Interference, Multipath, and Scintillation - Understanding GPS: Principles and Applications - page 297

Tf − p with p in the range of 2.0-3.0 [26] and where T is a strength parameter (in units
of rad 2 /Hz).
Scintillation can lead to intermittent tracking outages in two different ways.
First, if an amplitude fade is of sufficient depth and time duration, from a receiver
perspective the desired signal is absent, and loss of lock of the code and carrier phase
tracking loops is inevitable. If the desired signal is being received at a very high level,
such as 50 dB-Hz, a 20-dB fade is generally tolerable, but a much deeper fade will
typically cause an outage if the fade persists longer than the time constant of the
tracking loops. At low SNRs, even a 5-10-dB fade can cause a disruption in track-
ing. Second, strong phase scintillation can cause loss of phase lock within the
receiver if the phase variations introduce a level of dynamics that is greater than the
phase lock loop can accommodate (see discussion in Section 5.6.1).
Fortunately, scintillation rarely occurs on all visible satellites simultaneously.
The irregularities that cause scintillation are not generally present within the iono-
sphere at each of the points where the signals from the visible satellites intersect the
ionosphere. Thus, scintillation tends to only impact one or at most a few satellites
simultaneously.
Both S 4 index and phase standard deviation are a function of carrier frequency:

1

S ∝ (6.55)

4

f

15 .

1
f

σ φ ∝ (6.56)

so that when fading due to ionospheric scintillation occurs, the observed S 4 index
for a signal on L2 is approximately 1.45 times greater than the S 4 index for an L1
signal and observed σ φ for L2 is approximately 1.28 times greater than for L1. The

implication of this carrier frequency dependency is that scintillation would be much
more likely to cause outages for GPS signals on L2 and L5 than for L1 signals, if the
same signal was to be broadcast on all three frequencies at similar received power
levels. Variations in signal design and power levels between the GPS signals at L1,
L2, and L5 make it more difficult to make such a general statement.

References

[1] Spilker, J. J., and F. D. Natali, “Interference Effects and Mitigation Techniques,” Global

Positioning System: Theory and Applications , Vol. 1, American Institute of Aeronautics
and Astronautics, Washington, D.C., Vol. 163, 1996, p. 726.

[2] Van Dierendonck, A. J., “GPS Receivers,” Global Positioning System: Theory and Applica-

tions , Vol. 1, American Institute of Aeronautics and Astronautics, Washington, D.C.,
Vol. 163, 1996, pp. 354-355.

[3] Amoroso, F., “Adaptive A/D Converter to Suppress CW Interference in DSPN Spread-Spec-

trum Communications,” IEEE Trans. on Communications, Vol. COM-31, No. 10, Octo-
ber 1983, pp. 1117-1123.

[4] Amoroso, F., and J. L. Bricker, “Performance of the Adaptive A/D Converter in Combined

CW and Gaussian Interference,” IEEE Trans. on Communications , Vol. COM-34, No. 3,
March 1986, pp. 209-213.