Inertial Aiding Deeply Integrated Receiver Architecture
Background & Objectives
Hybridisation of GPS and INS units is a well-known concept, with strong developments in the field, especially in the United States for military applications. So far, hybridisation of GPS and INS units has been mainly limited, either to processing the navigation output of both systems (loose coupling) or to integrating measurements from both systems into a Kalman filter (tight coupling). Although there is ongoing research on INS/GPS integration of the tight level, the INS/Galileo integration has not yet been investigated. Recent studies concluded that new developments should focus on low-cost and fully integrated GNSS-IMU systems based on miniaturised sensors. IADIRA is a step further towards this objective, where a tightly coupled or deeply integrated architecture is used to aid the receiverâ€™s tracking loops, reducing the dynamic stress uncertainty, allowing a reduction of the loop bandwidth and an increase of the integration time from the typical figure of a few milliseconds. As a consequence, velocity and position accuracy improve substantially and the minimum C/N0 to track the signal is reduced even in high-dynamic environments. Furthermore, since the inertial aiding signal allows tracking loops to operate even during a signal blockage (both PLL and DLL), reacquisition times are very small and cycle-slip occurrences are reduced, therefore increasing service availability.
IADIRA focuses on the field of inertial aiding, referring to the use of inertial-derived position and velocity for the improvement of the GNSS receiver tracking loops, and inertial coasting, which in turn refers to the use of inertial-derived position and velocity to interpolate GNSS trajectories for short periods, either because of GNSS data gaps or just to interpolate between GNSS fixes.
The chosen study plan for IADIRA is as follows:
- Task 1: System engineering: This task corresponds to the Consolidation Phase defined in the statement of work. It defines in detail the objectives of the project and the system requirements.
- Task 2: Requirements engineering. This phase defines the technical specification and the architecture for the deeply integrated receiver, the inertial units evaluation, selection and characterisation, the target application architecture design, the test-bench architecture design and the algorithms selection and specification.
- Task 3: Design and implementation engineering. This phase comprises the detailed design and implementation of the receiver algorithms, in signal processing, navigation processing and GNSS/INS integration, and of the test bench. The test bench includes the customisation of the GRANADA software receiver.
- Task 4: Verification and validation. Verification is performed to guarantee that the software produced is conforming to the technical specification made during the requirements engineering task and that the software architecture and implementation is free of any faults. Validation testing will be performed by means of a demonstration to guarantee that the software produced meets the system requirements.
- Task 5: Technology transfer analysis: This phase concludes the study. It aims at transferring the achieved technical and innovation objectives to the identified market. It comprises the evaluation of the deeply integrated receiver functions for the target application, the analysis of the applicability of the concept to different receivers, the dissemination of results and the compilation of the synthesis and recommendations.
The main project goal is aimed at developing and testing the concept of an inertially enabled GNSS receiver through simulating tightly coupled integrating architecture. The simulation experience and the available software tool will help in designing an aided receiver. Such a receiver will be of superior performance in terms of satellite signal tracking, signal (re)acquisition and navigational accuracy in comparison to its unaided counterpart. The improved receiver performance should widen the spectrum of satellite positioning applications as well as enhance the existing ones.
Work performed & results
The IADIRA test bench allows the testing of the IADIRA concept in a friendly and intuitive environment, keeping a modular software approach. The test bench uses a modified version of GRANADA Bit-True to simulate the deeply integrated receiver including the developed combination of IMU/GNSS receiver algorithms. A test campaign was carried out in order to test the IADIRA concept and the IADIRA test bench. The reference trajectories were generated based on data collected in real environments â€“ a low-dynamic track switch simulation and a medium-dynamic urban rail layout simulation. Different receiver configurations and test scenarios have been used and the IADIRA concept validated. Tests conducted with this hybrid receiver have shown the following main benefits: - Carrier and code noise â€“ noise reduction when compared to the unaided solution; - Position and velocity accuracy â€“ accuracy increase when compared to the unaided solution; - Minimum required C/N0 for successful tracking â€“ possible operation under lower C/N0 when compared to the unaided solution; - Reacquisition time â€“ faster or even instantaneous reacquisition of signal lock when compared to the unaided solution; - Overall system robustness. IADIRA has demonstrated the potential of such technology by showing promising results. Envisaged applications should typically have stringent accuracy and integrity requirements and operate in high dynamics such as trains and aircraft.