Global Navigation Satellite System Corrections Research Articles

New Concept of Smart UAS-GCP: A Tool for Precise Positioning in Remote-Sensing Applications

Today, ground control points (GCPs) represent indispensable tools for products’ georeferencing in all the techniques concerning remote sensing (RS), particularly in monitoring activities from unmanned aircraft system (UAS) platforms. This work introduces an innovative tool, smart GCPs, which combines different georeferencing procedures, offering a range of advantages. It can serve three fundamental purposes concurrently: (1) as a drone takeoff platform; (2) as a base station, allowing the acquisition of raw global navigation satellite system (GNSS) data for post-processed kinematic (PPK) surveys or by providing real-time GNSS corrections for precision positioning; (3) as a rover in the network real-time kinematic (NRTK) mode, establishing its position in real time with centimetric precision. The prototype has undergone testing in a dedicated study area, yielding good results for all three geodetic correction techniques: PPK, RTK, and GCP, achieving centimeter-level accuracy. Nowadays, this versatile prototype represents a unique external instrument, which is also easily transportable and able to connect to the GNSS RING network, obtaining real-time positioning corrections for a wide range of applications that require precise positioning. This capability is essential for environmental applications that require a multitemporal UAS-based study. When the real-time RING data are accessible to the scientific community operating in RS surveying, this work could be a helpful guide for researchers approaching such investigations.

A Data Quality Assessment Approach for High-Precision GNSS Continuously Operating Reference Stations (CORS) with Case Studies in Hong Kong and Canada/USA

Centimeter-level or better positioning accuracy is needed in engineering surveying applications. When employing the Global Navigation Satellite System (GNSS) in engineering surveying, the high-precision real-time kinematic (RTK) positioning method must be used to achieve such positioning accuracy. Currently, precise point positioning (PPP) cannot reliably achieve the positioning accuracy needed for engineering surveying in real time. The high-precision RTK positioning method needs carrier-phase measurements and a reference station/network. Surveyors may not need a GNSS receiver in their organizations/companies to act as the reference station. Continuously Operating Reference Stations (CORS), run by international/national organizations/agencies or private companies such as GNSS receiver manufacturers, let users freely access the raw GNSS measurements or corrections for real-time and post-processing applications. The positioning accuracy of the GNSS rover is affected by the data quality of the reference stations, including virtual reference stations (VRS). The International GNSS Service (IGS) currently provides the number of cycle slips and the L1 and L2 average pseudorange multipath errors per station daily. The US National Geodetic Survey (NGS) provides daily station coordinate residuals. Carrier-phase data quality of the CORS stations is not provided by their organizations/agencies. Nowadays, many CORS stations track multi-GNSS satellites. This paper proposes a multi-GNSS and multi-frequency data quality assessment approach for CORS stations with a focus on carrier-phase data quality. The proposed approach is demonstrated with case studies on IGS/CORS networks in Hong Kong and Canada/USA. In other words, a strategy to obtain non-linear combined carrier-phase multipath errors and noise is proposed in this work. The data quality of a CORS station depends on the site environment, monument type and height, and GNSS receiver/antenna. An account of the data quality at some selected stations is given; the main focus of the paper is on the proposed data quality assessment approach.

A Low-Cost Global Navigation Satellite System Positioning Accuracy Assessment Method for Agricultural Machinery

The high-precision positioning and navigation of agricultural machinery represent a backbone for precision agriculture, while its worldwide implementation is in rapid growth. Previous studies improved low-cost global navigation satellite system (GNSS) hardware solutions and fused GNSS data with complementary sources, but there is still no affordable and flexible framework for positioning accuracy assessment of agricultural machinery. Such a low-cost method was proposed in this study, simulating the actual movement of the agricultural machinery during agrotechnical operations. Four of the most commonly used GNSS corrections in Croatia were evaluated in two repetitions: Croatian Positioning System (CROPOS), individual base station, Satellite-based Augmentation Systems (SBASs), and an absolute positioning method using a smartphone. CROPOS and base station produced the highest mean GNSS positioning accuracy of 2.4 and 2.9 cm, respectively, but both of these corrections produced lower accuracy than declared. All evaluated corrections produced significantly different median values in two repetitions, representing inconsistency of the positioning accuracy regarding field conditions. While the proposed method allowed flexible and effective application in the field, future studies will be directed towards the reduction of the operator’s subjective impact, mainly by implementing autosteering solutions in agricultural machinery.

Distribution of Reference Stations: Initial Explore on the Optimal Selection Strategy

The Satellite-based augmentation system (SBAS) is intended to provide real-time differential global navigation satellite system corrections with the high accuracy, availability, and integrity required for aviation applications. Since the performance of Satellite clock and ephemeris (SCE) corrections and Ionospheric range delay (IRD) corrections can vary dramatically depending on satellites and Ground reference stations (GRSs) geometry, therefore, we present a GRSs distribution optimized criteria and process to improve SBAS corrections performance. The present step-by-step optimized scheme using the average satellite surveillance dilution of precision and relative centroid metric availability of grid points as fitness values to determine the appropriate GRSs distribution to sufficiently meet the corrections requirements. The results show that the statistical mean RCM availability can reach more than 0.5518 for all IGPs and the coverage depth of GRSs in China and its surrounding areas is more than 25, which fully satisfies the requirement for solving SCE and IRD corrections.

Implementation and performance evaluation of an inertial navigation system/global navigation satellite system real‐time kinematic Ntrip navigation system aided by a robot operating system‐based emulated odometer for high‐accuracy land vehicle navigation in urban environments

AbstractAdvanced land‐vehicle navigation commonly uses integrated systems to counteract global navigation satellite system (GNSS) solution degradation. This occurs mainly in urban environments due to blockage of the satellite signals. This paper presents a loosely coupled inertial navigation system/GNSS navigation system that combines an attitude and heading reference system (AHRS) device with a dual‐frequency dual‐antenna GNSS heading receiver. The integrated navigation system is aided by a low‐cost odometer which replaces external wheel speed sensors usually installed in autonomous vehicles. The proposed odometer extracts the anti‐lock braking system‐generated pulses of rear wheels from vehicle controller area network messages. Following, it converts them into software‐generated signal pulses which are sent to the AHRS device through the serial port. The system platform uses a mobile internet data link to get differential GNSS corrections in real‐time from a public Ntrip—networked transport of Radio Technical Commission for Maritime Services via internet protocol—broadcaster in order to allow the GNSS receiver to operate in differential global positioning system/real‐time kinematic (RTK) modes. Thus, the integrated navigation system provides centimeter‐level positioning accuracy at 100 Hz. Since the positioning accuracy is severely affected by numerous factors, this work proposes a replicated 24 full factorial design with the purpose of evaluating the in‐field obtained positioning performance under different factors combinations. The experimental design chosen allows to know under which conditions it is feasible to replace the available GNSS velocity by the proposed odometry solution, when they are used as navigation aids, and knowing that the proposed odometry has a low resolution. The analysis of 32‐runs factorial design results, using a significance level of .05, demonstrated that the proposed odometry can overcome GNSS/RTK velocity.

Characterization of GNSS observations from a Nexus 9 Android tablet

Global navigation satellite system (GNSS) raw data were made available in the application programming interface (API) starting from version 7.0 of the Android operating system. This opens possibilities for precise positioning with Android devices, as externally generated GNSS corrections can now be included in the positioning estimation in a convenient way. The Nexus 9 tablet is a good candidate for an early assessment of the raw GNSS observables and the corresponding derived precise positions, as it also supports many of the optional features and observation types presented by the API, including carrier phase observations which play an important role in many precise positioning techniques. It is known from the previous studies that poor handling of multipath in smartphones and tablets is a big challenge when it comes to precise GNSS positioning with these kinds of devices. Hence, this study assesses the raw GNSS observations and the calculated precise positions of the Nexus 9 tablet in two experimental setups with different multipath impacts. In addition, various biases of the observations are determined, some of which is not present on high-grade geodetic receivers. The analysis is done for GPS and GLONASS, which are supported by the Nexus 9 tablet. The study shows that multipath plays an important role for the expected accuracy of the calculated precise positions, both due to the induced error on the measurements, and due to loss of lock of the GNSS signals, which significantly affects precise positioning from carrier-phase measurements. Position accuracy ranges from just below 1 m to a few decimeters between the experimental setups with moderate and low levels of multipath respectively, for positioning based on carrier-phase observations. It is, furthermore, demonstrated that consideration of code inter-system biases and code inter-frequency biases of the Nexus 9 tablet are crucial for differential GNSS with multiple GNSS systems and when GLONASS observations are used in the positioning solutions. Besides these expected biases, also other less expected behaviors were discovered in the Nexus 9 GNSS observations, including various drifts for the code and phase observations. This study also proposes some strategies to handle these.

NRCan’s Compliance Program for high accuracy, GNSS services: ensuring compatibility with the Canadian Spatial Reference System

There are currently over 700 Global Navigation Satellite System (GNSS) reference stations actively broadcasting corrections (Active Control Stations — ACSs) in Canada. This number has been consistently growing since the early 2000s. In 2009, the federal, provincial, and territorial members of the Canadian Council on Geomatics (CCOG) recognized that consumers of GNSS corrections data had very little ability to verify that service providers were following best practices to ensure the quality of their work. It is common for surveyors to delineate property boundaries or to define the location of civil infrastructure with significant economic value, so being dependent upon another party without quality assurance was perceived as a major risk. Additionally, this new dependence upon commercial ACSs for GNSS corrections posed a threat to the consistency of position values in Canada. To address this concern, CCOG tasked its Canadian Geodetic Reference System Committee (CGRSC) with developing a plan to describe, validate, and provide certification of the GNSS corrections services consumed by industry. This paper summarizes the development of Natural Resources Canada’s (NRCan) Compliance Program for High Accuracy, GNSS Services, and how it can benefit professional surveyors across Canada.

Fast Precise Point Positioning: A System to Provide Corrections for Single and Multi-frequency Navigation

Fast Precise Point Positioning (Fast-PPP) provides Global Navigation Satellite System corrections in real-time. Satellite orbits and clock corrections are shown to be accurate to a few centimeters and a few tenths of a nanosecond which, together with the determination of the fractional part of the ambiguities, enable global high-accuracy positioning with undifferenced Integer Ambiguity Resolution. The new global ionospheric model is shown to provide corrections accurate at the level of 1 Total Electron Content Unit over well-sounded areas and Differential Code Biases at the level of tenths of a nanosecond. These corrections are assessed with permanent receivers, treated as rovers, located at 100 to 800 kilometers from the reference stations of the ionospheric model. Fast-PPP achieves decimeter-level of accuracy after few minutes, several times faster than single- and dual-frequency ionospheric-free solutions, using a month of Global Positioning System data close to the last Solar Maximum and including equatorial rovers.