Browsing by Author "Guo, Jinyun"

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A crossover adjustment for improving sea surface height mapping from in-situ high rate ship-borne GNSS data using PPP technique
(2016-08-15)
Guo, Jinyun
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Dong, Zhenghua
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Tan, Zhengguang
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Liu, Xin
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Chen, Chuanfa
Ship-borne global navigation satellite system (GNSS) technique can overcome the weakness of satellite altimetry and tide gauge in measuring sea surface heights (SSHs) over coastal seas. Ship-borne GNSS technique can be used to calibrate SSHs determined by the satellite altimetry and tide gauge. The ship borne GNSS data are processed with the single-epoch precise point positioning (PPP) method to estimate SSHs which are filtered by the Gaussian filter to weaken and/or remove effects of sea wind and wave field. Tidal corrections are also taken into consideration to improve SSHs. One crossover adjustment method is put forward to calculate the bias and drift along the ship route and assess the accuracy of SSHs. We processed the in-situ ship-borne GPS data over the offshore sea around Keelung to compute precisely SSHs with the single-epoch PPP. Statistical results of SSH differences of crossover points indicate that the root mean squares error of SSHs determined by the ship-borne GPS is up to level of 12.9 cm over the offshore sea similar to 30 km far away to land. (C) 2016 Elsevier Ltd. All rights reserved.
Coastal gravity anomalies from retracked Geosat/GM altimetry: Improvement, limitation and the role of airborne gravity data
(2006-07-01)
Hwang, Cheinway
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Guo, Jinyun
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Deng, Xiaoli
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Hsu, Hsin-Ying
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Liu, Yuting
We process geophysical and waveform data records of the Geosat/GM (geodetic mission) satellite altimeter mission for waveform retracking and applications. An improved threshold retracker is developed. The performances of the Beta-5, threshold and improved threshold retrackers are assessed over waters around Taiwan. The improved threshold retracker outperforms the other two. The improvement in the accuracy of sea surface height (SSH) is investigated according to marine zone and the distance of waters to the shore. The improvement rate increases closer to the land, with the largest improvement rate of about 20% in waters within 10 km of the shore. Over waters around islands and coasts, there are still retracked SSHs with large errors. Least-squares collocation is used to compute gravity anomalies from the Geosat/GM altimeter data. Use of retracked SSHs improves the accuracy of gravity anomalies by about 11%. Adding airborne gravity data further improves the accuracy, especially in the immediate vicinity of the coasts. Tide model errors over coastal waters remain a problem in altimetry applications, even if the waveforms are properly retracked.
Decadal Variation in Surface Characteristics over Xinjiang, Western China, from TAP Altimetry Backscatter Coefficients: Evidence of Climate Change
(2013-08-01)
Guo, Jinyun
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Yang, Lei
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Liu, Xin
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Chang, Xiaotao
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Hwang, Cheinway
The backscatter coefficient, known as sigma0, is an important measurement of satellite radar altimetry and a key parameter for land altimetry because of its close relationship with the physical properties and geometric features of land coverage under global/regional climate change effects. Using the TOPEX/Poseidon GDR-M dataset from January 1993 to December 2004, we study the spatial and temporal distribution of sigma at bands Ku and C over Xinjiang, western China. The results show that the sigma0 is influenced by the water distribution over land and the time evolution of sigma0 has clear seasonal changes. River basins or deserts are classified over the spatial distribution based on different sigma0 values. For example, high sigma0 values are found in the Tarim River Basin and low values are found in the Taldimakan Desert. The periodic components of sigma0 time series are determined using the fast Fourier transformation method. The annual variation is the dominating cycle and the semi-annual variation is the secondary signal. The amplitudes of sigma0 time series at bands Ku and C are also given and most areas have quite low amplitudes except for the Tarim River Basin. Several areas including the Tarim River Basin, Tianshan Mountain and Taldimakan Desert are selected for sigma0 time series spacial analysis to discuss the reasons for variations in sigma0. The main factors are precipitation and vegetation growth, which are affected by the global/regional climate change. The correlation between the brightness temperature, which is related to the water-vapor content in the atmosphere measured by TMR at the 21 GHz channel and sigma0 at two bands, is analyzed.
How HY-2A/GM altimeter performs in marine gravity derivation: assessment in the South China Sea
(2019-11-01)
Zhu, Chengcheng
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Guo, Jinyun
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Hwang, Cheinway
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Gao, Jinyao
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Yuan, Jiajia
HY-2A is China's first satellite altimeter mission, launched in Aug. 2011. Its geodetic mission (GM) started from 2016 March 30 till present, collecting sea surface heights for about five 168-d cycles. To test how the HY-2A altimeter performs in marine gravity derivation, we use the least-squares collocation method to determine marine gravity anomalies on 1' x 1' grids around the SouthChina Sea (covering 0 degrees-30 degrees N, 105 degrees E-125 degrees E) from the HY-2A/GM-measured geoid gradients. We assess the qualities of the HY-2A/GM-derived gravity over different depths and areas using the bias and tilt-adjusted ship-borne gravity anomalies from the U.S. National Centers for Environmental Information (NCEI) and the Second Institute of Oceanography, Ministry of Natural Resources (MNR) of P. R. China. The RMS difference between the HY-2A/GM-derived and the NCEI ship-borne gravity is 5.91 mGal, and is 5.33 mGal when replacing the HY-2A value from the Scripps Institution of Oceanography (SIO) V23.1 value. The RMS difference between the HY-2A/GM-derived and the MNR ship-borne gravity is 2.90 mGal, and is 2.76 mGal when replacing the HY-2A value from the SIO V23.1 value. The RMS difference between the HY-2A and SIO V23.1 value is 3.57 mGal in open sea areas at least 20 km far away from the coast. In general, the difference between the HY-2A/GM-derived gravity and ship-borne gravity decreases with decreasing gravity field roughness and increasing depth. HY-2A results in the lowest gravity accuracy in areas with islands or reefs. Our assessment result suggests that HY-2A can compete with other Ku-band altimeter missions in marine gravity derivation.
Lake Level Variations Monitored With Satellite Altimetry Waveform Retracking
(2009-06-01)
Guo, Jinyun
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Chang, Xiaotao
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Gao, Yonggang
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Sun, Jialong
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Hwang, Cheinway
Over lake shores, altimetric waveforms are generally contaminated by lands, rough lake surfaces, and lag effects of the altimeter's automatic gain control. To improve altimeter ranging accuracy and in turn to get better surface height measurement, contaminated waveforms should be retracked against geophysical corrections. In this paper, an improved threshold retracker (ITR) is developed to retrack waveforms over lakes. ITR considers not only the physical characteristics of the reflecting surface, but also the stochastic feature of waveform, and two new retrackers, the N-Beta function model, and the N-5-Beta function model, are also put forward to develop the waveform retracking program of this study. TOPEX/POSEIDON waveforms over Hulun Lake in the North China are retracked to monitor the temporal lake level variations. A comparison with the in situ hydrological data indicates ITR is very efficient to monitor the lake level variations with the retracked altimetric data. The result of our study shows accurate seasonal level variations and the descending trend of Hulun Lake.
Marine gravity determined from multi-satellite GM/ERM altimeter data over the South China Sea: SCSGA V1.0
(2020-05-06)
Zhu, Chengcheng
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Guo, Jinyun
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Gao, Jinyao
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Liu, Xin
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Hwang, Cheinway
High-precision and high-resolution gravity fields can be derived from multi-source satellite altimeter data. A gravity anomaly model around the South China Sea (SCSGA) V1.0 on a 1 ' x 1 ' grid is established from sea surface heights (SSHs) of several geodetic missions (GMs) and exact repeat missions. Gridded deflections of the vertical are first calculated from SSHs by the least squares collocation (LSC) method and then used to derive gravity anomalies by the inverse Vening Meinesz formula. In gravity derivation processing, we establish an approximate relationship among the precision of altimetric gravity, precision of geoid gradients, and density of geoid gradients (the average number of geoid gradients per 1 ' x 1 ' region). The weights of geoid gradients from the Ka-band altimeter for the LSC are innovatively determined by an iterative method. Finally, SCSGA V1.0 is assessed by ship-borne gravity anomalies and marine gravity models. The performance of GMs in gravity derivation is evaluated. In general, the altimetric gravity precision in regions with many islands and reefs increases more obviously than those in other regions when the geoid gradient density increases. The standard deviation of SCSGA V1.0 is 2.78 mGal, which is slightly better than those of four recognized global marine gravity models around the SCS. CryoSat-2 is the most important dataset for SCSGA V1.0. Ka-band SARAL/AltiKa plays a major role in gravity derivation, and the contribution of Haiyang-2A is greater than those of other Ku-band satellites, except CryoSat-2. SCSGA V1.0 is concluded to reach an international advanced level for marine gravity from altimeter data around the SCS.
Precise orbit determination and accuracy analysis of HY-2A satellite using DORIS Doppler data
(2014-12-01)
Kong, Qiaoli
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Guo, Jinyun
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Hwang, Cheinway
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Gao, Fan
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Lin, Hsinfa
HY-2A is the first ocean dynamic environment monitoring satellite of China, and its precise radial orbit is a critical factor to its scientific research application, for example, its orbit can serve as a reference frame for altimeter measurement. In order to realize precise orbit determination, HY-2A is equipped with Doppler orbitography and radiopositioning integrated by satellite (DORIS), retro-reflector array for the satellite laser ranging (SLR) and global positioning system tracking systems. In this study, DORIS Doppler data is used for its good reputation for precise orbit positioning. We described the strategy of orbit determination using dynamic models in detail, and particularly analyzed the relationship between arc length, orbital determination accuracy, and consumed time, and then concluded that 3-day is the optimal arc length. We carried out detail accuracy assessments of external orbit comparison, overlap orbit validation and SLR range validation. Experimental results show that the radial accuracy can reach 0.0154 m for the 3-day arc, and the radial overlap validation accuracy is better than 0.0223 m, and the accuracy of SLR range validation is better than 0.0964 m. Results demonstrate that the DORIS system meets the orbit precision specifications of HY-2A satellite.
Precise orbit determination and accuracy analysis of HY-2A satellite using DORIS Doppler data (vol 49, 2014)
(2014-12-01)
Kong, Qiaoli
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Guo, Jinyun
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Hwang, Cheinway
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Gao, Fan
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Lin, Shinfa
Simulation of precise orbit determination of COSMIC from onboard GPS zero-difference phase data with kinematic method
(2007-01-01)
Guo, Jinyun
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Hwang, Cheinway
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Tseng, Zipang
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Chang, Xiaotao
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土木工程學系
COSMIC is a constellation mission to study the climate, ionosphere and geodesy. The main geodetic mission of COSMIC is to determine the global gravity field model and its temporal variations, which need the precise geometric orbit of COSMIC. GPS observations onboard COSMIC are simulated using the GPS precise final orbit and high-rate clock of CODE, the designed orbit of COSMIC and the GPS antennae for precise orbit determination (POD). The precise geometric orbits of COSMIC are determined with the kinematic method from the space-borne simulated observations to test the POD capability of GPS antennae. There are two POD GPS antennae onboard COSMIC, named as POD +X and -X. The orbit from POD -X antenna has the approximately same precision as that from POD +X antenna, and the errors of both are greater than the given random error while simulating GPS data. The main reason is that the designed positions of POD antennae are not good. There are the different angels between the boresight vector and zenith direction of two antennae. Another reason is that POD +X antenna is in the flying direction and POD -X antenna is in the inverse direction. In order to get the high precision of POD, a virtual antenna is constructed from POD +X and -X, whose center is the center of mass of COSMIC. Observations from POD +X and -X then are reduced to the virtual antenna. Comparing with the referenced orbit and the kinematic orbit from the virtual antenna, the precision of orbit is consistent to the given random error when simulating GPS data, up to centimeter level.
Zonal and meridional ocean currents at TOPEX/Poseidon and JASON-1 crossovers around Taiwan: Error analysis and limitation
(2008-04-01)
Hwang, Cheinway
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Shih, Hsuan-Chang
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Guo, Jinyun
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Hsiao, Yu-Shen
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土木工程學系
A crossover method for determining zonal and meridional ocean current components is examined using data at three crossovers of TOPEX/Poseidon and JASON- I ground tracks over 2002 - 2006. To implement this method, a geoid model around Taiwan is constructed using surface and airborne gravity data. The modeled and observed geoidal heights at coastal benchmarks are consistent to 5 cm RMS with the means removed. The error and limitation of this method are discussed, concluding that, in order to obtain current velocities at a 10 cm s(-1) accuracy and a 6-km resolution, the dynamic ocean topography (DOT) at a mm-level accuracy is needed, which is not possible to achieve today. By filtering DOT to a spatial scale of 100 km or coarser, a 10 cm s(-1) accuracy of velocity may be obtained. One crossover (A) is situated south of Taiwan and near the Kuroshio, the second (B) is at the axis of the Kuroshio and the third is located in the northern Taiwan Strait. These three crossovers feature different ocean current patterns. At a spatial scale of 120 km, the agreement among the altimeter, the Princeton Ocean Model (POM), and the drifter-derived velocities is the best at B, followed by that at A, and then C. In fact, at C the altimeter-derived velocities contradict the POM-derived values, and the tide model error is to be blamed. Further improvement on geoid modeling is suggested.