WASHINGTON – Bursts of energy from the Sun on microwave
radio frequencies can disrupt wireless cell communications several
times a year, according to scientists who have studied records
covering 40 years of such bursts. Solar bursts are most likely to
occur around solar maximum, the most active portion of the Sun’s
11 year cycle. One such maximum was recently passed, but
significant bursts may occur for several more years, according to
Louis J. Lanzerotti of Lucent Technologies’ Bell Labs in Murray
Hill, New Jersey, one of the researchers. The study not only
examines the effect on current systems, but also looks at higher
frequencies where future systems will operate, says Dale E. Gary,
associate professor of physics at New Jersey Institute of
Technology (NJIT), Newark, and principal investigator of the
project.
A good understanding of how solar bursts affect cellular
communications will help in the design of future generations of
wireless systems, the researchers say. Their report is published
March 7 and appears in the March-April issue of the journal, Radio
Science, published by the American Geophysical Union.
The study was possible only because of an archive of data on solar
radio bursts that has been assembled by the National Oceanic and
Atmospheric Administration (NOAA) from observations made
around the world by the U.S. Air Force and other entities and that
is now maintained by the National Geophysical Data Center (NGDC) of NOAA in Boulder, Colorado. The first detections of
solar radio bursts (at much lower frequencies) were made
inadvertently in 1942 by some of the earliest radars deployed
during World War II. After the war, solar radio studies became a
recognized field of astronomical research, and the Air Force was
active in collecting data, since the bursts continued to affect radar.
Because cellular communication has greatly expanded in recent
years, the researchers looked back at the last four decades (1960-
1999) of NGDC data in the context of noise levels found in
wireless communication systems. This data interval covered
slightly more than four solar cycles, including the solar maximum
in 1989-1991 which occurred before cellular communications
became ubiquitous around the world. The researchers note that the
number and location of collection points varied over time, and the
instruments used to measure solar radio bursts have improved
significantly since the early years. They do not believe that these
variations affect the main results of their study.
Radio wave energy received from the Sun is measured in solar flux
units (SFU), with one SFU equaling 10^-22 [1/10 followed by 22
zeros] watts per square meter of receptor area per hertz. During a
burst, the energy received may be as high as 100,000 SFU, with the
energy also depending upon the frequency measured. In the study,
the scientists at Bell Laboratories, together with Gary of NJIT,
sought to determine how often bursts of at least 1,000 SFU have
occurred over the years, this being the level that can potentially
disrupt cell communications by covering conversation with noise
or causing calls to be dropped.
Counting the number of solar bursts was difficult, since the same
event may have been recorded by several monitoring stations, often
on different frequencies, and separate events may also have
occurred close in time to one another. The researchers’ analysis
suggested that on 12 minutes was the minimum interval between
what they would regard as separate solar bursts, and they limited
their study to the frequency range of 1-20 gigahertz (Ghz). Most
present-day cell phone transmitters currently operate in the band
from 900 megahertz (MHz) to around 3 GHz.
The analysis of the data by the research team, which also included
Dr. Bala Balachandran and Dr. David Thomson, then of Bell Labs,
revealed that solar radio bursts of 1,000 SFU can occur on 10-20
days per year, on average, with higher rates and stronger bursts
during solar maximum periods and lower, weaker ones during solar
minimum periods. The effect of bursts on wireless communications
is dependent upon the orientation of cell antennas, with those
pointing east-west more susceptible mornings and evenings than at
noon. Therefore, any given cell site might be affected by solar
radio bursts only every 40-80 days, or several times per year on
average. But any single burst could affect a large service area, since
number of cell sites are likely to be pointed in the direction of the
Sun when an event occurs. Furthermore, the impacts on service, in
terms of increased noise levels and call disruptions, would be
expected to be more frequent during the years of maximum solar
activity.
The study was supported in part by Lucent Technologies and in part
by the Space Weather Program of the National Science Foundation
at the New Jersey Institute of Technology.
