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1. chinaXiv:201703.00289 [pdf]

Estimating the sea state bias of HY-2A radar altimeter by using a three-dimentional nonparametric model

Jiang, Maofei; Xu, Ke; Liu, Yalong; Wang, Lei
Subjects: Geosciences >> Space Physics

The sea state bias (SSB) has become the dominant source of error in satellite altimetry. The operational SSB correction models are two-dimensional (2-D) nonparametric models based on the wind speed (U) and the significant wave height (SWH) that can be directly measured by the altimeters. This paper estimates the sea state bias of HY-2A radar altimeter using a three-dimensional (3-D) nonparametric model based on SWH from HY-2A interim geophysical dataset records (IGDR), U and the mean wave period (MWP) from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis project ERA-Interim. The 3-D SSB estimates can increase the explained variance by 1.72 cm2, or 1.31 cm RMS relative to the traditional 2-D SSB estimates based on U and SWH. ?2016 IEEE.

submitted time 2017-03-10 Hits2461Downloads1074 Comment 0

2. chinaXiv:201612.00202 [pdf]

HY-2A Altimeter Time Tag Bias Estimation Using Reconstructive Transponder

Junzhi Wan; Wei Guo; Fei Zhao; Caiyun Wang
Subjects: Geosciences >> Space Physics

Independent clocks provide time tags for the precision orbit determination (POD) equipment and the radar altimeter onboard the HY-2A satellite, and a bias between POD data' time tag and corresponding range observation's time tag from the HY-2A altimeter exists. The time tag bias contributes a bias in the sea surface height observation due to the nonzero time rate of change of the HY-2A altimeter's height. A transponder for in-orbit radar altimeter calibration provides an approach to estimate the time tag bias. The altimeter receives the responding signals from the transponder and generates ranges. Pertinent reference ranges are obtained fromthe POD data and the transponder's coordinate. Using the ranges from the radar altimeter and the reference ranges, the time tag bias between the POD data and the altimeter observations can be estimated. During an in situ HY-2A altimeter calibration campaign using a reconstructive transponder from August 9, 2012, to July 20, 2014, 17 estimations of the altimeter's time tag bias were obtained. The preliminary results are presented in this letter.

submitted time 2016-12-26 Hits2409Downloads987 Comment 0

3. chinaXiv:201612.00199 [pdf]

Echo Signal Quality Analysis During HY-2A Radar Altimeter Calibration Campaign Using Reconstructive Transponder

Junzhi Wan; Wei Guo; Fei Zhao; Caiyun Wang; Peng Liu; Mingsen Lin; Hailong Peng; Chuan Xu
Subjects: Geosciences >> Space Physics

A reconstructive transponder has been utilized for the in-orbit calibration campaign of the HY-2A radar altimeter since March 2012. The precision of final calibration result is influenced by echo signal's quality in the HY-2A altimeter's range window. As an indicator of the signal's quality, echo signal dwell time is analyzed considering its influence on signal quality and its uncertainty. In HY-2A altimeter calibration, the echo signal dwell time is determined by the radial orbit prediction uncertainty and the real-time signal processing mechanism of the reconstructive transponder. The real-time signal processing mechanism of the reconstructive transponder utilizes some incoming signal samples without sending echo signals before transmitting. Comparing with the length of the HY-2A altimeter's range window, the radial orbit prediction uncertainty is large. Large radial orbit prediction uncertainty and signal processing mechanism of the reconstructive transponder are two main factors that limit the echo signal dwell time in HY-2A altimeter calibration. Finally, approaches for increasing echo signal dwell time are briefly proposed.

submitted time 2016-12-26 Hits2551Downloads914 Comment 0

4. chinaXiv:201612.00197 [pdf]

A Matching Method for Establishing Correspondence between Satellite Radar Altimeter Data and Transponder Data Generated during Calibration

Junzhi Wan; Wei Guo; Caiyun Wang; Fei Zhao
Subjects: Geosciences >> Space Physics

This letter presents a method for matching satellite radar altimeter data and transponder data generated during in situ calibration. The transponder generates a measurement error when it measures the arrival time of the altimeter’s transmitted signal and embeds the error in both the transponder’s recorded data and the altimeter’s recorded data. The secondorder finite difference sequence of this error sequence can be extracted from the raw data, thus, the correspondence between two identical but mismatched second-order difference sequences can be uniquely established. The measurement error is utilized,and a data matching method that can uniquely establish the correspondence between the altimeter’s recorded data sequence and the transponder’s recorded data sequence is presented.This post-processing method does not increase the real-time signal processing workload of the transponder. Furthermore,The principles underlying this method can be used for any transponder that can adjust the response signal delay during calibration.

submitted time 2016-12-26 Hits2035Downloads834 Comment 0

5. chinaXiv:201605.00286 [pdf]


Wang, Caiyun; Guo, Wei; Zhao, Fei; Wan, Junzhi; Xu, Ke
Subjects: Geosciences >> Space Physics

HY-2 satellite, the first ocean dynamic satellite of China, was launched in August 2011, carrying a dual frequency (Ku band and C band) altimeter, to measure the global mean sea level [1]. To provide truly sea level measurements, in-orbit calibration is essential. This paper concerns the transponder in-orbit calibration method. A signal reconstructive transponder for HY-2 altimeter in-orbit calibration is presented. Different from the existing bent-pipe ones, this kind of transponder can capture and track altimeter pulses, record them, and reconstruct the chirp pulses, transmit them back to the altimeter. It is carried on a SUV, reduces requirement for a permanent installation site. With the characteristics of capture and record altimeter signals, the HY-2 altimeter inner path delay and clock drift could be derived from the calibration data.

submitted time 2016-05-03 Hits1488Downloads854 Comment 0

6. chinaXiv:201605.00285 [pdf]


Xu, Xi-Yu; Ke, Xu; Hua, Shen; Wang, Zhen-Zhan; Liu, Ya-Long
Subjects: Geosciences >> Space Physics

An airborne delay Doppler altimeter was flown in autumn 2014. A calibration suite including a GNSS buoy, two tide gauges and two GPS stations were established. An experiment was carried out to test the GNSS buoy as a whole under dynamic ocean condition, to validate its performance and accuracy via the inter-comparison of tide gauges, and to demonstrate the altimetry calibration methods and to estimate the altimeter system bias. This paper presented some methods and results. The GNSS buoy system was proven to have the ability of measuring the sea surface height within 1cm, and the altimeter system bias had a consistency of centimeter level.

submitted time 2016-05-03 Hits924Downloads530 Comment 0

7. chinaXiv:201605.00278 [pdf]


Liu, Yalong; Xu, Ke; Zhang, Youguang; Wang, Lei; Xu, Xiyu; Chen, Hua
Subjects: Geosciences >> Space Physics

This paper is aimed to assess the accuracy of HY-2A altimetry system. To further improve the accuracy and performance of HY-2A observed SSHs, several new treatments including 4 parameters maximum likelihood estimation (MLE4) retracking for Ku and C band, non-parameter sea state bias (NPSSB) model, and reprocessed dual-frequency altimeter ionospheric correction are included. The evaluation from dual-crossover comparison of the fully reprocessed level-2 sensor geophysical dataset records (SGDRs) data suggests that the algorithm and model improvements mentioned above give rise to remarkable promotion to the accuracy of the forthcoming version GDRs. In this study we conclude that the standard deviation of 5.79 cm is achieved from crossover comparison between HY-2A and Jason-2 sea surface height (SSHs), suggesting a promising situation of HY-2A altimetry datasets.

submitted time 2016-05-03 Hits1735Downloads873 Comment 0

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