- loss of service;
- loss of accuracy;
- inability to estimate any position boundary (e.g., integrity information, also called Quality of Service, QoS, in mass market terminology).
2. Materials and Methods
- Alert Limit (AL): The alert limit for a given parameter measurement is the error tolerance not to be exceeded without issuing an alert. The horizontal alert limit is the maximum allowable horizontal positioning error beyond which the system should be declared unavailable for the intended application.
- Time to Alert (TA): The maximum allowable time elapsed from the onset of the navigation system being out of tolerance until the equipment enunciates the alert.
- Integrity Risk (IR): The probability that, at any moment, the position error exceeds the Alert Limit.
- Protection Level (PL): Statistical bound error computed so as to guarantee that the probability of the absolute position error exceeding said number is smaller than or equal to the target integrity risk. The horizontal protection level provides a bound on the horizontal positioning error with a probability derived from the integrity requirement.
- The GNSS pseudoranges and pseudorange rates are computed with the software receiver GNSS Environment Monitoring Station (GEMS)  using the Multi-Correlator (MC) tracking loop with the multipath detection activated.
- These measurements are fed into the EKF prediction step and the associated Receiver Autonomous Integrity Monitoring (RAIM) can thus exclude faulty satellites.
- For each satellite left, the elevation and the azimuth are computed and projected onto the camera reference frame using the vehicle heading computed from the reference trajectory. In an operational mode, the heading would be computed from the GNSS velocity. For our experiments, it is computed from the reference trajectory, so there is no error on the projection of the satellites on the camera reference frame and then the best possible contribution of the camera aiding can be assessed.
- A three-levels discretization of LoS probability map has been considered:
- LoS level: When the probability to be in LoS (pLoS) is between 0.75 and 1, the received signal is considered as the LoS, and the measurements are kept unchanged.
- Doubt level: When the pLoS is between 0.25 and 0.75 (most likely related to building edges or vegetation), there is doubt on the quality of the received signal, so an overweighting is applied on the measurements as:
- NLoS level: Finally, when pLoS is between 0 and 0.25, the received signal is considered as NLoS and the measurements are excluded if there are enough LoS space vehicles; otherwise, they are kept and overweighted as above.
- The remaining GNSS data go through the Kalman filter correction step, which provides the final PVT solution.
- The Horizontal Protection Level (HPL) is computed from the output of the Kalman filter.
- The Horizontal Positioning Error (HPE) and Miss-Integrity (MI) are computed from the output of the Kalman filter and the reference trajectory.
2.1. Data Acquisition
2.2. Image Processing Module
2.2.1. Image Segmentation
2.2.2. Image Processing Module Block Diagram
2.3. GNSS Signal Processing
3. Results and Discussion
3.1. Experimental Campaign
- Fisheye camera;
- GNSS Data Acquisition System (GDAS-2) used to capture and record GNSS raw signals;
- The live-sky testing platform, including the car carrying the equipment, was provided by GNSS Usage Innovation and Development of Excellence (GUIDE), a Toulousian test laboratory. It embeds a GBOX that provides the reference trajectory based on high-grade INS and GNSS differential corrections from the International GNSS Service (IGS).
3.2. Global Results
3.3.1. Focus on an Area Where the Camera Aiding Algorithm Improves the PVT
3.3.2. Statistic on Pseudorange Residuals
- Propose a criterion or a set of criteria in order to decide which data to exclude or to overweight in case the camera flags too many data as NLoS;
- Detect small and close “obstacles” (such as lamp poles) in order not to mark the related area as NLoS;
- Improve classification of vegetation;
- Feedback loop and additional criteria for adaptively tuning the image processing parameters regarding the threshold used on the probability map.
- Effect of small obstacles such as lamp poles that cause a NLoS flag by the camera while not significantly affecting the quality of the data;
- Difficult classification of the vegetation that can be considered a wrong source of NLoS, leading to the exclusion of too many measurements.
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
Appendix A. GNSS Basics
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- to realize a first estimation of their frequency and delay characteristics.
- to rest locked on the signals received from the acquisition sub-system;
- to decode the navigation message;
- to estimate the distance traveled by each signal to arrive at the receiver.
- to calculate the position of the receiver;
- to determine the bias of the receiver’s internal clock versus the reference system.
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