An Historical Perspective

Jupsat Pro Astronomy Software

Secrets of the Deep Sky

Get Instant Access

The author is not an historian, but he recognizes the benefit of retrospective examination of the great scientists, their achievements, and the role they play in the advance of space weather and communication technologies. A good deal of the information in this historical perspective is derived from the author's earlier book on HF Communication: Science & Technology (i.e., Goodman [1991]). Another excellent source is a series of articles in EOS by E.W. Cliver [1994a, 1994b, 1995] dealing with solar activity. A bibliography of additional reading is provided at the end of Chapter 1.

As we go through this brief history of the space weather discipline, it will become apparent that adaptive communication system developments and observations of space weather phenomena and the ionosphere are closely related. (See Figure 1-1.) This was especially true in the early years. Our knowledge of the ionosphere and the development of radio communications both derive from 20th Century science and technology. However some vestiges of space weather have been known for many centuries. In fact, auroras, long known for their visual beauty and complexity, have been chronicled since the dawn of recorded history. In the modern history context, auroras are also related to a variety of communication disturbances at high latitudes.

One of the most significant solar-terrestrial observables is the sunspot, a phenomenon related to solar activity. (See Chapter 2 for details.) Sunspots have been associated with certain terrestrial phenomena, including aurora, for more than a century, and the sunspot number has also been a convenient index for many 20th century climatological models of the ionosphere. The number of sunspots has exhibited an eleven-year periodicity for the last 250 years, as shown in Figure 1-2. The first telescopic observations of sunspots were made in 1611 by a number of observers, the most famous being Galileo. Sunspots have been monitored continuously since that time, although a pronounced minimum occurred between 1645 and 1715 (the Maunder minimum), during which time hardly any auroras were observed.

Figure 1-1: Relationship between space weather, the ionosphere, and communications system development. Adaptive telecommunication systems require either (i) organic methods for updating system parameters (viz., the frequency family to be exploited, or power-aperture product to be used) or (ii) non-organic methods from real-time interfaces. These real-time interfaces may involve access to data from NOAA-SEC or the ISES group of Warning Centers. It might also involve data procured from 3rd party vendors.

Figure 1-1: Relationship between space weather, the ionosphere, and communications system development. Adaptive telecommunication systems require either (i) organic methods for updating system parameters (viz., the frequency family to be exploited, or power-aperture product to be used) or (ii) non-organic methods from real-time interfaces. These real-time interfaces may involve access to data from NOAA-SEC or the ISES group of Warning Centers. It might also involve data procured from 3rd party vendors.

1850 1860 1870 1880 1890 1900 1910 1920 1930

Date

1850 1860 1870 1880 1890 1900 1910 1920 1930

Date

Fig. 1-2: Depiction of the sunspot number for the last 250 years.

While sunspots are indicative of many space weather phenomena, the role of geomagnetism in understanding the nature of the ionospheric personality is important, if not central, in many radio propagation applications. This is especially true of terrestrial systems that exploit the skywave mode of propagation at high latitudes (viz., MF/HF systems). The impact of the geomagnetic field on earth-space systems can also be significant, since geoplasma distributions are controlled by the magnetic field, and geophysical phenomena such as scintillation and Faraday fading are also influenced by it.

According to Sidney Chapman [1968], in his book Solar Plasma, Geomagnetism, and the Aurora, the first picture of the so-called auroral oval centered about the geomagnetic pole was drawn by Elias Loomis of Yale University in 1860. In 1878 Balfour Stewart suggested that ionization in the upper atmosphere would account for some of the magnetic field fluctuations that had been observed. By 1892 Stewart had identified the existence of an electrified layer in the upper atmosphere. Subsequently Arthur Schuster recognized this layer to be the origin of electric currents responsible for compass variations. Schuster, who coined the term "ring current", also developed a dynamo theory to explain the diurnal component of ionospheric currents, and he associated the currents with tidal motions of the neutral atmosphere. An aurora, as observed from a fixed terrestrial site, is displayed in Figure 1-3; and a depiction of the instantaneous auroral oval, as observed from space, is provided in Figure 1-4. Chapter 2 will provide more details about auroras and their consequences.

Fig. 1-3: Depiction of the aurora from Alaska. Adapted from image on website of the Alaska Vacation Store in Anchorage, Alaska. Courtesy of Alaska Department of Community and Economic Development

Fig. 1-4: Auroral Oval. IMAGE satellite images of the auroral oval during the Bastille Day storm of 15 July 2000. Note the dynamic behaviors, as each image is only minutes apart. From Lu et al. [2001], by permission.

Fig. 1-3: Depiction of the aurora from Alaska. Adapted from image on website of the Alaska Vacation Store in Anchorage, Alaska. Courtesy of Alaska Department of Community and Economic Development

Fig. 1-4: Auroral Oval. IMAGE satellite images of the auroral oval during the Bastille Day storm of 15 July 2000. Note the dynamic behaviors, as each image is only minutes apart. From Lu et al. [2001], by permission.

The geomagnetic field, as represented by the position of a compass needle, was observed to undergo transient fluctuations as early as the 1700s. Swedish scientist Anders Celsius discovered that magnetic storms exhibited a global characteristic and were not isolated events like tropospheric weather cells. He also discovered the correlation of optical auroras with magnetic activity, and in 1741 he determined that auroral forms were aligned with the geomagnetic field vector. These observations predated the first theory of magnetism developed by Simeon-Denis Poisson in 1824, and Johann Carl Friedrich Gauss made the first systematic measurements of the earth's magnetic field. In 1839 Gauss postulated the existence of ionized regions in the upper atmosphere in his work General Theory of Terrestrial Magnetism. Who actually should get credit for the first suggestion of ionospheric existence is rather controversial. This is because there are some reports that Michael Faraday (in 1832) and Lord William Thomson Kelvin (in 1860) made similar suggestions. Despite these early suggestions, Balfour Stewart generally gets the credit on the basis of his theory on diurnal variations of the geomagnetic field, which was published in 1878.

The most remarkable feature of the upper atmosphere is the visible aurora, a luminous display that appears in the nocturnal sky in high latitudes. Its generic designation is Aurora Polaris, but is termed Aurora Borealis in the Northern Hemisphere and Aurora Australis in the Southern Hemisphere. The Aurora Borealis is sometimes called the Northern Lights. It is now known that auroras occur at any time of day, but cannot be observed in the presence of competing sunlight. Carl Stormer developed one of the earliest explanations of auroral formations by 1911, and one of his most significant contributions was the theory of charged particle motion in the geomagnetic field.

In the years following the work of Stewart and Schuster, work on convenient indices of magnetic activity indices was undertaken. Although magnetic indices were being published by 1885, the first real step toward defining a geomagnetic index at an international level was not achieved until the early 1900s [Mayaud 1980], J. Bartels used magnetic activity indices in 1932 in connection with his discovery that the mysterious M-regions on the sun were associated with 27-day recurrence cycles of magnetic activity. These 27-day cycles were also shown to be related to the mean period of solar rotation. Many years later, magnetic indices have been used to explain the terrestrial effects caused by high-speed solar wind streams associated with the appearance of geoeffective coronal holes and coronal mass ejections (i.e., CMEs).

Let us now depart from classical space weather development and examine some important telecommunication milestones that are relevant to the issues of telecommunications and ionospheric effects. Half a century before Stewart made his suggestion about the existence of the ionosphere

(although that specific term was not used at the time), the English physicist Michael Faraday developed a theory of the electromagnetic field; and 17 years before Stewart's announcement, Maxwell had predicted the existence electromagnetic waves. Maxwell's work specifically dealt with the speed at which magnetic disturbances travel, but his equations are now the cornerstone of electromagnetic theory. Unfortunately his predictions about radiowaves could not be verified at that time, and experimental confirmation of the theory was left to the German physicist Heinrich Hertz. In 1887, Hertz developed the first radio transmitter and loop receiver. With this simple equipment he was able to determine the basic transmission properties of radiowaves.

In 1901 Italian inventor Guglielmo Marconi, transmitted the first long distance (trans-Atlantic) signals from a site at Poldu, England to Newfoundland. It is thought that he used a radio frequency of 313 kHz (in the MF band), a frequency appropriate for ionospheric bounce, and a phenomenon that was unknown at the time. Because of the startling nature of Marconi's result additional theoretical work in EM wave propagation was triggered. The trans-Atlantic experiment by Marconi created a puzzle since the earlier work of Hertz had conclusively demonstrated that radiowaves travel in straight lines unless some object deflects them. This brings us back to the 19th century suggestion by Stewart of a conducting stratum in the upper atmosphere, based upon magnetic disturbances. Perhaps this conducting medium could serve as such an obstacle for electromagnetic waves as well, a suggestion made independently by Arthur Kennelly and Oliver Heaviside in 1902. For many years, that which we now call the E region of the ionosphere was termed the Kennelly-Heaviside layer. It should be noted that the "reflection" mechanism for signal propagation for such long distances, and espoused by Kennelly and Heaviside was itself controversial. Noted physicists including Lord Rayleigh, Henri Poincare and Arthur Sommerfeld had concluded that diffraction around the surface of the earth was the mechanism, a theory that was ultimately disproved by precise field strength experiments. Marconi shared the Nobel Prize in physics with Karl Ferdinand Braun in 1909 in recognition of their contributions to the development of wireless telegraphy.

Following World War I more improvements were made in radio apparatus, and both theoretical and experimental studies continued. In the United Kingdom, Edward Appleton, who is currently associated with the equatorial fountain effect (i.e., Appleton Anomaly), made substantial contributions to magneto-ionic theory. In the United States, Gregory Breit and Merle Antony Tuve, of the Carnegie Institute in Washington DC, conducted landmark radio pulse experiments and later developed the well-known theorem that bears their names. The Breit and Tuve collaboration was the first known experimental verification of the ionosphere using the radio pulse method, and they discovered ionospheric layer height changes from day to night. In 1924, Appleton and Barnett unequivocally proved the existence of the ionosphere using a wave interference method, and this led to the development of new techniques for probing the region. Workers at the newly established Naval Research Laboratory (NRL) in the USA also contributed, using the HF pulse technique in 1925 that provided unique proof of multiple layers in the ionosphere. This technique was later applied by NRL in the development of radar. In the period between 1925-28 NRL investigators conducted additional experiments proving the existence of multiple hops and "skip zones" for oblique propagation of radio waves. Figure 1-5 is an example of "round-the-world" radio propagation, as well as the novel concept of "splashback" or signal backscatter. The backscatter phenomenon later led to the development of Over-the-Horizon-Radar (OTH-R). It should be noted that most of the experiments conducted during the 1920s were motivated by the need to communicate via the new wireless medium. But there was a growing synergy between emergent ionospheric scientists and radio communication engineers.

One of the more prominent scientists involved in early investigations of the ionosphere was Sidney Chapman, who in 1931 published a paper dealing with the Kennelly-Heaviside layer, and who, like E.O. Hulburt before him, provided a foundation for our current understanding of the ionosphere. To this day the Chapman hypothesis for ionized layer formation, while relatively simplistic, is a useful model, especially for the lower layers of the ionosphere.

The theory of radio wave propagation in ionized media has been fascinating from the beginning. In 1912, W.H. Eccles discovered that the refractive index of ionized gas was less than unity, leading to the interesting fact that radiowaves are bent away from the medium normal in a plasma environment and thus toward the horizontal. Joseph Larmor, in 1924, concluded that obliquely launched radiowaves at a specified frequency would be refracted downward, but could escape from the earth if the waves are launched above a certain critical angle. This leads to the notion of an ionospheric iris above a given transmitter through which waves may penetrate, and the existence of skip distances. From the theoretical vantage point, Joseph Larmor, Hendrik Lorentz, E.V. Appleton, and D.R. Hartree provided a clear understanding of radiowave propagation in magneto-ionic media, and the Appleton-Hartree formula for the radio refractive index is of fundamental importance in the analysis and prediction of media effects. In 1932, Appleton published a complete theory of radio propagation in magnetoionic media, such as the Kennelly-Heaviside layer, and he coined the term ionosphere to describe the ionized strata upon which MF and HF skywave propagation depend. Appleton also proposed the nomenclature we now employ for the multiplicity of ionospheric layers (i.e., D, E, F). Now, of course, we recognize the importance of the work of Appleton in a number of areas other than skywave propagation (e.g.; earth-space propagation). Sir Edward Victor Appleton was awarded the Nobel Prize in Physics in 1947 for "his investigations of the physics of the upper atmosphere especially for the discovery of the so-called Appleton Layer". The Appleton layer is now called the F layer, or more properly the F region.

A period of considerable activity in the realms of ionospheric physics and radio engineering characterized the years leading up to World War II and afterward. There were some extraordinary international campaigns that have provided vital information about the ionosphere and associated regions. For background purposes, it is noted that the 1st International Polar Year (IPY-1) was held in 1882-1883. Only a handful of countries were involved and the relevant topics included: solar radiation, the aurora, and geomagnetism. It was the first truly international collaborative scientific activity.

The 2nd International Polar Year (IPY-2) was held in 1932-1933 and was an activity of great significance. It was a multidisciplinary effort, but the science categories of main interest to us within the space weather constituency included: geomagnetism, auroral physics, aeronomy, and ionospheric physics. Appleton chaired the ionospheric science component and the sub-topics were comprised of (i) regular ionosonde measurements of electron density and layer heights, (ii) characteristics of radio propagation, and (iii) the effects of magnetic storms. The data sets obtained were not fully analyzed and some of the data were lost as the result of the ravages of World War II. It is understood that the final publication of the results came as late as 1950. While the negative impact on IPY-2 was significant, the experience gained and discipline associated with international cooperation on a large scale was not lost on the world scientific establishment. More campaigns would follow.

The 2nd World War, because of the importance of radio communication, target tracking and surveillance to the warring parties, invigorated the development of a number of new technologies. We previously alluded to radar, simultaneously invented in the UK and the USA. Communication systems and radio navigation aids were also developed, as well as rudimentaiy countermeasures. It was not long before OTH radar, based upon ionospheric bounce, was developed for long distance surveillance and targeting. Radio electronics blossomed following WWII and during the Cold War that followed. The field of radio astronomy, while directed at the cosmos, provided some interesting glimpses of refractive turbulence in the ionosphere (i.e., scintillation).

Figure 1-5: A typical chart display of various propagation phenomena in 1928. Si, S2, and S3 are pulses initiated by NRL's transmitter. Rb R2, and R3 are "splashback" echoes from the first reflection zone via the ionosphere. ASi and AS2 are received pulses that have traveled around the world. A2S| is the received pulse that has circled the earth twice. Figure adapted from Gebhard [1979],

Figure 1-5: A typical chart display of various propagation phenomena in 1928. Si, S2, and S3 are pulses initiated by NRL's transmitter. Rb R2, and R3 are "splashback" echoes from the first reflection zone via the ionosphere. ASi and AS2 are received pulses that have traveled around the world. A2S| is the received pulse that has circled the earth twice. Figure adapted from Gebhard [1979],

While rockets, developed by the Germans, were designed to have weaponized payloads, the technology of rocket science also found a use in ionospheric and space research following the war. NRL scientists used captured V2 rockets to probe the ionosphere and to explore solar emissions from the vantage point of the tenuous upper atmosphere. Early workers at NRL led by Herbert Friedman, made significant contributions in solar physics, aeronomy and ionospheric physics.

It is only logical that the value of space would be recognized in the Defense Department. By the early 1950's it was clear that artificial satellite systems were going to be developed, and these might be quite useful for surveillance purposes. Two specific activities were initiated during the Eisenhower administration (i.e., moon-bounce radar and earth satellites). Both the US Navy and the US Air Force were involved in lunar studies, with NRL leading the charge in the evaluation of passive reception of communication signals of potential adversaries. A large 600-foot dish antenna was partially constructed at Sugar Grove, West Virginia, an extremely quiet site, but the enormous project was abandoned, when it was determined that satellite techniques would be superior and less expensive. However, there was still a problem with artificial earth satellites as surveillance-gathering tools. The passage of a satellite over another country could be deemed to be violation of its airspace. Fortunately, President Eisenhower had his problem solved by the propositions of the International Geophysical year (IGY), including the stipulation that artificial earth satellites be launched for studies of the earth's surface.

The IGY was held in 1957-58, a period of maximum solar activity. The goals of the IGY were patterned from the IPY campaigns, but IGY emphasized worldwide studies. Additionally, rocket probe availability made it possible to conduct in situ measurements as well as vertical sounding measurements. As mentioned above, the IGY called for the launching of artificial earth satellites to be used as a tool for mapping the earth's surface. As an ingredient of the US component of the IGY scientific program, NRL was designated to place an artificial satellite in orbit, and the project was called Vanguard. As part of this project the first global network for satellite tracking was established.

As is well known, the satellite age began in the late 1950s with launch of the Russian Sputnik satellites, followed by the American Vanguard satellite, a matter we shall revisit below. Maximum solar activity having been investigated during the IGY, the international scientific community embarked upon another coordinated campaign during 1964-65, a period of minimum solar activity. Satellites were available during this period, and the campaign was dubbed The International Years of the Quiet Sun, (IQSY).

During the IGY (solar max: 1957-58) and the IQSY (solar min: 196465) a considerable amount of ionospheric sounding was accomplished at an increasing number of stations worldwide. There was also some solar minimum data obtained prior to the IGY, and the period 1954-1958 exhibited a strong rise from solar minimum activity to the largest epoch of sunspot activity in recorded history. During this 5-year period, a data archive of ionospheric sounding records was obtained from roughly 150 stations around the world. Because of a paucity of soundings from the Southern Hemisphere during the 1954 period of minimum activity, the data set was augmented by soundings obtained during the IQSY. From this augmented data sample it was possible to characterize the basic ionospheric parameters, including the layer maximum densities and heights. This led to the so-called CCIR Model of Ionospheric Characteristics, and the mapping methods employed were based upon the work of Jones and Gallet [1962, 1965], We shall mention the CCIR (now ITU-R) modeling methods in later chapters. It is noted that mapping issues arise from the use of a sparse network of sounding stations, especially if they are irregularly spaced. Topside sounding data can assist in the resolution of these difficulties.

In October of 1957, the Soviet Union was the first to successfully place a satellite in orbit. In fact, the USSR had two successful launches before the United States finally succeeded with Vanguard-I, which was placed into orbit on 17 March of 1958. One of the scientific achievements of the Vanguard program was the discovery that the earth is pear-shaped. The success of Sputnik, as an engineering feat, was a seminal event in space research history and had significant geopolitical implications as well. It created alarm in the United States, and the DoD responded by charging the

Redstone Arsenal team led by Werner von Braun to begin work on the Explorer Project, in parallel with the Vanguard program. Explorer-I, successfully launched in January of 1958, carried a scientific payload developed by James Van Allen, which led to the discovery of the earth's radiation belts (i.e., the Van Allen belts). The Explorer program was associated with a succession of small scientific satellites. The American response to the Sputnik launch was the creation of a civilian space agency in the United States, the National Aeronautics and Space Administration (NASA).

The United States and the Soviet Union engaged in vigorous space programs since the Sputnik-Vanguard days, driven by national pride and strategic concerns. Both countries have managed successful astronaut and cosmonaut programs, although each has experienced major human disasters along the way. There have also been striking successes of the manned and unmanned programs. It is beyond the scope of this short historical survey to itemize all the various space launches and scientific advances tied to them. It is worth mentioning a few key activities of special relevance to space weather and its impact on telecommunication systems.

The race to the moon was an exciting event during the 1960's. While the endeavor was largely driven by geopolitics, there was already keen interest in our nearest celestial neighbor, the moon. Before the age of satellites, the field of radar astronomy was emerging (see excellent book by Evans and Hagfors, [1968]). In the late 1940s, both the US Army (i.e. Project Diana) and NRL scientists succeeded in reflecting signals off the moon, but little information was ever published because of security concerns. In the late 1950s, John V. Evans, using the Jodrell Bank Radar Observatory in the UK, studied radar signals reflected from the moon and concluded that slow regular fading of the echoes was caused by the Faraday effect, probably the first observation of the phenomenon outside the field of magneto-optics. This enabled Evans and his team to derive the line integral of the electron population between the earth and the moon, the so-called cislunar electron content. Other attempts at measurement of the electron content using the moon bounce method followed, but the most important measurements of the ionospheric electron content were made using artificial earth satellites rather than lunar echoes.

In 1960, NASA launched Echo 1, a large metalized balloon that was designed to perform as a passive reflector of radiowave signals. It was used as a reflector of radio, television, and telephone signals, enabling intercontinental communication. Radar echoes from the large Echo 1 and 2 were also employed for calibration of earth-space and search radars and some rudimentary ionospheric studies. While the Echo program was significant in the development of various ground station and space tracking techniques, it was abandoned in favor of active satellites from a communications perspective.

A large number of low earth orbiting satellites (i.e., LEOs) were launched in the 1960s and beyond, and they were equipped with telemetry beacons, typically at VHF (i.e., 136 MHz). These beacons were used for propagation studies although that was not their original intent. Faraday rotation, dispersive Doppler, and hybrid methods were used to obtain information about horizontal structure of the ionosphere. Much was learned by these methods, but the most significant advance in ionospheric measurement by the middle 1960s was the so-called topside sounder.

The Alouette satellite was launched into a 1000 km circular orbit in September 1962. It was a polar orbiting satellite with an inclination of 80 degrees. The instrumentation consisted of a swept-frequency sounder from 0.5 to 11.5 MHz, as well as some auxiliary instruments. With Alouette data, the variation of the N(h) as a function of latitude can be determined. Figure 1-6 depicts the topside electron density distribution between 64°N and 45°S along the 75°W meridian. As suggested earlier, there were some horizontal inconsistencies in the global maps derived from ground-based ionosonde. Topside sounder data assists in the resolution of these problems, especially in regions such as the high latitude trough and the equatorial anomaly.

Geosynchronous satellites were an important advance in the art of ionospheric study. Since the satellites were positioned in the equatorial plane with a rotational period of 24 hours, they were almost geostationary. While LEOs that moved rapidly over a given field of view represented a "frozen" picture or virtual snapshot of the ionosphere, the so-called GEOs enabled ionospheric investigators to examine the time variation of the ionosphere.

The Mercury and Gemini programs preceded the Apollo program, which was associated with successful lunar landings in the late 1960s and early 1970s. The Gemini series of satellites provided several opportunities for propagation experiments to be conducted, including measurements of the subsatellite electron content using Faraday rotation [Goodman, 1967],

The first successful monitoring of solar x-ray and Lyman-alpha radiation by a satellite was performed using SOLRAD-1 in 1960. The Naval Research Laboratory launched a series of solar radiation satellites, culminating in SOLRAD 11A and 1 IB (i.e., a pair of satellites dubbed SOLRAD HI, at an altitude of 65,000 nautical miles) in the late 1970s. The only mission of the SOLRAD satellites was to monitor all aspects of the solar activity. The telemetry data from the satellite was received at the Blossum Point, Maryland and then conveyed to the NRL campus (Washington, D.C.) for processing and dissemination to users such as NOAA Space Environment Services Center (SESC; and currently NOAA-SEC) and the US Air Force Global Weather Central (or its equivalent). SOLRAD HI data was utilized in various near-real time prediction systems such as PROPHET (see below).

Light Height

Light Height

Figure 1-6: Alouette ionograms were analyzed along a pass along the 75°W meridian on 24 October 1962. The equatorial anomaly is evident in this early record. From Brown [1965], after Lockwood and Nelms [1964],

Probably the first use of space weather data to identify and solve realtime communications problems was carried out by U.S. Navy investigators [Argo and Rothmuller, 1979; Richter et al, 1976], The U.S. Navy was vitally interested in the performance of communication and navigation systems, and in the late 1970s, these systems included: the emergent Global Positioning Satellite System operating at SHF, the Fleet Satellite Communications System, HF and VLF Fleet Broadcast, and VLF and ELF strategic communications. These systems still exist is various forms. Engineers at Naval Ocean Systems Center (NOSC) in San Diego, CA developed a minicomputer platform, called PROPHET, which was used to develop realtime estimates of system effects derived from ionospheric and solar data sets. PROPHET used phenomenological, statistical, and semi-empirical models of the various phenomena; and these models and their outputs supported the OMEGA navigation system, satellite communication systems, and a number of HF communication and surveillance systems. Table 1-1 is a description of the forecasting models employed by PROPHET. The imbedded models were not all developed in-house, and a noteworthy example of this was a simplified scintillation model derived from the work of Fremouw and Rino [1973], Pope [1974] and LaBahn [1974],

Table 1-1: PROPHET was the first system to use Space Weather data to derive near-real-time performance predictions and operational guidance. Adapted from Argo and Rothmuller, [1976].

MODEL (SOURCE)

FUNCTION

CAUSATIVE SOURCE

SECTION

ACTION

Hare detection

Disturbance warning

VLF navigation

Warning

Flare detection estimate (NOSC)

Disturbance warning

VLF navigation

Warning

SID GRID (NOSC)

Disturbance warning-SWF

1 to 8A x-rays

— Reroute traffic HFDF

— Net impact assessment

PCA/VLF (NOSC)

Disturbance waming-PCA

10 MeV Particles

VLF navigation (Omega)

Correction factor for transpolar circuits

SPA/VLF (NOSC)

Disturbance warning-SPA

1 to 8A x-rays

VLF navigation (Omega)

Correction factor for sunlit circuits

LOF Split (NOSC)

Tactical (reduced intercept vulnerability)

Solar diumal transition 1 to 8A x-rays

Covert HF systems

Optimum frequency selection against known receivers

Scintillation grid (SRI)

Disturbance warni ng/tacti cal

Unknown (Statistical model)

VHF/UHF satellite communications

Advisory -dB fade probability based on location

PCA/HF (NOSC)

Disturbance warning-PCA

lOMeV particles

— Reroute traffic HFDF

— Net impact assessment

Quiet MUF (rrS/GMC)

MUF during normal times

All HF

HF comm normal operations

QLUF

LUF during normal times

All HF

HF comm normal operations

Raytrace

Receiver accessibility

.411 HF

HF comm normal operations

The PROPHET environmental prediction terminal is depicted in Figure 1-7. Data from a number of satellites such as SOLRAD-11 (e.g., SOLRAD HI), and from service centers such as the NOAA Space Environment Services Center (SESC) and the Air Force Global Weather Central (AFGWC), was conveyed to the NOSC data fusion center in San Diego for data sorting and quality control, and then disseminated to the individual (deployed) minicomputer platforms that hosted the PROPHET software. Engineers at NOSC also conducted field tests of the various products using selected communication stations (e.g., Stockton Naval Communication Station). The operating personnel found the system to be very useful and concluded that communication outages for the HF circuits were reduced by 15-20% using the near-real-time system.

Prophet Terminal j. NCS Honolulu

Forecasts r - Y^Ti«B¡iim¡r "f — '• ' " V-i —

Requests

Point, Maryland

NOAA Space Environment t. ,

Boulder, Colorado «'fr-^r SOLRAD

Air Force Global Weather Central Offutt AFB, Nebraska

69,000 mile Orbit

Figure 1-7: The PROPHET concept developed by US Navy Engineers at NOSC was the first attempt to use real-time space weather data operationally with success. The information flow for the system is shown. Adapted from Argo and Rothmuller [1976].

One of the monumental achievements of the 20th Century was the development of satellite navigation, culminating in the operational Global Positioning System (GPS). The Naval Center for Space technology at NRL developed the concept of passive ranging from space in 1964 and successfully demonstrated the feasibility of satellite navigation systems in

1967-68 with the launch of the Timation-1 and Timation-2. In 1973, management of the Timation program was assumed by the US Air Force to form the NAVSTAR/GPS program. The first NAVSTAR/GPS satellite was launched in 1978. The GPS space segment currently consists of a constellation of 24 operational satellites, with 4 satellites in each of 6 orbital planes, at an altitude of 11,000 nautical miles, and having an orbital period of 12 hours. Users can "see" between five and eight satellites from anywhere on the earth. The GPS system has been designed to eliminate the excess ionospheric group path delay since two frequencies are employed for compensation. Still there are ionospheric and space weather effects to be concerned with. Strong radiowave scintillation can cause GPS receivers to malfunction, and single-frequency users can suffer large errors during magnetic storms.

There are now two other systems in use or under development: GLONASS (Russia) and Galileo (European Space Agency). There are numerous military and civilian uses for satellite navigation systems, and we shall investigate the various applications in more detail in later chapters.

It should be noted that the GPS system is being exploited for ionospheric studies. Jules Aarons, currently with Boston University, has stimulated interest in global scintillation studies using GPS. Scintillation is discussed in the paragraph below. It should also come as no surprise that the GPS is also being used to investigate the Total Electron Content (TEC) on a global basis. The various mapping schemes using various GPS receiver networks will be covered in a later chapter. The GPS system is constantly finding additional uses, and upgrades are being developed for specific purposes such as precise landing. The U.S. FAA WAAS system is but one example.

It is well known that stars twinkle because of atmospheric turbulence resulting in refractive index fluctuations in the optical portion of the electromagnetic spectrum. Stars that emit radio waves are called radio stars, and have been used to examine the twinkling of radio signals. Such twinkling of signals received from radio stars is generally referred to as scintillation, and it was deduced that radio star scintillation was largely due to temporal and spatial variations in ionospheric refractivity. A number of investigators used radio star scintillation measurements to investigate ionospheric inhomogeneous structures such as spread-F in the equatorial region. J.P. Wild [1956] used signals from a source in the constellation Cygnus to derive dynamic spectra, and apparently observed focusing irregularities. Other early investigators included B.H. Briggs [1958] and M. Dagg [1957]. One of the major disadvantages associated with the use of radio stars in propagation studies is the paucity of discrete sources. There are only two sources that have proven to be very useful, and they are both located in the northern sky: Cygnus A and Cassiopeia A.

Radio and radar astronomy preceded satellite studies for ionospheric investigation. Jules Aarons [1963] provides a good summary of competing methodologies, circa 1962, in an Introduction to the volume Radio Astronomical and Satellite Studies of the Atmosphere. While analysis of radio stars and analysis of lunar reflections provided some interesting information about the intervening ionosphere, it is clear that satellite studies have provided more detailed and synoptic information. However, we should not lose sight of the contributions made by rocket probes, especially in the early days. Other techniques have also been brought to bear, including: incoherent backscatter radar (i.e., Thomson scatter), and terrestrial radars used for ionospheric monitoring. Robert Hunsucker, in his book Radio Techniques for Probing the Terrestrial Ionosphere, has discussed various experimental methods.

It wasn't long before investigators began to exploit satellite transmissions as a replacement for radio stars. Workers at the Air Force Cambridge Research Laboratory (AFCRL), now called the Air Force Geophysics Laboratory (AFGL), and led by Jules Aarons, have made significant contributions to this field, and much of our understanding of the climatology of ionospheric scintillation derives from investigations conducted by U.S. Air Force scientists and affiliated organizations. It should be noted that Jules Aarons was a pioneer in beacon satellite studies of the ionosphere, and is credited for promoting an international investigation of scintillation morphology on a global scale.

NASA obtained a considerable amount of telemetry data over the years using the Minitrack system, and the VHF signals exhibited disruptions due to scintillation, especially at certain equatorial and high latitude stations. Tom Golden, a NASA communication network engineer, recognized that a model of the worldwide scintillation activity would have operational merit. Other agencies subsequently developed an interest. A significant amount of satellite data, including data from a multi-beacon satellite DNA-002 (i.e., Wideband), was used to develop a semi-empirical model (Fremouw and Rino [1973]; Pope, [1974]). The firm Northwest Research Associates (NWRA) maintains the current operational model, WBMOD, for the U.S. Air Force with certain improvements in the equatorial and high latitude regions (Secan [1995, 1997]). This climatological model provides a baseline estimate of ionospheric scintillation in the absence of real-time methods.

An anecdote is in order. TACSAT-1 was the largest communications satellite ever put in orbit at the time of launch in 1969, and it served the mobile user needs of the US military. UHF was a critical band for tactical communications, especially during the Viet Nam War period. It should be noted that a period of maximum solar activity occurred in the period 19681972, leading to enhanced scintillation probability, especially over the equatorial regions (i.e., ± 20° from the magnetic equator). Conventional wisdom at the time suggested that the roll-off of scintillation with increasing frequency above VHF was relatively steep, being based upon an incomplete theory and limited data sets. But, alas, this was not the case. Significant outages were observed, and this led to increased emphasis on the development of scintillation countermeasures (i.e., diversity schemes) at UHF and the consideration of even higher frequencies. The significant level of scintillation activity at UHF also led to further investigations of the underlying theory of scintillation. These issues are covered more fully in Chapter 4.

It is clear that the various DoD mission areas are the primary drivers for many of the space weather applications relating to telecommunications (i.e., communications, navigation, and surveillance). While scientists at NRL can be credited with many firsts in the area of ionospheric phenomenology, it can be safely said that the geophysicists and engineers at the AFCRL/AFGL have contributed significant amounts of operationally useful information in the context of telecommunications and space weather. This is not to say that there have not been significant civilian and university contributions as well.

Since the 1980s, there are three things that have provided the greatest boost in our capabilities for assessment and prediction of space weather parameters, especially those of importance in real-time support of telecommunication systems. There are: (i) faster and more capable computers, (ii) the Internet and improved data accessibility, and (iii) more observational capability. There have been significant developments in data-driven modeling and data assimilation approaches, similar to those methods used in the tropospheric weather prediction business. Mapping technologies have been improved and novel methods for prediction have been advanced.

Was this article helpful?

0 0
Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment