Introduction
According to recent reports from the National Oceanic and Atmospheric Administration (NOAA), solar flare activity is increasing in frequency and is poised to present detrimental effects to critical Department of Defense communications-electronics (CE) equipment.
The purpose of this article is to briefly introduce the general technology-minded reader to the detrimental effects of solar flare activity and begin a dialogue by which the professional acquisition community can begin to plan mitigation methods that will reduce or completely negate the impending, damaging impact of solar flare activity on DoD's CE equipment.
The Department of the Navy Chief Information Officer, Office of the Chief of Naval Operations and Naval Air Systems Command's Electromagnetic Environmental Effects Division have already begun to examine this not so well-known phenomenon by opening the dialogue with prestigious universities such as Cornell University.
Cornell University has established a space weather research group dedicated to uncovering the harmful effects of solar flare activity, and they are working toward the development of successful mitigation methodologies.
Overview of Space Weather
Space weather begins at the sun. The sun exhibits an 11-year cycle of sunspots that are visible manifestations of an increased solar magnetic field. The last sunspot maximum (peak of activity) was in 2000, and the next one is expected in 2011.
The maxima are somewhat broad and last three to five years. During the sunspot maximum, the solar magnetic field is disrupted by solar flares (extremely large explosions) emitting solar ultraviolet light, x-rays, energetic particles (millionelectron-volt protons), coronal mass ejections (high temperature plasma gases which give a ring-like appearance around the sun or any other celestial body), and a "stormy" solar wind.
Certain larger flares produce solar radio bursts of broadband noise from 10 megahertz to 10 gigahertz that may directly affect Global Positioning System (GPS) receivers on the dayside of the Earth. Although larger solar flares produce solar radio bursts, a one-to-one relation between the size of a solar flare and the intensity of a solar radio burst does not exist.
Coronal mass ejections and stormy solar winds frequently reach the Earth, if they originate on the part of the sun facing the Earth. These ejections arrive as supersonic shock waves, frequently carrying high-energy particles.
Because the solar wind is fully ionized, it first encounters the Earth's magnetic field. The high-energy particles can directly reach the upper atmosphere over the north and south poles, endangering transpolar air flights.
Depending on how the solar magnetic field captured in the solar wind encounters the Earth's magnetic field, a magnetic storm may develop. In a magnetic storm the Van Allen radiation belts (the charged plasma particles surrounding the Earth) are rearranged, creating a doughnut that carries a ring current of 100 kiloelectron-volt plasma around the Earth.
This current creates a magnetic field opposite to the Earth's magnetic field at the surface of the Earth. The disturbance magnetic field may amount to 1 percent or more of the Earth's field, thus it is called a magnetic storm.
The radiation belts pose a hazard to medium Earth orbit and geostationary Earth orbit spacecraft because of spacecraft charging that may cause static discharges in delicate electronics in the short run and solar cell power reduction from radiation damage in the long run.
They are also potentially fatal to astronauts if exposed directly to the radiation. During these storms the rearrangement of the Earth's magnetic field and creation of the ring current drive disturbances in the ionosphere as well.
The ionosphere is the uppermost part of the atmosphere produced by solar ultraviolet light ionizing the thermosphere at about 350-kilometer altitude. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth and to signals between satellites and the ground.
An important aspect of the solar cycle is that the average solar ultraviolet light increases substantially at solar maximum. Since solar ultraviolet light produces the ionosphere by direct ionization and heats the thermosphere, the ionosphere is denser and thicker during solar maximum.
Hence GPS signals are more strongly affected by the ionosphere during solar maximum. For example, ranging signals will have larger errors and experience large/rapid amplitude and phase fluctuations (scintillation), leading to larger navigation errors or, in extreme cases, temporary failure to navigate. See Figure 1 for an illustration of this phenomenon.
Ionospheric space weather can be roughly organized into three categories: equatorial latitudes, mid-latitudes and high latitudes. At equatorial or tropical latitudes, it frequently will affect GPS signals with the intensity modulated by the solar ultraviolet light intensity, as noted above. However, the occurrence of ionospheric weather in the tropics is usually suppressed by solar and magnetic storms.
At mid-latitudes, ionospheric weather is dominated by magnetic storms. Large storms move the aurora (brilliant display of bands or streamers of light observed in the night sky, particularly in polar regions) equatorward over the United States, and all magnetic storms have the potential to move equatorial plasma poleward and create thicker ionospheres.
At high latitudes, the northern lights, as well as high density ionospheric structures called "blobs," occur frequently but usually do not have a major impact on GPS signals.
Mitigating Space Weather Effects on GPS Receiver Operation
The first step in mitigating the effects of space weather on GPS signals is monitoring. Scintillations and rapid changes in total electron content (the number of electrons in a one meter cross-section between the receiver and the transmitter) produced by the ionosphere have unique signatures that can be used to detect their presence.
Scintillations are a combination of destructive and constructive interference produced when small scale density irregularities in the ionosphere scatter electromagnetic signals. Similar phenomena can be observed when looking though jet engine exhaust.
In transiting the ionosphere, electromagnetic signals, such as GPS signals, slow down and the excess time lag introduced is proportional to the total electron content. Without monitoring, anomalous receiver performance cannot be properly diagnosed. For example, monitoring is helpful in distinguishing ionospheric scintillations from a flock of birds roosting on or near a receiving antenna.
Second, you can predict when space weather will occur. There are a variety of aids to help in this effort. NOAA's Space Environment Center Space Weather service is useful for both nowcasting and forecasting magnetic storms and solar flare activity.
Satellites, (located upstream at the L1 Lagrangian point — where the Earth's and the sun's gravity cancel each other), monitoring the solar wind can yield predictions up to an hour in advance.
Solar imaging satellites can detect the onset of coronal mass ejections, yielding substantially earlier predictions. These observations are being combined with models to predict the effect on the Earth's magnetosphere and ionosphere.
Third, we can design better GPS receivers. Current receivers are not designed for a scintillating environment nor are their performance evaluated in the presence of scintillations. They are not able to detect or report whether a GPS signal is scintillating. The noise bandwidth of a GPS receiver's frequency or phase lock loops is not optimized for a scintillating environment.
GPS software receivers may be particularly useful in this application since their operation can be flexible. The receiver tracking loop bandwidth can be increased when the signals are robust and decreased when the signals are scintillating.
Finally, remember that GPS signal scintillations are not the only space weather effect on GPS signals. Solar radio bursts reduce the signal to noise ratio by increasing the noise ratio, which can threaten GPS receiver operation. Fast-moving ionospheric gradients can produce rapid signal phase changes that endanger the receiver's ability to track GPS signals.
Dr. Kintner is a professor of electrical and computer engineering at Cornell University in Ithaca, N.Y. Kintner received a bachelor of science degree in physics from the University of Rochester and a Ph.D. in physics from the University of Minnesota.