44 GPS WORLD July 2000 www.gpsworld.com

called ions. Such an ionized gas is known asa plasma.

The ionosphere is not bound by specific,fixed limits, although the altitude at whichthe ionosphere begins to be detectable isabout 50 kilometers and stretches to heightsof 1,000 kilometers or more. The upperboundary of the ionosphere is not welldefined as the electron distribution thins outinto the plasmasphere (or protonosphere, asthe dominant positive ions there are protons)and subsequently into the interplanetaryplasma.

As altitude decreases, the ionosphere’sabsorption of EUV light increases as does thedensity of neutral atoms and molecules. Thenet result is the formation of a maximumelectron density layer. However, because dif-ferent atoms and molecules absorb EUV lightat varying rates, a series of distinct regions orlayers of electron density exist, termed D, E,F1, and F2. The F2 layer is usually where themaximum electron density occurs. The struc-ture of the ionosphere is not constant but iscontinually varying in response to changes insolar radiation and the Earth’s magnetic field.This variability impacts the signals from the GPS satellites as they pass through theionosphere on their way to users’ receivers.Before examining how the ionosphere affects GPS, let’s see how the Sun affects the ionosphere.

SOLAR ACTIVITY In addition to ultraviolet light and x-rays, theSun continuously emits a wide spectrum ofelectromagnetic radiation as well as particleradiation primarily in the form of electronsand protons, called the solar wind. The near-constant level of this radiation deviates byonly a small amount. But this variability isquite important, as it directly affects the par-ticles in the ionosphere. Heightened solaractivity is characterized by an increase in thenumber of flares, coronal holes, and coronalmass ejections.

A solar flare is a sudden energy release inthe solar atmosphere where electromagneticradiation, and sometimes energetic particlesand bulk plasma are emitted. A suddenincrease of x-ray emissions, resulting from a

The solar maximum is upon us. It brings withit higher and more variable ionospheric elec-tron densities that can try the souls of someGPS receivers. For any ranging system suchas GPS, the propagation speed of the signalsis of critical importance. This speed, whenmultiplied by the observed propagation timeinterval, provides a measure of the range. Inthe case where an electromagnetic signalpropagates in a vacuum, the speed of propa-gation is the vacuum speed of light — validfor all frequencies. However, the signalstransmitted by the GPS satellites must passthrough the Earth’s atmosphere on their wayto receivers on or near the Earth’s surface.These signals interact with the constituentcharged particles and neutral atoms and mol-ecules in the atmosphere, resulting in achanged propagation speed and direction —the signals are refracted.

When describing atmospheric refractioneffects on radio waves, such as GPS signals,it is convenient to separate the effects of neu-tral atoms and molecules primarily containedin the troposphere (the lowest portion of theatmosphere) from the charged particles prin-cipally contained in the ionosphere, the partof the upper atmosphere that is significantlyaffected by solar activity. In recent “Innova-tion” columns, such as “Enhancing GPS:Tropospheric Delay Prediction at the MasterControl Station” in the January 2000 issue ofGPS World, we have examined the tropos-phere’s influence on the GPS signals. This month, we turn our attention to theionosphere.

THE IONOSPHEREThe ionosphere is the region of the Earth’satmosphere where ionizing radiation primar-ily in the form of solar extreme ultraviolet(EUV) and x-ray emissions causes electronsto exist in sufficient enough quantities toaffect the propagation of radio waves. Whenthe photons that make up the radiationimpinge on the atoms and molecules in theupper atmosphere, their energy breaks someof the bonds that hold electrons to their par-ent atoms. The result is a large number offree, negatively charged electrons as well aspositively charged atoms and molecules

GPS, the Ionosphere, and theSolar MaximumRichard B. Langley University of New Brunswick

Oh, it was wild and weird and wan, and everin camp o’ nightsWe would watch and watch the silver dance ofthe mystic Northern Lights. And soft they danced from the Polar sky andswept in primrose haze; And swift they pranced with their silver feet,and pierced with a blinding blaze.

So wrote Canadian poet Robert W. Service inthe “Ballad of the Northern Lights.” Thenorthern lights, also known as the auroraborealis, are a product of the complexrelationship between the Sun and the Earth.

More frequent auroras at more southerlylatitudes are evidence of the period ofmaximum solar activity now upon us. Thesolar maximum, which occurs approximatelyevery 11 years, also brings with it more activeionospheric conditions. The more frequentejections of high-energy electromagneticradiation and particles from the Sun aroundthe time of the solar maximum results ingreater ionospheric electron densities andmore variable densities. And as the signalsfrom the GPS satellites must pass through thismore active ionosphere on their way to Earth-bound receivers, there are potential problemsfor GPS users. In this month’s column, we willlook at how solar activity affects the iono-sphere, how the ionosphere affects GPS, andhow these effects can be ameliorated to reducetheir impact.

I N N O V A T I O N

www.gpsworld.com GPS WORLD July 2000 45

flare, causes a large increase in ionization inthe lower regions of the ionosphere on thesunlit side of Earth. The x-ray radiation, trav-eling at the speed of light, takes approxi-mately eight minutes to arrive at the Earth,resulting in an effect soon felt after the flareoccurs. High-energy particles can reach theEarth in a fraction of an hour. The lower-energy particles flowing in the solar wind, onthe other hand, take two to three days to reachthe Earth. Solar flares are usually associatedwith the strong magnetic fields that manifestthemselves in sunspots — relatively coolareas that appear as dark blemishes on theSun’s face. The sunspots are formed whenmagnetic field lines just below the Sun’s sur-face become twisted and poke through thesolar photosphere — the visible solar surface.

Sunspots have been observed since at leastthe early 1600s with regular, daily sunspotcounts beginning in 1749. The number ofsunspots waxes and wanes with an approxi-mately 11-year periodicity called the solar

I N N O V A T I O N

cycle, shown in Fig-ure 1. Actually, ratherthan merely countingindividual sunspots,solar astronomerscompute a sunspot number based on thenumber of sunspotsand sunspot groups.The day-to-day sun-spot number can varywidely, so a runningaverage is typicallycomputed to showlong-term trends. Weare currently in cycle

23 and near its maximum, shown in Figure 2. Another sign of increased solar activity

are coronal mass ejections (CMEs), hugebubbles of gas threaded with magnetic fieldlines that are ejected from the Sun. (Thecorona is the outermost layer of the solaratmosphere). Coronal mass ejections areoften associated with solar flares and promi-nence eruptions. The frequency of CMEsvaries with the sunspot cycle. At solar mini-mum, we observe approximately one CMEper week. Near solar maximum, we observean average of two to three CMEs per day. If aCME is directed toward the Earth, a world-wide disturbance of the Earth’s magneticfield, called a geomagnetic storm, can occur.

Space weather. Geomagnetic storms oftenresult in ionospheric disturbances. Magneticstorms and associated ionospheric stormsoccur when high-energy charged particlesfrom solar flares, prominence eruptions, orcoronal holes (regions of exceptionally low-density that show up as dark areas in x-ray

images of the Sun) arrive at the Earth, caus-ing perturbations in the Earth’s magneticfield. The charged particles interact with theEarth’s neutral atmosphere, producingexcited ions and additional electrons. In fact,we can use measurements of the geomagneticactivity as a proxy for ionospheric activity.The strong electric fields that are generatedcause significant changes to the morphologyof the ionosphere, changing the propagationdelay of GPS signals within time intervals asshort as one minute. Such changes, primarilyin the polar and auroral ionospheres, can lastfor several hours.

As we noted in the introduction, solaractivity is also responsible for the colorfulauroras — the northern and southern lights.The Earth is surrounded by a magneticcocoon (the magnetosphere) created by theinteraction of the Earth’s magnetic field with that of the Sun. When the solar wind isparticularly strong, it appears to create tearsin the cocoon, allowing charged particlestrapped in the magnetosphere to enter theionosphere and spiral down along the Earth’s magnetic field lines. The particles,primarily electrons, interact with oxygenatoms or nitrogen molecules in the atmos-phere. Energy from an atom or moleculeexcited by fast electrons is released as red(oxygen or nitrogen) or green (just oxygen)light.

Figures 3 through 6 illustrate a recentCME and its effect on the Earth’s magneticfield and atmosphere. The CME spawnedG4-level magnetic storms (storms or fluctua-tions in the magnetic field are rated on a scalefrom 1, or minor, to 5, or extreme) as well asfrequent auroras.

Figure 1. Daily observations of sunspots have been carriedout since 1749. Initially conducted at the Swiss FederalObservatory in Zürich, the sunspot count is currently com-puted from observations by an international network ofobservers. Monthly averages of the International SunspotNumber show that the number of sunspots visible on the Sunwaxes and wanes with an approximate 11-year cycle.

1750 1760 1770 1780 1790 1800 1810 1820 1830 1840 1850DATE

0

100

200

300

SU

NS

PO

T N

UM

BE

R

1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950DATE

0

100

200

300

SU

NS

PO

T N

UM

BE

R

1950 1960 1970 1980 1990 2000 2010 2020 2030 2040 2050DATE

0

100

200

300

SU

NS

PO

T N

UM

BE

R

Figure 2. The current sunspot cycle, number 23, is predicted topeak this year. Shown in this figure are the observed monthlysunspot averages and the smoothed sunspot count predictedby scientists at NASA’s Marshall Space Flight Center. Alsoshown by dotted lines is the statistical uncertainty of theirpredictions.

1996 1998 2000 2002 2004 20060

50

100

150

200

46 GPS WORLD July 2000 www.gpsworld.com

REFRACTIVE INDEXSo how does the ionosphere’s behavior, governed by solar activity, affect GPS? Tomeasure the range between a GPS receiverand a satellite, we need to know preciselyhow fast the radio signals from a satellitepropagate. We can characterize the propaga-tion speed of a radio wave (a pure carrier) ina medium such as the ionosphere by itsrefractive index, n, which equals the ratio ofthe speed of propagation in a vacuum (thevacuum speed of light), c, to the speed in themedium, v:

(1)

The speed of propagation of a pure orunmodulated wave is actually the speed atwhich a particular phase of the wave propa-gates, or the phase speed, and we refer to therefractive index governing this speed as thephase refractive index. A medium may bedispersive, in which case the phase speed is afunction of the wave’s frequency.

A signal, or modulated carrier wave, canbe interpreted as the superposition of a groupof waves of different frequencies centered onthe carrier frequency. If the medium is dis-persive, each of the waves making up the

n = cv

I N N O V A T I O N

Figure 3. On April 4, 2000, at 1541Coordinated Universal Time (UTC), amagnetically complex sunspot regionnear the west limb of the Sun pro-duced a large flare that was accompa-nied by a coronal mass ejection. Thisimage, taken by the Extreme ultravio-let Imaging Telescope (EIT) on theSolar and Heliospheric Observatory(SOHO) spacecraft, shows the activeregion near the right-side limb atabout the two-o’clock position.

Figure 4. The halo coronal massejection (CME) which occurred onApril 4, 2000 was captured by theLarge Angle and Spectrometric Coro-nagraph Experiment (LASCO) C2coronagraph on the SOHO spacecraft.This running-difference image, thedifference between two sequentialimages, shows the CME when itsleading edge had already left the C2’sfield of view. The solid-colored disk inthe middle is an occulting disk thatblocks out the Sun’s intense light toreveal the faint corona. The whitecircle shows the size and position of the Sun.

Figure 5. The fast-moving material in the CME launched from the Sun on April 4,2000 arrived at the Earth two days later and generated a geomagnetic storm. Plotsof the geomagnetic field components at St. John’s, Newfoundland, show a suddenimpulse at about 1640 UTC on April 6. The field became quite disturbed shortlythereafter.

Figure 6. When the shock front fromthe April 4 CME reached the Earth, ittriggered auroras that were seen incentral Europe and as far south asFlorida in North America. This imageshows an estimate of the auroral ovalon April 7, 2000 at 0242 UTC com-puted from observations made by theNational Oceanic and AtmosphericAdministration (NOAA) Polar-orbitingOperational Environmental Satellite(POES), which continually monitorsthe power flux carried by the precipi-tating protons and electrons thatproduce the aurora.

www.gpsworld.com GPS WORLD July 2000 47

nighttime side, in the absence of solar radia-tion, free electrons and ions tend to recom-bine, thereby reducing the TEC as illustratedin Figure 7. The protonosphere, or upper-most region of the ionosphere, may con-tribute up to 50 percent of the electroncontent during the nighttime hours.

Typical nighttime values of vertical TECfor mid-latitude sites are of the order of 10 17

m-2 with corresponding daytime values of theorder of 1018 m-2. However, such typical day-time values can be exceeded by a factor oftwo or more, especially in near-equatorialregions. TEC also varies seasonally withhigher values during equinoxes.

Spatially, at latitudes about 20 degreeseither side of the geomagnetic equator, highTEC values are produced by a so-called“fountain effect.” The effect is caused bydrifting electrons interacting with the Earth’smagnetic field to produce large-scale move-ment of ionization. Polar and auroral regionsof the ionosphere also have anomalous TECbehavior but for a different reason. Globally,TEC is primarily determined by the Sun’sEUV radiation, however some electronscome from the magnetosphere and enter theionosphere in the auroral regions as men-tioned earlier.

Although the Sun emits a broad spectrumof radiation, as we’ve already discussed, theflux of the Sun’s radio emissions at a wave-length of 10.7 centimeters (2.8 GHz) is a use-ful indicator of solar activity. This particularwavelength was selected for monitoring pur-poses shortly after World War II, and dailyobservations have been carried out at obser-vatories in Canada ever since. Recentresearch has shown that while long-termchanges in the 10.7-centimeter solar fluxseem to correspond with long-term solarcycle changes in TEC, short-term TECchanges are not well correlated with the day-to-day changes. Only approximately 20 per-cent of the day-to-day fluctuations in TECcan be attributed to changes in solar EUV.The bulk of TEC changes result from varia-tions in temperature, composition, anddynamics of the ionosphere.

CORRECTIONS AND MODELS So how does a GPS receiver cope with theionosphere both when it is quiescent or well-behaved and when it is active?

As we noted, both pseudorange and car-rier-phase observations are biased by thepresence of the ionosphere. This bias must beaccounted for or position accuracy will suf-fer. Dual-frequency receivers take advantageof the ionosphere’s dispersive nature to cor-rect for its effect. A linear combination of the

n is about 0.9999838 at the GPS L1 frequency,and ng is about 1.0000162 — values onlyslightly different from unity, but neverthelesssignificant. These refractive index valuesindicate that the phase speed of a wave in theionosphere is slightly greater than the vac-uum speed of light, while the modulation orgroup speed is slightly smaller.

The propagation speed of the GPS signalsvaries all along their path through the iono-sphere as the refractive index varies. The sig-nals received by a GPS receiver on or nearthe Earth’s surface are affected by the cumu-lative or integrated effect of the ionosphere,so the time required for a signal to reach thereceiver is given by

(6)

where S is the path of the signal and n´ iseither the phase or group refractive index.We can convert this travel time to an equiva-lent distance traveled by multiplying it by thevacuum speed of light. Inserting the expres-sions for the phase and group refractiveindices gives us

rf 5 ctf

(7)

and

(8)

where r is the true satellite–receiver geomet-ric range. (We have ignored the small effectthe bending of the signal’s path has on themeasured range.) Carrier-phase measure-ments of the range are reduced by the pres-ence of the ionosphere (the phase isadvanced), whereas pseudorange measure-ments are increased (the signal is delayed) —by the same amount. *s Ne dSis the integratedelectron density along the signal path and iscalled the total electron content (TEC). TECis nothing more than a count of the number ofelectrons in a column stretching through theionosphere with a cross-sectional area of onesquare meter.

TEC VARIABILITYTEC is highly variable both temporally andspatially. The dominant variability is diurnalfollowing the variation in incident solar radi-ation. Maximum ionization occurs at approx-imately 1400 local time. On the ionosphere’s

ρp = ρ + 40.3f 2 Ne dS

S

= 1 –40.3 Ne

f 2 dS

S

= ρ – 40.3f 2 Ne

S

dS

τ = n'c

dSS

I N N O V A T I O N

group propagates at a different speed. The netresult is that the modulation of the signalpropagates with a different speed from that ofthe carrier; this is called the group speed.

Corresponding to the phase refractiveindex, n, we can define a group refractiveindex, ng:

(2)

It can be shown that

(3)

where the derivative, dn/df, describes howthe phase refractive index changes with fre-quency. In general, a medium will not behomogeneous, in which case, n and ng will befunctions of position in the medium.

Because of the varying refractive index,the signal’s path through a medium will bebent. The path bending is a direct conse-quence of Fermat’s principle of least timethat states out of all possible paths that aradio wave (and other electromagnetic wavessuch as light) might take, it will take the paththat requires the shortest amount of time.

The refractive index of the ionosphericplasma is described by a rather complicatedexpression developed by Edward Appletonand Douglas Hartree in the early 1930s. Theexpression assumes an equal number of posi-tive ions and free electrons exist but that theions (being relatively massive in comparison)have negligible effect on radio waves. Inaddition to several other constants includingthe mass and charge of the electron, therefractive index is determined by the electrondensity, the strength of the Earth’s magneticfield, and the frequency of the radio wave.For frequencies significantly higher than theplasma frequency (the “natural” frequency at which ionospheric electrons oscillate, ~ 15–30 MHz), and ignoring the effects ofthe magnetic field, the expression for thephase refractive index approximates to

(4)

where Ne is the electronic density in recipro-cal cubic meters and f is the frequency inhertz. Using equation 3, the group refractiveindex works out to

(5)

What are typical values for n and ng? As wehave mentioned, the density of electrons inthe ionosphere varies significantly in bothspace and time. But taking a relatively highvalue of Ne of 1012 electrons per cubic meter,

ng= 1 +40.3 Ne

f 2

n = 1 –40.3 Ne

f 2

ng = n + f dndf

ng = cvg

48 GPS WORLD July 2000 www.gpsworld.com

surement errors and other biases. In practice, N1 and N2 cannot be deter-

mined, but as long as the phase measure-ments are continuous (no cycle slips) theyremain constant. Hence, the carrier-phasemeasurements can be used to determine thevariation in the ionospheric delay — the so-called differential delay — but not theabsolute delay at any one epoch. The estima-tion of the differential delay in this way, hav-ing ignored third and higher order effects inthe expressions for the refractive indices, isaccurate to a few centimeters. For higheraccuracy we would need to account for thehigher order terms neglected in the first orderapproximation, including the geomagneticfield effect as well as ray path bending.

If measurements are made at only one car-rier frequency, then an alternative procedurefor correcting ionospheric bias must be used.The simplest approach, of course, is to ignorethe effect. Surveyors sometimes follow thisapproach by employing relative positioningover short distances using single frequencyGPS receivers. Differencing between theobservations by simultaneously observingreceivers removes that part of the iono-spheric range error that is common to themeasurements at both stations. The remain-

L1 and L2 pseudorange measurements maybe formed to estimate and subsequentlyremove almost all the ionospheric bias fromthe L1 measurements:

(9)

where f1 and f2 are the L1 and L2 carrier fre-quencies respectively, P1 and P2 are the L1and L2 pseudorange measurements, and erepresents random measurement errors andother biases such as satellite and receiverinter-frequency biases. (It is also possible todirectly compute an ionosphere-free combi-nation without the extra step of first comput-ing the ionospheric delay.) A similarapproach is used to correct carrier phase mea-surements with

(10)

where F1 and F2 are the L1 and L2 carrier-phase measurements (in units of length)respectively, l1 and l2 are the L1 and L2 car-rier wavelengths respectively, N1 and N2 arethe L1 and L2 integer cycle ambiguitiesrespectively, and e represents random mea-

f 22

f 22 – f 1

2λ1 N1 – λ2 N2 – Φ1 – Φ2 + ε

dion,1 =

dion,1 =f 2

2

f 22 – f 1

2 P1 – P2 + e

ing residual ionospheric range error resultsfrom the fact that the signals received at thetwo stations have passed through the iono-sphere at slightly different elevation angles.Therefore, the TEC along the two signalpaths is slightly different, even if the verticalionospheric profile is identical at the two stations.

In differential positioning, the main resultof this effect is a baseline shortening propor-tional to the TEC and proportional to thebaseline length. Such errors can introducesignificant scale and orientation biases in rel-ative coordinates. For example, at a typicalmid-latitude site using an elevation cut-offangle of 20 degrees, a horizontal scale bias of–0.63 parts per million is incurred for each 13 1017m-2 of TEC not accounted for. Never-theless, ignoring the ionospheric effect onvery short baselines may be preferable to theuse of the dual-frequency linear combinationand the attendant higher observation noise.

An empirical model also can be used tocorrect for ionospheric bias. The GPS broad-cast message, for example, includes the para-meters of a simple prediction model. Whilethis model can sometimes account forapproximately 70 to 90 percent of the day-time ionospheric delay, it was designed toremove only about 50 percent of the delay ona root-mean-square basis. More sophisticatedionospheric models, such as the Bent Iono-spheric Model and the International Refer-ence Ionosphere, may not perform muchbetter than the broadcast model in partbecause the top portion of the ionosphere isinaccurately represented. An estimate ofsolar activity drives these models. In the caseof the broadcast model, a running average ofthe observed solar flux values for the pastfive days is used. But as we noted, the day-to-day variability of the ionosphere is not wellcorrelated with variations in the flux.

Ionospheric delay corrections can be com-puted by a regional network of dual-fre-quency receivers and transmitted in real-timeto single-frequency users by radio transmis-sions or archived for data postprocessing.This technique is used, for example, by Nat-ural Resources Canada for their GPS•C ser-vice and will be used for the Federal AviationAdministration’s Wide Area AugmentationSystem.

IONOSPHERIC SCINTILLATIONIf the number of electrons along a signal pathfrom a satellite to a receiver changes rapidly,the resulting rapid change in the phase of thecarrier may present difficulties for the carriertracking loop in a GPS receiver. For a GPSreceiver tracking the L1 signal, a change of

I N N O V A T I O N

Figure 7. A “snapshot” of the global total electron content (TEC) at 1700 UTC onApril 6, 1995, clearly shows the ionosphere’s diurnal behavior and the equatorialanomaly spanning either side of the geomagnetic equator. Contours are in totalelectron content units (1 TECU = 1016 electrons per square meter). The map wascomputed using data from 74 stations of the International GPS Service network.

www.gpsworld.com GPS WORLD July 2000 49

only 1 radian of phase (corresponding to 0.193 1016 m-2 change in TEC, or only 0.2 per-cent of a typical 10 18 m-2 TEC) in a timeinterval equal to the inverse of the receiverbandwidth can be enough to cause problemsfor the receiver’s tracking loop. If thereceiver bandwidth is only 1 Hz (which isjust wide enough to accommodate the geo-metric Doppler shift), then when the secondderivative of the phase exceeds 1 Hz per sec-ond, loss of lock will result. During suchoccurrences, the signal amplitude also gener-ally fades.

These short-term (less than 15 seconds)variations in the signal’s amplitude and phaseare known as ionospheric scintillations. Scin-tillation effects are most pronounced in theearly evening hours and may affect GPSreceivers differently, depending on theirhardware and software differences. Somecodeless dual-frequency receivers, for exam-ple, are particularly susceptible to loss oflock problems on the L2 signal when rapidfluctuations exist in ionospheric electroncontent. Generally, receivers making carrier-phase measurements are more susceptible toscintillations than code-only receivers.

A temporary loss of lock results in a phasediscontinuity or cycle slip. A cycle slip must be repaired before the data followingthe slip can be used. Large variations inionospheric range bias during short time

intervals can hamper the determination of thecorrect number of integer cycles associatedwith these phase discontinuities. If the varia-tions of the ionospheric range bias exceedone half of a carrier cycle, they may bewrongly interpreted in the data processing asa cycle slip.

Signal fading. Two regions exist where irreg-ularities in the Earth’s ionosphere often occurcausing short-term signal fading that canseverely test the tracking capabilities of aGPS receiver: the region extending approxi-mately ± 20 degrees either side of the geo-magnetic equator and the auroral and polarregions. The fading can be so severe that thesignal level drops completely below thereceiver’s signal lock threshold. When thisoccurs, data are lost until the receiver reac-quires the signal. The process of loss andreacquisition of signals may go on for severalhours.

Such signal fading is also associated withgeomagnetic storms. Occasionally, magneticstorm effects extend to the mid-latitudes.During the storm that occurred in March1989, during the previous solar maximum,range-rate changes produced by rapid varia-tions in TEC exceeded 1 Hz in one second.As a result, GPS receivers with a narrow 1 Hzbandwidth continuously lost lock during theworst part of the storm because of theirinability to follow the changes.

I N N O V A T I O N

FURTHER READINGTo learn more about solar activity and theionosphere, see

n Handbook of Geophysics and the SpaceEnvironment, edited by A.S. Jursa, the AirForce Geophysics Laboratory, Hanscom AFB,Massachusetts, 1985.

n Introduction to the Space Environment,2nd edition, by T. F. Tascione, KriegerPublishing Company, Malabar, Florida, 1994.

n “Eye on the Ionosphere,” a column inGPS Solutions, published quarterly by JohnWiley & Sons, Inc., New York.

For previous “Innovation” columns aboutthe ionosphere, see

n “Ionospheric Effects on GPS,” by J.A.Klobuchar, GPS World, Vol. 2, No. 4, April1991, pp. 48–51.

n “GPS — Satellites of Opportunity forIonospheric Monitoring,” by D. Coco, GPSWorld, Vol. 2, No. 9, October 1991, pp.47–50.

n “Effects of the Equatorial Ionosphere onGPS” by L. Wanninger, GPS World, Vol. 4,No. 7, July 1993, pp. 48–54.

For a comprehensive analysis of how theionosphere affects GPS and how GPS canbe used to study the ionosphere, see

n “GPS and the Ionosphere,” by A.J.Mannucci, B.A. Iijima, U.L. Lindqwister, X. Pi,L. Sparks, and B.D. Wilson, Chapter 25 inReview of Radio Science: 1996-1999, editedby W.R. Stone for the International Union ofRadio Science, Oxford University Press, 1999;pp. 625-665.

There is a wealth of current and interestingmaterial about space weather and the solarmaximum on the World Wide Web, forexample

n Space Environment Center, NationalOceanic and Atmospheric Administration(providing space weather alerts and warningsand related educational material)<http://www.sec.noaa.gov/>.

n SpaceWeather.com (science news andinformation about the Sun-Earth environment)<http://www.spaceweather.com/>.

n Sunspots and the Solar Cycle (data andbackground information on cycle #23)<http://www.sunspotcycle.com/>.

n Solar Data Analysis Center at NASAGoddard Space Flight Center (the latestground- and space-based solar images)<http://umbra.nascom.nasa.gov/>.

n International Solar-Terrestrial Physics(exploring the Sun-Earth connection duringsolar maximum) <http://www-istp.gsfc.nasa.gov/>.

CONCLUSIONThe solar maximum seems to be upon us. Inrecent months, we have witnessed a signifi-cant increase in solar activity with frequentsolar outbursts and the attendant effects onthe Earth’s magnetic field and ionosphere.Although the auroras are the most visibleeffects of this solar activity, we also witnessthe impact on the various infrastructures ofour modern society: spurious currents inelectricity grids and pipelines, increasedatmospheric drag on low-orbiting spacecraft,and potential damage to the equipment oncommunications satellites. The signals fromGPS satellites also can be significantlyaffected. At best, predictive ionosphericmodels lose their accuracy; at worst, areceiver may not even be able to track thesignals.

But a silver lining for GPS does exist. Thehigher and more variable total electron con-tent values offer scientists an enhancedopportunity to study the ionosphere and itscomplex relationship with solar activity. Andwith better understanding of the ionosphereand further improvements in hardware andsoftware, at the next solar maximum in about11 years’ time, GPS receivers may be betterable to weather the space storms that it willinevitably bring.

ACKNOWLEDGMENTSThe figures used to illustrate this article werekindly provided by the Solar Physics Groupof Marshall Space Flight Center’s Space Sci-ence Department (Figures 1 and 2); theSOHO EIT and LASCO consortia (Figures 3and 4); the Geological Survey of Canada(Figure 5); the Space Environment Center,National Oceanic and Atmospheric Adminis-tration (Figure 6); Attila Komjathy, Univer-sity of Colorado (Figure 7). Thanks toAnthea Coster for helpful comments on adraft of this article. ■

“Innovation” is a regu-lar column featuringdiscussions aboutrecent advances inGPS technology andits applications as wellas the fundamentals ofGPS positioning. The

column is coordinated by Richard Langley ofthe Department of Geodesy and GeomaticsEngineering at the University of NewBrunswick, who appreciates receiving yourcomments as well as topic suggestions forfuture columns. To contact him, see the“Columnists” section on page 4 of this issue.