GPS Receiver-Related Subjects - Fundamentals of Global Positioning System Receivers: A Software Approach - page 329

0.35

0.3

0.25

0.2

Direct subtract

0.15

0.1

From projection method

0.05

0

43 44 45 46 47 48 49 50 51 52

C/N 0

FIGURE 12.19 Residue after the strong signal is subtracted from input.

TABLE 12.4 Weakest Signal Acquired with Strong Signal Presence

Strong Signal (S/N) 1 Weakest Signal Acquired (S/N) 2

− 13 dB ( C/N 0 = 50 dB) − 32 dB ( C/N 0 = 31 dB)

− 15 dB ( C/N 0 = 48 dB) − 33 dB ( C/N 0 = 30 dB)

− 17 dB ( C/N 0 = 46 dB) − 35 dB ( C/N 0 = 28 dB)

referenced to a 2-MHz bandwidth system. The weak signals shown in this
table are about 1 to 2 dB lower than the one shown in Figure 12.17. One
needs to keep in mind, however, that the result in Table 12.3 is the lowest
values that can be achieved in a number of simulation test runs. They are
not statistical averages.
3. If the strong signal in the range of (S/N) 1 =− 9 dB is removed

using the projection method, the weakest signal that can be acquired is
(S/N) 2 =−

13 ∼−

39 dB. Up to 3 strong signals were tested, and the weakest
signal could be acquired is still (S/N) 2 =− 39 dB. This represents an

improvement of 4 to 7 dB from Table 12.4.

12.7 SIMULATION OF GPS SIGNALS

It is convenient to use simulated results to evaluate a certain signal processing
method. Simulation can be performed easily, and the S / N results are accurate.
In data collected from real satellites, the S / N ratio cannot be easily controlled.
Collecting signals from a hardware simulator can be tedious, but sometimes a
hardware simulator must be used to confirm simulated results.
One way to generate simulated data is to consider the noise first. The normal
(Gaussian) distribution is usually used for the noise. The signal is generated
based on the noise amplitude. The noise and signal are added together as the