This effort emphasizes several important results pertaining to a mobile satellite system propagation experiment performed in Central Maryland during June 1987. Fade distributions due to multipath and roadside trees at L Band (1.5 GHz) during a period in which the deciduous trees were in full bloom are examined. The multipath statistics for roadside trees are compared with previous multipath measurements made in canyon terrain in North Central Colorado. Also examined is the repeatability of previous UHF measurements made in Central Maryland and the attenuation effects of foliage on trees at UHF. Fade duration for the multipath mode for fade levels of 5 dB and 10 dB is also presented

Goldhirsh, Julius

Vogel, Wolfhard J.

English

NASA Technical Reports Server

N89-11077
RESULTS OF 1987 MSS HELICOPTER PROPAGATION EXPERIMENT
AT UHF AND L BAND IN CENTRAL MARYLAND
Julius Goldhirsh
Applied Physics Laboratory, Johns Hopkins University, Johns Hopkins Road,
Laurel, MD 20707-6099
Wolfhard J. Vogel
Electrical Engineering Research Laboratory, University of Texas at Austin,
Austin, Texas 78758-4497
Abstract
This effort emphasizes several important results pertaining to a mobile
satellite system propagation experiment performed in Central Maryland during
June 1987. In particular, we examine fade distributions due to multipath
and roadside trees at L Band (1.5 GHz) during a period in which the
deciduous trees were in full blossom. We compare the multipath statistics
for roadside trees with previous multipath measurements made in canyon
terrain in North Central Colorado. Also examined is the repeatability of
previous UHF measurements made in Central Maryland and the attenuation
effects of foliage on trees at UHF. Fade duration for the multipath mode
for fade levels of 5 dB and i0 dB is also presented.
1.0 Introduction
Designers of planned mobile satellite systems require information as to
the extent of fading caused by shadowing and multipath from roadside trees
and terrain. The Applied Physics Laboratory of the Johns Hopkins University
and the Electrical Engineering Research Laboratory of the University of
Texas at Austin conducted the fifth in a series of mobile satellite system
experiments in June of 1987 in Central Maryland [Vogel and Goldhirsh, 19861
1988, Goldhirsh and Vogel 1987; 1988a; 1988b]. The objectives of the June
1987 test were as follows: (I) Determine roadside tree fade distribution
statistics for roadside tree shadowing and multipath geometries. (2)
Determine fade distributions for various elevation angles, road types, and
for lane of road driven. (3) Assess the variability of fade distribution
statistics for combined road cases at the various elevation angles. (4)
Express the overall fade distribution due to combined road cases in the form
of convenient and accurate functional forms. (5) Obtain a scaling factor
describing the ratio of UHF (870 MHz) and L Band (1.5 GHz) fades. (6)
Obtain fade duration statistics. (7) Establish repeatability with previous
measurements at UHF. (8) Compare L Band multipath measurements for roadside
trees and previous canyon measurements. We describe here the results of
objectives (5) through (8) as the others were previously presented by
Goldhirsh and Vogel [1988a].
]8
https://ntrs.nasa.gov/search.jsp?R=19890001706 2020-03-20T05:48:56+00:00Z
2.0 Background
The transmitter platform was a Bell Jet Ranger helicopter which carried
L Band and UHF antennas on a steerable mount below the helicopter fuselage.
The antennas transmitted right circular polarization and both had nominal
beamwidths of 60 ° . Also located on the mount was a video camera. An
observer inside the aircraft viewed the scene on a monitor and pointed
(with remote controls) the mount in the direction of the mobile van which
carried the receiving antennas and receivers operating at the corresponding
frequencies. The receiver antenna gains were approximately omnidirectional
in azimuth and the beamwidths (in elevation) were approximately 60 ° within
the interval 15 ° to 75 ° . The received signal levels were sampled at a one
kHz rate. The receiver gain settings and vehicle speeds were also measured.
Measurements were made along three stretches of roads in Central
Maryland. These were: (i) Route 295 north and south between Routes 175 and
450, a distance of 25 km. (2) Route 108 southwest and northeast between
Routes 32 and 97, a distance of 15 km. (3) Route 32 north and south between
Routes 108 and 70, a distance of 15 km. Route 295 is a popular four lane
highway (connecting Baltimore and Washington DC). This road contains pairs
of lanes carrying traffic in opposite directions with trees located about
75% of the extreme sides and along 35% of the median. Route 108 is a
relatively narrow two lane suburban road containing approximately 55%
roadside trees along the stretch examined. Route 32 is a two lane rural road
with more sparsely placed trees dislplaced further away from the sides and
containing approximately 30% trees.
Fade measurements due to shadowing were obtained for the geometry in
which the helicopter traveled along a trajectory parallel to the van and the
propagation path was normal to the line of roadside trees. Such a
configuration is considered as a "worst case" for corresponding satellite
paths as they give maximum attenuation. Fade measurements due to
"multipath" were generated for the geometry in which the helicopter followed
the van maintaining a nominal fixed geometry and a visible line of sight.
Repeated runs were made for both measurement modes for elevation angles of
30 ° , 45 ° , and 60 ° . The nominal height of the aircraft was 300 m for all
cases. The attenuation levels were derived by comparing the shadowed and
unshadowed signals.
3.0 Repeatability and Seasonal Effects on Measurements
We address here the following questions: (I) How repeatable were the
UHF measurements as compared to those derived in the previous years over
the same roads? (2) What are the relative seasonal effects on the
distributions for October 1985 which consisted of trees having 80% of full
blossom with branches and leaves having low water content, March 1986 in
which the deciduous trees were devoid of leaves, and June 1987
corresponding to 100% full blossom with branches and leaves having high
water content? In Figure I we compare the October 1985, March 1986, and
June 1987 UHF fade distributions for Route 295 south, right side of the road
driving. The vertical scale represents the percentage of the distance the
fade is greater than the abscissa and the ascissa corresponds to the fade
19
depth in dB. Since the vehicle speeds were nominally constant for the
individual runs, the ordinate may also represent the percentage of time the
fade is greater than the abscissa value. The abscissa fade depth is taken
between -6 dB and 28 dB where the negative fades represent constructive
interference due to multipath. Figure 2 depicts the October 1985 and June
1987 distributions for left side of the road driving. The March 1986
distribution is not shown in this figure since no March measurements were
implemented for this geometry. We note from Figure I that the October 1985
and June 1987 distributions are within 2 dB of each other at the I_ level
and are nearly coincident above 2_. Figure 2 shows the October and March
distributions to be nearly coincident. Since the dB differences of these
two sets of curves give a measure of both the repeatability of the
measurements and the relative seasonal effects, we may conclude that: (i)
negligible seasonal effects exist between the October and June period, and
(2) the repeatability of the measurements is generally smaller than one dB.
The March 1986 distribution in Figure i (no leaves case) gives fades which
are consistently smaller than those for October and June. For example, at
the i_ and 10g levels the fades are at most 3 and I dB smaller during March.
These represent seasonal fade reductions of less than 20_. We may conclude
from these results that, on the average, the effects of leaves on trees are
small for the dynamically acquired statistics. Stationary fade measurements
made on individual trees at UHF gave consistently similar results (e.g., 14_
to 40_ reduction) [Goldhirsh and Vogel, 1987]. Although the seasonal
effects on attenuation have been examined only at UHF, the results are not
expected to differ significantly at L Band.
4.0 L Band-UHF Scaling Factor
Figure 3 shows equal percentage values of the ratio of L Band to UHF
fades for eight runs at the elevation angle of 30 ° (solid points). The
dashed lines at the individual percentage values represent the +/- rms
levels relative to the average for the respective percentage. The runs
correspond to Routes 295 north and south for right and left lane driving and
Routes 108 and 32 for both directions. The total road length for the
combined runs (for each path angle) constitutes approximately 160 km. Shown
in this figure are the ratios at the equal percentage values at I, 2, 5, I0,
20, and 30_ levels. Also plotted is the best fit linear li_e through the
set of points. We note that these best fit ratios are approximately
independent of percentage in the interval I to 30g. Similar results were
derived for 45 ° and 60 ° . The overall ratio (all elevation angles) of L Band
to UHF fade was noted to be
or
Rf = 1.35 +/- 0.i (RMS) (i)
f(L Band) ]0.Rf = f(UHF)
551
(2)
20
5.0 Fade Distributions Due to Multipath Geometries
Fade distributions for multipath geometries in mountainous and canyon
terrain were previously measured by Vogel and Goldhirsh [1988]. To insure
that the phenomenon examined was fading due to multipath in this previous
effort, the helicopter followed the van maintaining an unshadowed line of
sight for the propagation paths. In a similar fashion, the multipath
interference effects caused by roadside trees were also measured in Central
Maryland during the June 1987 period. In Figure 4 are shown comparisons of
multipath fade distribution for the roadside tree case (Route 295 S, right
lane driving, 45 ° elevation), and the worst case fade distributions for the
canyon terrain (Big Thompson Canyon, into and out of canyon at 45 °
elevation). The cartoon in the figure depicts the helicopter following the
van at the fixed elevation angle. The roadside tree multipath fades at L
Band are noted to be approximately 6 dB and 3 dB at the 1P and 10P levels,
respectively. Virtually no differences were noted for the multipath
distributions for the 30 ° and 60 ° elevation cases. We note that at the 1P
level the canyon multipath fades flank those due to the roadside tree case
by approximately +/- i dB. On the other hand, at the 10P level the tree
fade exceeds both canyon cases by 1.3 dB. We thus observe that no excessive
multipath conditions exist in mountainous terrain and that the distributions
from roads having roadside trees may even exceed those for mountainous
terrain.
6.0 Multipath Versus Shadowing
The dramatic effects of attenuation caused by shadowing at L Band are
depicted in Figure 5. Shown is a comparison of the distributions
corresponding to multipath and shadowing geometries for Route 295 south
(right lane driving) at a path angle of 45 ° . We note 20 dB and i0 dB fades
at the 1P and lOP levels, respectvely, for the shadowing case as compared to
6 dB and 3 dB for the multipath roadside tree case. The enhanced
attenuation effects caused by shadowing relative to those from multipath
have also been measured by other investigators [Jongejans et al, 1986].
7.0 Fade Duration Statistics
Fade durations distributions are presented in Figure 6 and 7. The
curves in these figures correspond to the combined results of the eight
shadowing runs described above for each of the indicated elevation angles
and for 5 dB and I0 dB fade thresholds. In each of these figures two sets
of statistics are presented; namely, "faded" (solid) and "unfaded" (dashed).
These correspond to durations for which the attenuations are greater and
less than the indicated fade thresholds, respectively. The vertical scales
represent the percentage of the durations greater than the abscissa values
which have been normalized to number of wavelengths; one wavelength at L
band being 20 cm. The duration may alternately be expressed in terms of
time duration by dividing by the vehicle speed. Hence, for a4 speed of 88
km/hr (24.4 m/s), the abscissa ranges in values from 8.2 x I0 seconds (.i
_) to .82 seconds (I00 _). Also given in Figure 6 are the fade durations
due to multipath at the 5 dB fade threshold. These were derived by
combining the three elevation angle runs on Route 295 south (right side of
21
the road driving). Recall, these data sets correspond to the geometry in
which the helicopter followed the van while maintaining a visible line of
sight.
Figures 6 and 7 show the distributions to systematically depend on
elevation angle. That is, the lower the elevation angle, the greater the
fade duration at any fixed percentage. This result is consistent with the
fact that lower elevations result in more persistent shadowing. We also
note from Figure 6 that a large difference in fade durations exists when
comparing multipath with shadowing cases. This is attributed to the
different characteristics of randomness of the received signals associated
with multipath interference versus those associated with shadowing_ i.e.,
absorption and scattering from trees, branches, and foliage. In comparing
Figures 6 and 7, we note that the larger shadowing (I0 dB versus 5 dB)
thresholds result in smaller fade durations. This is again attributed to
the nature of the absorption and scattering phenomena where deep fades of
long duration are less likely than less intense fades of long duration.
This is also consistent with the fact that there is a smaller dependence of
elevation angle on fade duration for the I0 dB threshold than for the 5 dB
case.
8.0 Summary
We summarize the salient results of this effort as follows:
(i) Although foliage on trees at UHF produces enhanced fade depths
relative to the bare tree case (e.g., nominally 20% increase in
attenuation), the dominant attenuation effects are caused by the branches
and trunk of the trees. It is believed that similar results should apply at
L Band although no measurements at this wavelength have been made to verify
this.
(2) Repeatabilities of the measurements at UHF were found to be
smaller than 10% in fade at the 1% and greater exceedance levels.
(3) The overall average best fit ratio of L Band to UHF fades (scaling
factor) is 1.35 (+/- 0.I rms), and it is relatively insensitive to the path
angle and the exceedance percentage over the interval of 1% to 30%. This
ratio is approximately equal to the square root of the ratio of frequencies
(equation (I)). The above ratio was derived for an overall average road and
driving condition corresponding to 24 runs (comprised of Route 295 right
lane and left lane driving for north and south directions, Route 108
northeast and southwest directions, and Route 32 north and south directions
for 30 °, 45 ° , and 60 ° elevations (480 km of road).
(4) Attenuations caused by shadowing of roadside trees far exceed
those due to multipath. For example, fading due to multipath at the 1%
level was found to be approximately 6 dB as compared to fades as high as 25
dB due to shadowing at a 30 ° path angle.
22
(5) Fades due to multipath from roadside trees were found to be
comparable and in many cases slightly larger (e.g., i to 2 dB) than those
obtained in canyon terrain. This may be attributed to the fact that
roadside trees are nearby and tall and therefore give rise to more
appropriate scattering geometries for the given antenna pattern. The canopy
tops of the trees may also represent more of an isotropic scatterer than
nearby faceted canyon walls.
(6) Fade durations are elevation angle dependent for the shadowing
geometry. For the multipath case, fade durations are dramatically smaller
than those for shadowing cases at the same exceedance percentage.
9.0 Acknowledgements
The authors are grateful to J. R. Rowland for his ingenuity in
outfitting the helicopter transmitter platform and to G. W. Torrence for the
development of the receiver system. Many thanks are also extended to A. J.
Walker, Steve Babin, and Jack Miller for assisting the experiment. This
work was supported by NASA Headquarters Communications Division under
Contract #N00039-87-C-5301 for APL and by the Jet Propulsion Laboratory for
the University of Texas under Contract #JPL956520.
I0.0 References
Goldhirsh, J. and W. J. Vogel, "Roadside Tree Attenuation Measurements at
UHF for Land-Mobile Satellite Systems," IEEE Trans. on Antennas and
Propagat., vol AP-35, pp 589-596, May 1987.
Goldhirsh, J. and W. J. Vogel, "Propagation Effects by Roadside Trees
Measured at UHF and L-Band for Mobile Satellite Systems," Proceeding
of the Mobile Satellite Conference, Pasadena, California pp 87 - 94,
May 3-5, 1988a (Jet Propulsion Laboratory Publication 88-9, Jet
Propulsion Laboratory, Pasadena, California)
Goldhirsh, J. and W. J. Vogel, "Attenuation Statistics Due to Shadowing and
Multipath from Roadside Trees at UHF and L Band for Mobile Satellite
Systems," __--J°hnsHopkins University/Applied Physics Laboratory
Technical Re_9_e_t SIR88U 004, February, 1988b (Applied Physics
Laboratory, Johns Hopkins Road, Laurel MD 20707)
Jongejans, A. and A. Dissanayake, N. Hart, H. Haugli, C. Lois and R. Rogard,
"PROSAT - Phase i Report," European _ Ae_A_ Technical R_e_port ESA
STR-216, May 1986 (European Space Agency, 8-10 rue Mario-Nikis, 75738
Paris Cedex 15, France)
Vogel, W. J. and J. Goldhirsh, "Tree Attenuation At 869 MHz Derived from
Remotely Piloted Aircraft Measurements, "IEEE T ran___ss.Antennas and
Propagat., vol AP-34, pp 1460-1464, Dec. 1986.
Vogel, W. J. and J. Goldhirsh, "Fade Measurements at L Band and UHF in
Mountainous Terrain for Land Mobile Satellite Systems," I__EEE Trans.
Antennas and Propagat., vol AP-36, Jan. 1988.
23
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Results of 1987 MSS helicopter propagation experiment at UHF and L band in Central Maryland

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