Handbook of Propagation Effects for Vehicular and ... - Courses

9-2
Propagation Effects for Vehicular and Personal Mobile Satellite Systems
ranged from 15° to 0°. Measurements were performed over a period 40 hours during
which the ship was en route from Norfolk, VA to Texas City, Texas. Time-division
multiplexing (TDM) carriers for Teletype and voice carriers for telephone and data
transmissions were monitored and analyzed. These signals were monitored at the shore
station, Southbury, Connecticut, and the ship terminal. The frequencies for these signals
were within 1.537 and 1.541 GHz. At satellite elevation angles below 2°, the mean
carrier reduction and peak-to-peak fluctuations were noted to be severe. For example,
down to two degrees, the peak-to-peak fluctuations of the carrier to noise ratio were
smaller than 4 dB. These fluctuations followed the two-component multipath model
from a calm sea relatively well after a 1-dB bias was added for gaseous fading. Below 2°
elevation, the peak-to-peak fluctuations increased to levels as high as 10 dB and deviated
considerably from the two-component multipath model. Cumulative signal distributions
relative to the mean values demonstrated that peak-to-peak fluctuations exceeded 10 dB
with a probability of 42% in the angular interval of 0.5-2°. The spectrum properties of a
10-minute sample at 2° elevation indicated the presence of turbulent-type scattering in the
troposphere or ionosphere. Measurements showed that in passing from 10° to 5°
elevation angle, a mean carrier to noise ratio drop of less than 2 dB was observed. The
maximum fade level at 5° caused by signal fluctuations was less than 6.5 dB 99% of the
time. On the other hand, at an elevation angle of 10°, the fading was less than 4.4 dB for
99% of the time.
9.2.2 Evolution of a Simplified Multipath Fading Prediction Model
A number of investigators have analyzed the characteristics of multipath fading due to
sea surface reflections. Karasawa and Shiokawa [1984a] applied the Kirchhoff
approximation theory and developed a model for the coherent and incoherent scattered
power as a function of sea surface conditions for L-Band. Using their developed model,
fade depths were determined as a function of elevation angle, wave height and antenna
gain. They showed that at an elevation angle of 5°, the fade depth under rough sea
conditions was dependent on small antenna gains and had little dependence on wave
height. For example, at 5° elevation and 99% of the time, the fade depth varied between
4.5 to 6.5 dB for an antenna gain of 20 dBi; (BW ≈ 17°, BW is the half power
beamwidth). The fade depth varied between 7 to 9 dB for an antenna gain of 15 dBi
(BW ≈ 30°) and 8 dB to 10.5 dB for an antenna gain of 10 dBi (BW ≈ 53°). These results
assume wave heights smaller than 4 m. In a later paper, Karasawa and Shiokawa [1986]
reported on a series of measurements using shore-to-shore, satellite-to-shore and satelliteto-ship
paths and antennas with gains from 13 dBi (BW = 37°) to 21 dBi (BW = 15°).
Based on these and other measurements and previously developed concepts, simplified
prediction models were developed by them [Karasawa and Shiokawa; 1988, 1987] and
adopted by the ITU-R [1994, pp. 352-354]. These simplified models are described in
Section 9.3.