"CAN YOU HEAR ME NOW?" This familiar question is asked countless times each day by mobile phone users attempting to improve their signal. Terrain, buildings, and foliage can block or seriously impede the propagation of cell-phone signals. Users of GPS receivers suffer the same problems. While there have been some advances in improving the sensitivity of GPS receivers and developing techniques such as assisted GPS that permit a GPS receiver to use attenuated signals, the antenna of a conventional receiver must have a direct line of sight to the GPS satellites. In urban canyons, it may not be able to "see" a sufficient number of satellites with good geometry to determine a three-dimensional position fix. And in tunnels or in parking garages, the receiver will see no satellites at all. Consequently, continuous navigation in many cities is impossible for conventional GPS-only navigation systems. In this month's column, we look at how GPS can be combined with other sensors to provide continuous navigation in even the most-challenging environments.--R.B.L.
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Densely populated cities pose a problem for reliable navigation and vehicle tracking solutions using GPS. Coverage must include every street, alley, tunnel, underpass, multi-level road, indoor and underground parking facility. But narrow streets between high-rise buildings blocking GPS signal paths provide limited visibility to satellites and cause multipath effects, resulting in degraded navigation accuracy and reliability.
Reliable in-car navigation only becomes possible with high position accuracy combined with an up-to-date map database. Such capability is particularly important for emergency vehicles needing to reach the location of an incident quickly.
In addition, large cities often suffer from high crime rates that make asset tracking a mission-critical application requiring the same high-level positioning availability.
Furthermore, commercial transportation services such as taxi operators, freight and logistics, express delivery and security transports benefit from 100-percent position coverage for optimal vehicle dispatch, and the highest level of security is provided for drivers, the transported goods, and the fleet.
One-hundred-percent positioning availability is a must for implementing flexible and driver-convenient road pricing systems and automatic billing of car park usage.
Finally, envisioned advanced traffic safety projects aiming to reduce the number of traffic accident casualties require a high level of GPS coverage.
GPS receivers with high sensitivity and good multipath mitigation concepts alone cannot fulfill these demanding requirements, particularly when fewer than four satellites are visible. Additional means are necessary to achieve this, such as sensor-based dead reckoning with additional sensors detecting traveled distance and turn rate to supplement GPS and help provide uninterrupted positioning coverage. This article focuses on a sensor-based dead-reckoning solution with excellent potential for widespread use.
Challenging Environments
GPS has gained widespread acceptance for personal, commercial, and government applications requiring location awareness.
More than 24 satellites orbit the earth in six orbital planes to provide visibility to four or more satellites from any location on Earth. Four satellites are required to compute three-dimensional position fixes. In densely populated metropolitan areas, it is not always possible for a receiver to "see" the minimum number of four satellites. Nevertheless, such metropolitan areas are precisely where the need for location awareness is highest.
Today's GPS receivers may achieve fairly high coverage in densely populated cities, but in numerous cases, the visibility to the satellites is hindered so continuous navigation or tracking is not guaranteed. For example, tunnels, underpasses, indoor parking facilities, roofed logistics centers, covered driveways, dense foliage, multi-level roads, and narrow alleyways all block GPS signals to some extent. Limited sky views challenge accurate position calculation.
What can be done to provide navigation under such conditions? The following helpful approaches are available:
* GPS navigation with fewer than four satellites,
* SBAS making more satellites available for ranging, and
* Sensor-based dead reckoning.
Navigation with < 4 Satellites. As mentioned, at least four satellites are required for three-dimensional navigation.
If only three satellites are visible, then one dimension, typically the altitude, must be held constant to provide position fixes.
If the number of satellites drops to two, then another assumption must be made to continue navigation (for example, holding the clock offset constant or assuming a constant driving direction). If the clock offset is held constant, then the effective clock offset between satellites and GPS receiver will diverge and position accuracy deteriorates quickly. On the other hand, if the direction of travel is held constant, the GPS receiver can at least compute the position along the assumed trajectory. In reality, the vehicle will likely not travel along a perfectly straight path and navigation fails after the first turn.
For single-satellite navigation, three assumptions must be made concerning altitude, trajectory, and clock offset, but the navigation results are, at best, educated guesses.
To conclude, degraded navigation with fewer than four satellites is not a viable solution for tunnels, underpasses, roofed driveways and other areas with no view of the sky.
SBAS. Today's GPS receivers support satellite-based augmentation systems (SBAS) such as the Wide Area Augmentation System (WAAS) in North America and the European Geostationary Navigation Overlay Service (EGNOS) in Europe. These geostationary satellite systems provide GPS-like ranging signals, correction, and integrity information.
Additional ranging signals and correction of atmospheric influences provide some improvement in increased coverage and accuracy when navigating through cities, but SBAS provides no help when traveling through tunnels or in parking garages. In northern regions (Europe, the U.S., and Canada), the low elevation angle of the SBAS satellites typically precludes their use for improving navigation performance in urban canyons.
Sensor-Based Dead Reckoning. Given the limitations of navigating with fewer than four satellites and the limited usefulness of SBAS, is there any other option for providing continuous navigation in built-up areas? GPS in combination with a technique used by navigators for more than 500 years provides the solution.
By the end of the 15th century, prior to the full development of celestial navigation, deduced reckoning, also known as dead reckoning, was used in Europe as the primary maritime navigation method. Christopher Columbus used dead reckoning for his voyages across the Atlantic Ocean.
The navigators started sailing from a known position like a harbor or coastal landmark. They measured heading (with a magnetic compass or by observing the sun), predicted the distance traveled over a one-day period by careful observation of the winds and their speed through the water, and calculated the new position in their logbooks or on maps. Later, celestial navigation using sextants and fairly accurate clocks enabled absolute positioning, but the sailors had to refer back to dead reckoning on days with poor weather conditions.
Dead reckoning is equally useful as a fallback solution when GPS signals are not receivable. Similar to navigation in the past, dead reckoning starts with a known geodetic position, [p.sub.n]. The next position, [p.sub.n+1], is computed by identifying the displacement (distance and direction) from the current position. External sensors provide the distance and direction information. The principle of dead reckoning is illustrated in Figure 1.
Dead reckoning has one major drawback: The displacement and heading errors accumulate over time. The error depends on the accuracy of the sensors, the data format (quantization errors), and time granularity.
Short time-measurement intervals in the double-digit milliseconds range improve accuracy. Accurate sensors to track distance and direction are necessary for providing good dead-reckoning navigation. Figure 2 illustrates dead-reckoning error.
The following list shows possible sensors and information sources for dead reckoning:
Distance Sensing:
* Odometer pulses (absolute distance traveled),
* Digital speed information (distance is reconstructed from a single integration),
* Linear accelerometers (distance reconstructed from double-integrating acceleration), and
* Radar, optical, and acoustic sensors.
Direction Sensing:
* Turn rate sensor (gyroscopes),
* Linear accelerometers,
* Steering linkage angular sensor,
* Differential speed information,
* Magnetic compass.
