Satellite Signal Acquisition, Tracking, and Data Demodulation - Understanding GPS: Principles and Applications - page 186

code receivers than for P(Y) code receivers, assuming no signal jamming and that all
other things in the receiver designs are equal. It is another common misconception
that the carrier loop measurement is a velocity measurement, when actually it is a
range measurement. The PLL thermal noise error is in units of range because it is
part of the carrier Doppler phase measurement (i.e., a measurement of some fraction
of the carrier wavelength). The velocity is approximated using the change in carrier
Doppler phase between two carrier range measurements over a short time. Any sin-
gle PLL measurement is a precise phase measurement of the carrier Doppler phase
within one cycle. The differential measurement must also account for the integer
number of carrier Doppler phase cycles that occur between measurements.
Figure 5.22 illustrates the PLL thermal noise jitter plotted as a function of
( C / N 0 ) dB for three different noise bandwidths assuming the maximum 20-ms
predetection integration time. Even though the thermal noise jitter is not directly
dependent on the loop order, there is an indirect relationship. The loop order is sen-
sitive to the same order of dynamics (first order to velocity stress, second order to
acceleration stress, and third order to jerk stress), and the loop bandwidth must be
wide enough to accommodate these higher-order dynamics. In general, when the
loop order is made higher, there is an improvement in dynamic stress performance.
Thus, the thermal noise can be reduced for the same minimum C / N 0 by increasing
the loop order and reducing the noise bandwidth while also improving the dynamic
performance.

5.6.1.2 Vibration-Induced Oscillator Phase Noise
Vibration-induced oscillator phase noise is a complex analysis problem. In some
cases, the expected vibration environment is so severe that the reference oscillator

B

n = 2 Hz,

T = 20ms B

n = 10 Hz,

T = 20ms B

n =20Hz,

T =20ms

20

15

10

5

0

15 20 25 30 35

( / CN 0dB ) (dB-Hz)

Figure 5.22 PLL thermal noise jitter.