Global Positioning System Reference
In-Depth Information
Data Processing for Positioning
8.1
Navigation Data Recovery
The output from the tracking loop is the value of the in-phase arm of the tracking
block truncated to the values 1 and
1. Theoretically we could obtain a bit value
every ms. However, we deal with noisy and weak signals, so a mean value for
20 ms is computed and truncated to
1 or 1. One navigation bit durates 20 ms.
The bit rate of the navigation data is 50 bps. The sample rate of the output
from the tracking block is 1000 sps corresponding to a value each ms. Before
the navigation data can be decoded, the signal from the tracking block must be
converted from 1000 sps to 50 bps. That is, 20 consecutive values must be replaced
by only 1. This conversion procedure is referred to as bit synchronization .
8.1.1 Finding the Bit Transition Time and the Bit Values
The first task of the bit synchronization procedure is to find the time in a sequence
where bit transitions occur. First, a zero crossing is detected. A zero crossing is
where the output changes from 1 to
1, or vice versa. When a zero crossing is
located, the time of a bit transition is located. When the time of one bit transition
is known, it is possible to find all bit transition times. These are located 20 ms
apart beginning from the first detected bit transition. Figure 8.1 shows how all
bit transition times are found in a 200-ms sequence. The bit transition times are
marked by the arrows.
When the bit transition times are located, the 1000-bps signal must be converted
to a bit rate of 50 bps. To do this, 20 samples must be replaced by only 1 value.
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