Global Positioning System Reference
In-Depth Information
Figure 6.3
Simple common antennae configurations showing directionality.
To measure distances, the temporal aspects of the signal must be exploited.
There are two methods used. First, and more commonly, the transit or flight time
of a signal is measured. Many systems use narrow pulses or rely on the timing of
various epochs in digital modulation (i.e., the start and end of a pattern of binary
changes). From (6.1) it can be seen that in one microsecond the signal will travel
300m. The accuracy of the positioning system is then down to how accurately the
signal can be measured and how good an S/N ratio is present. Noise can be
thought of as power or voltage-level fluctuations but when these are passed
through a threshold detector in the receiver that is a part of the demodulation
process, they are translated into time jitters (see Figure 6.4). Further inaccuracy
occurs when the timing is sampled by the receiver's digital processor since there
is always a minimum sampling period usually determined by the clocking and
logic speed of the processor's digital electronics. A fast processor may be able to
sample to within a few nanoseconds, but this translates to 0.33m per nanosecond.
