI)image The chlorophyll a concentration in Taihu Lake derived from the maximum characteristic peak height(MPH)algorithm was used as a reference.The experimental results prove that the coherent reflection of the spaceborne GNSS-R reflection signal can be used to judge whether a cyanobacteria bloom(severe)occurs in a specific wind speed interval,and the chlorophyll a concentration can be retrieved through the power ratio.It confirmed the feasibility of GNSS-R detection of algae blooms in Taihu Lake under the condition of spaceborne,and made up for the gap of GNSS-R detection of cyanobacteria blooms in inland lakes.The main research content and experimental results of this paper:1.Introduced the GNSS-R scattering principle and related functions in detail;used the bistatic radar scattering cross section to characterize the roughness of the reflective surface,combined with the Z-V model to invert the relationship between the delayed Doppler unit and the scattering surface;used the power of DDM than to judge whether coherent reflection occurs;Specifically analyze the influence of wind speed on two aspects of cyanobacteria bloom.Wind speed will affect the vertical distribution of cyanobacterial blooms in Taihu Lake,and the DDM peak power value under different wind speeds is simulated using a simulation program according to the sea surface mean square slope formula.Combined with the simulation results,the relationship between the roughness of the reflective surface and the wind speed is analyzed.2.The US Cyclone Global Navigation Satellite System(CYGNSS)dataset,the Sentinel-3 remote sensing data and the reanalysis wind speed products of the European Center for Medium-Range Weather Forecast(ECMWF)were introduced respectively.The preprocessing of CYGNSS data,the method of Sentinel-3 inversion of chlorophyll a concentration,and the matching method between the three data.3.Using the CYGNSS data from April to August 2020,calculate the power ratio of the time-delay Doppler delay map of the CYGNSS specular reflection point,and use the maximum characteristic peak height(MPH)of the "Sentinel-3" satellite water color remote sensing instrument(OLCI)image)algorithm inverts the Taihu Lake chlorophyll a concentration as a reference,and performs time-space linear matching with ECMWF wind speed products.Finally,through analysis,it is found that in the range of wind speed1-2.5m/s,when the concentration of chlorophyll a reaches 0.1mg/L or more(severe water bloom),it is very easy to cause coherent reflection at the specular reflection point(PR≥2),and the power ratio The correlation coefficient with the concentration of chlorophyll a is 0.84,which has a good correlation.In the extrapolation verification experiment,it was shown that the exponential fitting model established according to the concentration of chlorophyll a and PR had a good extrapolation effect.The experimental results prove the research on the detection of cyanobacteria bloom in Taihu Lake by using the power ratio and correlation characteristics of the reflected signal of spaceborne GNSS-R.