Dead reckoning fills-in GPS navigation gap
To address this growing problem, u-blox, for example has integrated dead reckoning functionality into GPS receiver chip technology. Dead reckoning is actually a centuries old concept originally used by sailors and later aviators to extrapolate their position based on how far and in what direction they have travelled from a last known location, typically the last harbor.
Based on proprietary Sensor Fusion Dead Reckoning technology, the u-blox GPS receiver chip accomplishes the same task when travelling through regions of poor GPS reception. Based on the last known position, vehicle sensors feed information to the receiver indicating how far and in what direction the vehicle has travelled. The chip processes the sensor data and blends it with GPS positional readings. In this way, a better approximation of where the vehicle actually is can be extrapolated, regardless of GPS satellite visibility.
Dead reckoning has one drawback: The displacement and heading errors accumulate over time. The errors depend on the accuracy of the sensors, data quantization errors, and time granularity. It is the job of dead reckoning hardware and software to minimize the errors for as long as possible to provide accurate location information during short signal interruptions as well as long drives through tunnels.
The figure below illustrates accumulated dead-reckoning error.
The following list shows typical sensor and information sources used by dead reckoning technology to calculate a position:
- Odometer pulses (absolute distance traveled, this is most typical)
- Distance information from wheel ticks
- Digital speed information (distance is reconstructed from a single integration),
- Linear accelerometers (distance reconstructed from double-integrating acceleration)
- Radar, optical, and acoustic sensors.
Sensing distance traveled
There are a variety of sensor techniques for detecting distance travelled. Typically a direct connection to the vehicle’s odometer is enough. If this is not available, for example for after-market installations, wheel sensors can provide the raw information for distance travelled. The variable reluctance sensor and Hall Effect position sensor are two common types of electromagnetic sensors for this purpose.
- Turn rate sensor (gyroscopes, most typical)
- Linear accelerometers
- Steering linkage angular sensor
- Differential wheel speed information (between left and right wheels)
- Magnetic compass
Besides distance, direction or turn-rate information (measured, for example, in degrees per second) is also needed to extrapolate the travelled route. The easiest approach is to use a small turn-rate sensor, also called a gyroscope. Several types of gyroscopes exist: mechanical (a rotating mass suspended in gimbals), optical, and micro-electromechanical system (MEMS) vibrating structures.
The first two types of gyros provide excellent accuracy but are large and expensive. MEMS gyroscopes excel in their small size, good performance, and user-friendliness and are relatively inexpensive. They are also available as surface-mount devices easily installed on a printed circuit board.
In addition, a technique known as map matching can be used. Based on an actual map, application software knows to always report a position located on an actual road, even if the extrapolation calculated by the GPS receiver is slightly off due to accumulated position and heading error.
Ultimately, an approach of using the dead reckoning solution based on both GPS and sensor measurements simultaneously together with map matching delivers the best result in city environments where a wide range of signal conditions can be expected: Partial, reflected, and blocked GPS signals.