Virginia Tech has performed a comprehensive propagation experiment using the Olympus satellite beacons at 12.5, 19.77, and 29.66 GHz (which we refer to as 12, 20, and 30 GHz). Four receive terminals were designed and constructed, one terminal at each frequency plus a portable one with 20 and 30 GHz receivers for microscale and scintillation studies. Total power radiometers were included in each terminal in order to set the clear air reference level for each beacon and also to predict path attenuation. More details on the equipment and the experiment design are found elsewhere. Statistical results for one year of data collection were analyzed. In addition, the following studies were performed: a microdiversity experiment in which two closely spaced 20 GHz receivers were used; a comparison of total power and Dicke switched radiometer measurements, frequency scaling of scintillations, and adaptive power control algorithm development. Statistical results are reported

Ajaz, H.

Laster, J.

Nelson, B.

Pratt, Timothy

Safaai-Jazi, A.

Stutzman, Warren L.

English

NASA Technical Reports Server

N94-14658
STATISTICAL RESULTS FROM THE VIRGINIA TECH
PROPAGATION EXPERIMENT USING THE OLYMPUS 12, 20, AND
30 GHz SATELLITE BEACONS
W.L. Stutzman, A. Safaai-Jazi, T. Pratt, B. Nelson, J. Laster, and H. Ajaz
Satellite Communications Group
Bradley Department of Electrical Engineering
Virginia Polytechnic Institute and State University
Blacksburg, Virginia 24061-0111
1. INTRODUCTION
Virginia Tech has performed a comprehensive propagation experiment using the
Olympus satellite beacons at 12.5, 19.77, and 29.66 GHz (which we refer to as 12, 20,
and 30 GHz). Four receive terminals were designed and constructed, one terminal at
each frequency plus a portable one with 20 and 30 GHz receivers for microscale and
scintillation studies. Total power radiometers were included in each terminal in order
to set the clear air reference level for each beacon and also to predict path attenuation.
More details on the equipment and the experiment design are found in [1].
Statistical results for one year of data collection have been analyzed. In
addition, the following studies were performed: a microdiversity experiment in which
two closely spaced 20 GHz receivers were used [2]; a comparison of total power and
Dicke switched radiometer measurements, frequency scaling of scintillations [3], and
adaptive power control algorithm development. In this paper we report on statistical
results.
2. THE DATABASE
Data were collected from August 1990 through September 1992. The initial few
months of operation were used for calibration and equipment adjustment. Beacon data
for the period June through August, 1991, could not be used because the Olympus
spacecraft was out of its assigned geostationary orbit. Since fall 1992 the inclination
angle of the satellite has been high due to fuel conservation measures, no N-S
stationkeeping is performed. This leads to severe diurnal fluctuations at our location,
which renders the data of poor quality. The analysis year uses the following 12 months:
33
https://ntrs.nasa.gov/search.jsp?R=19940010185 2020-06-16T19:40:59+00:00Z
January- May, 1991; June - August, 1992; September - December, 1991
3. ATTENUATION STATISTICS
Perhaps the most important aspect of the Olympus propagation experiment is
the attenuation statistics. These statistics include exceedance data for attenuation with
respect to free-space (AFS), attenuation with respect to clear air (ACA), and
radiometer derived attenuation (ARD). The most important data sets are those for
which data exist simultaneously on all three frequencies. This permits direct
comparison, frequency scaling, etc. Numerous graphs and tables on attenuation
statistics have been generated.
Figure 1 shows exceedance plots of AFS and ARD for the analysis year for 12,
20, and 30 GHz. As is typical of radiometer data, ARD values are not accurate above
about 10 dB. Comparison of AFS and ARD data leads to the following conclusions: (1)
For fades of less than 3 dB, attenuation data derived from total power radiometer
measurements can match beacon attenuation measurements to within 0.1 dB. (2) For
fades up to 10 dB, attenuation data derived from total power radiometer measurements
match beacon attenuation measurements to within 1 dB. These data are for a common
time base; that is, data are used only when valid data exist on all three frequencies.
Exceedance plots for individual frequencies and pairs of frequencies 12/20, t2/30 and
20/30 have also been generated.
Measured clear air attenuation (ACA), which is the loss of signal strength due to
gases and water vapor present in tile earth's atmosphere, is shown in Fig. 2 for the
analysis year. Clear air attenuation predicted (from the CCIR model) and measured
(50% occurrence level for the analysis year) values are: 0.3 and 0.68 dB for 12.5 GHz,
1.53 and 1.58 dB for 19.77 GHz, and 1.36 and 1.42 dB for 29.66 GHz.
Table 1 shows attenuation data at many occurrence levels for all frequencies.
4. ATTENUATION RATIOS
A unique aspect of tile experiment is the controlled frequency dependence, i.e.,
all variables across the 12, 20, and 30 GHz receivers are identical. Therefore, any
differences are due to frequency effects. Frequency dependence is presented in terms of
attenuation ratio (RA). Attenuation with respect to clear air is used in RA
34
calculations, becausethe clear Mr and rain frequency dependencies are much different
and we wish to focus on the rain component. Plots of attenuation ratios for 30 to 20
GHz of a function of percent time of occurrence and as a function of the lower frequency
attenuation level are shown in Fig. 3 for the analysis year. It is emphasized that RA is
the instantaneous attenuation ratio and is obtained from the instantaneous data pairs at
the two frequencies. This ratio relates directly to frequency scaling applications such as
adaptive power control. It is noted that RA for 30]20 is very flat indicating potential
application to adaptive control. Note from Fig. 3b that the median for the 30/20 ratio
is about 2 at all levels with a spread from 1 to 99% between 1.4 and 2.6.
Statistical attenuation ratio (RAS), defined as the ratio of attenuation values
obtained from cumulative attenuation statistics at the same level of occurrence, was
also studied. The instantaneous nature of frequency scaling is not present in this
parameter. Figure 4 compares the median values of RA to RAS for 30/20; similar
results were obtained for 30]12 and 20/12. The excellent agreement between RA (50%)
and RAS indicates that RAS, which is easily computed from separate exceedance
statistics, is an accurate predictor of median instantaneous attenuation ratio.
5. SECONDARY STATISTICS
The secondary statistics include fade duration, interfade (non-fade) duration,
fade slope, and ultimate fade depth. These statistics are valuable to system designers
and also aid in the understanding of the nature of fading. Fade duration, FD, is defined
as the length of time in seconds for which the attenuation with respect to clear air,
ACA, exceeds a specified threshold level. Interfade duration is the complement of fade
duration.
Fade and interfade durations statistics have been generated for threshold levels
of-3, -1, 0, 1, 3, 5, 10, 15, 20, and 25 dB and durations exceeding 1, 6, 10, 20, 60, 600,
and 3600 seconds. At each frequency, plots of the number of fade/non-fade events and
the fade/interfade time as the percent time of the year for which data were collected
versus fade/interfade duration are given for several threshold values. In order to
eliminate the effect of scintillations on the number of fade/interfade events 10-second
block averages of attenuation have been used. Fade duration statistics for the month of
July 1992 are presented in Fig. 5 which illustrates the number of fades versus fade
duration at 12, 20, and 30 GHz for several threshold levels. As an example, for a
35
r
threshold level of 5 dB the number of fades with fade durations exceeding 10 seconds is
34 at 12 GHz, 87 at 20 GHz, and 164 at 30 GHz. It is observed that for the same fade
duration exceeded the number of fade events generally decreases with the threshold
level. A deviation from this trend occurs at 20 GHz for fade durations less than 60
seconds and threshold levels of 1 and 3 dB. In other words, the number of fade events
with duration exceeding 1, 6, 10, and 20 seconds at 3 dB exceeds that at 1 dB by few
hundreds for the 20 GHz beacon. Such behavior at low threshold levels and short fade
durations should not be surprising. Comparison of Figs. 5 a, b, and c, also indicates
that for the same threshold level (>3 dB) and the same fade duration exceeded, the
number of fade events at 12 GHz is less than that at 20 GHz which in turn is less that
at 30 GHz. For threshold levels of 1 and 3 dB no consistent behavior prevails.
Fade slope is the time rate of change of attenuation in units of dB/second. The
ESA definition of fade slope at a given threshold level fade is the difference between
attenuations (in dB) 5 seconds before the threshold is crossed and 5 seconds after the
threshold is crossed divided by 10 (seconds). In order to alleviate sensitivity to small
variations in the received signal, 10-second block averages of attenuation are used here
in the calculation of fade slope. Attenuation block average is calculated from stored
values of attenuation in dB in the i th 0.1-s interval from the beginning of the day
(00:00:00 UT), AFSi. The block averaged attenuation values are found from
m + 50
AFSB m 1 [dB]
= 100 _ AFSi
i = m-49
Fade slope is calculated from block averages at the i th sample point as
= r  /sl
where AFSi is the threshold attenuation with respect to freespace at the i th time sample
obtained by averaging over the central lO-s period:
i+50
AFSi = E AFSj [dB]
j=i-49
36
At each frequency we determine fade slope FSB for attenuation (AFS) thresholds
ranging from -8 to 39 dB in 1 dB steps. Fade slopes are sorted into 0.05 dB/second
bins. Plots of fade slope statistics for each channel and for several threshold levels have
been generated for the analysis year and are displayed in Fig. 6.
Figure 7 plots the fade slope and attenuation at 20 GHz in real time of one rain
event that occurred on May 14, 1991. The fade slope and attenuation for this plot were
generated using a 3-minute moving block average which eliminated small variations in
attenuation and made the first derivative relation between fade slope and attenuation
evident. Data points were plotted every 30 seconds beginning at 17:50 GMT and ending
at 18:32 GMT on May 14, 1991.
6. CONCLUSIONS
Results from a comprehensive propagation experiment involving beacon and
radiometric measurements at 12, 20, and 30 GHz for one year of data were presented.
Radiometer derived attenuations agree well with beacon measured attenuations for fade
levels below 10 dB. Attenuation statistics, attenuation ratios, fade and interfade
durations, and fade slope for all three frequencies have been examined.
[1]
REFERENCES
W.L. Stutzman et al., "Initial results from the 12, 20, and 30 GHz OLYMPUS
propagation experiment in Blacksburg, Virginia," IEEE AP-S Inter. Symp. Digest
(Chicago), pp. 736-739, July 1992.
[2] J.C. Cardoso, A. Safaai-Jazi, and W.L. Stutzman, "Microdiversity in satellite
communications," to appear in IEEE Trans. on Ant. and Prop..
[31
[4]
F. Haidara and C.W. Bostian, "Preliminary results on scintillation intensity
frequency scaling from the Virginia Tech OLYMPUS experiment," IEEE AP-S
Inter. Syrup. Digest (Chicago), pp. 301-304, July 1992.
W.L. Stutzman and K.M. Yon, "A simple rain attenuation model for earth-space
radio links operating at 10 - 35 GHz," Radio Science, vol. 21, pp. 65-72, Jan/Feb
1986.
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Statistical results from the Virginia Tech propagation experiment using the Olympus 12, 20, and 30 GHz satellite beacons

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