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9.6 Other Maritime
Investigations
9.6.1 Experimental
Measurements in Japan during 1980 and 1983
Ohmori
et al. [1985] report on experimental results at sea shores at Wakasa
Bay and Echizen Cape in Fukui Prefecture, Japan in 1980 and 1983. In the
1980 experiment, L-Band (1.54 GHz) and VHF (250 MHz) transmitter systems
were placed at the top of a 300 m mountain (Mt. Tennou-san). The receiver
was located on the shore (3800 m distance) of an intervening body of water.
The 1983 experiment involved reception of a time-division-multiplexing
carrier at 1.538 GHz radiated from the geostationary INMARSAT satellite
located above the Indian Ocean. In both experiments the elevation to the
radiating source was 5° and circular polarization was used. A wave
rider buoy measured the sea wave height. Measurements were performed by
these investigators in calm seas (RMS wave height h of approximately
0.06 m and rougher seas (h = 0.4 to 0.9 m). Cumulative fading distributions
at L-Band (using a short backfire antenna) for rougher sea conditions showed
that 99%, 95%, and 90% of the times the fading signals were greater than
–6 dB, -4 dB, and –3 dB, respectively for the case when h = 0.91
m. There results showed that the fading depth peaked at h = 0.4
m and diminished slightly at greater wave-heights.
9.6.2 Extension
of the Kirchhoff Classical Model
Sobieski
and Guissard [1993] extended the Kirchhoff classical approach for mobile
maritime satellite propagation using the boundary perturbation method (BPM)
[Brown, 1980; Guissard
and Sobieski, 1987]. They evaluated carrier to specular and carrier
to multipath ratios and compared these results with those derived by measurements
and by other models. Comparing their calculations with results from other
measurement campaigns, they obtained generally good agreement.
9.6.3 K/Ka-Band Maritime
Experiments
Perrins and
Rice [1997] describe shipboard measurements employing the Advanced
Communication Technology Satellite (ACTS) Mobile Terminal (AMT) [Abbe
et al., 1996]. The link was comprised of a ground terminal at the Jet
Propulsion Laboratory in Pasadena, California and the mobile terminal on
board the USS Princeton in the eastern Pacific Ocean (October, 1996), where
the ACTS steerable spot beam was employed. The transmit gain of the AMT
(30 GHz) is a minimum of 20 dBi with a 12° elevation beamwidth for
elevation angles between 30° and 60°. The receive gain at 20 GHz
is a minimum of 18.8 dBi over a 12° elevation beamwidth for elevation
angles between 30° and 60°. The minimum receive system G/T is better
than –6 dB/K over the elevation beamwidth. The AMT antenna is mechanically
steerable in both azimuth and elevation.
Analysis of pilot tone data received by the AMT showed that negligible
variations of the signal arose due to ocean multipath. This is consistent
with the relatively narrow beamwidth (12°) and the fact that the elevation
angle was always above 40°. During the measurement campaign, six significant
fades above 10 dB were noted. The deepest fades (in excess of 30 dB) occurred
just after the ship changed direction after a long turn; suggesting the
antenna tracker may have been the source of the fades [Perrins
and Rice, 1997]. For the ship oriented azimuth angles associated with
the maneuvers, the line-of-sight was clear for elevation angles as low
as 30°. Hence, it did not appear that shadowing by the superstructure
accounted for the observed fades.
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