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"Navigation (Astronautics) Data processing."
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The Apollo guidance computer : architecture and operation
\"The first book to fully describe the Apollo guidance computer's architecture, Executive software, and the programs used by the astronauts. It describes the full range of technologies required in order to fly the Apollo lunar missions, and which enabled the astronauts to fly to the moon, and back\"-- Provided by publisher.
Book
GAMP: An open-source software of multi-GNSS precise point positioning using undifferenced and uncombined observations
As the number of GNSS satellites and stations increases, GNSS data processing software should be developed that is easy to operate, efficient to run, and has a robust performance. To meet these requirements, we developed a new GNSS analysis software called GAMP (GNSS Analysis software for Multi-constellation and multi-frequency Precise positioning), which can perform multi-GNSS precise point positioning based on undifferenced and uncombined observations. GAMP is a secondary development based on RTKLIB but with many improvements, such as cycle slip detection, receiver clock jump repair, and handling of GLONASS pseudorange inter-frequency biases. A simple, but unified format of output files, including positioning results, number of satellites, satellite elevation angles, pseudorange and carrier phase residuals, and slant Total Electron Content, is defined for results analysis and plotting. Moreover, a new receiver-independent data exchange format called RCVEX is designed to improve computational efficiency for post-processing.
Journal Article
PRIDE PPP-AR: an open-source software for GPS PPP ambiguity resolution
The PRIDE Lab at GNSS Research Center of Wuhan University has developed an open-source software for GPS precise point positioning ambiguity resolution (PPP-AR) (i.e., PRIDE PPP-AR). Released under the terms of the GNU General Public License version 3 (GPLv3, http://www.gnu.org/licenses/gpl.html), PRIDE PPP-AR supports relevant research, application and development with GPS post-processing PPP-AR. PRIDE PPP-AR is mainly composed of two modules, undifferenced GPS processing and single-station ambiguity resolution. Undifferenced GPS processing provides float solutions with wide-lane and narrow-lane ambiguity estimates. Later, single-station ambiguity resolution makes use of the phase clock/bias products, which are released also by the PRIDE Lab at ftp://pridelab.whu.edu.cn/pub/whu/phasebias/, to recover the integer nature of single-station ambiguities and then carry out integer ambiguity resolution. PRIDE PPP-AR is based on a least-squares estimator to produce daily, sub-daily or kinematic solutions for various geophysical applications. To facilitate the usage of this software, a few user-friendly shell scripts for batch processing have also been provided along with PRIDE PPP-AR. In this article, we use 1 month of GPS data (days 001–031 in 2018) to demonstrate the performance of PRIDE PPP-AR software. The PRIDE Lab is committed to consistently improve the software package and keep users updated through our website.
Journal Article
Multi-GNSS precise point positioning with raw single-frequency and dual-frequency measurement models
The emergence of multiple satellite navigation systems, including BDS, Galileo, modernized GPS, and GLONASS, brings great opportunities and challenges for precise point positioning (PPP). We study the contributions of various GNSS combinations to PPP performance based on undifferenced or raw observations, in which the signal delays and ionospheric delays must be considered. A priori ionospheric knowledge, such as regional or global corrections, strengthens the estimation of ionospheric delay parameters. The undifferenced models are generally more suitable for single-, dual-, or multi-frequency data processing for single or combined GNSS constellations. Another advantage over ionospheric-free PPP models is that undifferenced models avoid noise amplification by linear combinations. Extensive performance evaluations are conducted with multi-GNSS data sets collected from 105 MGEX stations in July 2014. Dual-frequency PPP results from each single constellation show that the convergence time of undifferenced PPP solution is usually shorter than that of ionospheric-free PPP solutions, while the positioning accuracy of undifferenced PPP shows more improvement for the GLONASS system. In addition, the GLONASS undifferenced PPP results demonstrate performance advantages in high latitude areas, while this impact is less obvious in the GPS/GLONASS combined configuration. The results have also indicated that the BDS GEO satellites have negative impacts on the undifferenced PPP performance given the current “poor” orbit and clock knowledge of GEO satellites. More generally, the multi-GNSS undifferenced PPP results have shown improvements in the convergence time by more than 60 % in both the single- and dual-frequency PPP results, while the positioning accuracy after convergence indicates no significant improvements for the dual-frequency PPP solutions, but an improvement of about 25 % on average for the single-frequency PPP solutions.
Journal Article
GINav: a MATLAB-based software for the data processing and analysis of a GNSS/INS integrated navigation system
With the development of GNSS, many open-source software packages have become available for GNSS data processing. However, there are only a handful of open-source software that can handle GNSS/INS integrated data, even though GNSS/INS integration schemes have been widely used in vehicle navigation systems due to their high accuracy, stability, and continuity in harsh environments. Considering the above, we developed an open-source software, GINav, which focuses on the data processing and analysis of a GNSS/INS integrated navigation system. GINav is suitable for in-vehicle situations and aims to provide a useful tool for carrying out GNSS/INS-related research. It is a convenient platform for testing new algorithms and experimental functionalities. GINav is developed in the MATLAB environment. It provides a user-friendly graphical user interface (GUI) to facilitate learning how to use it quickly. A visualization tool, GINavPlot, is provided for solution presentation and error analysis. We have conducted experimental tests to validate and assess the performance of GINav. The results indicate that GINav can provide navigation solutions comparable to general GNSS/INS accuracy standards, and it can handle both suburban and urban GNSS/INS integrated datasets.
Journal Article
GPS-Based Precise Orbit Determination of LEO Satellites Using Space-Based Double-Differenced Observations
One of the important GPS applications in space is precise orbit determination (POD) of Low-Earth Orbit (LEO) satellites. Thousands of LEO satellites are currently in orbit. One of the challenges is how to efficiently and precisely determine the orbits to satisfy the relative and absolute accuracy needs of missions. Currently, GPS-based POD of LEO satellites can be performed using either un-differenced (UD) or ground-based double-differenced (DD) observations. The UD POD needs both precise GPS satellite orbits and clocks; the DD POD needs not only the precise orbits, but also global ground reference receivers. Therefore, the GPS-based LEO POD is based on either global ground stations or precise GNSS clocks, which are not convenient for near real-time or real-time data processing. The GPS orbits can be precisely predicted in certain time; the clocks are not. For some formation flying satellite missions (two or more LEO satellites), the absolute orbit accuracy requirements are not as stringent as the relative requirements. The problem is how to perform LEO POD without global station data and precise GPS clocks in near real-time or real-time to achieve the mission orbit accuracy requirement. Based on this motivation, we investigated the GPS-based LEO POD using space-based DD observations without using global ground station GPS data and precise GPS clock products. For this study, we processed the real GPS observations from two LEO satellites. The absolute and relative orbit accuracy is assessed using several tests, including analysis of the orbit fits, external orbit comparisons, Satellite Laser Ranging (SLR) and K-band Ranging (KBR) residuals.
Journal Article
LEO augmented precise point positioning using real observations from two CENTISPACE™ experimental satellites
LEO augmentation systems are attracting worldwide attention. However, their performance in real-world environments has not yet been reported, although several systems have already launched experimental satellites, such as the CENTISPACE™ system developed by Beijing Future Navigation Tech Co., Ltd. By using real observations from two experimental CENTISPACE™ satellites recorded by a regional network, we present the results of LEO-enhanced precise point positioning (PPP) performance. Based on a proposed three-step approach for LEO orbit determination and time synchronization, a framework for LEO augmentation data processing is established and validated. Two stations are selected for LEO augmentation evaluation, and static PPP is performed by combining the LEO augmentation observations with different GNSS systems, including GPS, BDS-3, and Galileo. The static PPP precision shows a few improvements attributed to LEO observations; the positioning errors with one, two, and three GNSS systems are 6.1, 4.6, and 4.6 cm, respectively, while they are reduced to 5.2, 3.9, and 3.8 cm by adding the LEO satellites, respectively, revealing improvements of 13.9, 16.4, and 18.8%, respectively. Moreover, PPP convergence is significantly enhanced. The average convergence times with one, two, and three GNSS systems are 32.7, 17.9, and 14.2 min, respectively, which are reduced to 16.7, 8.9, and 5.7 min, respectively, by adding LEO observations. This indicates that the convergence time is significantly shortened by approximately 53% with the augmentations from only two LEO satellites. Such results show the potential of LEO augmentation, and larger improvements can be expected with more LEO satellites to be deployed in the future.
Journal Article
A global zenith tropospheric delay model with ERA5 and GNSS-based ZTD difference correction
Highly accurate tropospheric delay information is essential for global navigation satellite system (GNSS) data processing. However, the current models still have limitations such as a lack of systematic difference correction and appropriate fitting functions. We investigated the performances of different order polynomials in developing ERA5 (the fifth generation of European Center for Medium-Range Weather Forecasts Reanalysis)-based zenith tropospheric delay (ZTD) vertical adjustment model, and it is noticed that a cubic polynomial fitting function is optimal. Considering there is a difference between the ERA5 and GNSS-based ZTD, its characteristic is analyzed using 5 years data. This difference is found to have a noticeable variation of the annual period and is modeled in each window, which depends on ERA5-based ZTD vertical adjustment model. An improved ZTD vertical adjustment model is constructed using the modeled difference, and it has a 20% improvement in accuracy compared with that of ERA5-based ZTD vertical adjustment model. When the proposed ZTD vertical adjustment model is used to establish a ZTD empirical global grid model, the model, which includes systematic differences correction, has an accuracy improvement of 6 and 2%, respectively, compared with GGZTD-H (global grid zenith tropospheric delay) and without correction models. The proposed model also exhibits superior performance in precise point positioning, particularly in the vertical direction. This ZTD empirical global grid model delivers highly accurate ZTD values and can thus be a viable option for GNSS precise positioning.
Journal Article
Reliable single-epoch ambiguity resolution for short baselines using combined GPS/BeiDou system
GNSS single-epoch real-time kinematic (RTK) positioning depends on correct ambiguity resolution. If the number of observed satellites in a single epoch is insufficient, which often happens with a standalone GNSS system, the ambiguity resolution is difficult to achieve. China’s BeiDou Navigation Satellite System has been providing continuous passive positioning, navigation and timing services since December 27, 2012, covering China and the surrounding area. This new system will increase the number of satellites in view and will have a significant effect on successful ambiguity resolution. Since the BeiDou system is similar to GPS, the procedure of data processing is easier than that for the Russian GLONASS system. We briefly introduce the time and the coordinate system of BeiDou and also the BeiDou satellite visibility in China, followed by the discussion on the combined GPS/BeiDou single-epoch algorithm. Experiments were conducted and are presented here, in which the GPS/BeiDou dual-frequency static data were collected in Wuhan with the baseline distance varying from 5 to 13 km, and processed in separate and combined modes. The results indicate that, compared to a standalone GPS or BeiDou system, the combined GNSS system can increase the successful ambiguity fixing rate for single epochs and can also improve the precision of short baselines determination.
Journal Article
Characteristics of Beidou-2 flex power and its impact on precise point positioning with ambiguity resolution
The modernization of the Global Navigation Satellite Systems (GNSS) has brought many new features and capabilities, one of which is programmable power output capability, also known as flex power. Flex power capability allows for an increase in the signal strength of the individual signals to better fulfil operational constraints, but it may also cause biases in the pseudorange and carrier phase observations. This study focuses on the flex power capability characteristics of the second-generation Beidou Navigation and Positioning System (Beidou-2). We summarized the Beidou-2 flex power activation periods from January 2021 to April 2024 and analyzed the impact of flex power on phase biases and precise point positioning with ambiguity resolution (PPP-AR). The results show that Beidou-2 flex power would affect both pseudorange and phase observations on the B3I signals simultaneously. In addition, Hatch–Melbourne–Wübbena (HMW) combinations with single-epoch exhibit obvious discontinuities due to this mode of Beidou-2 flex power. Furthermore, the differences in estimated wide-lane (WL) biases can reach up to approximately 0.4 cycles when the Beidou-2 flex power is switched on or off. During this time, regarding the WL biases estimation with the daily constant strategy, the WL ambiguity residuals of Beidou-2 PPP-AR users are only approximately 70% within ± 0.25 cycles. In contrast, with the piecewise constant strategy, the WL ambiguity residuals above the threshold can reach approximately 90%. Considering flex power in kinematic PPP-AR, the position biases root mean square (RMS) values of 0.8, 0.8 and 2.5 cm can be achieved for the east, north and up components, respectively, while the corresponding position biases RMS without careful consideration of flex power are 1.0, 0.9 and 2.8 cm. Therefore, to achieve more reliable positioning results, it is advisable to incorporate flex power into high-precision GNSS data processing, especially for bias products of PPP-AR.
Journal Article