首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
The current study aims to analyze the wind and wave parameters over Indian Ocean region obtained from first Ka –band altimeter AltiKa onboard SARAL, a collaborative mission of Indian Space Research Organization (ISRO) and Centre National d'Etudes Spatiales (CNES), France. It also demonstrates a real time application of SARAL data by assimilating the wave height in a wave model operational at the Space Applications Centre, ISRO. State-of–the art coastal wave model Simulating Wave Near shore (SWAN) is used for this purpose. The well-tested optimal interpolation technique is adopted for assimilation. Before proceeding to the assimilation per se, SARAL/AltiKa Wind and Significant Wave Height (SWH) have been validated using in- situ observations and WAVEWATCH III model. Apart from assessment of wind and wave data quality, this also served the purpose of providing error covariance to be used in assimilation. Supremacy of the assimilation run over parallel control run without assimilation has been judged by comparing the results with buoy observations at Indian National Centre for Ocean Information System (INCOIS). The statistics of validation of the assimilation run has been found to be extremely encouraging and interesting.  相似文献   

2.
SARAL/AltiKa surface wind speed (WS) and significant wave height (SWH) measurements are monitored and validated against operational European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric and wave model results in addition to available in-situ observations to access their suitability for various applications, especially SWH data assimilation. The quality of SWH is very high while that of WS is very good except for an underestimation of high wind speeds. The impact of assimilating SWH in the ECMWF Integrated Forecast System was assessed using several numerical experiments. The results show positive impact. Operational assimilation of SWH at ECMWF model is part of the forthcoming model change.  相似文献   

3.
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   

4.
High-precision satellite altimeters help in measuring the variations in sea level since the early 1990s. After a number of such successful altimetry missions such as Topex/Poseidon, Jason-1, Jason-2, and Envisat, SARAL/AltiKa, a high resolution altimetry mission based on the Ka frequency band that can also cover high latitudinal zones, was launched in February 2013. Even though the data set available from this recent mission is not yet suitable for climate research owing to its short duration, in this study we perform a preliminary validation of SARAL/AltiKa sea-level data. The first part of the validation is the comparison of SARAL/AltiKa and Jason-2 sea-level data between March 2013 and August 2014 in terms of temporal mean spatial pattern. Comparisons in terms of global mean sea-level time series and latitudinal band-based mean time series are also performed. The second part of the validation is the comparison of the SARAL/AltiKa sea-level based time series with several tide gauge records covering the period of our study. Finally, an analysis of the annual sea-level budget with SARAL/AltiKa data, steric sea level, and ocean mass is performed. Results of these preliminary comparisons show good agreement with other sea-level data.  相似文献   

5.
The CNES/ISRO mission SARAL/AltiKa was successfully launched on 25 February 2013. It reached its nominal orbit on 13 March 2013. AltiKa is the first altimeter using the Ka-band frequency. This article presents the results of the calibration and validation activities perfromed on the first year of the SARAL/AltiKa mission. The main objective of the article is to assess the SARAL/AltiKa data quality and to estimate the altimeter system performance using GDR products. To achieve this goal, we present mono-mission metrics and compare them with Jason-2 over the same period. Even if these missions do not have the same ground track, precise comparisons are still possible. They allow assessing parameter discrepancies and SSH consistency between both missions in order to detect geographically correlated biases, jumps or drifts. These results show that SARAL/AltiKa data quality is excellent: ocean data coverage is greater than 99.5%, standard deviation at cross-overs is 5.4 cm. The mission therefore fulfills the requirements of high precision altimetry and can be used (in conjunction with Jason-2) to monitor the global mean sea level, ensuring the continuity of the record over ERS/Envisat historical ground track. Possible improvements and open issues are also identified, foreseeing an even better mission performance.  相似文献   

6.
SARAL/AltiKa GDR-T are analyzed to assess the quality of the significant wave height (SWH) measurements. SARAL along-track SWH plots reveal cases of erroneous data, more or less isolated, not detected by the quality flags. The anomalies are often correlated with strong attenuation of the Ka-band backscatter coefficient, sensitive to clouds and rain. A quality test based on the 1 Hz standard deviation is proposed to detect such anomalies. From buoy comparison, it is shown that SARAL SWH is more accurate than Jason-2, particularly at low SWH, and globally does not require any correction. Results are better with open ocean than with coastal buoys. The scatter and the number of outliers are much larger for coastal buoys. SARAL is then compared with Jason-2 and Cryosat-2. The altimeter data are extracted from the global altimeter SWH Ifremer data base, including specific corrections to calibrate the various altimeters. The comparison confirms the high quality of SARAL SWH. The 1 Hz standard deviation is much less than for Jason-2 and Cryosat-2, particularly at low SWH. Furthermore, results show that the corrections applied to Jason-2 and to Cryosat-2, in the data base, are efficient, improving the global agreement between the three altimeters.  相似文献   

7.
With the launch of SARAL/AltiKa altimeter, efforts have been made to develop wind speed retrieval algorithms. Here we present two algorithms for estimating and validating wind speed from AltiKa. The first method is based on a theoretical Geophysical Model Function (GMF) using forward model simulations for Ka band specifications. The second is the model function developed using the matched database of input and output vectors of Normalized Radar Cross Section (NRCS) from AltiKa and wind speed measurements from concurrent Jason-2 altimeters. Since the NRCS depends on both the surface roughness due to surface wind speed and on mean square slope of the surfaces, the significant wave height is used along with wind speed for model development as an proxy variable. Both the theoretical and empirical GMFs are evaluated for retrieval of wind speed from AltiKa and validated with NDBC buoys data. The empirical model provide wind speed retrieval accuracy of 1.4 m/s. The accuracy of wind retrievals from theoretical model is also in the similar range (1.6 m/s), indicating the sound physical basis applicable for the future altimeters with various incidence angles. The retrieved wind speed is applied for various case studies, bringing out all the regional and global features quite well.  相似文献   

8.
The impact of SARAL/AltiKa derived sea level anomaly (SLA) has been studied by assimilating it along with Jason-2 and Cryosat-2 SLA in the Princeton Ocean model (POM) using ensemble optimal interpolation (EnOI) technique. For isolating the extra benefit brought by SARAKL/Altika, a parallel run with assimilation of only Jason-2 and Cryosat-2 SLA has also been conducted. The importance of SARAL SLA in a data assimilative ocean prediction system has been evaluated with special emphasis on the improvement in thermocline depth, depth of the 20° isotherm, subsurface temperature and currents. Comparison with RAMA buoy has shown a positive impact of up to 13% for 20°C isotherm and up to 17% for thermocline depth after assimilating SARAL SLA. An overall improvement in temperature profile is also observed when compared with analogous profiles from RAMA buoys and Argo floats. Improvement in zonal currents away from the equator has also been noticed.  相似文献   

9.
In the absence of many gauging stations in the major and mighty river systems, there is a need for satellite-based observations to estimate temporal variations in the river water storage and associated water management. In this study, SARAL/AltiKa application for setting up hydraulic model (HEC-RAS) and river flow simulations over Tapi River India has been discussed. Waveform data of 40 Hz from Ka band altimeter has been used for water levels retrieval in the Tapi river. SARAL/AltiKa retrieved water levels were converted to discharge in the upstream location (track-926) using the rating curve available for the nearby gauging site and using linear spatial interpolation technique. Steady state simulations were done for various flow conditions in the upstream. Validation of river flow model was done in the downstream location (track-367) by comparing simulated and altimeter retrieved water levels (RMSE 0.67 m). Validated model was used to develop rating curve between water levels and simulated discharge for the downstream location which enables to monitor discharge variations from satellite platform in the absence of in situ observations. It has been demonstrated that SARAL/AltiKa data has potential for river flow monitoring and modeling which will feed for flood disaster forecasting, management and planning.  相似文献   

10.
The India-France SARAL/AltiKa mission is the first Ka-band altimetric mission dedi-cated to oceanography. The mission objectives are primarily the observation of the oceanic mesoscales but also include coastal oceanography, global and regional sea level monitoring, data assimilation, and operational oceanography. Secondary objectives include ice sheet and inland waters monitoring. One year after launch, the results widely confirm the nominal expectations in terms of accuracy, data quality and data availability in general.

Today's performances are compliant with specifications with an overall observed performance for the Sea Surface Height RMS of 3.4 cm to be compared to a 4 cm requirement. Some scientific examples are provided that illustrate some salient features of today's SARAL/AltiKa data with regard to standard altimetry: data availability, data accuracy at the mesoscales, data usefulness in costal area, over ice sheet, and for inland waters.  相似文献   

11.
We present an initial assessment of SARAL/AltiKa data in the coastal band. The study focuses on the Ibiza Channel where the north-south water exchanges play a key role in controlling the circulation variability in the western Mediterranean. In this area, the track 16 of SARAL/AltiKa intercepts the domain covered by a coastal high-frequency (HF) radar system, which provides surface currents with a range up to 60 km. We evaluate the performance of the SARAL/AltiKa Ssalto/Duacs delayed-time along-track products compared to the HF radar surface velocity fields. SARAL/AltiKa data are retrieved at a distance of only 7 km from the coast, putting in evidence the emerging capabilities of the new altimeter. The derived velocities resolved the general features of the seasonal mesoscale variability with reasonable agreement with HF radar fields (significant correlations of 0.54). However, some discrepancies appear, which might be caused by instrumental hardware radar errors, ageostrophic velocities as well as inaccurate corrections and editing in the altimeter data. Root mean square (rms) differences between the estimated SARAL/AltiKa and the HF radar velocities are about 13 cm/s. These results are consistent with recent studies in other parts of the ocean applying similar approaches to Topex/Poseidon and Jason-1 missions and using coastal altimeter corrections.  相似文献   

12.
This paper presents an assessment of SARAL/AltiKa satellite altimeter for the monitoring of a tropical western boundary current in the south-western Pacific Ocean: the East Caledonian Current. We compare surface geostrophic current estimates obtained from two versions of AltiKa along-track sea level height (AVISO 1 Hz and PEACHI 40 Hz) with two kinds of dedicated in situ datasets harvested along the satellite ground tracks: one deep-ocean current-meter mooring deployed in the core of the boundary current and five glider transects. It is concluded that the AltiKa-derived current successfully captures the velocity of the boundary current, with a standard error of 11 cm/s with respect to the in situ data. It also appears important to reference AltiKa sea level anomaly to the latest mean dynamic topography available in our area. Doing so, Ka-band altimetry provides a satisfactory representation of the western boundary current. Thereby, it usefully contributes to observing its variability in such a remote and under-observed ocean region. However, the rather long repeat period of SARAL (35 days) in comparison to the high frequency variability seen in the flow velocity of the boundary current calls for a combined use of SARAL with the other satellite altimetry missions.  相似文献   

13.
This work presents the first calibration results for the SARAL/AltiKa altimetric mission using the Gavdos permanent calibration facilities. The results cover one year of altimetric observations from April 2013 to March 2014 and include 11 calibration values for the altimeter bias. The reference ascending orbit No. 571 of SARAL/AltiKa has been used for this altimeter assessment. This satellite pass is coming from south and nears Gavdos, where it finally passes through its west coastal tip, only 6 km off the main calibration location. The selected calibration regions in the south sea of Gavdos range from about 8 km to 20 km south off the point of closest approach. Several reference surfaces have been chosen for this altimeter evaluation based on gravimetric, but detailed regional geoid, as well as combination of it with other altimetric models.

Based on these observations and the gravimetric geoid model, the altimeter bias for the SARAL/AltiKa is determined as mean value of ?46mm ±10mm, and a median of ?42 mm ±10 mm, using GDR-T data at 40 Hz rate. A preliminary cross-over analysis of the sea surface heights at a location south of Gavdos showed that SARAL/AltiKa measure less than Jason-2 by 4.6 cm. These bias values are consistent with those provided by Corsica, Harvest, and Karavatti Cal/Val sites. The wet troposphere and the ionosphere delay values of satellite altimetric measurements are also compared against in-situ observations (?5 mm difference in wet troposphere and almost the same for the ionosphere) determined by a local array of permanent GNSS receivers, and meteorological sensors.  相似文献   

14.
Satellite altimetry has been proven as an effective technology to accurately measure water level, ice elevation, and flat land surface changes since the 1990s. To overcome limitations of pulse-limited altimetry, new altimetric missions such as Cryosat-2 and Satellite with ARgos and AltiKa (SARAL/AltiKa), have been designed to have higher along-track spatial resolution to measure more accurately inland water levels for small water bodies, and coastal sea level changes. In this study, we evaluate the performance of Cryosat-2 low-resolution (LRM) and SARin modes and SARAL/AltiKa Ka-band data on two connected lakes in central Tibetan Plateau, and in the coastal region of Taiwan. Results are compared with in situ tide gauge data in Taiwan and altimetric lake level time series from the CNES Hydroweb database. Our results show that water level change trends observed by Cryosat-2 20-Hz retracked observations, the SARAL/AltiKa 40-Hz Ice-1 retracked data, and the Hydroweb measurements are consistent with the estimated water level trend of ～0.30?m/y, during 2011–2017, and 2013–2015, for the Tibetan Migriggyangzham Co and Dorsoidong Co, respectively. For the coastal region, the performance of SARAL/AltiKa is better than that of Cryosat-2 LRM data in Taiwan. This finding demonstrates the superiority of the Ka-band over Ku-band radar altimetry.  相似文献   

15.
The Kavaratti calibration-validation site in India at Lakshadweep Sea has been improved to carry out absolute calibration of SARAL/AltiKa altimeter. This site is augmented with a down-looking radar gauge and a permanent GPS receiver. The Kavaratti Island is located near a repeating ground track of SARAL/AltiKa and ～12 km away from the point of closest measurement of Jason-2, SARAL/AltiKa crossover point. Additionally, the altimeter and radiometer footprints do not experience any land contamination. This article aims at presenting the initial calibration-validation results over cycles 001-011 of AltiKa. The absolute sea surface height bias has been found to be ?48 mm at Kavaratti calibration site. In this preliminary study the effect of environmental variables such as winds and pressure are not considered in calculations.  相似文献   

16.
The SARAL/AltiKa project is based on a single Ka band altimeter (35.75 GHz), which is the first oceanography altimeter to operate at such a high frequency. Ka band offers reduced radar footprint in comparison to traditional Ku band altimeters and negligible ionospheric effects. In this paper we present and evaluate benefits of AltiKa altimeter applied in the study of lakes in Andean chain in South America. Water levels time series obtained with Envisat/RA-2 and SARAL/AltiKa altimeters over 17 lakes of various sizes are calculated and compared to in situ observations. SARAL/AltiKa measurements tend to be extremely well correlated with in situ measurements and offer significant improvements compared to the Envisat mission.  相似文献   

17.
Radar altimetry has demonstrated strong capabilities for the monitoring of water levels of lakes, rivers and wetlands over the last 20 years. The Indo-French SARAL/AltiKa mission, launched in February 2013, is the first satellite radar altimetry mission to carry onboard a Ka-band sensor. We propose here to evaluate the potential of this new instrument for land hydrology through comparisons with other altimetry-derived stages and discharges in the Ganges-Brahmaputra and Irrawaddy river basins using its first year of data. Due to the lack of concomitant in situ measurements for the current period, Jason-2 data, previously evaluated against in situ gauge records, were used as reference. Comparisons between Jason-2 and SARAL-derived water levels and discharges, and Jason-2 and Envisat (which flew the same orbit as SARAL from 2002 to 2010)-derived ones, was performed. Time-series of only one year of SARAL-derived water levels and discharges present better performances (lower RMSE and higher R, generally greater than 0.95) than the ones derived from Envisat when compared with Jason-2.  相似文献   

18.
SARAL/AltiKa completed its first year in orbit in March 2014. The 1 Hz GDR-T data of the first 10 cycles of the mission are used to perform a comprehensive quality assessment by means of a global multi-mission crossover analysis. Within this approach, SARAL sea surface heights are compared with data from other current missions, mainly Jason-2 and Cryosat-2, to reveal its accuracy and consistency with the other altimeter systems. Alongside with global mean range bias and instrumental drifts, investigations on geographically correlated errors as well as on the realization of the systems origin are performed. The study proves the high quality and reliability of SARAL. The mission shows only a small range bias of about ?5 cm with respect to Jason-2 and neither significant time-tag bias nor instrumental drifts. With 1.3 cm the scatter of radial errors is in the same order of magnitude as for Cryosat-2 and Jason-1 GM and will probably further improve using an enhanced sea state bias (SSB) model. However, the wet tropospheric corrections from SARAL radiometer still show some systematic effects influencing the range bias as well as geographically correlated error patterns and the z-component of the origin. Improved inflight calibration will be necessary to overcome these effects.  相似文献   

19.
The resolution of seamount geoid anomalies by the SARAL/AltiKa Ka-band radar altimeter is compared with the Envisat RA2 Ku-band altimeter using cross-spectral analysis of exact-repeat profiles. Noise spectra show white noise floors at root-mean-square levels around 8 mm per root-Hz for AltiKa and 19 mm per root-Hz for RA2, and are colored at wavelengths longer than a few km, with a spectral hump similar to that seen in Jason-2 data. The AltiKa noise level is lower than the RA2 noise level by more than one would expect from the ratio of their pulse repetition frequencies. Large outliers are present in data from both altimeters, always of one sign (range too long), and show little correlation with rain or other error flags. Seamount anomaly signal to noise ratios are 30 to 10 dB for AltiKa and 3 to 8 dB less for RA2, decreasing as seamount size decreases. Seamounts as small as 1.35 km tall are resolved by both instruments, with significantly better performance by AltiKa due to its lower noise level. If AltiKa can fly a geodetic mission, it will find many presently unknown seamounts.  相似文献   

20.
Comparison of ENVISAT and SARAL missions data shows that AltiKa can be successfully used for ice discrimination methodology and extension of ice conditions time series. Due to shorter wavelength and large bandwidth (480 MHz) which leads to a higher sensitivity to different surface conditions, AltiKa shows more clearly the separation between open water and various ice types. We observe significant decrease of backscatter (25–30 dB) in late spring for both ENVISAT and SARAL and discuss it in the context of ice metamorphism. There is a clear need to continue and expand our dedicated field studies of lake Baikal ice cover to better assess influence of ice structure on altimetric signal.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号