Search

Objectives: Atmospheric water vapor is one of the important components in troposphere. It is of great significance to study the factors and accuracy of influencing water vapor. This paper mainly studies the influencing factors of precipitable water vapor (PWV) in the Loess Plateau and evaluates its actual accuracy. Methods: Firstly, the pressure, temperature data of the fifth-generation atmospheric reanalysis data of EC-MWF (ERA5) and global navigation satellite system (GNSS) derived zenith troposphere delay (ZTD) are evaluated. Secondly, according to the pressure and temperature data of ERA5 and ZTD data of crustal movement observation network of China (CMONOC), the PWV with 1-hour time resolution is calculated, and the theoretical error of PWV is deduced by using error propagation theory, and then the accuracy of PWV in the Loess Plateau is analyzed by comparing with the actual calculation error of PWV. Results: The results show that the average root mean square(RMS) and bias of GNSS-derived ZTD calculated by GAMIT/GLOBK and PANDA are 4.05 mm and −0.46 mm, respectively. The average RMS and bias of ERA5-provided pressure and temperature are 3.36 hPa/1.97 K and − 0.01 hPa/0.04 K, respectively. The theoretical error of PWV in the Loess Plateau is 1.51 mm, and the actual error is 1.94 mm. Conclusions: The accuracy of PWV calculated in this paper is high, which is of great significance to the investigation of water vapor distribution and climate monitoring. [ABSTRACT FROM AUTHOR]

Objectives: On July 31, 2020, China's BeiDou-3 satellite navigation system (BDS-3), which operates independently, has been fully completed and officially opened for service. The signal system of BDS-3 has been redesigned to provide public service signals of five frequencies: B1I, B3I, B1C, B2a and B2b. Methods: Based on the observations of international global navigation system monitoring and assessment service and multi-GNSS experiment (MGEX) tracking stations, this paper analyzes the characteristics of pseudoranges multipath error, signal to noise ratio, geometry-free ionosphere-free phase(GFIFP) combined observations of the new signal system, as well as the precise orbit determination (POD) performance evaluation of BDS-3 satellites. Results and Conclusions: The results show that the multipath noise level of BDS-3 satellites signal is better than that of BeiDou-2 navigation satellite system satellites, and there is no systematic deviation related to altitude angle, B1C is more significantly affected by multipath and noise. The GFIFP sequences of different signal combinations of BDS-3 satellites show periodic systematic error related to satellite, and the peak value is about 2 cm. For BDS-3 satellites, the one-step POD method is used. The two frequencies ionospheric free combination of B1I & B3I and B1C & B2a is adopted respectively. The orbit boundary discontinuity and satellite laser ranging carried out an orbit accuracy check, and the results show that when the number of available observations is less than B1I & B3I, the orbit accuracy of B1C & B2a is equivalent to that of B1I & B3I, and the radial internal coincidence accuracy is 6.1 cm and 6.6 cm respectively. [ABSTRACT FROM AUTHOR]

Objectives: North China is an important social and economic area in China but suffers from severe surface deformation. Monitoring the surface displacement effectively and timely can provide data basis for understanding the deformation mechanism and predicting disasters. However, the surface vertical displacement in North China shows significant seasonal fluctuations. Hence, this paper aims to analyze the spatiotemporal feature of seasonal vertical displacement and determine the driving sources. Methods: Firstly, the vertical time series of 20 global navigation satellite system (GNSS) stations from January 2011 to November 2019 are decomposed by independent component analysis. Secondly, the first six independent components (ICs) are derived and compared with the vertical displacements caused by environmental loadings. Results: The results show that GNSS-IC1 presents obvious annual variation, as well as a relative uniform spatial response, which is highly consistent with the displacement time series derived from atmospheric loading, and the average correlation coefficient at all stations is 0.60. GNSS-IC2 and GNSS-IC3 are both the mixed signals constrained by annual and intra-annual periods, while the spatial responses reflect locally clustered characteristics, which corresponds to the distribution of water resources in North China. After combining GNSS-IC2 and GNSS-IC3 as GNSS-IC2/3, the average correlation coefficient between GNSS-IC2/3 and the hydrological loading displacement time series at all stations is 0.50. Conclusions: The temporal component of each IC is generally less clustered by different time cycles and the spatial response shows distinct localized characteristics. Atmospheric and hydrological loadings are the primary driving sources of seasonal vertical displacement in North China. However, the seasonal displacements induced by other effects, including non-tide ocean loading and thermal expansion effect of bedrock and observation pillar, are not extracted and need further analysis. [ABSTRACT FROM AUTHOR]

Objectives: Deep space antennas are widely used in deep space and radio astronomical observations, and their precise position coordinates are required for research needs. In many cases, this precise position is given by very long baseline interferometry (VLBI) based on an initial position with millimeter-scale accuracy. Global positioning system (GPS) alone is merely not sufficient to obtain the initial position of newly-built antennas. Therefore, a method using other objects as reference is needed to calibrate observatory positions, especially antenna arrays, and this paper presents such a method for deep space and radio astronomical observations. Methods: This method employs an azimuth-elevation telescope with an accurate position as a reference object to measure the center position of a target telescope. Other reference objects with satisfying position accuracy can also be used in this method. GPS receivers are adopted to establish a reference network with given scale and orientation by differential measurement. The next step is measuring the local positions of two telescopes (or objective and reference) in the reference network using total station instruments deployed in several different locations. Calculations are induced during the transfer of positions in the reference system to unify data from those measurements. The position of the target to the reference object in a local coordinate system is obtained. The final step is the calculation of the coordinates of the tar⁃ get telescope through the accurate position of the reference telescope. A series of processes are used to con⁃ vert coordinate positions from the local reference system to the international terrestrial reference frame 2014 (ITRF2014) reference system. Results: Experimental measurement was carried out to obtain the coordinates of the 40 m diameter radio telescope in national observatory of China (NAOC) Miyun observatory with the 50 m diameter radio telescope also there as a reference object. Total station measurement was con⁃ ducted at three locations around the reference telescope and four locations around the target telescope. Two reflecting prisms calibrated by GPS devices were used as scale and orientation reference. Total station targets were placed on azimuth and elevation structures of both telescopes. The target position data were obtained when the telescope rotated and tilted at multiple angles. More than 400 times of measurement were taken during a two ⁃ day field experiment. Round ⁃ fitting and sphere ⁃ fitting achieved by the trust-region method were used to process measured points, and the coordinates of the 40 m diameter radio telescope in NAOC Miyun observatory were given with a calibration error of 2.312 mm (root mean square error， RMSE) in the ITRF2014 system. Conclusions: This method provides center point location in mm or sub-mm-scale precision in complex terrain situations with simple tools and low time cost. The measurement of total station instruments towards reference objects or target objects could be the direct or centering measurement. This method can be applied to obtain positions of single or multiple antennas for observation and further calibration. Observatories can apply this method in complex terrain locations such as mountains, rocky fields, and even a meteor crater on an extra-earth planet. Subsequent observations show that this calibration has met the need. [ABSTRACT FROM AUTHOR]

Objectives: The global navigation satellite system (GNSS)radio occultation (RO) signals can be reflected on the earth surface during the propagation process. It is of great value to explore atmospheric information contained in the reflected occultation signal for improving the accuracy of weather forecast and climate monitoring. Methods: Using the constellation observing system for meteorology, ionosphere and climate (COSMIC) occultation data collected from January 2011 to December 2015 in China, this paper extracted the occultation profile containing the reflected signals, which were strong at oceans and weak at inland and also showed seasonal changes. It was found that the inland snowy areas and the ocean surface were easier to reflect the occultation signals. Further, the offshore areas of China were set into 5°×5° grids and the correlation between the percentage of occultation reflected signals and the reanalysis data from European Centre for Medium‑Range Weather Forecasts in each grid was analyzed. Results: Around the offshore areas to the north of 10°N, the water vapor and temperature are negatively correlated with the percentage of occultation profile reflected signal, with a correlation coefficient from − 0.28 to − 0.58. However, around the offshore areas to the south of 10°N, affected by the monsoon at the South China Sea, the percentage of occultation profile reflection signal is positively correlated with the reanalyzed water vapor data, with a correlation coefficient from 0.32 to 0.34, but not significantly correlated with the reanalyzed temperature data. Conclusions: The RO reflected signals associate with atmospheric conditions, and it can be applied to the direct signals as supplementary data. [ABSTRACT FROM AUTHOR]

This paper introduces a method which utilizes global navigation satellite system (GNSS) based precision point positioning (PPP) for conducting long-term tilt and settlement monitoring of high-rise buildings in North China. Two-year continuous GNSS observations from two high-rise buildings near a deep foundation pit in Beijing are used for the case study. The method comprises four steps: ① Calculating the geocentric coordinates (XYZ) of the GNSS antenna with respect to a global reference frame (e.g., IGS08) with the PPP method; ② transforming the geocentric coordinates to a regional reference frame − the North China reference frame (NChina16); ③ converting the geocentric XYZ coordinates to a station-centric East -North- Up (ENU) coordinates; ④ removing seasonal ground-surface deformation from the ENU time series and restoring the true displacements (tilt and settlement) of the building. Experimental result indicates that the PPP method is able to achieve 2-3 mm horizontal accuracy(root mean square error) and 6-9 mm vertical accuracy (root mean square error)for 24 - hour continuous observations in North China. The combination of PPP, NChina16, and the North China seasonal ground deformation model can perform millimeter-accuracy, nearreal time, and long term static structural health monitoring of high-rise buildings and other large-scale infrastructure projects in North China. [ABSTRACT FROM AUTHOR]

The satellite Fengyun 3C(FY3C) launched in 2013 and carried a global navigation satellite system (GNSS) occultation sounder, which is the first BDS/GPS compatible radio occultation sounder in the world and records both GPS and BDS measurements. It provides an opportunity for investigating the precision realtime orbit determination based on space‐borne GPS/BDS measurements. In order to realize the real‐time orbit determination based on space‐borne GPS/BDS measurements with decimeter‐level accuracy, this paper carries out in‐depth research in the following aspects. Firstly, the real‐time orbit determination mathematical model based on the space‐borne GPS/BDS carrier phase observations is introduced. Then the properties of the errors in the signal’s propagation path that originates from GPS/BDS broadcast ephemeris are analyzed, and the signal‐in ‐propagation path error(SIPPE) statistics of different kinds of GPS/BDS satellites are also presented. Based on the analysis, a pseudo‐ambiguity is modeled as a stochastic process and estimated in the real‐time orbit determmination(RTOD) filter to absorb the SIPPE error, and then the decimeter‐ level accuracy can be realized. The space‐borne GPS/BDS data from China’ s FY3C satellite are em ‐ ployed for the simulative RTOD test using the SATODS software. Results have demonstrated that the position and velocity accuracy of 30 cm and 0.3 mm/s, respectively, are achieved when using GPS/BDS carrierphase data and broadcast ephemeris. Although the accuracy of BDS broadcast ephemeris is obviously worse than that of GPS, orbit results of the GPS/BDS fusion solution with reasonable stochastic models for pseudo‐ambiguities will be slightly superior to that using single GPS data. For example, when the FY3C satellite flew over Asia/Pacific region where more BDS satellites can be tracked, the orbit accuracy can be improved by 10% with the GPS/BDS fusion solution. [ABSTRACT FROM AUTHOR]