Tracking Weak Signals - Fundamentals of Global Positioning System Receivers: A Software Approach - page 298

5000 pt data 5000 pt data 5000 pt data 5000 pt data 5000 pt data 5000 pt data

B

A

Initial point of C/A code

FIGURE 11.17 Initial C/A code moving toward the beginning of the data due to positive
Doppler shifting.

of the local C/A code must be shifted from the beginning to the end in two
consecutive ms. The input data in block A can be considered as the additional
ms of data received, and it must be processed twice. The first time it is processed
with a local initial C/A code point at the beginning, and the second time with a
local C/A code initial point at the end. When block B is processed, the local C/A
code initial point is already at the end. This operation processes one additional
set of 5000-point data.
When the Doppler frequency is 5 kHz, the initial C/A code point in the data

moves toward the end of the grabbed data, as shown in Figure 11.18. After some
time the initial C/A code point will line up with the data selected. Because the
time interval between two adjacent initial C/A code points is slightly longer than
5000 data points in one set of data, it may not contain an initial C/A code point,
as shown in block B in Figure 11.18. In this case the local C/A code initial point
must switch from the end to the beginning in two adjacent ms. The local code
will multiply with input data in blocks A and C , but skip B because 1 ms of
data is missed in every 315 seconds.
The discussion above can be summarized as follows: When the Doppler fre-
quency is positive, one C/A code length is less than 5000 points of input data. In
one set of 5000 input data, it may contain two initial C/A code points. This data
set should be processed twice. When the Doppler frequency is negative, one C/A

−

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5000 pt data 5000 pt data

B

A C

Initial point of C/A code
(a) xy Plane. (b) Altitude variation versus time.
FIGURE 11.18 Initial C/A code moving toward the end of the data due to negative
Doppler shifting.