GNSS USB Modem

5G Hub's GNSS USB modem is a GNSS with dead reckoning (DR) and real-time kinetics (RTK). It is a dual-band and multi-constellation GNSS module supporting multiple global positioning constellations, such as GPS, GLONASS, Galileo, BDS, and QZSS. It includes an integrated 6-axis inertial measurement unit (IMU). It supports sophisticated dead-reckoning algorithms, fusing the IMU data with the GNSS data to provide a continuous tracking solution in GNSS-impaired environments. It also includes an RTK position engine to provide centimeter-level positioning accuracy.

The GNSS USB supports two DR modes: automotive dead reckoning (ADR) and untethered dead reckoning (UDR). In ADR mode, the module relies on speed data from the vehicle and the onboard 6-axis sensor for enhanced accuracy in environments with no GNSS coverage. The UDR mode does not require speed data. The firmware automatically switches to UDR mode if no speed data is injected upon module power-up. In ADR mode, it obtains vehicle speed data through wheel ticks or direct vehicle speed data output (m/s). There are two wheel-tick injection methods: injection through the WHEELTICK pin, with a maximum distance increment of 0.05 m per pulse, or cumulative wheel-tick injection through the UART interface, with a minimum injection frequency of 10 Hz and a maximum distance increment of 0.05 m per pulse. The direct vehicle speed output can only be injected through the UART interface, with a minimum injection frequency of 20 Hz and a maximum error of 0.1 m/s between the injected and actual speeds.

The GNSS USB modem supports the DR technology. By combining satellite navigation data with wheel speed, gyroscope, and accelerometer data, the module obtains continuous and high-accuracy positioning in weak signal environments such as tunnels and urban canyons when the vehicle state (e.g., speed, forward direction, or vertical displacement) changes, or even when the satellite signal is partially or completely blocked.

The GNSS DR algorithm uses GNSS and an internal navigation system (INS) in a fused solution to provide continuous high-accuracy positioning. If the GNSS visibility drops due to obstructions, the INS will provide the information until the satellite visibility improves. This technology allows the device to get full coverage positioning or navigation even in parking garages, tunnels, and urban canyons.

The diagram below shows how GNSS with DR works when a car is driving under an elevated road. The GNSS signal is cut off because of the elevated road. The car position is accurate when using the GNSS with DR, while it is off when the GNSS is used without DR.

The GNSS RTK algorithm is a differential GNSS technique that uses real-time corrections from a nearby base station or reference network to achieve centimeter-level accuracy. It corrects for satellite orbit and clock errors, atmospheric delays, and multipath effects. RTK depends on the spatial correlation of these errors between the base station and the rover receiver. RTK typically achieves high centimeter-level accuracy.

The diagram below shows how GNSS with RTK works when a car is driving in an open-sky environment. The car position is highly accurate when using the GNSS with RTK, while it is less accurate when the GNSS is used without RTK.