Ku– and Ka-band Altimeter Data in the Northwestern Mediterranean Sea: Impact on the Observation of the Coastal Ocean Variability

The strong increase in altimeter measurement errors near land surfaces is a limiting factor for coastal applications. We analyze the performance of the new Ka-band SARAL/AltiKa (SRL) mission in the northwestern Mediterranean Sea. SRL sea surface height (SSH) measurements are compared with those from the Jason-2 Ku-band satellite mission. The results show a significant increase in both quantity and quality of SSH data available near coastlines when using SRL data. Available edited data are 95.1% of SRL compared with 88.6% for Jason-2. Closer than 10 km to the coastline, available SRL data are still about 60% and only about 31% for Jason-2. Comparisons of the altimeter sea level variations are made with available coastal tide gauge data. The differences obtained between altimeter and tide gauge SLA time series are reduced for SRL (3.3 cm in average) compared with Jason-2 (4.2 cm in average), especially closer than 30 km to the land. It results in higher correlations (by 30%) obtained with SRL data. The coastal circulation derived from altimetry using SRL data shows an offshore meandering, which is more stable in time and with larger velocities close to the coast than that derived from Jason-2 observations.     

Absolute Calibration of SARAL/AltiKa in Kavaratti During its Initial Calibration-Validation Phase

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.     

SARAL/AltiKa Global Statistical Assessment and Cross-Calibration with Jason-2

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.     

First Calibration Results for the SARAL/AltiKa Altimetric Mission Using the Gavdos Permanent Facilities

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.     

Resolution of Seamount Geoid Anomalies Achieved by the SARAL/AltiKa and Envisat RA2 Satellite Radar Altimeters

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.     

基于GNSS浮标和验潮资料的HY-2A卫星高度计绝对定标

为探测我国HY-2A卫星高度计海面高测量绝对偏差及其在轨运行状态,本文利用GNSS浮标星下点同步测量和验潮资料海面高传递方法在山东千里岩和珠海担杆岛海域开展定标研究。为验证GNSS浮标定标方法的准确性,还对国外卫星Jason-2和Saral进行了定标实验。实验表明GNSS浮标绝对海面高测量精度达2 cm,对Jason-2和Saral高度计多个周期定标得到的海面高偏差均值分别为5.7 cm和-2.3 cm,与国际专门定标场的结果符合较好。2014年9月和2015年5月HY-2A卫星高度计浮标定标结果分别是-65 cm和-91 cm,因两次结果差异显著,故又利用千里岩验潮站资料对HY-2A卫星高度计第56至73周期进行了定标分析,结果证明HY-2A卫星海面高存在约-51 cm/a的漂移,置信度为95%的回归分析表明浮标和验潮定标结果符合。本文研究结果表明在我国尚无专门定标场的情况下,可利用GNSS浮标对我国高度计实施灵活、精准的在轨绝对定标,在有高度计轨迹经过验潮站的情况下可使用验潮资料结合精密大地水准面模型进行绝对定标。     

TOPEX/Poseidon and Jason-1: Absolute Calibration in Bass Strait, Australia

Updated absolute calibration results from Bass Strait, Australia, are presented for the TOPEX/Poseidon (T/P) and Jason-1 altimeter missions. Data from an oceanographic mooring array and coastal tide gauge have been used in addition to the previously described episodic GPS buoy deployments. The results represent a significant improvement in absolute bias estimates for the Bass Strait site. The extended methodology has allowed comparison between the altimeter and in situ data on a cycle-by-cycle basis over the duration of the dedicated calibration phase (formation flight period) of the Jason-1 mission. In addition, it has allowed absolute bias results to be extended to include all cycles since the T/P launch, and all Jason-1 data up to cycle 60. Updated estimates and formal 1-sigma uncertainties of the absolute bias computed throughout the formation flight period are 0 ± 14 mm for T/P and +152 + 13 mm for Jason-1 (for the GDR POE orbits). When JPL GPS orbits are used for cycles 1 to 60, the Jason-1 bias estimate is 131 mm, virtually identical to the NASA estimate from the Harvest Platform off California calculated with the GPS orbits and not significantly different to the CNES estimate from Corsica. The inference of geographically correlated errors in the GDR POE orbits (estimated to be approximately 17 mm at Bass Strait) highlights the importance of maintaining globally distributed verification sites and makes it clear that further work is required to improve our understanding of the Jason-1 instrument and algorithm behavior.     

Absolute Calibration of Jason-1 and TOPEX/Poseidon Altimeters in Corsica Special Issue: Jason-1 Calibration/Validation

The double geodetic Corsica site, which includes Ajaccio-Aspretto and Cape Senetosa (40 km south Ajaccio) in the western Mediterranean area, has been chosen to permit the absolute calibration of radar altimeters. It has been developed since 1998 at Cape Senetosa and, in addition to the use of classical tide gauges, a GPS buoy is deployed every 10 days under the satellites ground track (10 km off shore) since 2000. The 2002 absolute calibration campaign made from January to September in Corsica revealed the necessity of deploying different geodetic techniques on a dedicated site to reach an accuracy level of a few mm: in particular, the French Transportable Laser Ranging System (FTLRS) for accurate orbit determination, and various geodetic equipment as well as a local marine geoid, for monitoring the local sea level and mean sea level. TOPEX/Poseidon altimeter calibration has been performed from cycle 208 to 365 using M-GDR products, whereas Jason-1 altimeter calibration used cycles from 1 to 45 using I-GDR products. For Jason-1, improved estimates of sea-state bias and columnar atmospheric wet path delay as well as the most precise orbits available have been used. The goal of this article is to give synthetic results of the analysis of the different error sources for the tandem phase and for the whole studied period, as geophysical corrections, orbits and reference frame, sea level, and finally altimeter biases. Results are at the millimeter level when considering one year of continuous monitoring; they show a great consistency between both satellites with biases of 6 ± 3 mm (ALT-B) and 120 ± 7 mm, respectively, for TOPEX/Poseidon and Jason-1.     

Absolute Calibration of TOPEX/Poseidon and Jason-1 Using GPS Buoys in Bass Strait, Australia

An absolute calibration of the TOPEX/Poseidon (T/P) and Jason-1 altimeters has been undertaken during the dedicated calibration phase of the Jason-1 mission, in Bass Strait, Australia. The present study incorporates several improvements to the earlier calibration methodology used for Bass Strait, namely the use of GPS buoys and the determination of absolute bias in a purely geometrical sense, without the necessity of estimating a marine geoid. This article focuses on technical issues surrounding the GPS buoy methodology for use in altimeter calibration studies. We present absolute bias estimates computed solely from the GPS buoy deployments and derive formal uncertainty estimates for bias calculation from a single overflight at the 40-45 mm level. Estimates of the absolute bias derived from the GPS buoys is -10 ± 19 mm for T/P and +147 ± 21 mm for Jason-1 (MOE orbit) and +131 ± 21 mm for Jason-1 (GPS orbit). Considering the estimated error budget, our bias values are equivalent to other determinations from the dedicated NASA and CNES calibration sites.     

Validation of SWH and SSHA from SARAL/AltiKa Using Jason-2 and In-Situ Observations

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.     

Global Calibration of SARAL/AltiKa Using Multi-Mission Sea Surface Height Crossovers

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.     

Absolute Calibration of Jason-1 and TOPEX/Poseidon Altimeters in Corsica

The double geodetic Corsica site, which includes Ajaccio-Aspretto and Cape Senetosa (40 km south Ajaccio) in the western Mediterranean area, has been chosen to permit the absolute calibration of radar altimeters. It has been developed since 1998 at Cape Senetosa and, in addition to the use of classical tide gauges, a GPS buoy is deployed every 10 days under the satellites ground track (10 km off shore) since 2000. The 2002 absolute calibration campaign made from January to September in Corsica revealed the necessity of deploying different geodetic techniques on a dedicated site to reach an accuracy level of a few mm: in particular, the French Transportable Laser Ranging System (FTLRS) for accurate orbit determination, and various geodetic equipment as well as a local marine geoid, for monitoring the local sea level and mean sea level. TOPEX/Poseidon altimeter calibration has been performed from cycle 208 to 365 using M-GDR products, whereas Jason-1 altimeter calibration used cycles from 1 to 45 using I-GDR products. For Jason-1, improved estimates of sea-state bias and columnar atmospheric wet path delay as well as the most precise orbits available have been used. The goal of this article is to give synthetic results of the analysis of the different error sources for the tandem phase and for the whole studied period, as geophysical corrections, orbits and reference frame, sea level, and finally altimeter biases. Results are at the millimeter level when considering one year of continuous monitoring; they show a great consistency between both satellites with biases of 6 ± 3 mm (ALT-B) and 120 ± 7 mm, respectively, for TOPEX/Poseidon and Jason-1.     

Assessing SARAL/AltiKa Data in the Coastal Zone: Comparisons with HF Radar Observations

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.     

Metocean Comparisons of Jason-2 and AltiKa—A Method to Develop a New Wind Speed Algorithm

As well as range, the AltiKa altimeter provides estimates of wave height, H_s and normalized backscatter, σ⁰, that need to be assessed prior to statistics based on them being included in climate databases. An analysis of crossovers with the Jason-2 altimeter shows AltiKa H_s values to be biased high by only ?0.05m, with a standard deviation (s.d.) of ?0.1m for seven-point averages. AltiKa's σ⁰ values are 2.5–3 dB less than those from Jason-2, with a s.d. of ?0.3 dB, with these relatively large mismatches to be expected as AltiKa measures a different part of the spectrum of sea surface roughness. A new wind speed algorithm is developed through matching a histogram of σ⁰ values to that for Jason-2 wind speeds. The algorithm is robust to the use of short durations of data, with a consistency at roughly the 0.1 m/s level. Incorporation of H_s as a secondary input reduces the assessed error at crossovers from 0.82 m/s to 0.71 m/s. A comparison across all altimeter frequencies used to date demonstrates that the lowest wind speeds preferentially develop the shortest scales of roughness.     

AltiKa Radiometer: Instrument Description and In–Flight Performance

On 25 February 2013, the Satellite for Argos and AltiKa (SARAL) was launched from the Indian Sriharikota launch site. The AltiKa payload consisted of an altimeter and a radiometer. This paper describes the AltiKa radiometer. This instrument has been studied for several years by CNES, TAS-F, ASTRIUM-F and a set of science laboratories, and AltiKa is the first compact instrument embedding simultaneously the altimeter and radiometer functions. AltiKa radiometer is a dual frequency instrument working in K (23.8 GHz) and Ka band (37 GHz), it is based on the total power principle, with direct detection receivers. On-ground acceptance tests exhibited a very high level of performance: less than 0.2 dB has been estimated for both sensitivity and absolute accuracy in both frequencies. This paper focuses on the in-flight performances that have been observed since the launch. All the instrument observable characterizations are nominal, and in-flight sensitivity has been estimated lower than 0.2 K.     

Global Statistical Assessment and Cross-Calibration with Jason-2 for Reprocessed HY-2 A Altimeter Data

HY-2 A (Haiyang-2 A) satellite was launched on August 16, 2011 and radar altimeter is one of its main payloads. We reprocessed two years of HY-2 A altimeter sensor geophysical dataset records (SGDR) data. This paper presents the main results in terms of reprocessed HY-2 A altimeter data quality: verification of data availability and validity, monitoring several relevant altimeter parameters, and assessment of the HY-2 A altimeter system performances. A cross-calibration analysis of reprocessed HY-2 A altimeter data with Jason-2 was conducted. The reprocessed HY-2 A altimeter data show good quality and have a low level of noise with respect to Jason-2. The same geophysical correction methods were used to calculate the sea surface height (SSH) for the two missions. The mean standard deviations of the crossover differences for HY-2 A and Jason-2 are 5.24 cm and 5.34 cm, respectively. The mean standard deviation of the crossover differences between HY-2 A and Jason-2 is 5.37 cm. These show that HY-2 A can provide SSH measurements at almost the same level of accuracy as Jason-2. The relative SSH bias between HY-2 A and Jason-2 due to the Ultra Stable Oscillator (USO) drift is obviously observed, and it can affect the calculation of mean sea level and should be further studied and corrected.     

The 2013 Ibiza Calibration Campaign of Jason-2 and SARAL Altimeters

This study presents the results of the 2013 Ibiza (Western Mediterranean) calibration campaign of Jason-2 and SARAL altimeters. It took place from 14 to 16 September 2013 and comprised two phases: the calibration of the GNSS (Global Navigation Satellite System) buoys to estimate the antenna height of each of them and the absolute calibration to estimate the altimeter bias (i.e., the difference of sea level measured by radar altimetry and GNSS). The first one was achieved in the Ibiza harbor at a close vicinity of the Ibiza tide gauge and the second one was performed at ～ 40 km at the northwest of Ibiza Island at a crossover point of Jason-2 and SARAL nominal groundtracks. Five buoys were used to delineate the crossover region and their measurements interpolated at the exact location of each overflight. The overflights occurred two consecutive days: 15 and 16 September 2013 for Jason-2 and SARAL, respectively. The GNSS data were processed using precise point positioning technique. The biases found are of (?0.1 ± 0.9) and (?3.1 ± 1.5) cm for Jason-2 and SARAL, respectively.     

Comparison of Sea Surface Heights Derived from Satellite Altimetry and from Ocean Bottom Pressure Gauges: The SW Pacific MOTEVAS Project

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.     

Sea-Level Variations Measured by the New Altimetry Mission SARAL/AltiKa and its Validation Based on Spatial Patterns and Temporal Curves Using Jason-2, Tide Gauge Data and an Overview of the Annual Sea Level Budget

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.     

Retracking of SARAL/AltiKa Radar Altimetry Waveforms for Optimal Gravity Field Recovery

The accuracy of the marine gravity field derived from satellite altimetry depends on dense track spacing as well as high range precision. Here, we investigate the range precision that can be achieved using a new shorter wavelength Ka-band altimeter AltiKa aboard the SARAL spacecraft. We agree with a previous study that found that the range precision given in the SARAL/AltiKa Geophysical Data Records is more precise than that of Ku-band altimeter by a factor of two. Moreover, we show that two-pass retracking can further improve the range precision by a factor of 1.7 with respect to the 40 Hz-retracked data (item of range_40 hz) provided in the Geophysical Data Records. The important conclusion is that a dedicated Ka-band altimeter-mapping mission could substantially improve the global accuracy of the marine gravity field with complete coverage and a track spacing of <6 km achievable in ～1.3 years. This would reveal thousands of uncharted seamounts on the ocean floor as well as important tectonic features such as microplates and abyssal hill fabric.