Integration of GPS with Other Sensors and Network Assistance - Understanding GPS: Principles and Applications - page 531

Measurements in high-rise buildings included a reference receiver on the roof. The
resulting fade data was a differential measurement from the two receivers. The
preprocessed data output that is of interest here is fade magnitude versus time.
Traces were typically 4 to 8 minutes long and can be interpreted as signal attenua-
tion versus time relative to an unattenuated outdoor received signal. Plots of two
such traces are shown in Figure 9.37. Note the relatively high frequency variation in
the traces, corresponding to vehicle motion through its signal environment.
Two CDFs corresponding to the traces in Figure 9.37 are shown in Figure 9.38.
The mobile curve on the left has an extremely steep slope with very few fades
exceeding 8 dB. The in-vehicle curve on the right has a gentle slope characteristic of
a greater standard deviation of fade value.
An alternate way to look at the signal attenuation profile is to directly use GPS
signals detected by a high-sensitivity receiver. In most cases, 8-12 satellite signals
are available for measurement at any one time. In order to profile the signal attenua-
tion characteristics in a particular environment, 12 to 24 hours of data needs to be
mapped out to capture the effects of the GPS satellite constellation repeat time. The
GPS receiver-reported SNR for each satellite (typically in units of dB-Hz) is col-
lected and then translated to an equivalent signal power on the antenna in units of
dBm by making an estimate of the receiver noise figure and equivalent bandwidth. If
desired, one can map the attenuation profile of the environment as a function of sat-
ellite azimuth and elevation angle to provide even more detail on the environment
and identify directions of low and high attenuation. Signal power CDF curves are
produced and used to predict the availability of location fix within the environment
by determining the probability of at least four satellites offering signal power above
the receiver's raw detection threshold.
Figure 9.39 shows the signal power CDF curves for the eight strongest satellites
detected from a rooftop antenna over a 12-hour period. As can be seen, the 95%

Fade magnitude

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Time (secs)

Figure 9.37 Typical fade magnitude versus time for mobile (top line) and in-vehicle (bottom line).