Low-cost Global Navigation Satellite System Receiver Research Articles

Enhancing Real-Time Kinematic Relative Positioning for Unmanned Aerial Vehicles

Real-time kinematic (RTK) positioning of the global navigation satellite systems (GNSS) is used to provide centimeter-level positioning accuracy. There are several ways to implement RTK but a Kalman filter-based RTK is preferred because of its superior capability to resolve GNSS carrier phase integer ambiguities. However, the positioning performance of the Kalman filter-based RTK is often compromised by various factors when it comes to determining a precise relative position vector between moving unmanned aerial vehicles (UAVs) equipped with low-cost GNSS receivers and antennas, where the locations of both GNSS antennas are not accurately known and change over time. Some of the critical factors that lead to a high rate of incorrect resolutions of carrier phase integer ambiguities are measurement time differences between GNSS receivers, frequent cycle slips with high noise in code and carrier phase measurements, and an improper Kalman filter gain due to a newly risen satellite. In this paper, effective methods to deal with those factors to achieve a seamless Kalman filter-based RTK performance in moving UAVs are presented. Using our extensive 45 flight tests data sets, conducted over a duration of 3 to 12 min, the RTK positioning results showed that the root-mean-square position error (RMSE) decreased by up to 95.13%, with an average of 65.31%, and that the percentage of epochs that passed the ratio test, which is the most common method for validating double differenced carrier phase integer ambiguity resolution, increased by up to 130%, with an average of 23.54%.

Comparison of Low-Cost GNSS Receivers for Time Transfer using Zero-Length Baseline

Abstract A comparison between low-cost single-frequency and dual-frequency Global Navigation Satellite System (GNSS) receiver timing modules is presented, focusing on their suitability for time transfer applications. The study uses a zero-length baseline measurement approach to assess their performance and highlights the advantages of dual-frequency receivers. The clock comparison residuals between these low-cost devices and a reference receiver are analyzed. In particular, it is shown that the use of averages longer than 200s can effectively mitigate the quantization error inherent in pulse per second outputs of the timing modules. The results showcase sub-nanosecond time deviation instabilities between the reference receiver and the dual-frequency timing module. In contrast, the single-frequency module exhibits time deviations of 3.3ns at a one-day averaging interval. This research provides insights into the selection and utilization of GNSS timing modules for time transfer applications, where such modules can serve as attractive, cost-effective alternatives for applications requiring moderate accuracy.

Analysis of Water Level Monitoring using GNSS Interferometric Reflectometry in River Waters

GNSS Interferometric Reflectometry (GNSS-IR) is one of the newest explorations of Global Navigation Satellite System (GNSS) signals which utilizes multipath signals to calculate the vertical distance from the reflecting surface to the geodetic antenna/receiver. However, scientific-grade or geodetic GNSS instruments are expensive, which is a limiting factor for their prompt and more widespread deployment as a dedicated environmental sensor. Hence, in this study, low-cost GNSS receiver devices were used to monitor the surface level of PT Garam River in Pamekasan Regency using the GNSS-IR method. The study location of PT Garam River which is quite close to the sea causes the water level to vary following the tides at the sea. This is because there are needs for advanced methodologies to limit GNSS observations in a much narrower area. The river in this case is a more challenging study location in terms of monitoring the water level. Calculation of river water level height with GNSS-IR observations is based on determining the peak frequency on the periodogram resulting from signal-to-noise ratio (SNR) data extraction. The two-weeks GNSS-IR observation in this study were applied by installing sideway orientation of GNSS antenna towards the river. This will be done to see how effective the strength of the reflected signal received by the low-cost GNSS devices are in narrow footprint area. Conventional tide gauge near the GNSS-IR site was used to evaluate the accuracy of GNSS-IR in river water level monitoring. Tide analysis is carried out to obtain tidal constituents and tide predictions using least square harmonics estimation (LSE). The resulting accuracy of the validation data is at an RMSE of 15.35 cm, with the correlation value of 0.94. The type of river tides in the study location based on the calculation results is mixed tide prevailing semidiurnal. Tide prediction for 20 days gives promising results with an RMSE of 16 cm. These results indicate that low-cost GNSS device has a promising capability for water level monitoring using GNSS-IR method in a narrow reflector area.

Unscented Kalman filter for a low-cost GNSS/IMU-based mobile mapping application under demanding conditions

For the last two decades, low-cost Global Navigation Satellite System (GNSS) receivers have been used in various applications. These receivers are mini-size, less expensive than geodetic-grade receivers, and in high demand. Irrespective of these outstanding features, low-cost GNSS receivers are potentially poorer hardwares with internal signal processing, resulting in lower quality. They typically come with low-cost GNSS antenna that has lower performance than their counterparts, particularly for multipath mitigation. Therefore, this research evaluated the low-cost GNSS device performance using a high-rate kinematic survey. For this purpose, these receivers were assembled with an Inertial Measurement Unit (IMU) sensor, which actively transmited data on acceleration and orientation rate during the observation. The position and navigation parameter data were obtained from the IMU readings, even without GNSS signals via the U-blox F9R GNSS/IMU device mounted on a vehicle. This research was conducted in an area with demanding conditions, such as an open sky area, an urban environment, and a shopping mall basement, to examine the device's performance. The data were processed by two approaches: the Single Point Positioning-IMU (SPP/IMU) and the Differential GNSS-IMU (DGNSS/IMU). The Unscented Kalman Filter (UKF) was selected as a filtering algorithm due to its excellent performance in handling nonlinear system models. The result showed that integrating GNSS/IMU in SPP processing mode could increase the accuracy in eastward and northward components up to 68.28% and 66.64%. Integration of DGNSS/IMU increased the accuracy in eastward and northward components to 93.02% and 93.03% compared to the positioning of standalone GNSS. In addition, the positioning accuracy can be improved by reducing the IMU noise using low-pass and high-pass filters. This application could still not gain the expected position accuracy under signal outage conditions.

On low-cost GNSS observables under different grades of antennas: Receiver-related biases and RTK results

Low-cost receivers are becoming mainstream devices for providing positioning, navigation and timing (PNT) services that are accessible to the public. In contrast to existing advances, it is inappropriate for less-rigorous functional models and rough systemic error models, including the unknown characteristics of receiver-related biases (RRB) caused by internal hardware and the indefinite impact of an external antenna, to restrain the development of low-cost global navigation satellite system (GNSS) receivers. In this work, a comparative analysis was conducted to assess the respective effects of the internal hardware and an external antenna on low-cost receiver observables. We reparameterized a single-differenced geometry-based functional model and adopted a least squares filtering method to estimate RRB, including inter system biases (ISB), differential code biases (DCB), differential phase biases (DPB). First, an abnormal phenomenon was reported that the C099-F9P receiver showed better performance than the Trimble receiver on RRB when a U-blox ANN-MB antenna was used. Second, comparing the DCB and the DPB results, we found that the negative effect of the external antenna on the code observables is stronger than that on the phase observables. The standard deviation (STD) of the L1-L2 and E1-E5b DCB from the C099-F9P receiver reach 0.160 and 0.129 m, respectively, when the ANN-MB antenna is used. When connected to a higher-grade antenna, the low-cost receivers show DCB estimates stability similar to the Trimble receivers, where the L1-L2 and E1-E5b DCB STDs can reach 0.078 and 0.076 m, respectively. Third, it should be noted that the L1-L2 and E1-E5b DCB will change if a low-cost receiver restarts or is powered off. Fourth, of the DPB STDs of the low-cost and Trimble receivers connected to three different grades of antennas maintain agreement and reach 0.007 cycles or better. The phase observables can be free from the effect of the antenna in short-term case. Above all, the comparison results illustrate that low-cost GNSS receivers have the potential for high-precision applications and achieve an accuracy level and ability similar to high-grade receivers in mass applications when a survey-grade or higher-quality antenna is equipped.

Low-Cost Dual-Frequency GNSS Receivers and Antennas for Surveying in Urban Areas.

Low-cost dual-frequency global navigation satellite system (GNSS) receivers have recently been tested in various positioning applications. Considering that these sensors can now provide high positioning accuracy at a lower cost, they can be considered an alternative to high-quality geodetic GNSS devices. The main objectives of this work were to analyze the differences between geodetic and low-cost calibrated antennas on the quality of observations from low-cost GNSS receivers and to evaluate the performance of low-cost GNSS devices in urban areas. In this study, a simple RTK2B V1 board u-blox ZED-F9P (Thalwil, Switzerland) was tested in combination with a low-cost calibrated and geodetic antenna in open-sky and adverse conditions in urban areas, while a high-quality geodetic GNSS device was used as a reference for comparison. The results of the observation quality check show that low-cost GNSS instruments have a lower carrier-to-noise ratio (C/N0) than geodetic instruments, especially in the urban areas where the difference is larger and in favor of the geodetic GNSS instruments. The root-mean-square error (RMSE) of the multipath error in the open sky is twice as high for low-cost as for geodetic instruments, while this difference is up to four times greater in urban areas. The use of a geodetic GNSS antenna does not show a significant improvement in the C/N0 and multipath of low-cost GNSS receivers. However, the ambiguity fix ratio is larger when geodetic antennas are used, with a difference of 1.5% and 18.4% for the open-sky and urban conditions, respectively. It should be noted that float solutions may become more evident when low-cost equipment is used, especially for short sessions and in urban areas with more multipath. In relative positioning mode, low-cost GNSS devices were able to provide horizontal accuracy lower than 10 mm in urban areas in 85% of sessions, while the vertical and spatial accuracy was lower than 15 mm in 82.5% and 77.5% of the sessions, respectively. In the open sky, low-cost GNSS receivers achieve a horizontal, vertical, and spatial accuracy of 5 mm for all sessions considered. In RTK mode, positioning accuracy varies between 10-30 mm in the open-sky and urban areas, while better performance is demonstrated for the former.

A Hybrid State Representation-Based GNSS Filtering Model to Improve Vehicular Positioning Performance

In the era of intelligence, there is a huge demand for precise positioning in the fields such as automatic driving, assisted driving, and vehicle to everything. Due to the continuous and high-precision positioning capability, the global navigation satellite system (GNSS) has been an important foundation for large-scale location applications in open-sky conditions. For land vehicle kinematic navigation, the GNSS positioning method based on Kalman filtering (KF) has also become the primary choice. However, the general state model based on velocity and acceleration in KF lacks mining and utilization of the physical laws of vehicle motion, resulting in the insufficient performance of GNSS KF positioning in urban conditions. Thus, we constructed a hybrid state model, where the vehicle kinematics are classified into nonlinear and quasi-linear motions, and the corresponding velocity constraint (VC) and heading constraint (HC) models are constructed. Then, the reparameterization between VC-model and HC-model is derived through a strict formula, and the hybrid state model can switch adaptively by vehicle motion recognition. Field vehicle test data was collected with a low-cost GNSS receiver and processed by different methods. The test results of different observation conditions and different motions show that the hybrid state model has better positioning performance, and the root-mean-square (rms) errors in the occlusion condition are reduced by 15% compared with only the VC-model. Furthermore, the test results using the public data of the Google Decimeter Challenge project also verify the effectiveness and practicability of the filtering model in vehicular positioning.

Utilization of iPad Pro LiDAR and Low-Cost GNSS Ublox F9R for Data Collection of Road Assets in the Form of 3D Models

The development of mapping survey equipment technology products is growing very rapidly. These products include Global Navigation Satellite System (GNSS) and Light Detection and Ranging (LiDAR) technology. The current development of GNSS receivers can be seen in terms of accuracy and price. The GNSS receiver has very high accuracy, especially with the integration with the inertial measurement unit (IMU). This integration system can provide benefits such as estimation of GNSS position data and GNSS data stability. One of the GNSS receiver products that has been integrated with IMU, has an accuracy below 1 meter, and at an economical price is the low-cost GNSS receiver U-blox F9R series. This receiver has a positional accuracy of 0.01 + 1 ppm. LiDAR products are commonly used for the manufacture of 3D objects and distance determination on Autonomous Vehicles, however, in 2020 the company Apple inc. has made a smartphone device that is integrated with LiDAR technology. One of the goals of using this integration system is to visualize 2D objects into 3D Augmented Reality (AR). In this study, iPad Pro LiDAR technology was used to create a 3D road model. This activity continues from previous studies that have used the iPad Pro LiDAR for making 3D buildings and measuring tree diameters. In research related to the manufacture of 3D buildings, the iPad Pro LiDAR has been analyzed and it is stated that it has an accuracy of 2 cm. However, this observation using the iPad Pro LiDAR still does not provide actual position information. One way to get the true position value is to use position data from the GNSS receiver. In this study, the GNSS observation method receiver used is Real Time Kinematic Networked Transported of RTCM via Internet Protocol (RTK-NTRIP). This method utilizes the Global System for Mobile Communications (GSM) network to transmit corrections from several nearby Continuously Operating Reference Stations (CORS) in Radio Technical Commission for Maritime Services (RTCM) format to the user’s receiver. This activity aims to create a database of road assets in 3D with real positions by utilizing the integration between GNSS, IMU, and iPad Pro LiDAR. From this study, it was found that the volume measurement data using GNSS data has a value of 4,930 m3. Meanwhile, processing using 3D model data from the iPad Pro 2020 resulted in 4,422 m3. The results of the two data have a volume difference of 0.5 m3 or 10% of the validation data.

Low Cost GNSS Trimble BD982 and U-blox Performance Test Analysis F9 Series for Several Measurement Methods (Case Study: Sidoarjo Regency)

Global Navigation Satellite System (GNSS) is a tool that can assist community activities, especially in navigation and positioning for mapping surveys. This tool has an accuracy of up to millimeters for determining the position of latitude and longitude, but to get this accuracy requires an expensive cost, which is more than 200 million rupiah. Currently the development of GNSS receivers is very rapid, one of which is a low-cost GNSS product. The advantages of this receiver are that it has a low cost, light weight and an accuracy of up to centimeters for determining latitude and longitude positions. In this study, we have conducted trials using a low-cost GNSS receiver from several products such as U-blox, Trimble, and Comnav. For products from U-blox in this study, the F9P and F9R series were used. The F9R series is one of the receivers that has been integrated with the inertial measurement unit (IMU)/inertial navigation system (INS), the benefit of this integration is to improve position accuracy when data collection using the kinematic method. Products from U-blox have position accuracy below 1 meter. Meanwhile, the series used in Trimble and Comnav products are Trimble BD982 and Comnav Oem Board K708 which have accuracy below 1 centimeter. The purpose of this activity is to compare all these receivers by using Comnav OEM Board K708 data as validation data. Several parameters in this study will be analyzed such as the number of satellites obtained by the receiver, position accuracy and various other analyzes. The measurement methods that will be carried out in this study are static differential and Real Time Kinematic (RTK) measurements that utilize direct corrections from Continuously Operating Reference Stations (CORS) and deformation observations. The results of the first analysis obtained in this activity are the number of satellites obtained by the U-blox F9 series receiver which is more stable to receive satellites, namely as many as 24-29 satellites, then Trimble BD982 as many as 2-32 satellites, and Comnav K708 receivers as many as 3-28 satellites. Meanwhile, the Dilution Of Precision (DOP) values for 4 receivers, namely K708, F9P, and F9R, have stable values ranging from 0.5-1 compared to the BD982 receiver which fluctuated in several observation epochs, especially in the last observation epoch. In observations using the static differential method, using corrections from CORS for positioning this receiver can be used because the difference in position data is in the range of 0.009 m - 0.040 m from reference data (receiver K708). Meanwhile, the data receiver that has been integrated with IMU/INS (F9R) kinematic method has the smallest position difference value (K708) compared to other receivers, which is from 0.001 m - 8.091 m. In testing for deformation measurements, further research needs to be done because the data obtained is still in the form of a simulation.

Time-Correlated Window-Carrier-Phase-Aided GNSS Positioning Using Factor Graph Optimization for Urban Positioning

This article proposes an improved global navigation satellite system (GNSS) positioning method that explores the time correlation between consecutive epochs of the code and carrier-phase measurements, which significantly increases the robustness against outlier measurements. Instead of relying on the time difference carrier phase which only considers two neighboring epochs using an extended Kalman filter estimator, this article proposed to employ the carrier-phase measurements inside a window, the so-called window carrier phase (WCP), to constrain the states inside a factor graph. A left null space matrix is employed to eliminate the shared unknown ambiguity variables and, therefore, correlate the associated states inside the WCP. Then, the pseudorange, Doppler, and the constructed WCP measurements are integrated simultaneously using factor graph optimization to estimate the state of the GNSS receiver. We evaluated the performance of the proposed method in two typical urban canyons in Hong Kong, achieving the mean positioning errors of 1.76 and 2.96 m, respectively, using the automobile-level GNSS receiver. Meanwhile, the effectiveness of the proposed method is further evaluated using a low-cost smartphone-level GNSS receiver, and similar improvement is also obtained when compared with several existing GNSS positioning methods.

Evaluation of low-cost RTK GNSS receiver in motion under demanding conditions

The capabilities of global navigation satellite systems (GNSS) are still increasing. This is due to their continuous development - new satellites are sent into space, new constellations are created, and they transmit new signals on previously unused frequencies. Receiving devices - receivers with antennas - are also improved. They are smaller, consume less power, calculate their position faster and better, and are also getting much cheaper and thus more accessible.The main objective of this paper was the evaluation of u-blox ZED-F9P, low-cost RTK (real time kinematic) GNSS receiver, which is relatively new in comparison to geodetic ones. The tests were conducted in motion, under demanding conditions, in conjunction with a low-cost patch antenna u-blox ANN-MB-00 and a Leica AS10 high-precision geodetic one. A total of three test runs were performed:·two short ones (approx. 900 m each) in which reference data was provided by a Leica MS50 robotic total station and the 360-degree prism, coaxially mounted with the tested antenna,·a long one (approx. 12 km long) in which positioning was simultaneously performed using a Leica GS18T high-end geodetic receiver.As a result, it is concluded that the low-cost GNSS receivers, in particular equipped with a geodetic-grade antenna, constitute a valuable alternative to the high-quality positioning receivers. Moreover, they might act even better in some situations, which are described in the article. Thanks to comprehensive tests, it is shown that the u-blox ZED-F9P receiver can be successfully used in all projects requiring positioning accuracy of approximately ± 2 cm.

Cooperative D-GNSS Aided with Multi Attribute Decision Making Module: A Rigorous Comparative Analysis

Satellite positioning lies within the very core of numerous Intelligent Transportation Systems (ITS) and Future Internet applications. With the emergence of connected vehicles, the performance requirements of Global Navigation Satellite Systems (GNSS) are constantly pushed to their limits. To this end, Cooperative Positioning (CP) solutions have attracted attention in order to enhance the accuracy and reliability of low-cost GNSS receivers, especially in complex propagation environments. In this paper, the problem of efficient and robust CP employing low-cost GNSS receivers is investigated over critical ITS scenarios. By adopting a Cooperative-Differential GNSS (C-DGNSS) framework, the target’s vehicle receiver can obtain Position–Velocity–Time (PVT) corrections from a neighboring vehicle and update its own position in real-time. A ranking module based on multi-attribute decision-making (MADM) algorithms is proposed for the neighboring vehicle rating and optimal selection. The considered MADM techniques are simulated with various weightings, normalization techniques, and criteria associated with positioning accuracy and reliability. The obtained criteria values are experimental GNSS measurements from several low-cost receivers. A comparative and sensitivity analysis are provided by evaluating the MADM algorithms in terms of ranking performance and robustness. The positioning data time series and the numerical results are then presented, and comments are made. Scoring-based and distance-based MADM methods perform better, while L1 RMS, HDOP, and Hz std are the most critical criteria. The multi-purpose applicability of the proposed scheme, not only for land vehicles, is also discussed.

Improved attitude estimation accuracy by data fusion of a MEMS MARG sensor and a low-cost GNSS receiver

To obtain precise and continuous attitude information, this paper proposed a data fusion scheme that exploits the complementary advantages of a microelectromechanical system (MEMS) magnetic, angular rate, and gravity (MARG) sensor and a low-cost global navigation satellite system (GNSS) receiver. A quaternion-based error state Kalman filter is designed to integrate the different data sources, where the attitude error and gyro bias variation are taken as the error states. The gyro-measured angular rates are continuously integrated to propagate the attitude quaternion, while the accelerometer and magnetometer readings or the GNSS pitch and heading angles estimated by the precise carrier phase observations are utilized to perform the measurement update. The system kinematic model remains constant when the aiding source changes, only the measurement model alternates accordingly. A vehicular dynamical test was conducted to evaluate the proposed algorithm, the test results show that the attitude estimation accuracy of the integrated algorithm is apparently improved.

Evaluation of low-cost GNSS receivers for speed monitoring

Accidents caused by exceeding the driving speed limit are one of the main reasons for premature deaths and permanent injuries worldwide. As such, the search for actions seeking to reduce traffic accidents or their severity becomes increasingly crucial as these accidents become more relevant. The objective of this work is to present a low-cost solution based on the GNSS (Global Navigation Satellite System) technology for vehicle monitoring and detection of road sections where drivers exceed speed limits. This allows classifying roads by their driver’s behavior, helping traffic authorities to manage these road sections. For this purpose, two devices were developed to measure GPS speed and to ensure the same tracking conditions for these devices. A fixing structure was built to accommodate the devices as well as a smartphone, which recorded the speedometer on video. Considering that the spatial data analysis showed no significant difference in speed measurements, the device can be chosen by the lowest cost. The comparison of measured speeds with devices in relation to speeds considered by traffic laws showed that device present values are closer to the vehicle speedometer (mainly at slower speeds). These results indicate that the sensation of speed difference on the part of the driver is considerable, and that tolerances established by the legislation need to be revised so as not to cause a feeling of impunity. Therefore, the use of low-cost GNSS receivers in automatic speed control and monitoring applications in road vehicles shows great potential. It is of great value especially on roads with tracking conditions similar to our experiments. This potential can be explored by educational measures to modify the road geometric designs where excesses are frequently identified.

Monitoring the Water Vapor Content at High Spatio-Temporal Resolution Using a Network of Low-Cost Multi-GNSS Receivers

The Global Navigation Satellite System (GNSS) has the capacity for remote sensing of water vapor content in the atmosphere. Post-processing of GNSS data can provide Integrated Water Vapor (IWV) with accuracies comparable to measurements of traditional sensors, i.e. water vapor radiometers. While GNSS meteorology benefits from thousands of permanent GNSS stations operating worldwide the spatial resolution of GNSS-derived IWV is limited to tens of kilometers. Further densification of GNSS networks is achievable with low-cost GNSS receivers. We investigated the feasibility of low-cost multi-GNSS receivers for monitoring IWV. The post-processing and the real-time solution are validated against 1) the results from a geodetic-grade GNSS receiver, 2) co-located water vapor radiometer, and 3) numerical weather model (NWM). Despite the high variability of the IWV during the validation period, the standard deviation of IWV differences with respect to the water vapor radiometer was 1.0 kg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 1.5 kg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in post-processing and real-time, respectively. The city-scale variability of water vapor content in the atmosphere was monitored by a network of 16 low-cost GNSS receivers deployed in the city of Wroclaw, Poland. During rapidly changing weather conditions the disagreement between the low-cost GNSS-derived IWV field and the NWM reached up to 5.4 kg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and inter-station IWV differences exceeded 5 kg/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . It has been demonstrated that low-cost GNSS receivers are reliable tools for precise determination of IWV, also in real-time. This study is the first to measure the water vapor content with a spatial resolution of single kilometers and to present a significantly diversified city-scale IWV field.

Carrier-Phase-Based Initial Heading Alignment for Land Vehicular MEMS GNSS/INS Navigation System

For a micro-electromechanical system (MEMS)-based inertial navigation system (INS) and global navigation satellite system (GNSS) integrated system in land vehicular applications, rapid and accurate initial heading alignment is still a challenge. We propose an initial heading alignment method for MEMS INS using the GNSS carrier-phase measurement, based on the basic principle of trajectory similarity. The proposed method performs “trajectory matching” in the line-of-sight direction of satellites, where the angle is obtained by comparing the actual observed time-differenced carrier phase (TDCP) and INS-derived TDCP. Experimental results show that the initial heading could be determined accurate to 0.65° and 1.68° at a 95% confidence level within 5 s under open sky conditions and in typical urban environments, respectively, using a typical MEMS inertial measurement unit (e.g., STIM300, Safran Sensing Technologies Norway) and high-quality GNSS receiver (NovAtel OEM6). The proposed method is also verified using low-cost GNSS receivers and low-cost IMU chip under different environment. The proposed method performs better than existing approaches in terms of both time efficiency and accuracy.

Sensing Real-Time Water Vapor Over Oceans With Low-Cost GNSS Receivers

Water vapor over oceans is significant for numerical weather prediction (NWP) and climate research. Ocean platform-based global navigation satellite system (GNSS) which can sense the atmospheric water vapor is becoming an important supplement for water vapor measurements over oceans. However, the application of ocean platform-based GNSS meteorology is normally based on geodetic GNSS receivers, which implies the high cost of hardware. In this contribution, we investigate the potential of retrieving real-time water vapor over oceans with a low-cost receiver (u-blox F9P), and a geodetic GNSS receiver (Trimble NetR9) is also equipped in the experiment vessel. The post-processed Trimble NetR9 zenith total delay (ZTD) estimates and European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 precipitable water vapor (PWV) products are used for the validation of real-time ZTDs and PWV values. The results show that the real-time ZTDs derived from the low-cost multi-GNSS (GPS + Galileo) observations obtain a difference of over 2.13 cm in root-mean-square (RMS) compared to the post-processed ZTDs with an averaged initialization time of approximately 40 mins. In addition, compared to ERA5 PWV, the real-time PWV derived from u-blox F9P multi-GNSS observations shows a difference in RMS of approximately 4 mm. Although u-blox F9P multi-GNSS performs relatively worse than Trimble NetR9 multi-GNSS in real-time ZTD/PWV estimates, the accuracy of low-cost GNSS receivers derived water vapor over oceans can still meet the requirements for NWP and nowcasting, which demonstrates promising prospects in supplementing the measurements of water vapor over oceans.

GNSS Outlier Mitigation via Graduated Non-Convexity Factor Graph Optimization

Accurate and globally referenced global navigation satellite system (GNSS) based vehicular positioning can be achieved in outlier-free open areas. However, the performance of GNSS can be significantly degraded by outlier measurements, such as multipath effects and non-line-of-sight (NLOS) receptions arising from signal reflections of buildings. Inspired by the advantage of batch historical data in resisting outlier measurements, in this paper, we propose a graduated non-convexity factor graph optimization (FGO-GNC) to improve the GNSS positioning performance, where the impact of GNSS outliers is mitigated by estimating the optimal weightings of GNSS measurements. Different from the existing local solutions, the proposed FGO-GNC employs the non-convex Geman McClure (GM) function to globally estimate the weightings of GNSS measurements via a coarse-to-fine relaxation. The effectiveness of the proposed method is verified through several challenging datasets collected in urban canyons of Hong Kong using automobile level and low-cost smartphone level GNSS receivers.

Carrier Phase-Based Precise Heading and Pitch Estimation Using a Low-Cost GNSS Receiver

Attitude and heading estimation methods using the global navigation satellite system (GNSS) are generally based on multi-antenna deployment, where the installation space and system cost increase with the increase in the number of antennas. Since the single-antenna receiver is still the major choice of the mass market, we focus on precise and reliable heading and pitch estimation using a low-cost GNSS receiver. Carrier phase observations are precise but have an ambiguity problem. A single difference between consecutive epochs can eliminate ambiguity and reduce the measurement errors. In this work, a measurement model based on the time-differenced carrier phases (TDCPs) is utilized to estimate the precise delta position of the antenna between two consecutive epochs. Then, considering the motion constraint, the heading and pitch angles of a moving land vehicle can be determined by the components of the estimated receiver delta position. A threshold on the length of the delta position is selected to avoid large errors in static periods. To improve the reliability of the algorithm, the Doppler-aided cycle slip detection method is applied to exclude carrier phases with possible cycle slips. A real vehicular dynamic experiment using a low-cost, single-frequency GNSS receiver is conducted to evaluate the proposed algorithm. The experimental results show that the proposed algorithm is capable of providing precise vehicular heading and pitch estimates, with both the root mean square errors being better than 1.5°. This also indicates that the cycle slip exclusion is indispensable to avoid unexpected large errors.

Performance evaluation of GNSS receiver clock modelling in urban navigation using geodetic and high-sensitivity receivers

AbstractIn urban areas, the Global Navigation Satellite System (GNSS) can lead to position errors of tens of meters due to signal obstruction and severe multipath effects. In cases of 3D-positioning, the vertical coordinate is estimated less accurately than are the horizontal coordinates. Multisensor systems can enhance navigation performance in terms of accuracy, availability, continuity and integrity. However, the addition of multiple sensors increases the system cost, and thereby the applicability to low-cost applications is limited. By using the concept of receiver clock modelling (RCM), the position estimation can be made more robust; the use of high-sensitivity (HS) GNSS receivers can improve the system availability and continuity. This paper investigates the integration of a low-cost HS GNSS receiver with an external clock in urban conditions; subsequently, the gain in the navigation performance is evaluated. GNSS kinematic data is recorded in an urban environment with multiple geodetic-grade and HS receivers. The external clock stability information is incorporated through the process noise matrix in a Kalman filter when estimating the position, velocity and time states. Results shows that the improvement in the precision of the height component and vertical velocity with both receivers is about 70% with RCM compared with the estimates obtained without applying RCM. Pertaining accuracy, the improvement in height with RCM is found to be about 70% and 50% with geodetic and HS receivers, respectively. In terms of availability, the HS receiver delivers an 100% output compared with a geodetic receiver, which provides an output 99⋅4% of the total experiment duration.