Global Positioning System Reference
In-Depth Information
1
0.5
0
−0.5
−1
−1.5
−1
−0.5
0
0.5
1
Time Delay [chip]
FIGURE 2.8. Coherent (full line) and noncoherent (dashed line) discriminator functions.
The input signal is split into two paths and correlated with two versions, an early
and a late, of a locally generated PRN code. The two versions are equally spaced,
typically
5 chip, about the prompt PRN code. This is done for both the in-
phase
I
branch and the quadrature
Q
branch. The estimation of the ACF in all six
signals
I
E
,
±
0
.
Q
P
,where
E
denotes early,
L
denotes late, and
P
denotes promptly, is based on a summation of the respective signals over an inter-
val of time
T
.Often
T
equals
I
L
,
I
P
,
Q
E
,
Q
L
,
1
50
s corresponding to the bit duration of the navi-
gation bits. In this time interval the data bit is constant. The GPS signal structure
implies that the duration of one navigation data bit divided by one chip duration
is an integer (20,460).
The ACF for the signals discussed above can be used to set up the correspond-
ing code discriminators. The early minus late discriminator is obtained by sub-
tracting a late copy of the correlation function from an early copy. The correlator
spacing
d
between the early and the late code is set to 0
.
5 chip; see Figure 2.7.
The code tracking loop is a device that estimates the signed time difference be-
tween the received and the reference code. The zero of this detector is the
pseu-
dorange
or code phase observation. Null tracking is enabled by subtracting the
late correlation from the early correlation. The resulting difference is called the
discriminator function
. For a coherent DLL we have
2
.
For a noncoherent DLL, the discriminator function is
D
R
t
2
−
R
t
d
d
(
)
=
+
−
D
t
R
2
t
2
−
R
2
t
2
;
d
d
(
t
)
=
+
−
see Figure 2.8.
2.6
Navigation Data
The navigation data are transmitted on the L1 frequency with the earlier men-
tioned bit rate of 50 bps. This section describes the structure and contents of the
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