The geoid height method and the deflection of the vertical method are two mainstream methods for inversion of Marine gravity field based on satellite altimetry. Each of the two methods has more advantages in different types of sea areas, but it is difficult for a single method to provide high-precision ocean gravity field information in complex sea areas. Different from the fusion method based on distance to coastline used in EGM08 model, a new fusion method based on water depth is proposed in this paper. According to the NCEI ship gravity data, It can be seen that in the whole sea area, the accuracy of the new model based on depth fusion is 3.45mgal, which is better than that of the SIO28 model and the DTU17 model. Especially in complex sea conditions such as offshore and shallow water, the accuracy improvement is more significant.

The radar altimeter onboard China's HY-2B satellite was launched in October 2018. In this paper, we processed the HY-2B L1 altimetry data from October 2020 to April 2021. The radar freeboard was preliminarily estimated from the HY-2B altimetry data over the Arctic sea ice. We validated the HY-2B radar freeboard estimates using the radar freeboard products from the Alfred Wegener Institute (AWI). The overall difference between the HY-2B radar freeboard estimates and the AWI data is$0.088\pm 0.057\ \mathrm{m}$.

The radar altimeter onboard China's HY-2B satellite was launched in October 2018. In this paper, the precision of the HY-2B elevation measurements over the Antarctica ice sheet (AIS) and the Greenland ice sheet (GrIS) is assessed through crossover analysis. The TFMRA (threshold first maximum re-tracker) algorithms is applied to retrack the waveforms of HY-2B altimeter. The precision of the HY-2B altimeter elevations is found to vary as a function of the threshold of the surface slope from$\sim 48 \text{ cm}$to$\sim 51\text{ cm}$over the AIS, and from$\sim 23\text{ cm}$to$\sim 34 \text{ cm}$over the GrIS. When compared with results obtained from time-coincident SARAL altimeter, the standard deviation of the crossover differences between the SARAL and HY-2B altimeters varies from$\sim 68 \text{ cm}$to$\sim 101 \text{ cm}$over the AIS, and from$\sim 56\text{ cm}$to$\sim 102\text{ cm}$over the GrIS.

In the grid processing of the deflection of the vertical (DOV) for marine gravity recovery, the traditional least squares collocation (LSC) gridding method only considers the accuracy difference between different satellite radar altimeters, but ignores the accuracy difference between different observation points of the same radar altimeter. In order to solve this problem, a new LSC gridding method weighted by the accuracy of observation points is proposed in this paper. The new method sets the quality parameters of each observed value according to the dispersion of the data and gives corresponding weights to each residual along-track DOV for LSC gridding based on the error propagation law. Then, based on the 1’ × 1’ gridded DOV obtained by the traditional method and the new method, the inverse Vening Meinesz formula with one-dimensional FFT and the remove and restore method were used to recover the 1’ × 1’ gridded gravity field in the South China Sea. The validation of ship-measured gravity data shows that the new DOV gridding method can effectively improve the accuracy of the marine gravity field derived from HY-2A and CryoSat-2 data. The new method is also applicable to other similar satellite altimeter data, and will bring significant improvements to the recovery of higher-precision ocean gravity fields.

This paper assesses the sea surface height (SSH), significant wave height (SWH) and wind speed (U) measurements derived from Haiyang-2B (HY-2B) altimeter products from April 2019 to September 2019. Crossover analysis is used to assess to SSH measurements. The mean standard deviations of the SSH crossover differences for HY-2B is 5.09 cm, which is lower than Jason-2 (5.31 cm) and Jason-3 (5.27 cm). Compared with the National Data Buoy Center (NDBC) observations, the HY-2B SWH measurements show a root-mean-square error (RMSE) of 0.205 m with a positive bias of 0.150 m. The HY-2B wind speed measurements show a RMSE of 1.139 m/s with a negative bias of 0.563 m/s. The SSH, SWH and wind speed measurements from HY-2B products show high accuracy, but should be further calibrated.

HY-2A (Haiyang-2A) is China's the first ocean dynamic environment satellite and is equipped with a radar altimeter as one of the primary payloads. HY-2A shifted the geodetic mission (GM) in March 2016. In this paper, we present the gravity field inverted by the HY-2A/GM data from March 2016 to December 2017 in the South China Sea (105°-125°E, 5°-25°N) with a resolution of 1'* 1'. The gravity anomaly is computed by inverse Vening-Meinesz formula with a one-dimensional FFT method during remove-restore procedure with the EGM2008 model as the reference model. In addition, the same period data of Cryosat-2 was inverted using the same method, and the NGDC shipboard gravity data was used for verification. The results show that the accuracy of HY-2A altimetry-derived gravity field is comparable to that of Cryosat-2. Meanwhile, the standard deviation between HY-2A and Cryosat-2 marine gravity field is 3.744mGal.

Since March 2016, the HY-2A satellite has swifted to the drift orbit with a repeat cycle of 168 days and has accumulated geodetic data for more than two years. This paper assesses the reprocessed sea surface height (SSH) measurements derived from the HY-2A geodetic mission (GM) data. The standard deviation of the SSH crossover differences for reprocessed HY-2A/GM data is 5.22 cm, which is close to the results from Jason-2 (5.49 cm) and Jason-3 (5.35 cm). The standard deviations of the SSH crossover differences between HY-2A and Jason-2 and Jason-3 are 5.25 cm and 5.31 cm, respectively. Sea level anomaly (SLA) power spectra analysis shows that the reprocessed HY-2A data have low noise level with respect to Jason-2 and Jason-3.

Haiyang-2B (HY-2B) is the second ocean dynamic environment satellite of China and was launched on launched on October 26th, 2018. Radar altimeter is one of HY-2B's main payloads and mainly used to provide sea surface height (SSH), significant wave height (SWH) and sea surface wind speed measurements. In this paper, the range noise level of HY-2B radar altimeter is estimated and compared with Jason-2 and Jason-3 altimeters. The noise level of HY-2B radar altimeter for an SWH of 2 m is estimated to be 1.38 cm, which is lower than those of Jason-2 and Jason-3 (1.60 cm). Sea level anomaly (SLA) power spectra analysis also shows that the HY-2B altimeter has low noise level with respect to Jason-2 and Jason-3. Therefore, the HY-2B radar altimeter has the potential to provide sea surface height measurements of high precision.

HY-2 satellites are the ocean dynamic environmental satellite series in China and the radar altimeters onboard are land-compatible ones particularly suitable for the coastal applications. In this paper, the HY-2A altimetry data are extracted and reprocessed offshore Hong Kong. The results show good agreement with the Jason-2 and tide gauge measurements, indicating the potential of HY-2A satellite altimetry in coastal applications.

This paper assesses the reprocessed sea surface height (SSH) and significant wave height (SWH, or Hs) measurements derived from Haiyang-2A (HY-2A) radar altimeter using Jason-2 and buoy observations. The mean standard deviations of the SSH crossover differences for HY-2A and Jason-2 are 5.123 cm and 5.202 cm, respectively. The mean standard deviation of the SSH crossover differences between HY-2A and Jason-2 is 5.23 cm, Compared with the National Data Buoy Center (NDBC) SWH measurements, the reprocessed HY-2A SWH measurements show a root-mean-square error (RMSE) of 0.221 m with a positive bias of 0.149 m. After calibrated with the two-branched corrections, the RMSE for the reprocessed HY-2A SWH measurements is reduced to 0.160 m, which is even lower than that for Jason-2 (0.228 m). Therefore, the reprocessed HY-2A SSH and SWH measurements are of high accuracy.

The radar altimeter onboard China's HY-2A satellite was launched in August 2011 and it can provide the surface elevation measurements from 80.6 °N to 80.6 °S. In this letter, the precision of the HY-2A elevation measurements over the Antarctica ice sheet (AIS) and the Greenland ice sheet (GrIS) is assessed through crossover analysis. We process the HY-2A sensor geophysical dataset records (SGDR) data from June 1, 2014 to June 14, 2014. The OCOG, Ice-1 and Ice-2 algorithms are applied to retrack the waveforms of HY-2A altimeter over the AIS and GrIS. Crossover analysis shows that the Ice-2 algorithm performs best among the three algorithms. The precision of the HY-2A altimeter elevations are found to vary as a function of the threshold of the surface slope from ~56 cm to ~63 cm over the AIS, and from ~29 cm to ~50 cm over the GrIS.

HY-2B, the second China's satellite for oceanic dynamic environment measurement, is scheduled to launch at 2018. To improve the capability, some new methods are adopted in HY-2B altimeter design. By these means, the long term stability of the instrument is improved compared with HY-2A altimeter. In this paper, the improved design based on a rubidium atomic clock for HY-2B altimeter is discussed in detail.

The sea state bias (SSB) has become the dominant source of error in satellite altimetry. The operational SSB correction models are two-dimensional (2-D) nonparametric models based on the wind speed (U) and the significant wave height (SWH) that can be directly measured by the altimeters. This paper estimates the sea state bias of HY-2A radar altimeter using a three-dimensional (3-D) nonparametric model based on SWH from HY_2A interim geophysical dataset records (IGDR), U and the mean wave period (MWP) from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis project ERA-Interim. The 3-D SSB estimates can increase the explained variance by 1.72 cm2, or 1.31 cm RMS relative to the traditional 2-D SSB estimates based on U and SWH.

Since the launch, HY-2A has observing global ocean dynamic environment data over the last 5 years including sea surface height (SSH), significant wave height (SWH) and corresponding auxiliary data (wind fields and water vapor). The in-orbit validation and crossover comparison of which suggest that the various technical indicators cater for basic needs in practicality. However, for the most important ocean dynamic environment parameter SSH, the accurate, reliable and standard processing for absolute calibration has not achieved yet, significantly limiting quantitative application of altimetry production. To evaluate the accuracy of HY-2A, we perform absolute calibration at California offshore Harvest Oil Platform. HY-2A interim geophysical dataset records (IGDR) are used to perform comparison. All corrections are estimated independently avoiding instruments aging. According to results, there are two significant linear trends for HY-2A derived SSHs consisting of -0.001083m/day for cycle 1~99 and 0.001475m/day afterward. The results indicate that HY-2A observations are low by -40.5 ±1.2 cm.

Altimeter provides a unique perspective to observe global ocean frequently which is never happened before. Whereas the altimeter measures the ocean in a narrow track, it is inconvenient in studying the ocean structures such as Kuroshio and Gulf Stream, let alone the mesoscale structures which are important ways to transport mass and energy. These ocean structures determined by sea surface topography (SST) can be estimated using altimeter data. In this study, we present a fusion result of altimetry data by combining HY-2A and Jason-2. The merged data, generated using optimal interpolation (OI), present a detailed description for mesoscale structures in Kuroshio area. In this study, (1) altimetry data for both HY-2A and Jason-2 employed to determine sea surface height anomaly (SSHA) are corrected using collected independent source including European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data, TPXO ocean tide model etc. (2) in order to further enrich the details of mesoscale structures, altimetry data from other ongoing mission including Cryosate-2 and SARAL/Altika are take into account. (3) to ensure accuracy of the fusion, observation bias of sea surface heights from HY-2A are corrected to align with the reference mission Jason-2 using cross-calibration, since Jason-2 have been calibrated stringently. The relative biases for HY-2A, SARAL/Altika and Cryosate-2 are 37.36cm, -4.54cm and - 72.5cm and the corresponding standard deviations are 5.58cm, 5.26cm and 5.02 cm respectively, which suggest accurate observations and reliable reprocessing. (4) optimal interpolation is used to perform the interpolation, thus generate SSHA with grid format which are easy to distinguish the mesoscale structures. (5) more details are introduced from multi-missions rather than one.

The synthetic aperture Altimeter (SARA) introduces synthetic aperture processing into the conventional radar altimeter (CRA) in the along-track dimension, so both the spatial resolution and precision of SARA are improved. The sub-looks alignment is a key technique in the SARA data processing. In this paper, a method for sub-looks alignment is proposed to correct the varieties of the satellite orbit height and the reference height, and a simulation is given to demonstrate the effectiveness of the method.

Significant wave height (SWH) is one of the major ocean altimetry products. This paper proposes a fast algorithm in retrieving the SWH from altimeter waveform. The idea is to differentiate the echo waveform and acquire a Gaussian-shape waveform. Then the standard deviation of the Gaussian function is computed directly and the standard deviation of the scatter probability density function (hence, SWH) is retrieved. The performance of the algorithm is verified by Jason-2 altimeter data. This algorithm can be employed onboard or in the near-real-time processing in future altimeter missions.

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

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

Compared with conventional radar altimeter (CRA), synthetic aperture radar altimeter (SARAL) is a new generation radar altimeter and has better height precision and spatial resolution. As using synthetic aperture technique, SARAL data processing is very different from CRA. To verify the performance and the processing algorithm, an airborne experimental system of SARAL is developed. The experimental system was installed in Y-12 aircraft, and a lot of data was obtained in the October 2014. The echo signal with SAR mode and conventional mode are obtained at the same time, so the standard deviations of the two modes are compared. The results indicate that the height precision of SARAL is improved about 1 times. The significant wave height value extracted from the SARAL waveform is very close to that of the buoy deployed at the experiment area.

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

HY-2A, the China's first satellite for oceanic dynamic environment measurement, was launched on August 16th, 2011. As one of main payloads of HY-2A satellite, HY-2A radar altimeter is a dual-frequency (Ku and C band) altimeter with a conventional pulse-limited method. The altimeter can measure sea surface height, significant wave height, and sea surface wind speed. The accuracy of the altimeter is about 2cm (@4m SWH) or 4cm (@20m SWH). The system techniques and capabilities of the HY-2A altimeter are discussed in this paper.

HY-2A, the China's first satellite for oceanic dynamic environment measurement, was launched on August 16th, 2011. As one of main payloads of HY-2A, HY-2A radar altimeter (RA/HY-2A) has been operating stably since the launch, and obtained a lot of scientific data. RA/HY-2A is a dual-frequency (Ku and C band) radar altimeter with a conventional pulse-limited method, and some new techniques such as model compatible tracker are adopted in it. The accuracy of RA/HY-2A is better than 2cm (@4m SWH) or 4cm (@20m SWH), and the inversion results show that RA/HY-2A meets the design requirements. In this paper, the design and in flight results of RA/HY-2A are discussed.

In this paper the PTR and LPF of China's HY-2A satellite altimeter were extracted and their features were analyzed. Then Look-Up Tables (LUTs) of the range (height), SWH and amplitude (AGC) were generated based on the PTR and LPF by Monte-Carlo simulation. The actual PTR and the Bessel FSIR expression were included in the simulation model to acquire the most accurate results. The biases with respect to the SWH and attitude angle were tabulated by very fine grids, and the pertaining biases must be corrected in the pre-processing routine to get better sea surface height products.

This paper is concerned with the theoretical model for SAR altimeter or Delay-Doppler radar altimeter. Work with this aim, first, the data process of the SAR altimeter is introduced and some assumptions of SAR altimeter systems are introduced. And this result in that the mean echo model of SAR altimeter could expressed as three terms convolution as that of conventional radar altimeter. Second, the sub-fan-beam pattern formed by along track signal process is analyzed and approximated with one simplified Gaussian expression. Third, the SAR altimeter measurement geometry is described and the mean echo model in common sense is described with parameters echo delay, sea wave height, scatter coefficients and radar antenna pointing angle. Some applications of these results are presented at the last section, include some features of mean echo model and proposal for future SAR altimeter system.

A theoretical analysis of the altimeter split-gate height tracker error is carried out in this paper. The principles of altimeter split-gate tracker are reviewed, and the discriminative curve of the tracker is derived. It is shown that the antenna parameters have little influence on the slope of discriminative curve. The height noise of the tracker is analyzed, assuming there is no other noise in the instrument. It shows that the AGC gate noise contributes much in the overall tracker noise in high sea state condition, so a long AGC gate is preferable. Finally, the 1saveraged height noise level in a typical sea state (SWH=4m) is calculated, and the error specification (≫2cm) is proved to be met.

Satellite radar altimeter is an active microwave remote sensor for obtaining ocean topography, as well as the marine and land ice mass fluxes at a global scale. Because the echo properties of ocean, land and pole ice are very different, the on-board tracker of radar altimeter should have the ability to handle the distinct signals automatically. In this paper, a new tracker for ocean land compatible, the model compatible tracker (MCT), is proposed. In MCT, two track algorithms, MLE (Maximum likelihood Estimate) and OCOG (off center of gravity), cooperate in parallel. Therefore, not only the good accuracy but also the robustness can be realized in real time on board. So MCT is very suitable for the ocean-land compatible radar altimeter. The principle of MCT, the cooperation strategy for the two algorithms, the adaptive bandwidth technique, and the realization of MCT are discussed in this paper.

This presentation describes the specifications and parameters of the radar altimeter and scatterometer for China's Oceanic Dynamic Environment Mission (ODEM) (HY-2, Haiyang-2. "Haiyang" means ocean in Chinese). Some details about the design of these two instruments are also presented. The instrument on-board HY-2 includes a microwave imager (MWI), a dual-band radar altimeter (RA) and Ku-band radar scatterometer (SCAT).

The control system plays a key role in the spaceborne radar altimeter, such as it should implement track function to provide the height from the satellite to the sea surface, the significant wave height (SWH) and the backscatter coefficient. In this paper, the design of the China spaceborne radar altimeter control system (RACS), which is one part of Multi-mode Microwave Remote Sensor (M/sup 3/RS), is discussed in detail. RACS can realize the function of noise-bias measurement, inner calibration, return wave acquisition-track process, and data transfer. A new algorithm to implement the IF simulation signal is brought forward, and an IF simulator is developed to test RACS. RACS is also successfully tested in the airborne flight experiment.

An airborne multi-mode microwave sensor has been developed in order to verify the design and performance of future Chinese spaceborne multi-mode microwave sensor. Like spaceborne system, the airborne system also includes altimetry, scatterometry and radiometry functions. There are five frequency channels in radiometry mode. Altimetry mode, scatterometry mode and one of radiometry channels operate at the same Ku band. Flight experiments have been conducted on southern sea of China. The results show that future Chinese spaceborne multi-mode microwave sensor is practicable. In this paper, the principle of spaceborne and airborne multi-mode microwave sensor is briefly introduced. Flight experimental details and results are described.