Over the past three decades GPS has evolved from a system designed to provide meter-level positioning for military applications to one that is used for a diverse range of unforeseen applications, for instance in engineering setting out and deformation monitoring, estimating atmospheric water vapor content, in-vehicle navigation systems, and to provide precise timing to control internet and mobile phone communications. This evolution has been both driven and underpinned by fundamental research, especially in the fields of error modeling, receiver design, and sensor integration. However, GPS and its current augmentations still cannot satisfy the ever-increasing demands for higher performance. For instance, it is not accurate or responsive enough to control a number of machine-guided operations in engineering; it does not have sufficient integrity for it to be used for gate-to-gate aircraft navigation and it is not sufficiently available in cities and indoors for a wide range of location-based services such as emergency services.
Over the next few years, the current satellite navigation systems will continue to evolve into new modernized forms. Modernized GPS and GLONASS bring new signals to complement those that we have been using from GPS for many years, and Europe’s GALILEO and China’s COMPASS-BEIDOU will add to these. From this proliferation of systems, we can avail a new and broad portfolio of signals from which to choose. These new signals have the potential to extend the applications of GNSS into those areas that GPS alone cannot satisfy. They should also enable the invention of new positioning concepts to significantly increase the efficiency of positioning for many of today’s applications and open up the market for new ones. This special issue consisting of eight papers presents a sample of the recent developments in exploiting GNSS, including its integration with terrestrial sensors.
The first paper, “Integrity monitoring of fixed ambiguity Precise Point Positioning (PPP) solutions”, by Jokinen, Feng, Schuster, and Ochieng, demonstrates an ingenious way of how GPS and GLONASS measurements can be used together to improve the performance of single receiver-based positioning employing the concept of PPP. GPS and GLONASS are used together in the first instance to estimate the float position solution before using this to resolve and fix the GPS integer ambiguities. Furthermore, the paper uses and extends an existing integrity monitoring method to cater for the failures of base-satellites used to form the between-satellite-differenced measurements, enabling PPP to be used for mission critical applications. PPP has a number of advantages over conventional Real-Time Kinematic (RTK) positioning, including the potential for centimeter level positioning without the need for a dense network of local reference stations making it more cost-effective, and therefore able to support many applications at sea and in remote regions of the world.
The second paper, “GNSS vulnerabilities: Simulation, verification and mitigation platform design”, by Zhao, Zhan, and Yan, addresses the problem of GNSS signal interference (natural and artificial) through the development of a platform for research on the different types of interference in the signal propagation domain (including electromagnetic, atmospheric, and multipath). The paper concentrates on the design of the main system modules and testing through an experimental analysis. The platform should enable the development of effective detection and mitigation strategies and methods to maximize GNSS service availability.
The third paper, “Integration of GPS with a WiFi high accuracy ranging functionality”, by Nur, Feng, Ling, and Ochieng, proposes an innovative positioning and navigation capability based on integrating GPS (or GNSS) with measurements from a new high accuracy WiFi ranging system in the measurement domain. This is attractive because (1) WiFi networks are prevalent in indoor and dense urban areas, (2) GPS and WiFi functions exist in most mobile devices, and (3) The integration removes GPS outages and improves the overall positioning accuracy to the meter-level. These characteristics make this system a cost-effective way of supporting LBSs in indoor and dense urban environments.
The fourth paper, “INS stochastic error detection during kinematic tests and impacts on INS/GNSS performance” by Hasnur, Kealy, and Morelande, addresses the limitations of modeling INS stochastic errors from Allan variance (AV) analysis using static data for kinematic/dynamic applications. The paper proposes the use of Dynamic Allan variance applied to a kinematic data-set, and shows that for dynamic applications this improves INS/GNSS performance compared to the use of AV.
The fifth paper, “Ubiquitous indoor vision navigation using a smart device”, by Huang and Gao, proposes a vision-based indoor positioning and navigation system. The proposed vision-based system relies on a single camera, widely available on smart phones and tablets. The derivation of the absolute 3D position from 2D snapshots of a single camera requires the use of an external geo-reference database. A ubiquitous floor plan database is used to provide accurate geodetic information. Unlike other popular geo-reference databases, the database used can easily be generated with existing resources. The proposed system was developed as an iOS App and tested on iPad for various indoor scenarios.
The sixth paper, “Numerical weather modeling based slant tropospheric delay estimation and its enhancement by GNSS data”, by Yang, Hill, and Moore, presents a new method to deal with the unmitigated tropospheric delay, which is a major error source in PPP. The method involves the direct estimation of Slant Tropospheric Delay (STD) from the combination of high resolution Numerical Weather Modeling information and multi-GNSS observations from a network of local Continuously Operating Reference Stations. The results demonstrate that the new method is very effective in improving the accuracy and quality of the STD estimation with the positive effect of improving the accuracy and reducing the convergence time of PPP.
The seventh paper, “New methods for dual-constellation single receiver positioning and integrity monitoring”, by Feng, Jokinen, Milner, and Ochieng, uses GPS and GLONASS to highlight and demonstrate the benefits of using multiple constellations for single receiver-based positioning and integrity monitoring with single and dual frequency measurements. The single frequency pseudorange-based dual constellation positioning method employs a new cross constellation single difference scheme to benefit from the similarities whilst addressing the differences between the constellations. The second technique uses dual frequency carrier phase measurements from GLONASS and GPS for PPP. The results show significant improvements both in positioning accuracy and integrity monitoring as a result of the use of two constellations, and can be extended to the combined use of more constellations.
The eighth paper, “Group delay and phase delay in GNSS systems”, by Berry, Mattos, and Kale, addresses GNSS signal modulation using Binary Phase Shift Keying and its alternative/replacement, Binary Offset Carrier (BOC). It focuses on a number of concerns with how the BOC signals might be affected when passed through a surface acoustic wave filter including those arising from the split spectrum nature of BOC signals suggesting different delays on the upper and lower side-lobes, and a delay in magnification effect.