Conference Agenda

This research and collaboration project aims at the calibration and validation (Cal/Val) of the European Sentinel-3, Sentinel-6 and the Chinese HY-2 satellite altimeters using two permanent Cal/Val facilities: (1) the Permanent Facility for Altimetry Calibration established by ESA in Crete, Greece and (2) the National Altimetry Calibration Cooperation Plan of China. Other satellites, such as the Guanlan, CryoSat-2, CFOSAT, CRISTAL, etc., may also be supported by these Cal/Val infrastructures.

Satellites are being calibrated and monitored using uniform, standardized procedures and protocols while exploiting trusted and indisputable reference standards at both Cal/Val infrastructures in Europe and China. At present, the PFAC, Greece implements the action plan established by ESA for Fiducial Reference Measurements for Altimetry and reports its Cal/Val results along with their FRM uncertainty.

Through the ESA Dragon-5 project, the FRM procedures, protocols and best practices, will be updated, upgraded and followed at both Cal/Val facilities in Europe and China. Calibration of altimeters is accomplished by examining satellite observations in open seas against reference measurements. Comparisons are established through precise satellite positioning, water level observations, GPS buoys and reference models (geoid, mean dynamic topography, earth tides, troposphere and ionosphere) all defined by Cal/Val sites. The final uncertainty (FRM status) for altimeter bias will be attributed to several individual error sources, coming from observations in water level, atmosphere, absolute positioning, reference surface models, transfer of heights from Cal/Val sites to satellite observations, etc.

During this second Dragon5 year, the following tasks are being carried out:

• Two European Cal/Val sites in the west and south coast of Crete, Greece have been employed for the calibration of HY-2B satellite altimeter;
• Both sites already attain the FRM strategy in terms of their infrastructure, instrumentation, operations, and data processing and analysis;
• The Wanshan Cal/Val facility comprises four sites at Wailingding, Dangan, Zhiwan and Miaowan islands. Multi-disciplinary scientific and ancillary equipment (i.e., GNSS receivers, tide gauges, GNSS buoys, meterorological stations, solar photometers, etc.) support the calibration and validation services;
• Absolute calibration of the HY-2B sea-surface height bias has been determined by European and Chinese Cal/Val infrastructures following diverse and independent methodologies;
• The ESA FRM4ALT guidelines are put into action to estimate the FRM uncertainty for each Cal/Val site;
• Relative calibration of the HY-2B altimeter is carried out with respect to Sentinel-6A MF and Sentinel-3 altimetry mission via crossover analysis.
• Cal/Val results are cross-examined at the two Cal/Val facilities.

The main outcomes and conclusions of this Dragon5 joint work for the 2^nd year of collaboration, are:

• Both European and Chinese Cal/Val sites have become operational and implement independent calibration methodologies for European and Chinese satellite altimetry missions (i.e., Sentinel-6A MF, Sentinel-3A/B, HY-2B/C, etc.);
• Standardization of operations and data processing are followed by both Cal/Val infrastructures in Europe and China using different instrumentation/models and processing algorithms/strategies, while deviations because of geographic location (i.e., bathymetry, tides, etc.) are reported;
• Products of HY-2B satellite altimeter differ depending on the data repository used.;
• The performance of the HY-2B and HY-2C satellite altimeter is within its mission requirements.

Respect to the objectives and schedule of our project, the mid-term report will include on-going activities and results of Bufeng-1 data processing, calibration workflow, and validation of the calibrated results on hurricane winds, soil moisture, and sea level measurements. The presentation has three parts. Firstly, a short introduction will be given about Bufeng-1 and recent Chinese GNSS-R missions. Secondly, by utilizing the Bufeng-1 Normalized Bistatic Radar Cross Section (NBRCS), earth reflectivity, and range measurements, the preliminary results show that BuFeng-1 has a high agreement compared with other observations on severe sea surface winds, soil moisture, and sea level. In this presentation, the measurements of Bufeng-1 will be aligned with SFMR collected hurricanes, SMAP derived soil moisture, and DTU18 sea level models. Then, the validations of the accuracy and correlation coefficients will be analyzed to discuss the limitations and issues for the future research. For the last part, we will give the outlook about our future works of the objectives and the future plan of Chinese GNSS-R missiions.

This report provides an overview of the recent progress on the cross-calibration and validation of CSES/Swarm satellite magnetic field and plasma measurements. The main results are as follows:

(1) The first comprehensive comparison of ion density (Ni) in the topside ionosphere measured by the Langmuir probe (LP) and faceplate (FP) of the thermal ion imager on board Swarm satellites were performed. Results show a systematic difference between the LP and FP derived Ni values, and the systematic difference shows prominent dependences on solar flux, local time, and season. Although both Ni datasets show generally good linear regression with electron density (Ne) measurements from the incoherent scatter radar (ISR) located at Jicamarca, the Ni derived from LP shows an additional dependence on the solar flux, while such dependence cannot be seen in the FP-derived Ni. More light ions (e.g., H +), diffusing down from the plasmasphere to the Swarm altitude, seem to cause the overestimation of Ni from LP during low solar activity. A linear relation between the Swarm LP-derived Ni and ISR Ne is derived, and such a function is recommended to be implemented into further updates of the Swarm LP plasma density data.

(2) A detailed analysis for the correlation between electron density (Ne) and temperature (Te) at the topside ionosphere were carried out. In situ measurements from four satellites have been utilized, including the China Seismo-Electromagnetic Satellite (CSES), Swarm A and B, as well as the earlier Challenging Minisatellite Payload (CHAMP) satellite. Observations from the four satellites show generally consistent relationship between the Ne and Te at the topside ionosphere. When Ne is low, the Te is negative correlated with Ne, while the slop of negative relation becomes shallower or even reverses to a positive relation after Ne exceeds a certain threshold. Interestingly, two abnormal features of the Swarm Te measurements are observed: a) when Ne is lower than 1×10¹¹m⁻³, Te sometimes becomes very scatter at low and middle latitudes; b) when Ne is larger than 1×10¹¹m⁻³, Te is grouped into two branches at the equatorial and low latitudes. Further analysis reveals that the flags used in the Swarm Level-1 B plasma density product cannot well distinguish the two abnormal features of Te, implying further efforts are needed for the Swarm Te data calibration.

(3) Based on the in-orbit magnetic field data of China Seismo-Electromagnetic Satellite (CSES) and Swarm Bravo satellite, some researches on the cross calibration and correction technology were carried out. The condition applied is that two satellites pass by in a relatively short period of time and through spatial location at a relatively close range, and set different spatial-temporal scale standards, combined with Kp index to screen for geomagnetic quiet periods. Then with the help of CHAOS model, indirect analysis was realized. Furthermore, the difference between the in-orbit data and model value was visualized, and the phenomenon and possible reason of data variation with time and geomagnetic latitude variation were analyzed. According to the analysis results from 2019 to 2020, the scalar magnetic field detection payloads of the two satellites have maintained long-term stability in-orbit. Both scalar magnetic field data are in good agreement with CHAOS model and relatively consistent and stable. The difference between the data and the model is mainly distributed in the geomagnetic high latitude region. The results of the study can evaluate the reliability of the satellite magnetic field data and the consistency of multiple satellites detection results. Applying them to the field of in-orbit data processing and analysis may improve data accuracy and reliability, and further optimize the data processing method, which may provide a methodological reference for doing similar evaluation and analyzation subsequently.