This paper focuses on the precise point positioning (PPP) ambiguity resolution (AR) using the observations acquired from four systems: GPS, BDS, GLONASS, and Galileo (GCRE). A GCRE four-system uncalibrated phase delay (UPD) estimation model and multi-GNSS undifferenced PPP AR method were developed in order to utilize the observations from all systems. For UPD estimation, the GCRE-combined PPP solutions of the globally distributed MGEX and IGS stations are performed to obtain four-system float ambiguities and then UPDs of GCRE satellites can be precisely estimated from these ambiguities. The quality of UPD products in terms of temporal stability and residual distributions is investigated for GPS, BDS, GLONASS, and Galileo satellites, respectively. The BDS satellite-induced code biases were corrected for GEO, IGSO, and MEO satellites before the UPD estimation. The UPD results of global and regional networks were also evaluated for Galileo and BDS, respectively. As a result of the frequency-division multiple-access strategy of GLONASS, the UPD estimation was performed using a network of homogeneous receivers including three commonly used GNSS receivers (TRIMBLE NETR9, JAVAD TRE_G3TH DELTA, and LEICA). Data recorded from 140 MGEX and IGS stations for a 30-day period in January in 2017 were used to validate the proposed GCRE UPD estimation and multi-GNSS dual-frequency PPP AR. Our results show that GCRE four-system PPP AR enables the fastest time to first fix (TTFF) solutions and the highest accuracy for all three coordinate components compared to the single and dual system. An average TTFF of 9.21 min with $$7{^{\circ }}$$
 cutoff elevation angle can be achieved for GCRE PPP AR, which is much shorter than that of GPS (18.07 min), GR (12.10 min), GE (15.36 min) and GC (13.21 min). With observations length of 10 min, the positioning accuracy of the GCRE fixed solution is 1.84, 1.11, and 1.53 cm, while the GPS-only result is 2.25, 1.29, and 9.73 cm for the east, north, and vertical components, respectively. When the cutoff elevation angle is increased to $$30{^{\circ }}$$
 , the GPS-only PPP AR results are very unreliable, while 13.44 min of TTFF is still achievable for GCRE four-system solutions.

Multi-GNSS phase delay estimation and PPP ambiguity resolution: GPS, BDS, GLONASS, Galileo

Unmanned aerial vehicles (UAVs) play a primary role in a plethora of technical and scientific fields owing to their wide range of applications. In particular, the provision of emergency services during the occurrence of a crisis event is a vital application domain where such aerial robots can contribute, sending out valuable assistance to both distressed humans and rescue teams. Bearing in mind that time constraints constitute a crucial parameter in search and rescue (SAR) missions, the punctual and precise detection of humans in peril is of paramount importance. The paper in hand deals with real-time human detection onboard a fully autonomous rescue UAV. Using deep learning techniques, the implemented embedded system was capable of detecting open water swimmers. This allowed the UAV to provide assistance accurately in a fully unsupervised manner, thus enhancing first responder operational capabilities. The novelty of the proposed system is the combination of global navigation satellite system (GNSS) techniques and computer vision algorithms for both precise human detection and rescue apparatus release. Details about hardware configuration as well as the system’s performance evaluation are fully discussed.

Unsupervised Human Detection with an Embedded Vision System on a Fully Autonomous UAV for Search and Rescue Operations

This paper focuses on the contribution of the global positioning system (GPS) and BeiDou navigation satellite system (BDS) observations to precise point positioning (PPP) ambiguity resolution (AR). A GPS + BDS fractional cycle bias (FCB) estimation method and a PPP AR model were developed using integrated GPS and BDS observations. For FCB estimation, the GPS + BDS combined PPP float solutions of the globally distributed IGS MGEX were first performed. When integrating GPS observations, the BDS ambiguities can be precisely estimated with less than four tracked BDS satellites. The FCBs of both GPS and BDS satellites can then be estimated from these precise ambiguities. For the GPS + BDS combined AR, one GPS and one BDS IGSO or MEO satellite were first chosen as the reference satellite for GPS and BDS, respectively, to form inner-system single-differenced ambiguities. The single-differenced GPS and BDS ambiguities were then fused by partial ambiguity resolution to increase the possibility of fixing a subset of decorrelated ambiguities with high confidence. To verify the correctness of the FCB estimation and the effectiveness of the GPS + BDS PPP AR, data recorded from about 75 IGS MGEX stations during the period of DOY 123-151 (May 3 to May 31) in 2015 were used for validation. Data were processed with three strategies: BDS-only AR, GPS-only AR and GPS + BDS AR. Numerous experimental results show that the time to first fix (TTFF) is longer than 6 h for the BDS AR in general and that the fixing rate is usually less than 35 % for both static and kinematic PPP. An average TTFF of 21.7 min and 33.6 min together with a fixing rate of 98.6 and 97.0 % in static and kinematic PPP, respectively, can be achieved for GPS-only ambiguity fixing. For the combined GPS + BDS AR, the average TTFF can be shortened to 16.9 min and 24.6 min and the fixing rate can be increased to 99.5 and 99.0 % in static and kinematic PPP, respectively. Results also show that GPS + BDS PPP AR outperforms single-system PPP AR in terms of convergence time and position accuracy.

Ambiguity resolved precise point positioning with GPS and BeiDou

The focus of this study is on proper modeling of the dynamics for inter-system biases (ISBs) in multi-constellation Global Navigation Satellite System (GNSS) precise point positioning (PPP) processing. First, the theoretical derivation demonstrates that the ISBs originate from not only the receiver-dependent hardware delay differences among different GNSSs but also the receiver-independent time differences caused by the different clock datum constraints among different GNSS satellite clock products. Afterward, a comprehensive evaluation of the influence of ISB stochastic modeling on undifferenced and uncombined PPP performance is conducted, i.e., random constant, random walk process, and white noise process are considered. We use data based on a 1-month period (September 2017) Multi-GNSS Experiment (MGEX) precise orbit and clock products from four analysis centers (CODE, GFZ, CNES, and WHU) and 160 MGEX tracking stations. The results demonstrate that generally, the positioning performance of PPP in terms of convergence time and positioning accuracy with the final products from CODE, CNES, and WHU is comparable among the three ISB handling schemes. However, estimating ISBs as random walk process or white noise process outperforms that as the random constant when using the GFZ products. These results indicate that the traditional estimation of ISBs as the random constant may not always be reasonable in multi-GNSS PPP processing. To achieve more reliable positioning results, it is highly recommended to consider the ISBs as random walk process or white noise process in multi-GNSS PPP processing.

Influence of stochastic modeling for inter-system biases on multi-GNSS undifferenced and uncombined precise point positioning

In the past few decades, global navigation satellite system (GNSS) technology has been widely used in structural health monitoring (SHM), and the monitoring mode has evolved from long-term deformation monitoring to dynamic monitoring. This paper gives an overview of GNSS-based dynamic monitoring technologies for SHM. The review is classified into three parts, which include GNSS-based dynamic monitoring technologies for SHM, the improvement of GNSS-based dynamic monitoring technologies for SHM, as well as denoising and detrending algorithms. The significance and progress of Real-Time Kinematic (RTK), Precise Point Position (PPP), and direct displacement measurement techniques, as well as single-frequency technology for dynamic monitoring, are summarized, and the comparison of these technologies is given. The improvement of GNSS-based dynamic monitoring technologies for SHM is given from the perspective of multi-GNSS, a high-rate GNSS receiver, and the integration between the GNSS and accelerometer. In addition, the denoising and detrending algorithms for GNSS-based observations for SHM and corresponding applications are summarized. Challenges of low-cost and widely covered GNSS-based technologies for SHM are discussed, and problems are posed for future research.

https://www.mdpi.com/2072-4292/11/9/1001/pdf

A Review of Global Navigation Satellite System (GNSS)-Based Dynamic Monitoring Technologies for Structural Health Monitoring


 It has been acknowledged that the Doppler is beneficial to the GNSS positioning of smartphones. However, analysis of Doppler precision on smartphones is insufficient. In this paper, we focus on the characteristic analysis of the raw Doppler measurement from Android smartphones. A comprehensive investigation of the Doppler was conducted. The results illustrate that the availability of Doppler is stable and higher than that of carrier measurements, which means that the Doppler-smoothed code (DSC) method is more effective. However, there is a constant bias between the Doppler and the code rate in Xiaomi MI8, which indicates that extra processing of the DSC method is necessary for this phone. Additionally, it is demonstrated that the relationship between the Doppler and C/N0 can be expressed as an exponential function, and the fitting parameters are provided. The numerical experiment in car-borne and hand-held scenes was conducted for evaluating the performance of the Doppler-aided positioning algorithm. For positioning, the improvement reaches 37 ⋅ 69%/37 ⋅ 14%/26 ⋅ 61% in the east, north and up components, respectively, after applying the Doppler aiding. For velocity estimation, the improvement reaches 29 ⋅ 62%/39 ⋅ 63%/29 ⋅ 37% in the three components, respectively.

Characterisation and position-aiding performance analysis of Doppler observation from android smartphones

With the development of GNSS technology, the higher demand for calculation is put forward in the field of the whole network baseline solution. The conventional baseline solution is a three-step method, which is complex and relies on the success rate of ambiguity solution at each step. A modified strategy for baseline ambiguity resolution is proposed in the paper, using un-combined and un-differenced model. The equality constraint is also used to recover the integer property of ambiguities. By the constraint of the fixed ambiguities, the baseline solution could be obtained. The data sets from various lengths of baselines are conducted to investigate the performance of the UD strategy. Depending on the empirical fixed success rate and convergence time, the modified strategy has a better AR performance on the rising satellites and weakens the unknown coefficient of correlation between ambiguities.

A Baseline Ambiguity Resolution Using Un-combined and Un-differenced Model with Equality Constraint

A new method for the extraction and application of un-differenced atmospheric delays with un-combined precise point positioning technique, is proposed based on the regional continuous operational reference system (CORS) network The estimable state vector, which lets in the receiver clock offset, the ionospheric slant delay and zenith tropospheric wet delay (ZWD), is estimated epoch by epoch at every reference station In addition, the least squares and the ionospheric regional polynomial model is also used to separate the DCB and the ionospheric delay For improving the convergence time of the PPP, a new PPP algorithm, which was suited for single-frequency and dual-frequency receivers, was put forward The algorithm took full use of the interpolation of regional atmospheric delay value as the quasi-observable, and the un-combined satellite-different observations were used to obtain the precise positioning results Nevertheless, the method mentioned above is investigated and examined on the IGS tracking stations The results show that all the satellites’ DCB are solved and compared with the products published by CODE, the disparity between them is mostly not greater than 02 ns Furthermore, with the aim of the real-time fast precise point positioning based on the sparse regional CORS, results from American CORS data set is introduced and discussed The positioning accuracy can reach 10 cm in the plane within 30 min for the single-frequency PPP user and the plane position result in 1–2 cm also can be accomplished after the convergence of the filter For dual-frequency receiver, it takes approximately 10 min to get positions with accuracy better than 10 cm at the three directions for the dual-frequency PPP user On the other hands, due to its epoch-by-epoch nature, the method proposed above is also suited for dynamic precise point positioning

Extraction and Application of Un-differenced Atmospheric Delays with Un-combined Precise Point Positioning Technique

Research on the precise positioning of smartphones mainly depends on the output carrier phase data. However, many new versions of smartphones currently do not support carrier data, which poses challenges to the application of various precision positioning algorithms. Based on the above background, the Doppler frequency shift data quality of smartphones is analyzed. According to the characteristics of Doppler data, a method of Doppler integration to form a pseudo-carrier instead of a carrier for precise positioning is creatively proposed and verified through dynamic experiments. The results show that the Doppler integration method has the potential to replace the carrier to complete precise positioning, and they are consistent in positioning capability and results. This method provides a new way of thinking to solve the problems of missing carriers and the inability of carriers to be effectively used in current new models.

Research on Dynamic Positioning of Android Smartphone Based on Doppler Integration

The development of multi-GNSS remarkably increased the number of available satellites, but how to solve the multi-dimensional ambiguity parameters quickly and accurately in network RTK technology is still an issue full of perplexity and significance. To ensure the virtual station have more available satellites under the occlusion environment, a fast ambiguity resolution method for base station based on ambiguity tight constraint was proposed in this paper. Firstly, the optimal subset of ambiguity is selected by partial ambiguity resolution (PAR) strategy, and then impose strong constraints on the ambiguities of these satellites. Finally, update the filter equation and assist in fixing the ambiguity of other satellites. The real measured baseline data which contain GPS, BDS and GLONASS from Tianjin CORS and Curtin University was used in the experiments, and the results illustrated that this method could significantly shorten the initialization time of ambiguity between base stations, accelerate the convergence speed of newly-arisen satellites, and increase the number of available satellites of RTK virtual observations (especially low-elevation angle satellites), that providing a reliable guarantee for high-precision positioning in the occlusion environment, such as the roads in cities.

Chengfa Gao

Papers

Characterisation and position-aiding performance analysis of Doppler observation from android smartphones

A Baseline Ambiguity Resolution Using Un-combined and Un-differenced Model with Equality Constraint

Extraction and Application of Un-differenced Atmospheric Delays with Un-combined Precise Point Positioning Technique

Research on Dynamic Positioning of Android Smartphone Based on Doppler Integration

Rapid Ambiguity Resolution Algorithm for Multi-constellation Between Reference Stations Based on Ambiguity Tight Constraint