Global Positioning System Reference
In-Depth Information
Table 9.6 presents a very coarse estimate for typical relative frequencies of
E-911 calls based in part on speculation. Unfortunately, a more detailed mapping of
call probabilities to reception environment is not available, as such data is not nor-
mally logged for emergency calls. As evident from the table, and fortunately for GPS
usage, calls generally occur less frequently as signal reception becomes more difficult.
For example, in the heavy indoor environment, in the basement of a high-rise build-
ing, where signal attenuations at L-band can easily reach 40 dB (note its median
attenuation of 30 dB in Table 9.5), acquisition of the requisite signals required for a
fix is very difficult. The results of Table 9.6 can be used in the following way to test
adherence to the E-911 requirement: perform N fix attempts in each environment,
and note the number whose errors are less than 50m ( m ) and 150m ( n ). Composite
probabilities are then computed using:
(
)
( )
1
P
=
p m
N
(9.38)
c
i
i
(
)
(
)
2
P
=
p n
N
(9.39)
c
i
i
where the summations are indexed over the test environments, with p i representing
the frequency of occurrence in the environment, and m i and n i the number of suc-
cesses in each environment. The test results would be considered a success for the
candidate GPS receiver if the composite probabilities were greater than the specified
values (i.e., 0.67 for P c (1) , and 0.95 for P c (2) ).
However, the approach described earlier, resulting in the use of (9.38) and
(9.39), is only one way of interpreting the FCC mandate and demonstrating success
or failure. Alternate approaches for testing adherence have been proposed (e.g., the
so-called dart board test, where an area of cell phone coverage is given and test sites
are selected randomly within that area by assuming a uniform distribution of emer-
gency calls within that area).
TTFFs in excess of reasonable response times for an emergency call (e.g., TTFFs
of several minutes time) are obviously unacceptable. The FCC guidelines [64] discuss
a 30-second TTFF that is location method neutral, but leaves just enough room for
autonomous GPS given its 30-second ephemeris data demodulation time. A consor-
tium of CDMA cell phone vendors and suppliers, together with representatives from
cell phone carriers, have defined a set of minimum performance tests that must be
met [73]. These tests define specific signal simulation scenarios and requirements for
both position accuracy and TTFF. For example, the sensitivity test of the IS-916 spec-
ification requires that the GPS function embedded in a CDMA telephone acquire
Table 9.6 E-911 Call Distribution by
Environment
Call
Probability
Outdoors or in-vehicle 0.70
Light indoor or in-vehicle 0.15
Moderate indoor
Environment
0.10
Heavy indoor
0.05
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