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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 months column, we look at how GPS can bined with other sensors to provide continuous navigation in even the most-challenging environments.--R.B.L. 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 es possible with high position bined 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. mercial 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 mercial, and government applications requiring location awareness.

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