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Early measurements of multipath effects at 1.5 GHz for ship to satellite
communications scenarios were reported by Fang
et al. [1982a, b]. The measurements were executed with a terminal on
a ship transmitting to and receiving from the MARISAT F-1 satellite located
at 15°-west longitude (over the Atlantic Ocean). The antenna diameter
was 1.2 m (approximately 12° beamwidth) and the system G/T was –4 dB/K.
Elevation angles 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.
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 (BW30°)
and 8 dB to 10.5 dB for an antenna gain of 10 dBi (BW53°).
These results assume wave heights smaller than 4 m. In a later paper, Karasawa
et al. [1986] reported on a series of measurements using shore-to-shore,
satellite-to-shore and satellite-to-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.
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