We have all had our challenges using a ‘sat nav’ in our cars. Whether it is a factory-fitted device or an after-market unit, they have all had their limitations. Several examples spring to mind, but the most noticeable is that of limited or poor navigational accuracy when driving through a city district where there are many tall buildings. In the so called “urban canyon”, users typically experience poor and interrupted line of sight to the GNSS constellations - this being at a time when accurate and reliable navigation is most desired! Tall buildings can also reflect GNSS signals leading to further inaccuracies. Another common scenario is driving through a tunnel, resulting in the lost of navigation data and it usually taking a period of time to re-establish the correct position once out of the tunnel.
Recent innovations in global navigation satellite systems (GNSS) however, are now starting to appear across the automotive market. Initially only to be found in top-end automobiles, the concept known as automotive dead reckoning (ADR) combines GNSS data with positional information gathered directly from sensors fitted in the vehicle and a road wheel. In this way, using a simple dead reckoning technique, the current position can be calculated relative to the last GNSS fix. The degree of integration required to the vehicle’s own data network is high making it only possible for fitting at the time the vehicle is built.
Promising to deliver a performance better than using GNSS alone and close to ADR but using an approach that does not require any connection to the vehicle network is untethered dead reckoning (UDR). This works by combining inertial sensing data with GNSS data – see Figure 1.