5 INDOOR APPLICATIONS
The demand for personal navigation and location-based services is driving research and
development of enhanced civilian GPS receivers for use in increasingly difficult
operational environments. Receivers with longer signal integration times and external
means of acquiring the navigation message are lowering the acquisition and tracking
power thresholds to levels at which even indoor operation is possible. The enhanced
availability of measurements in environments where signals are highly attenuated
benefits solution availability in urban canyons and under heavy foliage.
However, interference in such environments can introduce large measurement errors.
GPS signal deterioration occurs by signal masking caused by natural (e.g. foliage) and
man-made (e.g. buildings) obstructions, interference due to reflected signals, signal selfinterference,
jamming, antenna effects, and receiver implementation losses. The impact
of any one of these can result in partial to total loss of signal tracking and/or tracking
errors, depending on the severity of the effect and the receiver tracking characteristics.
Tracking errors, especially if undetected by the receiver firmware, can result in large
position errors. Partial loss of tracking results in geometry degradation, which in turn
affects position accuracy