Global Positioning System Reference
In-Depth Information
Table 9.9
Maximum Search Times
Signal
dBm
PDI
(seconds)
T
dwell
per bin
(seconds)
N
cp
per SV
(half-chips)
N
dopp
per SV
T
search
12
(seconds)
T
search
32K
(seconds)
N
c
for 8 SVs
-130
0.001
0.002
~300
1
1,636
0.27
0.002
-145
0.006
0.050
~300
1
1,636
6.8
0.05
-150
0.012
1.0
~300
2
1,841
153
1
-155
0.020
5.0
~300
4
3,682
1,535
5
Time uncertainty = 100
s; frequency uncertainty = 0.05 ppm; 8 satellites; position uncertainty = 30 km; taking advantage of
reduced code-phase and Doppler search range after finding a first satellite; number of code phase delays and Doppler bins is
reduced after finding first satellite and reflected in total
N
c
.
µ
(
)
T
=
T
×
NN
(9.49)
search
dwell
c
corr
For a particular scenario in which the time uncertainty is 1 ms or more, full code
phase search of 2,046 half-chips is required to find the first satellite. Given a condi-
tion of a 0.5-ppm oscillator, the number of Doppler bins is dominated by the oscilla-
tor uncertainty; thus, column
N
dopp
in Table 9.7 indicates the number of Doppler
bins per satellite and
N
cp
the number of code phase search bins. The initial conditions
of time, position, and frequency uncertainty are shown. Column
N
c
indicates the
number of Doppler-codephase search bins for an eight-satellite case in which the
receiver does not take advantage of the code phase learned from a first detected sat-
ellite to reduce the code phase search range on the remaining. Finally, two condi-
tions are highlighted—the total search time
T
search
using (9.49) for two cases: that of a
typical automotive-grade receiver containing 12 searchers, and a modern high-per-
formance flash correlator receiver that can search up to 32,000 bins simultaneously.
As described earlier, when a first satellite is detected, it's possible to substan-
tially reduce the code phase and Doppler search range for the remaining
N
sv
- 1 satel-
lites. Table 9.8 illustrates the gain achieved as reflected in reduced
N
c
and
T
search
cases
by using the full code phase and Doppler search space to find the first satellite and
reducing the remaining seven satellite uncertainties to approximately 300 half-chips
in code phase and 100 Hz in Doppler.
Finally, Table 9.9 illustrates further reductions in the search space and search
time by changing the reference oscillator to 0.05 ppm (for example, taking advan-
tage of handset AFC tuning) and reducing the time uncertainty to 100
µ
s (such as
taking advantage of precise time transfer).
9.4.4 GPS Receiver Integration in Cellular Phones—Assistance Data from
Handsets
As shown in (9.42), much of the total code-Doppler uncertainty space for
N
satel-
lites is represented by common-mode error terms of time error and oscillator fre-
quency error. Typical low-cost reference oscillators are in the 0.5- to 1-ppm stability
range, and at this level, the oscillator frequency uncertainty is by far the largest ele-
ment of the total Doppler uncertainty search space. Likewise, a 1-ms or more time
uncertainty is common mode across all satellites and forces full code phase (2,046
half-chips) scan for each satellite, as time error is the largest of the possible contribu-
Search WWH ::
Custom Search