Global Positioning System Reference
In-Depth Information
The stream of samples at the ADC output (i.e.
[ ]
I
y n
) is correlated with the local code and
with two carriers, one in phase and one in quadrature, respectively. At the end of each
integration period, the values of correlation are used to generate feedback control signals,
one for the DLL and one for the PLL.
Early minus Late
DLLs use additional replicas of the
local code, shifted of 0.5 chips earlier and later than the reference one, which is referred as
Prompt code
. The Early and Late correlations are combined to generate the DLL feedback on
the basis of a proper discrimination function. Such a feedback is filtered to smooth the noise
effect and is used to steer the code generator, that prepares the local code for the next loop
iteration. In such a way the DLL continues to track the correlation peak in the time domain.
The PLL works in a similar way. Generally, the in-phase and quadrature Prompt
correlations are passed to a
Costas-PLL
(that is not sensitive to navigation bits transitions)
(Kaplan & Hegarty, 2006; Misra & Enge, 2001) that generates the loop control signal. This is
filtered and applied to the local carrier generator, that prepares the local carrier for the next
iteration. This process repeats over time, making the receiver able to track the correlation
peak in frequency domain.
When both the DLL and PLL are locked, the incoming signal is despread and converted to
baseband. The navigation data bits appear at the output of the in-phase Prompt correlator
and can be decoded. In addition, with the DLL locked, the local and the incoming codes are
aligned. Referring to the local code, the receiver exactly knows when a new code period
starts and is able to recognize navigation data bits and boundaries of the navigation message.
The receivers stays synchronized to the tracked satellites, continuously counting the number
of received chips, full code periods, navigation bits and message frames. These counters are
fundamental to measure the misalignment over different channels, tracking different
satellites, and are used to compute the pseudoranges. For sake of completeness, note that
real receivers generally use architecture more complex than that reported in Fig. 2. For
example, a Frequency Lock Loop (FLL) is employed to refine the rough estimate performed
by the signal acquisition and ease the PLL lock, reducing the transient time between the
signal acquisition and the steady-state carrier/code tracking. Recently new techniques based
on digital signal processing have been developed in order to obtain higher precision and
reduced computational load, improving the robustness against noise and interference. In
this section, we have recalled only some fundamentals of code and carrier tracking, with the
goal of providing the necessary background for the following part of the chapter.
3.3 Navigation message demodulation, frame and page synchronization
Once the tracking loops are locked (i.e. the local code keeps the alignment with the incoming
code and the local carrier is exactly a replica of the received one), the navigation data bits
appear at the output of the Prompt correlator, on the in-phase branch of the tracking loops.
Considering the GPS L1 C/A code, using an integration time equal to the code period, we
obtain a bit value every ms. However, due to the low signal power, real receivers usually set
the integration time to 20 ms, which is the inverse of the navigation data rate (i.e. 50 Hz).
Fig. 4.a shows 1 second of normalized navigation data bits at the Prompt correlator output,
using an integration time of both 1 ms (blue) and 20 ms (red). The same example could be
repeated considering the Galileo E1-B signal. In this case, a proper value of integration time
is 4 ms, that corresponds to either the code period and the inverse of the navigation data
rate. An example of navigation data bits, recovered processing the signal tranmitted by a
simulated Galileo satellite, is shown in Fig. 4.b.
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