Global Positioning System Reference
In-Depth Information
the total Doppler error from all sources must not exceed the Doppler bin width, and
this width is determined by the dwell time for each coherent set of code correla-
tions). Unless data wipeoff information is provided (and this is seldom used), this
limits the coherent dwell times to 20 ms or less depending on the sophistication of
the aiding. The coherent results are summed together noncoherently for as long as
possible—typically less than 600 ms. The greatest limitation to the amount of
noncoherent integration that can take place in the rover receiver is the stability of the
reference oscillator, which is usually provided by the cellular phone. These reference
oscillators are not normally specified with the stringent stability requirements of
GPS receivers, but there is more tolerance in this case because there is no attempt to
achieve phase lock with the satellite signals.
The methods for obtaining pseudorange measurements in indoor applications
are similar to the techniques for acquisition described in Section 5.8. A major differ-
ence is that, as described earlier, the communications aiding typically results in a
very narrow window in the two-dimensional codephase/Doppler parameter space.
This restricted space allows much longer dwells than would be practical for an
unaided receiver. As an example of the sensitivities that can be achieved by various
levels of coherent and further noncoherent integration, Figure 5.45 presents the
probabilities of detecting a GPS signal as a function of (
C
/
N
0
)
dB
for various values of
the coherent integration period,
T
, and the number,
K
, of successive I and Q outputs
10
0
K=50
T=20ms
K=20
T=20ms
K=20
T=1ms
K=1
T=1ms
10
−
1
K=1
T=20ms
K=50
T=1ms
K = 100
T=1ms
10
−
2
10
−
3
10
−
4
15
20
25
30
35
40
45
50
(
C/N
)
(dB-Hz)
0dB
Figure 5.45
Probability of detection as a function of (
C
/
N
0
)
dB
using
K
noncoherent integrations at
T
ms dwells for 10
−
4
probability of false alarm.
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