H5 low pathogenic avian influenza viruses maintained in wild birds in China.

Low pathogenic avian influenza virus, H5 or H7 subtype, possesses the potential capability to change to highly pathogenic variant, which damages wild waterfowl, domestic poultry, and mammalian hosts. In regular active surveillance of avian influenza virus from wild birds in China in 2020, we isolated six H5 avian influenza viruses, including one H5N2, two H5N3, and three H5N8. Phylogenetic analysis indicated that the H5N2 and H5N3 isolates clustered into Eurasian lineage, whereas the H5N8 viruses were originated in North America. The HA proteins of six viruses carried the cleavage-site motif PQRETR↓GLF, which indicated low pathogenicity of the viruses in chickens. However, the N30D, I43M, and T215A mutations in M1 protein and the P42S, I106M, and C138F residues changed in NS1 protein, implying all viruses could exhibit increased virulence in mice. Viral replication kinetics in mammalian cells demonstrated that the three representative viruses had the ability to replicate in both MDCK cells and A549 cells with low titers. Even though two of three representatives, WS/SX/S3-620/2020(H5N3) and ML/AH/A3-770/2020(H5N8), did not replicate and transmit efficiently in poultry (chickens), they did replicate and transmit efficiently in waterfowl (ducks). Viral pathogenicity in mice indicated that both H5N2 and H5N3 viruses are able to replicate in the nasal turbinates and lungs of mice without prior adaptation, while the H5N8 virus could not. The intercontinental and cross-species transmission of viruses may continuously exist in China, thereby providing constant opportunities for virus reassortment with local resident AIVs. Thus, it is crucial to continuously monitor migration routes for AIVs by systematic surveillance.

A Novel Method for Precise Onboard Real-Time Orbit Determination with a Standalone GPS Receiver.

Satellite remote sensing systems require accurate, autonomous and real-time orbit determinations (RTOD) for geo-referencing. Onboard Global Positioning System (GPS) has widely been used to undertake such tasks. In this paper, a novel RTOD method achieving decimeter precision using GPS carrier phases, required by China's HY2A and ZY3 missions, is presented. A key to the algorithm success is the introduction of a new parameter, termed pseudo-ambiguity. This parameter combines the phase ambiguity, the orbit, and clock offset errors of the GPS broadcast ephemeris together to absorb a large part of the combined error. Based on the analysis of the characteristics of the orbit and clock offset errors, the pseudo-ambiguity can be modeled as a random walk, and estimated in an extended Kalman filter. Experiments of processing real data from HY2A and ZY3, simulating onboard operational scenarios of these two missions, are performed using the developed software SATODS. Results have demonstrated that the position and velocity accuracy (3D RMS) of 0.2-0.4 m and 0.2-0.4 mm/s, respectively, are achieved using dual-frequency carrier phases for HY2A, and slightly worse results for ZY3. These results show it is feasible to obtain orbit accuracy at decimeter level of 3-5 dm for position and 0.3-0.5 mm/s for velocity with this RTOD method.

GPS/GLONASS Combined Precise Point Positioning with Receiver Clock Modeling.

Research has demonstrated that receiver clock modeling can reduce the correlation coefficients among the parameters of receiver clock bias, station height and zenith tropospheric delay. This paper introduces the receiver clock modeling to GPS/GLONASS combined precise point positioning (PPP), aiming to better separate the receiver clock bias and station coordinates and therefore improve positioning accuracy. Firstly, the basic mathematic models including the GPS/GLONASS observation equations, stochastic model, and receiver clock model are briefly introduced. Then datasets from several IGS stations equipped with high-stability atomic clocks are used for kinematic PPP tests. To investigate the performance of PPP, including the positioning accuracy and convergence time, a week of (1-7 January 2014) GPS/GLONASS data retrieved from these IGS stations are processed with different schemes. The results indicate that the positioning accuracy as well as convergence time can benefit from the receiver clock modeling. This is particularly pronounced for the vertical component. Statistic RMSs show that the average improvement of three-dimensional positioning accuracy reaches up to 30%-40%. Sometimes, it even reaches over 60% for specific stations. Compared to the GPS-only PPP, solutions of the GPS/GLONASS combined PPP are much better no matter if the receiver clock offsets are modeled or not, indicating that the positioning accuracy and reliability are significantly improved with the additional GLONASS satellites in the case of insufficient number of GPS satellites or poor geometry conditions. In addition to the receiver clock modeling, the impacts of different inter-system timing bias (ISB) models are investigated. For the case of a sufficient number of satellites with fairly good geometry, the PPP performances are not seriously affected by the ISB model due to the low correlation between the ISB and the other parameters. However, the refinement of ISB model weakens the correlation between coordinates and ISB estimates and finally enhance the PPP performance in the case of poor observation conditions.