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GSTAR Software: A New Framework For Processing Space Geodetic Data At The Observation Level
To meet the demands for the data combination with GNSS and multiple space geodetic techniques at the observation level, this paper presented the structure and performance of a new software platform developed by Beihang University, named Geodetic SpatioTemporal data Analysis and Research software (GSTAR). Most modules of the software are written in C++ language (version 11) while a small portion is written in ANSI C language. All modules are elaborately designed and implemented using the object-oriented method which obeys the rule of high cohesion and low coupling. Compared to the traditional software coded in FORTRAN language, the GSTAR software allows users to develop their own application programs without knowing mathematical details and make it easy for secondary development. The GSTAR software can be run on both Windows and Linux operating systems, which is applicable for various users with different usages and behaviors.
At present, the key features of the GSTAR software are as below:
1. Satellite precise orbit and clock determination, which includes quad-system GNSS satellites (GPS, GLONASS, Galileo, BDS) and numerous LEO satellites.
2. Precise positioning for both network and PPP solution in either of static or kinematic mode.
3. Precise estimation of geodetic parameters, which includes ERP, geocenter motion, troposphere delay and ionosphere delay.
4. Support of both ionosphere-free combination and uncombined approach in the multi-frequency context using all available GNSS observation types.
5. Support of combination of GNSS, ISL and SLR techniques at the observation level, while DORIS and VLBI techniques are still under development.
6. Support of ambiguity resolution using either double-difference or PPP-AR approach.
7. Support of parallel computing to improve data processing efficiency.
1) New software framework for future geodetic applications
The layered modular theory is adopted to develop the GSTAR software. Considering that future satellite is assumed to be packed with any number of sensors of GNSS, SLR, DORIS, ISL, VLBI (e.g., GENESIS satellite), the antenna rather than the satellite or the ground station is taken as the core unit in the software. Different with the physical antenna that is only used to send or receive signals, the antenna defined in the GSTAR acts as an independent processing unit which obtains and preprocesses observations before the normal equation is formed. The free and flexible combination of numerous antenna objects forms an observing network that covers space, sky and ground. Since antenna objects are fully independent of each other, this design makes it convenient for parallel programming. In addition, the spacetime event-driving mechanism and demand constraints are implemented to realize flexible processing of parameters.
2) High computation efficiency for large-scale data processing
A series of rapid data processing algorithms are designed and implemented for both serial and parallel computing in the GSTAR software. First, a rapid orbit integration algorithm is developed to improve the efficiency of orbit integration for a large-scale constellation without relying on parallel computing. In this algorithm, an adaptive step-changed Admas integration method and a synchronous integration algorithm are proposed. Second, the modified LSQ and SRIF algorithms realized synchronous elimination of parameters in a parallel program which can exploit both the CPU and GPU power of a multi-core system. To significantly reduce the number of memory and disk operations, an intelligence parameter organization algorithm is also proposed according to parameter’s life cycle. Third, a parallel algorithm is developed for data preprocessing at each antenna. In conclusion, the operation speed of the GSTAR software is 6 times faster than that of the Positioning and Navigation Data Analyst (PANDA) software for the quad-system precise orbit determination procedure when data from 160+ stations are processed simultaneously.
3) Applications on geodetic products using GNSS and other space geodetic techniques
On one hand, the GSTAR software is applied for generating GNSS-based products in Beihang University. Taking the products released by European Space Agency (ESA) as reference, the Three-Dimension (3D) Root-Mean-Squares (RMS) of the orbit differences are 2.7/6.7/3.3/7.7/21.0 cm and the STandard Deviations (STD) of the clock differences are 19/48/16/32/25 ps for GPS, GLONASS, Galileo, BDS MEO, and BDS IGSO satellites, respectively. The mean values of the X and Y components of the polar coordinate and the LOD with respect to the IERS 14 C04 products are -17.6 µas, 9.2 µas, and 14.0 µs/d. The RMSs of the differences of the tropospheric ZPD, the north gradients, and the east gradients are 5.8, 0.9, and 0.9 mm with respect to the IGS products.
To meet the demands for the data combination with GNSS and multiple space geodetic techniques at the observation level, this paper presented the structure and performance of a new software platform developed by Beihang University, named Geodetic SpatioTemporal data Analysis and Research software (GSTAR). Most modules of the software are written in C++ language (version 11) while a small portion is written in ANSI C language. All modules are elaborately designed and implemented using the object-oriented method which obeys the rule of high cohesion and low coupling. Compared to the traditional software coded in FORTRAN language, the GSTAR software allows users to develop their own application programs without knowing mathematical details and make it easy for secondary development. The GSTAR software can be run on both Windows and Linux operating systems, which is applicable for various users with different usages and behaviors.
At present, the key features of the GSTAR software are as below:
1. Satellite precise orbit and clock determination, which includes quad-system GNSS satellites (GPS, GLONASS, Galileo, BDS) and numerous LEO satellites.
2. Precise positioning for both network and PPP solution in either of static or kinematic mode.
3. Precise estimation of geodetic parameters, which includes ERP, geocenter motion, troposphere delay and ionosphere delay.
4. Support of both ionosphere-free combination and uncombined approach in the multi-frequency context using all available GNSS observation types.
5. Support of combination of GNSS, ISL and SLR techniques at the observation level, while DORIS and VLBI techniques are still under development.
6. Support of ambiguity resolution using either double-difference or PPP-AR approach.
7. Support of parallel computing to improve data processing efficiency.
1) New software framework for future geodetic applications
The layered modular theory is adopted to develop the GSTAR software. Considering that future satellite is assumed to be packed with any number of sensors of GNSS, SLR, DORIS, ISL, VLBI (e.g., GENESIS satellite), the antenna rather than the satellite or the ground station is taken as the core unit in the software. Different with the physical antenna that is only used to send or receive signals, the antenna defined in the GSTAR acts as an independent processing unit which obtains and preprocesses observations before the normal equation is formed. The free and flexible combination of numerous antenna objects forms an observing network that covers space, sky and ground. Since antenna objects are fully independent of each other, this design makes it convenient for parallel programming. In addition, the spacetime event-driving mechanism and demand constraints are implemented to realize flexible processing of parameters.
2) High computation efficiency for large-scale data processing
A series of rapid data processing algorithms are designed and implemented for both serial and parallel computing in the GSTAR software. First, a rapid orbit integration algorithm is developed to improve the efficiency of orbit integration for a large-scale constellation without relying on parallel computing. In this algorithm, an adaptive step-changed Admas integration method and a synchronous integration algorithm are proposed. Second, the modified LSQ and SRIF algorithms realized synchronous elimination of parameters in a parallel program which can exploit both the CPU and GPU power of a multi-core system. To significantly reduce the number of memory and disk operations, an intelligence parameter organization algorithm is also proposed according to parameter’s life cycle. Third, a parallel algorithm is developed for data preprocessing at each antenna. In conclusion, the operation speed of the GSTAR software is 6 times faster than that of the Positioning and Navigation Data Analyst (PANDA) software for the quad-system precise orbit determination procedure when data from 160+ stations are processed simultaneously.
3) Applications on geodetic products using GNSS and other space geodetic techniques
On one hand, the GSTAR software is applied for generating GNSS-based products in Beihang University. Taking the products released by European Space Agency (ESA) as reference, the Three-Dimension (3D) Root-Mean-Squares (RMS) of the orbit differences are 2.7/6.7/3.3/7.7/21.0 cm and the STandard Deviations (STD) of the clock differences are 19/48/16/32/25 ps for GPS, GLONASS, Galileo, BDS MEO, and BDS IGSO satellites, respectively. The mean values of the X and Y components of the polar coordinate and the LOD with respect to the IERS 14 C04 products are -17.6 µas, 9.2 µas, and 14.0 µs/d. The RMSs of the differences of the tropospheric ZPD, the north gradients, and the east gradients are 5.8, 0.9, and 0.9 mm with respect to the IGS products.
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