# OBU for Accurate Navigation through Sensor Fusion in the Framework of the EMERGE Project
## 3 Sensor Fusion: Loosely Coupled Algorithm Implementation
### The Initialisation Loop
The main elements to be stored for future analysis are GNSS information, inertial measurements, timing information, sensor fusion process results, and Kalman filter status information. This experiment has been conducted in the following sequence. * _Startup phase_ 1. Power-up of the whole system: vehicle and OBU 2. Initialisation of the acquisition SW 1. Initialise libraries, dependencies, SW modules and objects: MQTT client/server, parser/deparser UBX, XDA, PP credentials and files 2. Sensor (interface) detection 3. Calibrate and/or configure sensors 4. Prepare for data handling (event-based), KF and synchronisation structures 5. Change operation mode (in sensors) to measurement mode
* _Processing and Measurement phase_ 3. Handle incoming data: queuing, synchronisation, data availability and status of the fusion algorithm 4. Write the information (synchronously) to the logger files 5. Verify the conditions for the end of the experiment
* _Exit and Close phase_ 6. Save the final status of the system elements: GNSS, Inertial Unit and SF data 7. Close open devices and files 8. Clear and release all the structures used 9. Print the successful experiment message

The On-road experiment has been conducted in the parking area of the Tecnopolo of Abruzzo, city of L'Aquila, Italy, and lasted 240 s (4 min). During the test, a trajectory along the marked tracks has been followed, with acceleration intervals and stops, to simulate a typical urban scenario. For this experiment, the inertial sensor samples the data with a frequency of 100 Hz, and the GNSS receives one updated PVT message every second. Figure 7: Coordinate system, frame and hardware for final Integration Test. ## 5 Results

The most relevant results in evaluating the sensors lie in the accuracy of the satellite navigation system and the improvement obtained using an external GNSS augmentation service. In addition, considering the automotive application, we provide some statistics on the power consumption of the OBU. This section concludes by showing the final results of the sensor fusion application. ### GNSS Performance

The first GNSS key issue analysed was the TTFF measured starting from a \"coldstart\" signal to the receiver until it reaches a 3D Fix. Experimental results show a mean value slightly below 30 s. This result has been verified and is consistent with the data provided by the manufacturer. However, even if this result is satisfactory, it is important to note that a lower TTFF may be required in automotive applications. This is why the OBU foresees the storage of the last position/status of the vehicle after it has been powered off, functionalities supported thanks to the internally incorporated Power Management and UPS system. The backup battery connected to the Protected 18650 Li-Ion Separable Battery Holder has a capacity of 3000 mAh (at 3.7 V). Assuming OBU consumption in the high operating mode (average power of 5 W and average current of 1 A), the system autonomy, regarding energy, is about 2 h.

Table 1 compiles the results obtained in the evaluation process by accumulating data for more than 5000 s (approximately one and a half hours) at different fixed known positions and times along a given day. The first two columns of the table match the key performance indicators (KPI) with the corresponding measured parameters. Two measurement sets have been collected: one using two GNSS constellations (GPS + GALILEO) without GNSS corrections and the other, including the PointPerfect Augmentation service and using all four GNSS constellations. Measurements collected at different times during multiple trials, are summarized statistically through their mean and variance. Accuracy measures are improved by using the augmentation service, as expected: performance obtained using the GALILEO and GPS constellations simultaneously, but without atmospheric and clock correction, are significantly lower than the ones obtained when the full potential of the GNSS receiver is used.