Global Positioning System Reference
In-Depth Information
FIGURE 6.1 ( continued )
If the input to the receiver is an antenna pointing at the sky, the thermal noise is
lower than room temperature, such as 50 K.
For the C/A code signal, the null-to-null bandwidth is about 2 (or 2.046) MHz,
thus, the noise floor is at
111 dBm (
114
+
10 log2). Supposing that the GPS
111 ) below the noise floor.
One cannot expect to see the signal in the collected data. The amplification
needed depends on the analog-to-digital converter (ADC) used to generate the
data. A simple rule is to amplify the signal to the maximum range of the ADC.
However, this approach should not be applied to the GPS signal, because the
signal is below the noise floor. If the signal level is brought to the maximum
range of the ADC, the noise will saturate the ADC. Therefore, in this design the
noise floor rather than the signal level should be raised close to the maximum
range of the ADC.
A personal computer (PC)-based card ( 7 ) with two ADCs is used to collect
data. This card can operate at a maximum speed of 60 MHz with two 12-bit
ADCs. If both ADCs operate simultaneously, the maximum operating speed is
50 MHz. The maximum voltage to exercise all the levels of the ADC is about
100 mv and the corresponding power is:
signal is at
130 dBm, the signal is 19 dB (
130
+
( 0 . 1 ) 2
2 × 50 =
P
=
0 . 0001 watt
=
0 . 1mw
=−
10 dBm
( 6 . 3 )
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