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1.
We present the results of retracking 18 cycles (15 from the Jason-TOPEX collinear period) of Jason-1 data. We used the retracking method of Rodriguez which simultaneously solves for all relevant waveform parameters using a 26 Gaussian model of the altimeter point target response. We find significant differences from the Jason-1 Project retracking in the key parameters of range and significant wave height (SWH) in the second version of the Project SGDRs. The differences from the Jason-1 data have a strong dependence on off-nadir angle and some dependence on SWH. The dependence of range on SWH is what is called sea state bias. The retracking technique also estimates surface skewness. For Jason-1 with its very clean waveforms we make the first direct estimates of the skewness effect on altimeter data. We believe that the differences found here and thus in overall sea surface height are the result of the standard project processing using a single Gaussian approximation to the Point Target Response (PTR) and not solving simultaneously for off nadir angle. We believe that the relatively large sea state bias errors estimated empirically for Jason-1 during the cal/val phase result from sensitivity of quantities, particularly SWH, in project GDRs to off nadir angle. The TOPEX-Jason-1 bias can be determined only when a full retracking of Jason-1 is done for the collinear period.  相似文献   

2.
Jason-1 Altimeter Ground Processing Look-Up Correction Tables   总被引：1，自引：0，他引：1  
Poseidon-2 is the dual frequency radar altimeter embarked on the CNES/NASA oceanographic satellite Jason-1 that was launched on 7 December 2001. The primary objective of the Jason-1 mission is to continue the high accuracy time series of altimeter measurements that began with TOPEX in 1992. To achieve this goal, it is necessary to improve each component of the ground processing continually. Among these components are the look-up correction tables that are used to correct the estimations (range, significant waveheight, and sigma naught) issued from the retracking algorithms (on-board and ground). Look-up tables were first computed taking into account the prelaunch characteristics of the altimeter. They have to be updated to take into account better all the in-flight characteristics of the altimeter and all the updated ground algorithms that can impact the estimation process. The aim of this article is to describe the radar altimeter simulator of performances that has been used to compute look-up tables, to display the freshly computed look-up tables, and to discuss the consequences of these new corrections on the products provided to the users. The updated look-up correction tables allow improvement of SWH estimation, in particular with respect to TOPEX SWH data. It is also shown that no range dependency on SWH has to be looked for in these tables, and that the on-board TOPEX and Poseidon-2 tracking systems may contain the differences explaining the relative sea state bias between both altimeters.  相似文献   

3.
Poseidon-2 is the dual frequency radar altimeter embarked on the CNES/NASA oceanographic satellite Jason-1 that was launched on 7 December 2001. The primary objective of the Jason-1 mission is to continue the high accuracy time series of altimeter measurements that began with TOPEX in 1992. To achieve this goal, it is necessary to improve each component of the ground processing continually. Among these components are the look-up correction tables that are used to correct the estimations (range, significant waveheight, and sigma naught) issued from the retracking algorithms (on-board and ground). Look-up tables were first computed taking into account the prelaunch characteristics of the altimeter. They have to be updated to take into account better all the in-flight characteristics of the altimeter and all the updated ground algorithms that can impact the estimation process. The aim of this article is to describe the radar altimeter simulator of performances that has been used to compute look-up tables, to display the freshly computed look-up tables, and to discuss the consequences of these new corrections on the products provided to the users. The updated look-up correction tables allow improvement of SWH estimation, in particular with respect to TOPEX SWH data. It is also shown that no range dependency on SWH has to be looked for in these tables, and that the on-board TOPEX and Poseidon-2 tracking systems may contain the differences explaining the relative sea state bias between both altimeters.  相似文献   

4.
The Jason-1 verification phase has proven to be a unique and successful calibration experiment to quantify the agreement with its predecessor TOPEX/Poseidon. Although both missions have met prescribed error budgets, comparison of the mean and time-varying sea surface height profiles from near simultaneous observations derived from the missions' Geophysical Data Records exhibit significant basin scale differences. Several suspected sources causing this disagreement are identified and improved upon, including (a) replacement of TOPEX and Jason project POE with enhanced orbits computed at GSFC within a consistent ITRF2000 terrestrial reference frame, (b) application of waveform retracking corrections to TOPEX significant wave height and sea surface heights, (c) resultant improved efficacy of the TOPEX sea state bias estimation from the value added sea surface height, and (d) estimation of Jason-1 sea state bias employing dual TOPEX/Jason crossover and collinear sea surface height residuals unique to the validation mission. The resultant mean sea surface height comparison shows improved agreement at better than 60 percent level of variance reduction with a standard deviation less then 0.5 cm.  相似文献   

5.
The Jason-1 verification phase has proven to be a unique and successful calibration experiment to quantify the agreement with its predecessor TOPEX/Poseidon. Although both missions have met prescribed error budgets, comparison of the mean and time-varying sea surface height profiles from near simultaneous observations derived from the missions' Geophysical Data Records exhibit significant basin scale differences. Several suspected sources causing this disagreement are identified and improved upon, including (a) replacement of TOPEX and Jason project POE with enhanced orbits computed at GSFC within a consistent ITRF2000 terrestrial reference frame, (b) application of waveform retracking corrections to TOPEX significant wave height and sea surface heights, (c) resultant improved efficacy of the TOPEX sea state bias estimation from the value added sea surface height, and (d) estimation of Jason-1 sea state bias employing dual TOPEX/Jason crossover and collinear sea surface height residuals unique to the validation mission. The resultant mean sea surface height comparison shows improved agreement at better than 60 percent level of variance reduction with a standard deviation less then 0.5 cm.  相似文献   

6.
中国HY-2卫星雷达高度计有效波高真实性检验   总被引：9，自引：5，他引：4  
Chinese Haiyang-2(HY-2) satellite is the first Chinese marine dynamic environment satellite. The dual-frequency(Ku and C band) radar altimeter onboard HY-2 has been working effective to provide operational significant wave height(SWH) for more than three years(October 1, 2011 to present).We validated along-track Ku-band SWH data of HY-2 satellite against National Data Buoy Center(NDBC) in-situ measurements over a time period of three years from October 1, 2011 to September 30, 2014, the root mean square error(RMSE) and mean bias of HY-2SWH is 0.38 m and(–0.13±0.35) m, respectively. We also did cross validation against Jason-2 altimeter SWH data,the RMSE and the mean bias is 0.36 m and(–0.22±0.28) m, respectively. In order to compare the statistical results between HY-2 and Jason-2 satellite SWH data, we validated the Jason-2 satellite radar altimeter along-track Ku-band SWH data against NDBC measurements using the same method. The results demonstrate the validation method in this study is scientific and the RMSE and mean bias of Jason-2 SWH data is 0.26 m and(0.00±0.26) m,respectively. We also validated both HY-2 and Jason-2 SWH data every month, the mean bias of Jason-2 SWH data almost equaled to zero all the time, while the mean bias of HY-2 SWH data was no less than –0.31 m before April2013 and dropped to zero after that time. These results indicate that the statistical results for HY-2 altimeter SWH are reliable and HY-2 altimeter along-track SWH data were steady and of high quality in the last three years. The results also indicate that HY-2 SWH data have greatly been improved and have the same accuracy with Jason-2SWH data after April, 2013. SWH data provided by HY-2 satellite radar altimeter are useful and acceptable for ocean operational applications.  相似文献   

7.
本文基于Amarouche的二阶理论回波模型,导出了带有偏度系数的二阶理论回波模型;针对HY-2A卫星高度计波形特点,引入了奇异值分解滤波,并根据最大似然估计算法反演参数的不同得到6种重跟踪方案;利用其中的五参数方案处理该波形数据,获得海面散射点高度概率密度函数中偏度的合理取值为0.15;将结果分别与浮标、Jason-1和HY-2AIDR有效波高对比,分析6种方案反演有效波高的准确度,确定了MLE4_SVD(波形重跟踪之前进行滤波)对HY-2A高度计重跟踪更适合反演有效波高。  相似文献   

8.
The impact of Stokes drift on the mixed layer temperature variation was estimated by taking into account an advective heat transport term induced by the Stokes drift in the equation of mixed layer temperature and using the oceanic and wave parameters from a global ocean circulation model （HYCOM） and a wave model （Wave Watch III）. The dimensional analysis and quantitative estimation method were conducted to assess the importance of the effect induced by the Stokes drift and to analyze its spatial distribution and seasonal variation characteristics. Results show that the contribution of the Stokes drift to the mixed layer temperature variation at mid-to-high latitudes is comparable with that of the mean current, and a substantial part of mixed layer temperature change is induced by taking the Stokes drift effect into account. Although the advection heat transport induced by the Stokes drift is not the leading term for the mixed layer temperature equation, it cannot be neglected and even becomes critical in some regions for the simulation of the upperocean temperature.  相似文献   

9.
A bottom pressure gauge (BPG) was installed in proximity (3.7 km at closest approach) of Jason-1 and formerly TOPEX/Poseidon (T/P) ground track No. 238 at the Wusi site, located ∼ 10 km offshore off the west coast of Santo Island, Vanuatu, Southwest (SW) Pacific. Sea level variations are inferred from the bottom pressure, seawater temperature, and salinity, corrected for the measured surface atmospheric pressure. The expansion of the water column (steric increase in sea surface height, SSH) due to temperature and salinity changes is approximated by the equation of state. We compare time series of SSH derived from T/P Side B altimeter Geophysical Data Records (GDR) and Jason-1 Interim Geophysical Data Records (IGDR), with the gauge-inferred sea level variations. Since altimeter SSH is a geocentric measurement, whereas the gauge-inferred observation is a relative sea level measurement, SSH comparison is conducted with the means of both series removed in this study. In addition, high-rate (1-Hz) bottom pressure implied wave heights (H1/3) are compared with the significant wave height (SWH) measured by Jason-1. Noticeable discrepancy is found in this comparison for high waves, however the differences do not contribute significantly to the difference in sea level variations observed between the altimeter and the pressure gauge. In situ atmospheric pressure measurements are also used to verify the inverse barometer (IB) and the dry troposphere corrections (DTC) used in the Jason IGDR. We observe a bias between the IGDR corrections and those derived from the local sensors. Standard deviations of the sea level differences between T/P and BPG is 52 mm and is 48 mm between Jason and BPG, indicating that both altimeters have similar performance at the Wusi site and that it is feasible to conduct long-term monitoring of altimetry at such a site.  相似文献   

10.
ENVISAT Radar Altimeter Tracker Bias   总被引：1，自引：0，他引：1  
In the past, errors in the determination of the orbit were dominant in radar altimeter missions, but technical advances have improved the orbit accuracy and hence, other sources of error have become more important. Sea-state bias is now the main source of error and can be divided into three sea-state dependent errors: skewness, electromagnetic bias, and tracker bias. We estimated the magnitude of the third term, by retracking ocean waveforms from ENVISAT RA-2. The retracking algorithm used is based on Maximum Likelihood Estimation. Tracker bias shows a seasonal and geographical dependence related to the distribution of significant wave height (SWH) and time origin differences. We estimated a mean value of 0.13 ± 0.07 %SWH. Temporal and regional dependent errors are introduced when using a linear retracker processing approach.  相似文献   

11.
TOPEX/Poseidon is a well known success, with the operational altimeter (TOPEX) and the experimental one (Poseidon-1), providing data of unprecedented quality. However, there are two major differences between the TOPEX and Poseidon-1 radar altimeters on board TOPEX/Poseidon. The first is related to the estimated range noise; the second is linked to the sea-state bias (SSB) model estimates. Since the recent launch of the Jason-1 radar altimeter (also called Poseidon-2), we have been cross-comparing these three systems to better characterize each of them. Analyzing standard user products, we have found that Jason-1 is behaving like Poseidon-1 and thus shows the same observed differences when compared with TOPEX. A comparative analysis of their features was performed, starting from the on-board acquisition of the ocean return and ending with the ground generation of the high level accuracy oceanographic product. The results lead us to believe that the sources for these differences lie in both the waveform tracking processing and the presence or abscence of a retracking procedure whether on-board or on ground. Because Poseidon-1 and Jason-1 waveforms are retracked while TOPEX waveforms are not in the products distributed to the users, we have applied the same ground retracking algorithm to the waveforms of the three radar altimeters to get consistent data sets. The analysis of the outputs has shown that: (a) the noise level for the three radar altimeters is definitively the same, and (b) the source of the relative SSB between Jason-1 and TOPEX lies in the different behavior of the on-board tracking softwares.  相似文献   

12.
《Marine Geodesy》2013,36(3-4):201-238
TOPEX/Poseidon is a well known success, with the operational altimeter (TOPEX) and the experimental one (Poseidon-1), providing data of unprecedented quality. However, there are two major differences between the TOPEX and Poseidon-1 radar altimeters on board TOPEX/Poseidon. The first is related to the estimated range noise; the second is linked to the sea-state bias (SSB) model estimates. Since the recent launch of the Jason-1 radar altimeter (also called Poseidon-2), we have been cross-comparing these three systems to better characterize each of them. Analyzing standard user products, we have found that Jason-1 is behaving like Poseidon-1 and thus shows the same observed differences when compared with TOPEX. A comparative analysis of their features was performed, starting from the on-board acquisition of the ocean return and ending with the ground generation of the high level accuracy oceanographic product. The results lead us to believe that the sources for these differences lie in both the waveform tracking processing and the presence or abscence of a retracking procedure whether on-board or on ground. Because Poseidon-1 and Jason-1 waveforms are retracked while TOPEX waveforms are not in the products distributed to the users, we have applied the same ground retracking algorithm to the waveforms of the three radar altimeters to get consistent data sets. The analysis of the outputs has shown that: (a) the noise level for the three radar altimeters is definitively the same, and (b) the source of the relative SSB between Jason-1 and TOPEX lies in the different behavior of the on-board tracking softwares.  相似文献   

13.
After two years of verification and validation activities of the Jason-1 altimeter data, it appears that all the mission specifications are completely fulfilled. Performances of all instruments embarked onboard the platform meet all the requirements of the mission. However, the star tracker system has shown some occasional abnormal behavior leading to mispointing angles out of the range of Jason-1 system specification which states that the altimeter antenna shall be pointed to the nadir direction with an accuracy below 0.2 degree (3 sigma). This article discusses the platform attitude angle and its consequences on the altimetric estimates. We propose improvements of the Jason-1 retracking process to better account for attitude effects.

The first star tracker anomalies for the Jason-1 mission were detected in April 2002. The Poseidon-2 algorithms were specified assuming an antenna off-nadir angle smaller than 0.3 degree. For higher values, the current method to estimate the ocean parameters is known to be inaccurate. Thus, the algorithm has to be reviewed, and more specifically, the present altimeter echo model has to be modified to meet the desired instrument performance.

Therefore, we derive a second order analytical model of the altimeter echo to take into account attitude angles up to 0.8 degree, and consequently, we adapt the retracking algorithm. This new model is tested on theoretical simulated data using a maximum likelihood estimation. Biases and noise performance characteristics are computed for the different estimated parameters. They are compared to the ones obtained with the current algorithm. This new method provides highly improved estimations for high attitude angles. It is statistically validated on real data by applying it on several cycles of Poseidon-2 raw measurements. The results are found to be consistent with those obtained from simulations. They also fully agree with the TOPEX estimates when flying along the same ground track. Finally, the estimates are also in agreement with the ones available in the current I/GDR (Intermediate Geophysical Data Record) products when mispointing lies in the mission specifications.  相似文献   

14.
After two years of verification and validation activities of the Jason-1 altimeter data, it appears that all the mission specifications are completely fulfilled. Performances of all instruments embarked onboard the platform meet all the requirements of the mission. However, the star tracker system has shown some occasional abnormal behavior leading to mispointing angles out of the range of Jason-1 system specification which states that the altimeter antenna shall be pointed to the nadir direction with an accuracy below 0.2 degree (3 sigma). This article discusses the platform attitude angle and its consequences on the altimetric estimates. We propose improvements of the Jason-1 retracking process to better account for attitude effects.

The first star tracker anomalies for the Jason-1 mission were detected in April 2002. The Poseidon-2 algorithms were specified assuming an antenna off-nadir angle smaller than 0.3 degree. For higher values, the current method to estimate the ocean parameters is known to be inaccurate. Thus, the algorithm has to be reviewed, and more specifically, the present altimeter echo model has to be modified to meet the desired instrument performance.

Therefore, we derive a second order analytical model of the altimeter echo to take into account attitude angles up to 0.8 degree, and consequently, we adapt the retracking algorithm. This new model is tested on theoretical simulated data using a maximum likelihood estimation. Biases and noise performance characteristics are computed for the different estimated parameters. They are compared to the ones obtained with the current algorithm. This new method provides highly improved estimations for high attitude angles. It is statistically validated on real data by applying it on several cycles of Poseidon-2 raw measurements. The results are found to be consistent with those obtained from simulations. They also fully agree with the TOPEX estimates when flying along the same ground track. Finally, the estimates are also in agreement with the ones available in the current I/GDR (Intermediate Geophysical Data Record) products when mispointing lies in the mission specifications.  相似文献   

15.
S. CALMANT  K. CHENG  G. JAN  C. KUO  C. SHUM  Y. YI 《Marine Geodesy》2013,36(3-4):597-613
A bottom pressure gauge (BPG) was installed in proximity (3.7 km at closest approach) of Jason-1 and formerly TOPEX/Poseidon (T/P) ground track No. 238 at the Wusi site, located ～ 10 km offshore off the west coast of Santo Island, Vanuatu, Southwest (SW) Pacific. Sea level variations are inferred from the bottom pressure, seawater temperature, and salinity, corrected for the measured surface atmospheric pressure. The expansion of the water column (steric increase in sea surface height, SSH) due to temperature and salinity changes is approximated by the equation of state. We compare time series of SSH derived from T/P Side B altimeter Geophysical Data Records (GDR) and Jason-1 Interim Geophysical Data Records (IGDR), with the gauge-inferred sea level variations. Since altimeter SSH is a geocentric measurement, whereas the gauge-inferred observation is a relative sea level measurement, SSH comparison is conducted with the means of both series removed in this study. In addition, high-rate (1-Hz) bottom pressure implied wave heights (H 1/3 ) are compared with the significant wave height (SWH) measured by Jason-1. Noticeable discrepancy is found in this comparison for high waves, however the differences do not contribute significantly to the difference in sea level variations observed between the altimeter and the pressure gauge. In situ atmospheric pressure measurements are also used to verify the inverse barometer (IB) and the dry troposphere corrections (DTC) used in the Jason IGDR. We observe a bias between the IGDR corrections and those derived from the local sensors. Standard deviations of the sea level differences between T/P and BPG is 52 mm and is 48 mm between Jason and BPG, indicating that both altimeters have similar performance at the Wusi site and that it is feasible to conduct long-term monitoring of altimetry at such a site.  相似文献   

16.
1IntroductionSZ-4spaceborne,launched on30December2002,is the fourth unmanned spaceborne of China.Multimode microwave remote sensor is its primaryload,of which the altimeter mode is the most impor-tant one.During about five months,a large numberofSZ-4altim…  相似文献   

17.
The focus of this study is the validation of significant wave height (SWH) and sea surface height anomaly (SSHA) obtained from the first Ka-band altimeter AltiKa onboard SARAL (Satellite for ARGOS and Altimeters). It is a collaborative mission of the Indian Space Research Organization and Centre National d'Etudes Spatiales (CNES). This is done using in-situ observations from buoy and Jason-2 measurements. Validation using buoy observations are at particular locations while that using Jason-2 altimeter is an attempt towards global validation of Altika products. The results clearly indicate that the SARAL/AltiKa provide high-quality data and the errors are within a predefined range of accuracy. A parallel validation of SWH from other altimeters, which monitored ocean since last decade, like EnviSAT and Jason-2 was also performed with buoy observations. The results clearly show that the accuracy of AltiKa SWH is much better than EnviSAT and comparable to reference mission Jason-2. The accuracy is quite good for the calm sea while in the rough seas the accuracy degrades some. The inter-comparison of SARAL/AltiKa SSHA with Jason-2 indicates a fair match between them. These validation exercises demonstrate the high quality of AltiKa products, usable for practical applications.  相似文献   

18.
利用Jason-3卫星在国外4个主要卫星高度计定标场所在海域的测量数据,分析了这些海域的有效波高变化特性。在我国大陆沿岸选取了Jason-3卫星经过的12个海域,分析了它们的有效波高变化特性,给出了在我国大陆沿岸建设卫星测高定标场的选址建议。Bass、Corsica、Gavdos等3个定标场海域的平均有效波高约为1.1 m,说明平均有效波高1.1 m及以下的海域适用于建设定标场。选定的12个沿岸海域中,渤海及黄海海域平均有效波高小于0.8 m,显著小于东海及南海1.3～1.5 m平均有效波高,单独从有效波高的角度渤海及黄海海域更适合于建设卫星高度计定标场。  相似文献   

19.
The Jason-1 sea state bias (SSB) is analyzed in depth from the first year of GDR products. Compared to previous missions, this work benefits from two aspects of the empirical determination of the SSB from the altimetric data themselves. First, from a methodological point of view, a nonparametric technique (NP) has been developed and largely tested on TOPEX/Poseidon 1, GFO and Envisat data. The NP estimator has proven to be a useful tool in the SSB estimation, and it is now mature enough to be used for a refined analysis. On the other hand, the SSB can be extracted from three different data sets (crossovers, collinear data, and residuals) with different characteristics. It is then possible to cross calibrate various estimations of the SSB models and to determine the most accurate one. A systematic comparison is made between these different estimates for the Jason-1 altimeter. The collinear and crossover data sets yield very similar estimates despite their difference of spatial and temporal distributions. These SSB models assure consistency with the TOPEX mission when comparing Jason-1 and TOPEX residuals during the tandem phase. Thanks to the present work, the impact of the short wavelengths filtering on the SSB estimation is evidenced. More generally, our understanding of potential errors affecting the sea surface height and their impact onto the SSB estimation is also improved.  相似文献   

20.
The Jason-1 sea state bias (SSB) is analyzed in depth from the first year of GDR products. Compared to previous missions, this work benefits from two aspects of the empirical determination of the SSB from the altimetric data themselves. First, from a methodological point of view, a nonparametric technique (NP) has been developed and largely tested on TOPEX/Poseidon 1, GFO and Envisat data. The NP estimator has proven to be a useful tool in the SSB estimation, and it is now mature enough to be used for a refined analysis. On the other hand, the SSB can be extracted from three different data sets (crossovers, collinear data, and residuals) with different characteristics. It is then possible to cross calibrate various estimations of the SSB models and to determine the most accurate one. A systematic comparison is made between these different estimates for the Jason-1 altimeter. The collinear and crossover data sets yield very similar estimates despite their difference of spatial and temporal distributions. These SSB models assure consistency with the TOPEX mission when comparing Jason-1 and TOPEX residuals during the tandem phase. Thanks to the present work, the impact of the short wavelengths filtering on the SSB estimation is evidenced. More generally, our understanding of potential errors affecting the sea surface height and their impact onto the SSB estimation is also improved.  相似文献   

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