| Radar altimeter is an active microwave remote sensor,which can be used to measure the sea surface height(SSH),the significant wave height(SWH)and the backscatter coefficient by transmitting electromagnetic waves and receiving echoes returned from the surface.The backscatter coefficient can be further used to obtain the wind speed.The data measured by the radar altimeter can be further used for study of marine gravity anomalies,geoids,sea topography,seabed topography,ocean tides,wind-wave fields and ocean dynamics.With the development of technology,radar altimetry data can be used not only for oceanographic research but also for inland water level changes,land dynamics,sea ice thickness changes,glacier mass balance and abnormal climate impacts.In order to meet the application requirements,the measurements derived from the radar altimeter must reach a very high precision,which sets extremely high demands on the operational mechanism,system design and data processing technology for the radar altimeter.Error correction and echo signal processing technologies are important parts of the radar altimeter data processing technology,and they are of great significance to improve the accuracy of range measurements for radar altimetry.The HY-2A radar altimeter is the first spaceborne radar altimeter of China.Since its operation in 2011,a large amount of observation data have been accumulated.The analysis,calibration,modeling and application of these data are the goal of satellite engineering.This paper focuses on the HY-2A radar altimeter and studies the error correction and sea-land echo signal processing technology.The HY-2A radar altimeter data are reprocessed.The reprocessed SSH and SWH products are evaluated and preliminary applications are conducted using the reprocessed altimeter data.The main work and innovations are as follows:(1)Based on the fourth order Stokes wave theory,the relationship between the sea state bias and the wave period is discussed.The mean wave period from the ECMWF reanalysis ERA-interim is introduced to the sea state bias estination.A method based on the crossover data and a three-dimensional nonparametric model is proposed.Compared with the traditional two-dimensional sea state biasestimates based on the wind speed and the significant wave height,the three-dimensional sea state bias estimates based on the wind speed,significant wave height and mean wave period can increase the accuracy of the SSH products by 1.15 cm.(2)An improved local linear regression estimator is proposed for the sea state bias estimation.The improved algorithm reduces the time complexity from O(n2)to O(n).(3)The time tag biases of dual-frequency range measurements from the HY-2A radar altimeter are estimated and corrected.The corrected range measuements are applied to calculate the HY-2A dual-frequency ionosphere correction,significantly improving the accuracy of the HY-2A dual-frequency ionosphere correction.(4)The whole process of data processing from the original waveform processing,error correction to product quality assessment has been conducted for the HY-2A radar altimeter.The accuracy of the sea surface height is better than 5.5 cm and the accuracy of the significant wave height is better than 0.2 m.(5)The HY-2A radar altimter data are used to obtain global mean sea level change,El Ni?o,elevation maps of the Antarctica and Greenland ice sheets,and water level change of the Qinghai Lake,and these lay the foundation for subsequent application of the HY-2A radar altimeter data. |
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