Global Positioning System Reference
In-Depth Information
Satellite signal strength CDFs
site B
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I
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VIII
0.9
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155
150
145
140
135
130
125
SV signal power (dBm)
Figure 9.40
In-building (moderate indoor) CDFs for the strongest eight satellites over 12 hours.
of a three-story apartment building, in the center of the main living room away from
windows, with wood and brick construction. In this environment, the 95% proba-
bility for the fourth strongest satellite is at
152 dBm. Other notable items
include that the strongest signal is approximately 10 dB below that from the roof
antenna (50% point), and the spread from the strongest to the weakest is much
larger, on the order of 20 dB or more in this particular environment. The large
spread implies that the algorithm to detect indoor signals should be adaptive as the
integration dwell time to detect the stronger signals can be shorter than the dwell
time to detect the weaker signals. As will be shown later, common-mode error
parameters associated with each satellite signal (code phase error due to time error
and Doppler error due to oscillator error) can be exploited to reduce the total search
space after one or more satellites are detected; thus, the already detected stronger
signals can be used to further reduce the search space in order to detect the weaker
signals.
For every environment tested in this way, a unique set of CDF curves will be
produced. Thus, it is very difficult to project success or failure in a particular envi-
ronment based on Table 9.5 or Figures 9.37 through 9.40 without first collecting
data and generating CDF curves within the environment in question. The data in the
table and figures should only be used as an example of the specific location tested
and should not be used to project other environments, although the trends shown
are useful. Of course, it's impossible to collect data from every location that might
host an emergency cellular telephone call, and there is insufficient space to include
the complete set of measured CDF curves here. Thus, some form of statistical aver-
aging or site weighting is generally used to extend or predict the coverage from one
environment to another, the weight generally determined by estimates of emergency
calling patterns.
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