# OBU for Accurate Navigation through Sensor Fusion in the Framework of the EMERGE Project
## 2 EMERGE Onboard System Architecture
### EMERGE Navigation System Implementation
#### 2.1.1 Sensors and Services  
The GNSS receiver uses the u-blox ZED-F9P positioning module [21] that provides multi-band GNSS for high-volume industrial applications. The ZED-F9P integrates multi-band RTK and PPP-RTK (Precise Point Positioning--Real Time Kinematic) for centimetre accuracy. The module is ideal for automotive and UAV applications due to its low power consumption, accuracy and integration capability. Another strength of this receiver is the ability to handle multiple channels dedicated to different constellations and frequency bands. In addition, the ZED-F9P module features native compatibility with the PointPerfect (PP) augmentation service [22].  
PointPerfect is a GNSS augmentation service that supports PPP-RTK for position estimation, with 99.9% availability using the Internet or satellite (L-band) connection. The PP service has reported high availability and high levels of accuracy, making this product optimal for critical applications such as automotive. Specifically, in our configuration, we receive corrections over an IP connection. The GNSS corrections are received in SPARTN 2.0 format transparently and cost-effectively via the MQTT broker offered by the u-blox Thingsream platform [23]. This method of receiving GNSS corrections ensures efficient use of bandwidth, improves GNSS receiver initialisation times and reduces the power  
Figure 1: EMERGE Onboard System Architecture.  
consumption of the navigation system. It is important to note that the specific PointPerfect IP service (Unlimited access to IP data stream) costs 19.00 USD per month at the moment of this research.  
The inertial sensor component used is the Xsens-MTi-630 AHRS (Attitude and Heading Reference System) [24], which contains a 3-axis gyroscope, a 3-axis accelerometer, a 3-axis barometer magnetometer, a high-precision oscillator and a low-power microcontroller unit (MCU). The MCU applies calibration models (unique to each sensor and including orientation, gain and polarisation offsets, plus more advanced relationships such as non-linear temperature effects and other higher order terms) and runs Xsens' optimised strapdown algorithm, which performs high-speed dead-reckoning calculations up to 2 kHz, enabling accurate capture of high-frequency motions and compensation for coning and sculling. The MTi-600's output data is easily configured and customised according to the application's requirements and can be set to use one of several inertial navigation filter profiles [25]. In particular, the MTi-630 AHRS device enhances the process of determining referenced magnetic north and 3D acceleration, velocity and heading calibration data. The project requirements motivating the use of this sensor are low in-run bias stability (accelerometer: 10 (x,y) and 15 (z) [micro-g]; gyroscope: 8 [deg/h]), low noise density (accelerometers: 60 [micro-g/sqrt(Hz)]) and the ability to output data at a sufficiently high frequency for the sensor fusion process. Successive models of the MTi-600 family (i.e., MTi-670/80) support the INS/GNSS integration process internally; in our case, they do not; therefore, the INS/GNSS synchronisation and integration process is performed by SW.