The use of Multiple Sensors in Improving Satellite Navigation Performance

Today most commercial receivers are GPS L1 C/A code only. However, for many applications, the single frequency GPS performance is inadequate and many technology developers are turning to other sensors to compliment GPS.

Today we have many forms and functions, depending on the application and hence needs. Integrated in-vehicle navigation systems often compliment the GPS position with dead reckoning navigation information from wheel rotation sensors. Often these are the same sensors used for anti-lock braking systems. The vehicle’s navigation system uses the GPS and wheel sensor information to compute the vehicle position. Additional sensors can also be used, for example, accelerometers and digital compass.

The beauty of this approach is that the GPS and additional sensors are complimentary to each other. When GPS performance is poor, such as in a tunnel or urban canyon, the additional sensors can take over to maintain a useful position solution. The additional sensors are open-loop systems, which become increasingly inaccurate over time, and are well suited to short GPS outages such as these.

Conversely in open sky situations, such as long highway drives, GPS generally works perfectly and there is no need to defer to the additional sensors. It’s not only cars that have additional sensors; aircraft systems typically use much more accurate inertial sensors than vehicles. Accuracy of inertial sensors is measured in cumulative drift in degrees per hour. Aircraft system sensors are typically accurate to a few degrees per hour. This can be accurate enough, in fact, to reverse the role of GPS and additional sensors so that the sensors are the main navigation mechanism and GPS is used only as a periodic correction mechanism to the open loop inertial system. Military systems are a degree more sophisticated than this, using so called tightly coupled technology where the GPS and inertial sensors effectively learn from each other to optimise the position solution.

At the opposite end of the GNSS applications spectrum, mobile phone handsets are increasingly packed full of sensors of one type or other. Mobile phone video capture has an anti-shake sensor so that your videos are smoother than your shaky hand. Your phone can flip the display from portrait to landscape when you turn it. There is some evidence of work being undertaken to use these sensors as an aid to navigation. Even in early 2008, at the Mobile World Congress in Barcelona, mobile phones were on display where the built-in digital compass could orient the user to assist with navigation. The logic is that it’s difficult to read a map on a small phone display, particularly when you have just emerged from an underground subway station, for example. The digital compass orientates the phone, the GPS provides the position and a picture of the intersection you are at is downloaded automatically over the phone’s internet connection. By the time you have programmed your destination, the navigation system in your phone is showing you a picture of the corner of the intersection you need to walk towards, perhaps annotated with an arrow or instruction or street name.

All this would be even easier, of course, if GNSS worked everywhere, even indoors or underground. This is a tricky problem that is yet to be adequately solved by an experimental, let alone commercially viable, solution.


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