# OBU for Accurate Navigation through Sensor Fusion in the Framework of the EMERGE Project
## 3 Sensor Fusion: Loosely Coupled Algorithm Implementation
### The Initialisation Loop
provides the relevant atmospheric and clock corrections. The sensor verification process allows the performance to be characterised locally through appropriate configuration. For example, it can evaluate satellite availability and coverage (DOP), the average Time To First Fix (TTFF), and the most effective combinations of constellations and frequencies in the case of GNSS. ### Xsens MTi630 AHR

The tests conducted on the inertial navigation component focused on the calibration process and the correct functionality of the sensor. For this, we used the Xsens Development Kit MTi630 connected to a PC and a SW dedicated to the configuration and storage of data measured by the sensor. The experiment configuration just involves connecting the PC and the MTi-630-DK using the corresponding USB interface. The acquisition SW is implemented in Python and uses the manufacturer open-source API (Xsens Device API) as a library. To summarise the flow followed by the acquisition SW: a first \"startup\" phase dedicated to the creation of the XDA library objects, scanning of the ports/interfaces and configuration of the device; a second \"processing and storage\" step dedicated to the manipulation and storage of the inertial data; and finally the step of closing and disconnection of the objects, ports and files used. In addition, a summary of the IMU configuration profile, including motion filters, calibration information and inertial system data, is stored. ### GNSS and Augmentation Service

The whole satellite navigation system testing process is conducted in two steps: one dedicated only to verify the correct functionality of the GNSS receiver, and the other one to evaluate the impact of using the PointPerfect service. For the first step, the HW involved includes the GNSS antenna, the GNSS receiver, and a PC that connects to the receiver via the USB port. In the second step, an Internet connection is needed to access the PointPerfect service. This test is performed in a controlled outdoor environment with the antenna placed on a vehicle at rest. The tests described in this section focus on the evaluation of the performance of the satellite navigation system, considering the most important parameters in automotive positioning: TTFF, Accuracy, Integrity and satellite availability metrics. The GNSS data acquisition process (with and without corrections) is carried out using an acquisition SW that has the following flow: _startup_ (configuration of interfaces, objects, devices and services); _processing_ (main cycle in charge of decoding the UBX data coming from the GNSS sensor, the corrections coming from the PointPerfect service, and the file storage process); and finally the _close_ and _disconnection_ phase of the elements used. In the case of the TTFF tests, repetitions are performed from a \"_coldstart_\" and, afterwards, the mean is applied to the measured values. ### Sensor Integration: On-Road Navigation Test

The integration or fusion process is the core of the multi-sensor approach to navigation. This section describes the verification process applied to the algorithm described in Section 3. Having characterised the sensors/services in terms of noise, accuracy and response times, the improvements introduced by the integration process can be quantified. In this case we use a Raspberry Pi 4 Model B (On Board Controller), the satellite navigation system described in the previous section (Antenna, Receiver and GNSS Augmentation Service), the MTi630-DK, everything installed within a compact city car. Figure 7 specifies the coordinate system and reference frame used. In this case, we match the inertial reference frame (i-frame) with the body/vehicle reference frame (b-frame). The conversion process to a unique navigation system (n-frame) uses the corresponding transformations (i.e., Rotation Matrix). Likewise, the reference system used by the ground-referenced GNSS (e-frame) shall be in a compatible reference system to facilitate the fusion process. The navigation information is captured during the experiment using a data-logger script.