Single-frequency Real-time Kinematic Research Articles

Low-cost multi-GNSS, single-frequency RTK averaging for marine applications: accurate stationary positioning and vertical tide measurements

The use of Real Time Kinematic (RTK) Global Navigation Satellite System (GNSS) for accurate horizontal and vertical measurements in the marine environment has been considered since the late-1980’s and tested from the 1990’s when GPS and GLONASS were the only operational constellations available and high-cost multi-frequency receiver equipment was required. This paper modernizes the conversation using multi-constellation, low-cost, single-frequency RTK GNSS measurements and proves their value with accurate positioning and tide measurements. Our tests show average stationary horizontal positioning measurements using this equipment are suitable (95% CI) for the most stringent International Hydrographic Organization (IHO) Standard S-44 ‘Exclusive Order’ at base station ranges of up to 27 km. Vertical observations on a moving platform, smoothed using a baseline distance-dependent moving average filter show the equipment and method are comparable with traditional electronic tide gauge observations over the same base station range. All of our measurement results show the potential to improve total uncertainty calculations undertaken by hydrographers, engineers and scientists in the marine realm, while the low-cost equipment raises the possibility that more measurements can be taken, leading to improvements in monitoring, modelling and understanding the marine environment.

Tightly Coupled Integration of BDS-3 B2b RTK, IMU, Odometer, and Dual-Antenna Attitude

Real Time Kinematic (RTK) based on single-frequency observation is still widely used in many fields due to the characteristics of low-cost and low-power consumption. However, the positioning performance of single-frequency RTK in terms of accuracy, stability, and continuity would be significantly degraded during the harsh satellite environments. To improve the performance, this contribution presented a model of multi-sensor and analytical observations augmented the single frequency RTK tightly based on a modified Psi-angle state model. In such a model, the single frequency observations of the new signal of BDS-3 B2b are tightly integrated with inertial measurements, odometer data, dual-antenna attitude, and non-holonomic constraint. To evaluate the presented model, the typical navigation performance and the ambiguity resolution performance are analyzed based on a set of vehicle-borne data. Results illustrated that the Inertial Navigation System (INS) would bring about 13.5%, 16.2%, and 12.3% position enhancements to the BDS-3 B2b RTK mode. Such improvements could be up to 15.9%, 16.2%, and 25.2% while adding the non-holonomic constraint and odometer data. Besides, non-holonomic constraint and odometer also upgrade the attitude accuracy visibly in pitch and heading directions, with enhancements of about 16.9% and 62.9%. In contrast, augmentations from the dual-antenna attitude are mainly presented in terms of heading angle with about 29.5% compared to the RTK/INS/Odometer/NHC tight integration mode. Besides, the convergence time of yaw angle is visibly enhanced while using the odometer/NHC, the dual-antenna heading, or the two together. Moreover, the ambiguity resolution performance could also be improved while using the presented model. Due to the enhancements in position and attitude brought by different sensors, the ADOP performance would be improved to varying degrees. Besides, the fixed rate and reliability of ambiguity resolution could also be enhanced.

A Cycle Slip Detection Framework for Reliable Single Frequency RTK Positioning.

Single frequency real-time kinematic (RTK) positioning is expected to be the leading implementation platform for a variety of emerging GNSS mass-market applications. During RTK positioning, the most common source of measurement errors is carrier-phase cycle slips (CS). The presence of CS in carrier-phase measurements is tested by a CS detection technique and correspondingly taken care of. While using CS prone measurement data, positioning reliability is an area of concern for RTK users. Reliability can be linked with the CS detection scheme through a least squares (LS) adjustment process. This paper proposes a CS detection framework for reliable RTK positioning using single-frequency GNSS receivers. The scheme uses double differenced measurements for CS detection via LS adjustment using a detection, identification, and adaptation approach. For reliable positioning, the procedure to link the detection and identification stages is described. Through tests conducted on kinematic data, internal and external reliability are theoretically determined by calculating minimal detectable bias (MDB) and marginally detectable errors, respectively. After introducing CS, the actual values of MDB are found to be four cycles, which are higher than the theoretically obtained values of one and two cycles. Although CS detection for reliable positioning is implemented for single-frequency RTK users, the proposed procedure is generic and can be used whenever CS are detected through statistical tests during LS adjustment.

RTK with the Assistance of an IMU-Based Pedestrian Navigation Algorithm for Smartphones.

Real-time kinematic (RTK) technique is widely used in modern society because of its high accuracy and real-time positioning. The appearance of Android P and the application of BCM47755 chipset make it possible to use single-frequency RTK and dual-frequency RTK on smartphones. The Xiaomi Mi 8 is the first dual-frequency Global Navigation Satellite System (GNSS) smartphone equipped with BCM47755 chipset. However, the performance of RTK in urban areas is much poorer compared with its performance under the open sky because the satellite signals can be blocked by the buildings and trees. RTK can’t provide the positioning results in some specific areas such as the urban canyons and the crossings under an overpass. This paper combines RTK with an IMU-based pedestrian navigation algorithm. We utilize attitude and heading reference system (AHRS) algorithm and zero velocity update (ZUPT) algorithm based on micro electro mechanical systems (MEMS) inertial measurement unit (IMU) in smartphones to assist RTK for the sake of improving positioning performance in urban areas. Some tests are carried out to verify the performance of RTK on the Xiaomi Mi 8 and we respectively assess the performances of RTK with and without the assistance of an IMU-based pedestrian navigation algorithm in urban areas. Results on actual tests show RTK with the assistance of an IMU-based pedestrian navigation algorithm is more robust and adaptable to complex environments than that without it.

Tightly combined GPS and GLONASS for RTK positioning with consideration of differential inter-system phase bias

A tightly combined inter-system model between two global navigation satellite systems can improve the performance of real-time kinematic (RTK) positioning, if the associated differential inter-system biases (DISBs) are processed well, especially for the severely obstructed environment. In recent years, tightly combined models among code division multiple access (CDMA) systems (e.g. Global Positioning System (GPS), BeiDou Navigation Satellite System (BDS), Quasi-Zenith Satellite System (QZSS)) have been extensively studied. However, when there is tightly combined use of CDMA systems and Global Navigation Satellite System (GLONASS), which adopts frequency-division multiple access (FDMA), we need to process not only the DISBs between systems, but the problem caused by the different wavelengths of the GLONASS satellites. In this paper, we propose a new model for the tight combination of GPS and GLONASS. First, we adopt the model with two reference satellites to maintain the integer ambiguity resolution of GLONASS. Then, an inter-system model between GPS and GLONASS is proposed with a total of three ‘reference satellites’ involved. Using several zero and ultra-short baselines, the stability of GPS and GLONASS differential inter-system phase biases (DISPBs) will be evaluated. The results indicated that the DISPBs between GPS and GLONASS are generally stable, although low-frequency variations may appear in the baselines with different receiver types. Using the stability of DISPBs, the combined GPS and GLONASS single-frequency RTK positioning performance is evaluated with simulated obstructed environments with no more than four GPS and four GLONASS satellites visible. It will be shown that compared with the intra-system model, the proposed inter-system model can improve the positioning accuracies by 13.5%, 15.0% and 46.2%, respectively, in three directions.

Combined GPS and BDS for single-frequency continuous RTK positioning through real-time estimation of differential inter-system biases

Double-differenced (DD) ambiguities between overlapping frequencies from different GNSS constellations can be fixed to integers if the associated differential inter-system biases (DISBs) are well known. In this case, only one common pivot satellite is sufficient for inter-system ambiguity resolution. This will be beneficial to ambiguity resolution (AR) and real-time kinematic (RTK) positioning especially when only a few satellites are observed. However, for GPS and current operational BDS-2, there are no overlapping frequencies. Due to the influence of different frequencies, the inter-system DD ambiguities still cannot be fixed to integers even if the DISBs are precisely known. In this contribution, we present an inter-system differencing model for combined GPS and BDS single-frequency RTK positioning through real-time estimation of DISBs. The stability of GPS L1 and BDS B1 DISBs is analyzed with different receiver types. Along with parameterization and using the short-term stability of DISBs, the DD ambiguities between GPS and BDS pivot satellites and the between-receiver single-difference ambiguity of the GPS pivot satellite can be estimable jointly with the differential phase DISB term from epoch to epoch. Then the inter-system differencing model can benefit from the near time-constant DISB parameters and thus has better multi-epoch positioning performance than the classical intra-system differencing model. The combined GPS and BDS single-frequency RTK positioning performance is evaluated with various simulated satellite visibilities. It will be shown that compared with the classical intra-system differencing model, the proposed model can effectively improve the positioning accuracy and reliability, especially for severely obstructed situations with only a few satellites observed.

High-Precision Vehicle Navigation in Urban Environments Using an MEM's IMU and Single-Frequency GPS Receiver

Many applications demand high-precision navigation in urban environments. Two frequency real-time kinematic (RTK) Global Positioning System (GPS) receivers are too expensive for low-cost or consumer-grade projects. As single-frequency GPS receivers are getting less expensive and more capable, more people are utilizing single-frequency RTK GPS techniques to achieve high accuracy in such applications. However, compared with dual-frequency receivers, it is much more difficult to resolve the integer ambiguity vector using single-frequency phase measurements and therefore more difficult to achieve reliable high-precision navigation. This paper presents a real-time sliding-window estimator that tightly integrates differential GPS and an inertial measurement unit to achieve reliable high-precision navigation performance in GPS-challenged urban environments using low-cost single-frequency GPS receivers. Moreover, this paper proposes a novel method to utilize the phase measurements, without resolving the integer ambiguity vector. Experimental results demonstrate real-time position estimation performance at the decimeter level. Furthermore, the novel use of phase measurements improves the robustness of the estimator to pseudorange multipath error.

Results of Real-Time Kinematic Positioning Based on Real GPS L5 Data

The Global Positioning System (GPS) is currently being upgraded from a dual- to a triple-frequency constellation. Although a full triple-frequency constellation will not be realized before 2021, we can already make use of these new signals for positioning. This letter presents the initial results of real-time kinematic (RTK) ambiguity resolution and positioning based on real L5 measurements combined with L1 and L2C measurements. The L2C is the new second civil signal that is like L5, also transmitted by the GPS Block IIF satellites. The results, obtained for a zero baseline and short baselines between some MGEX receivers, demonstrate that the L5 data indeed improve the time-to-first-fix ambiguities, with 24%–49% with respect to dual $\text{L}1+\text{L}2\text{C}$ , to, on average, less than 1 min based on $\text{L}1+\text{L}2\text{C}+\text{L}5$ with four to five satellites. It is also shown that single-frequency RTK is performing better when, instead of L1, the L5 frequency is used, provided that the baseline is not too long. Positioning accuracy, especially the float, is improved when going from dual- to triple-frequency measurements, despite the large position errors that occur at some times, caused by a poor geometry based on only four satellites. In the near future, with more Block IIF satellites launched, these geometry defects will occur less frequently.

Combined BDS, Galileo, QZSS and GPS single-frequency RTK

We will focus on single-frequency single-baseline real-time kinematic (RTK) combining four Code Division Multiple Access (CDMA) satellite systems. We will combine observations from the Chinese BeiD...

Cycle slips detection algorithm for low cost single frequency GPS RTK positioning

Conventionally, cycle slips are detected by combining phase observations or phase/code observations. However, this is unsuitable for single frequency receivers in real time kinematic (RTK) positioning. Therefore, this study introduces an algorithm based on outlier detection concept to the detection and repair of cycle slip during GPS RTK positioning. The efficacy of the algorithm was verified on a low cost single frequency GPS receiver. To investigate the ability of the algorithm to detect error within a single cycle, minimum detectable bias (MDB) of less than one cycle was used as the index of success. Experiments verified the availability of the algorithm up to 96·12% (10° mask angle). The algorithm was able to accurately detect the time when cycle slips occur and precisely estimate their size in various simulated scenarios. Finally, tests were performed based on real data, and the results confirm that the proposed algorithm is applicable for single frequency RTK positioning.

Case Study of Precision of GPS Differential Correction Strategies: Influence on aDcp Velocity and Discharge Estimates

The precision of four differential global positioning systems (DGPS) was evaluated in the context of fluvial water velocity and discharge measurement. DGPS is used to resolve water velocities measured with an acoustic Doppler current profiler (aDcp) into earth coordinates if bottom tracking is unavailable. The DGPS systems assessed were: (1) the dual frequency real time kinematic (RTKL1L2); (2) the single frequency real time kinematic (RTKL1); (3) the code-phase Canadian Coast Guard (CG); and (4) the code-phase Wide Area Augmentation System (WAAS). Repeat discharge surveys (n=22) were conducted at a transect of the Gatineau River, Canada, simultaneously collecting bottom track boat velocity ( vBT ) and boat velocity from all four DGPS ( vDGPS ) . The mean absolute single ping differences between vBT and vDGPS were 3.1 (RTKL1L2), 3.2 (RTKL1), 8.9 (CG), and 9.8 cm∕s (WAAS). Errors were observed more often near channel margins, presumably due to obstruction and multipath associated with riverbank vegetation ...