ACTS counters fading by resource sharing between the users. It provides a large margin only for those terminals which are at risk by unfavorable atmospheric conditions. ACTS, as an experimental satellite, provides a 5 dB clear weather margin and 10 dB additional margin via rate reduction and encoding. For the uplink, this margin may be increased by exercising uplink power control. Some of the challenges faced by the radiowave propagation community are listed. The issue of needs for the satellite are listed, both general and specific

Davarian, Faramaz

English

NASA Technical Reports Server

N )O- 17963
ACTS PROPAGATION CONCERNS, ISSUES, AND PLANS
Faramaz Davarian
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California 91109
ACTS counters fading by resource sharing between the users. It provides a large
margin only for those terminals which are at risk by unfavorable atmospheric
conditions. A moderate power margin, known as the clear weather margin, is
provided to all active users. Each user monitors one of the ACTS beacons
continually to assess atmospheric conditions. If a severe reduction in the
received beacon power is detected, the network master control station (MCS) is
notified via orderwire Channels. The MCS responds by instructing the terminal to
reduce its bit rate and invoke coding. The MCS will also notify the terminal at
the other end of the llnk as well as the satellite on-board processor. Both
measures reduce the error rate in the data flow caused by atmospheric fading.
ACTS, as an experimental satellite, provides a 5-dB clear weather margin and 10-dB
additional margin via rate reduction and encoding. For the upllnk, this margin may
be increased by exercising upllnk power control.
In achieving these goals, the radlowave propagation community faces a number of
challenges. Among them are:
A) The 5-dB clear weather margin will be used for fade condition detection.
Is this margin sufficient? Could we reduce this margin? For example,
if our research shows that this margin can be reduced to 2 dB, a power
saving of approximately 50% will result. This results in substantial
reductions in cost for operational systems.
B) To invoke fade countermeasure, the system must determine that a fade is
imminent. A conservative approach (delayed decision) will prove
detrimental to the llnk experiencing a fade, whereas, overreactlon can
overburden an operational system. Hence, we need to develop techniques
which can accurately predict fades in real time. Figure 1 depicts two
fade scenarios that require two different responses. This figure shows
the received signal power as a function of time (note that the time
scale has been intentionally left out). The solid llne shows a fade
which requires a system response to counter the effect of fading,
whereas, the dashed llne shows a fade which does not require a system
response. A 5-dB margin is also marked on Figure I.
c) To facilitate the resource sharing feature of ACTS, natlon-wide fade
statistics are needed.
Our studies will focus on two issues: general needs and ACTS-speciflc needs.
general needs include:
A)
B)
C)
D)
Propagation data for VSAT with small power margin
Propagation data on short-term fades and fade slope
Fade countermeasure techniques
Natlon-wide fade statistics
The
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TIME SERIES OF SIGNAL POWER
Figure I. Fade Dynamics
217
To address ACTS-specific needs, an environment will be cultivated wherein
propagation experts can develop, test and refine their models and schemes in a
unified fashion. Furthermore, advice and assistance will be given to ACTS users.
What is required in the context of ACTS propagation needs is a convenient
environment that propagation experts can conduct propagation studies. This
environment should provide the expertswith statistically significant
observations. Data must be taken in climatologically diverse areas for long time
durations. Fade countermeasure schemes must be tested thoroughly. Data
collected and analyzed by different centers must be presented with a uniform and
consistent format. ACTS experimenters in other areas, i.e., communications, data
transmission, protocol, etc., should not be burdened with propagation issues. ACTS
users should be able to receive advice and assistance from the ACTS propagation
experimenters group. This must be performed in a manner convenient to ACTS users.
Although many propagation research centers will participate in this task, a single
organization must oversee the effort for cohesiveness of the endeavor.
NASA's Propagation Program is organizing a cohesive effort to respond to ACTS
propagation needs. Our plans call for the development of low-cost propagation
terminals consisting of beacon receivers, radiometers, and data acquisition
systems, which will be loaned to different research centers and universities for
data collection and analysis. The Propagation Program has already begun this
effort by participating in the Olympus experiment, which is described in the
article by Professor Stutzman of Virginia Tech. Table i shows the chronological
order of events.
Table i. Chronological Order of Events
Year Activity
1989
1990
1991
1992
Construct beacon receivers, radiometers, and a data acquisition
system for the OLYMPUS experiment
Perform OLYMPUS propagation experiments and collect data
Design and build a prototype ACTS beacon receiver system
Build a CODE* terminal
Complete the OLYMPUS experiments
Evaluate ACTS prototype propagation terminal using OLYMPUS
Build 8 to i0 ACTS propagation terminals
Start ACTS experiments
*CODE is an acronym for the cooperative OLYMPUS data exchange, a feature
which allows an experimenter to exchange data with other OLYMPUS
experimenters.
218
A reasonable question to ask is where should the ACTS propagation terminals be
placed. In attempting to answer this question, we note that although ACTS beacons
are received anywhere in the U.S. mainland, the spot beams are not available
everywhere. To enjoy the potential benefits of proximity to a communications
terminal, one may suggest to place the propagation terminals where communications
coverage is also available. Of course, selecting a site based on spot beam
availability alone is hardly a sufficient reason. Therefore, we note the Global
Rain Climate map of the U.S., which includes an overlay of ACTS coverage areas as
depicted in Figure 2. The map in Figure 2 is a clue for the answer to the above
question.
1 -i i
C
F
D
E
Figure 2. Rain Rate Regions for the Continental U.S.: Crane
Global Model, 1980
However, before selecting the locations for ACTS propagation terminals we need to
consider one more parameter, namely the existing propagation data. For this
purpose, we may refer to Figure 3, where the 20/30-GHz data availability in the
U.S. is shown. Figure 3 reveals that most of the past data were taken at the east
coast and the south. There are no data available from the west, southwest, north,
and the Rockies. Hence, we need to make sure that the areas that lack propagation
data receive attention.
219
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With considerations given to Figures 2 and 3, a suggested set of locations for
placing ACTS propagation terminals emerges. Table 2 shows these locations. Note
that according to the Global Rain-Zone Model there are 8 rain zones in the U.S.
Table 2 suggests one terminal per rain zone with the exception of zone DI, which
is allotted two terminals. Also note that with the exception of the terminal in
zone BI, all the other terminals are in areas that receive ACTS communications
coverage.
The Propagation Program will sponsor a two-day workshop on ACTS propagation in
Fall 1989. It is hoped that most of the issues addressed in this article will be
discussed thoroughly in the workshop. The theme of the workshop is "Planning ACTS
Propagation Studies, and Standardization of Propagation Data Collection and
Reduction."
Table 2. Suggested Set of Locations Using Crane's Zones
Number of
Zone Location Stations
BI Idaho i
B2 Denver i
F Los Angeles/Phoenlx/White Sands i
C San Franclsco/Seattle i
DI New Hampshlre/Michigan 2
D2 Blacksburg i
D3 Atlanta i
E New Orleans/Tampa/Miami/Houston i
Total 9
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