Surveying Colocated GNSS, VLBI, and SLR Stations in China

The local tie vectors between different space geodesy instruments in colocated sites, such as the global navigation satellite system (GNSS), very long baseline interferometry (VLBI), and satellite laser ranging (SLR), are essential for combination with the international terrestrial reference frame (ITRF). This paper introduces the surveying method and data processing model for determining the tie vectors in the seven colocated sites in Shanghai, Wuhan, Kunming, Beijing, Xian, Changchun, and Urumqi, and presents the values and full variance-covariance of these local ties. The surveying methodology and data processing method of the current work are rigorously determined to guarantee the relative positional precision of reference points (RPs) of different instruments in each colocation site to be a few millimeters. This paper compares the new tie vectors with those derived from ITRF 2008 products to consider the discrepancies at tie epoch. By comparing the new results to the previous results by other organizations, the new tie vector at the Wuhan site is in good agreement, but the vertical coordinate difference of the tie vector at the Shanghai site is as large as 2.24 cm. Therefore, the tie vector at the Shanghai site may have changed about 2 cm from 2003 to 2011

Modeling and Assessment of GPS/BDS Combined Precise Point Positioning

Precise Point Positioning (PPP) technique enables stand-alone receivers to obtain cm-level positioning accuracy. Observations from multi-GNSS systems can augment users with improved positioning accuracy, reliability and availability. In this paper, we present and evaluate the GPS/BDS combined PPP models, including the traditional model and a simplified model, where the inter-system bias (ISB) is treated in different way. To evaluate the performance of combined GPS/BDS PPP, kinematic and static PPP positions are compared to the IGS daily estimates, where 1 month GPS/BDS data of 11 IGS Multi-GNSS Experiment (MGEX) stations are used. The results indicate apparent improvement of GPS/BDS combined PPP solutions in both static and kinematic cases, where much smaller standard deviations are presented in the magnitude distribution of coordinates RMS statistics. Comparisons between the traditional and simplified combined PPP models show no difference in coordinate estimations, and the inter system biases between the GPS/BDS system are assimilated into receiver clock, ambiguities and pseudo-range residuals accordingly

Accuracy evaluation and improvement strategies of BeiDou broadcast clock error model

The spaceborne atomic clock is the core equipment of Beidou Satellite Navigation System (BDS). Clock error model is an important part of BDS broadcast messages, and its accuracy directly affects the service performance of BDS. In this paper, the accuracy of the BDS clock model is evaluated using the TWSTF clock error measurement as a reference. The results show that the accuracy of the BDS clock model is better than 2 ns (data age less than 12 h). Aiming at the problem of calculating the clock error parameters of BDS, combined with the clock error model solution strategy of the BDS ground segment, this paper proposes a variety of strategies to further improve the accuracy. For the short-term forecast of 1 h duration, this paper proposes a clock error forecast method of weighted linear model and mixed interval linear model. The accuracy of short-term forecast can be increased by more than 20%. For medium and long-term (>6 h), clock error time series is analyzed by spectral method and prediction model is constructed according to the spectral analysis results. Compared with simple quadratic polynomial model, the prediction accuracy of the clock error model is improved by 13% in 6 h and 21% in 12 h on average. For IGSO/MEO satellite, this paper proposes a strategy of using TWSTF equipment delay as parameters to estimate, which improves the accuracy of clock error model by 18% after equipment switching. Finally, using the improvement strategy proposed in this paper, the clock error data from January to June 2017 is reprocessed to obtain a new broadcast clock error model time series. Using the new broadcast clock error model to locate the BDS monitoring receiver, the results show that, the direction accuracy of N, E and U is increased by 14.22%, 29.39% and 14.91% respectively, which significantly improves the service performance of BDS broadcast clock error model

Changes in health behaviors and conditions during COVID-19 pandemic strict campus lockdown among Chinese university students

Objective: To explore how a stringent campus lockdown affects the physical activity (PA), sleep and mental health of Chinese university students living in student dormitories during the COVID-19 pandemic. Methods: Data on PA, sleep and mental health were collected between 24 March and 4 April 2022 from 2084 university students (mean age = 22.4 years, 61.1% male students) via an online questionnaire distributed by the students’ advisers of each dormitory. The Chinese short version of the International Physical Activity Questionnaire (IPAQ-C), Athens Insomnia Scale (CAIS) and General Health Questionnaire 12-item (GHQ-12) were applied. The Mann–Whitney test and Kruskal-Wallis tests were used to evaluate the PA profile differences between genders, before and during the lockdown period and between students’ living environments. Chi-squared (χ2) or Fisher’s exact test was used to assess changes in health behaviors by gender and students’ living environment compared to before the lockdown. A mediation model was used to examine whether sleep disorder mediated the relationship between PA and mental health in different students’ living environments. Results: Participants reported a significant decrease in weekly total PA levels (63.9%). Mean daily sedentary time increased by 21.4% and daily lying time increased by 10.7% compared to before lockdown. Among the participants, 21.2% had experienced insomnia, and 39.0% reported having high mental distress. Female students reported 10% higher rates of sleep disorders than male students (p < 0.001), and also experienced a higher incidence of mental disorders (p < 0.001). Students living with three roommates had a larger decrease in frequencies and durations of participation in light PA than other students (p < 0.001). PA was negatively associated with sleep and mental health, and sleep disorder was a mediating factor between PA and mental health in the students living with two and three roommates. Conclusion: This study showed that strict lockdowns within university dormitories during the COVID-19 pandemic had a negative effect on the health of university students by changing their health behaviors, physical activity and sleep. Our findings indicate a need for strategies to promote an active lifestyle for students in space-limited dormitories in order to maintain health during a prolonged lockdown.peerReviewe

A Pseudo-range and Phase Combined SBAS Differential Correction Model

BeiDou Navigation Satellite System (BDS) integrates legacy PNT service and authorized SBAS service, where SBAS service provides differential corrections and integrity information for authorized users. Current BDS differential corrections include one-dimensional equivalent satellite clock and ionospheric grids corrections, which is derived based on CNMC-smoothed pseudo-range observations. To improve the performance of BDS SBAS service, an algorithm is developed to estimate the real-time satellite clock and orbit corrections. In the algorithm, phase smoothed pseudo-range observation is used to define absolute value, and epoch differenced phase observation is used to define epoch-wise variation of the satellite clock/orbit corrections. It is analyzed that the impact of data sampling rate and station number on the correction parameter estimation. Real-time data of the BDS tracking network in China is used to validate the new algorithm. Compared with the current system performance:① mean UDRE using the new algorithm is reduced by 27%,35% and 27% for GEO, IGSO and MEO satellites, ② mean positioning error using the new algorithm is reduced by 23%,32% and 52% in the North, East and Up components, respectively. Implementing the new algorithm, BDS SBAS service supports the user positioning requirement with accuracy better than 1 m

Analysis and Application of BDS Broadcast Ephemeris Bias

We analyze the BeiDou broadcast ephemeris biases starting from February 2014 to October 2016 by using the IGS post-processed precise orbit and clock as references. Results show that the bias of BeiDou broadcast ephemeris can reach up to 2 meter. To further verify this finding, we perform precise point positioning for about 18 MGEX stations using BeiDou broadcast ephemeris and wide-area differential correction parameters. Data of 97 days are analyzed and pseudo-range residuals of each frequency for each station are derived. Systematic biases are found in the residuals using broadcast ephemeris. Comparing the BEB applying the differential correction parameters with the TGD difference between IGS and BeiDou broadcast ephemeris, we find that the correlation coefficient of TGD differences and BEB values could reach up to 0.89. This proves that the BEB applying the differential correction parameters is mainly due to the biases of the TGD parameters in broadcast ephemeris. By correcting the biases into TGD parameters, we performed point positioning at different stations. Results show that after applying the biases, the pseudo-range error reduces much, and the positioning precision can improve about 14.9%, 28.4%, 15.5% in N,E,U, direction for dual-frequency users, respectively

Mathematical model for computing precise local tie vectors for CMONOC co-located GNSS/VLBI/SLR stations

The seven co-located sites of the Crustal Movement Observation Network of China (CMONOC) in Shanghai, Wuhan, Kunming, Beijing, Xi'an, Changchun, and Urumqi are equipped with Global Navigation Satellite System (GNSS), very long baseline interferometry (VLBI), and satellite laser ranging (SLR) equipment. Co-location surveying of these sites was performed in 2012 and the accuracies of the solved tie vectors are approximately 5 mm. This paper proposes a mathematical model that handles the least squares adjustment of the 3D control network and calculates the tie vectors in one step, using all the available constraints in the adjustment. Using the new mathematical model, local tie vectors can be more precisely determined and their covariance more reasonably estimated