RadioScience, Volume 32, Number4, Pages1499-1512,July-August1997
A comparative study of ionospheric total electron content measurements using global ionospheric maps of GPS, TOPEX radar, and the Bent model C. M. Ho, B. D. Wilson, A. J. Mannucci,U. J. Lindqwister,and D. N. Yuan Jet PropulsionLaboratory, California Institute of Technology,Pasadena
Abstract. Globalionospheric mapping(GIM) is a newandemergingtechniquefor determining globalionospheric TEC (total electroncontent)basedon measurements from a worldwide networkof GlobalPositioningSystem(GPS)receivers. In thisstudy,GIM accuracyin specifyingTEC is investigated by comparison with directionospheric measurements fromthe TOPEX altimeter.A climatological model(Bentmodel)is alsousedto comparewith the TOPEX altimeterdata.We find thatthe GIM techniquehasmuchbetteragreement with TOPEX in TEC measurements, comparedwith the predictionsof the climatologicalmodel. The differencebetweenGIM andTOPEX in TEC measurements is very small(lessthan 1.5 TEC units(TECU)) withina 1500-kmrangefroma referenceGPS station.The RMS gradually increases with increasingdistancefromthe station,while the Bentmodelshowsa constantlarge RMS, unrelatedto any stationlocation. Within a 1000-kmdistanceof a GPS site(elevation angle> 25ø), GIM hasa goodcorrelation(R > 0.93) to TOPEX with respectto TEC measurements. The slopeof the linearfittinglineto the datasetfromtwo TOPEX cyclesis 44.5ø (nearthe ideal45ø). In the northernhemispheric regions,ionospheric specification by GIM appearsto be accurateto within3-10 TECU up to 2000+ km awayfromnearestGPS station (corresponding to -1 o elevationanglecutoff). Beyond2000km, GIM accuracy, on average,is reducedto the Bentmodellevels. In the equatorialregion,the Bentmodelpredictionsare systematically lower(-5.0 TECU) thanTOPEX valuesandoftenshowa saturation at largeTEC values. Duringionospheric disturbedperiods,GIM sometimes showsdifferences from TOPEX valuesdueto transientvariationsof the ionosphere.Suchproblemsmaybe improvedby the continuousadditionof new GPS stationsin data-sparse regions. Thus,over a GPS station's measurement realm (up to 2000 km in radius),GIM canproducegenerallyaccurateTEC values. Througha spatialandtemporalextrapolation of GPS-derivedTEC measurements, the GIM techniqueprovidesa powerfultool for monitoringglobalionospheric featuresin nearreal time. 1.
Introduction
Recently,a powerfultechniquefor specifyingglobal ionospherictotal electron content (TEC) has been developed[Mannucciet al., 1993; Wilsonet al., 1992]. This techniqueis basedon measurements from more than 60 Global Positioning System (GPS) stations which are distributed worldwide and continuously receive dual-frequencysignalsfrom GPS satellites. Using the dispersionof the signals,we canmeasurethe ionospheric TEC along -480 + lines of sight and generateglobal TEC fits with resolutionsof a few minutes. The global TEC maps are very useful in monitoring (and possibly forecasting) ionospheric
stormsand perturbations[Coker et al., 1995; Hunsucker et al., 1995; Hoet al., 1996; Kelley et al., 1996]. Lanyi and Roth [1988] comparedGPS measurements with beaconsatelliteobservations. They foundthatthe differencebetweenthe GPSandFaradaymeasurements
is 1 TEC unit (TECU) (1 TECU = 1016el/m2). However,in mostrecentstudies[e.g., Coker et al., 1995; Kelley et al., 1996], TEC values are calculated along the line of sight (LOS) basedon the received signaldelay from a singleor a few GPS stationsand
thenaresimplyprojected vertically.Theaccuracy of the GPStechnique in determining ionospheric TEC wasnot thoroughlyevaluated.Severalresearchers [Brownet al.,
Copyright1997by theAmericanGeophysical Union.
1991;Szuszczewicz et al., 1996] have alsoperformed comparative studies of the measured ionospheric parameterswith variousmodelpredictions.While these models,in general,may describethe diurnalvariations
Papernumber97RS00580.
of theionosphere, thepredicted parameters usually exhibitlargevariations fromthemeasured ionospheric
0048-6604/97/97RS-00580511.00
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The globalionosphericmap (GIM) is a data-driven model basedon direct measurements by GPS stations distributed worldwide. Currently, GPS provides excellent sky coverage with a full 24-satellite constellationand a relatively densegroundnetwork. Mannucci et al. [1994] have previously made a comparison study between GPS and TOPEX measurements basedon a small data set. In their study, comparisonswere made for the intervals when the TOPEX satellite was directly above a GPS station (within 5ø of the zenith angle). Overflights were examined for a total of 18 sites, using 36 separate overflights and resultingin 174 data points. On the basisof this restricteddataset,they foundthat the GIM measurements of verticalTEC were in agreementwith the TOPEX
altimeter data at the 2-3 TECU
level in the
low (4.0 TECU. The RMS is almost constant(6.5 TECU), independent of distancevariation. TEC averagevaluespredictedby the Bentmodelin the two cyclesare relativelylower thanvaluesmeasuredby TOPEX. Becausethere is no upstreamsolarwind and geomagneticindex as input [Bent et al., 1976], the historical model performssignificantlyworse during active ionospheric conditions, which may become significantas we approachthe next solarmaximum. We furtherdividecycle 18 into threedifferentlatitude regionsto seehow the accuracyof GIM and the Bent modeldependon distancewithin a 3000-kmrange.We havebinnedthe globaldataby the geomagnetic latitude: northern hemispheric region (geomagneticlatitude >30ø), equatorialregion (30ø> geomagneticlatitude> -30ø), and southernhemisphericregion (geomagnetic latitude 0.95) andthe scatter(S) is small.As a comparison, the Bent model predictionshave lower correlation coefficientswith the TOPEX measurements, as shown
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in Figures9 and 10. A significantfeatureis that the valuespredicted by Bentareoftensystematically lower at larger TEC values. We can see this cutoff phenomenon clearlyin the equatorialregionsfor the
two cycles. In cycle 18 the Bent model is cut off at about80 TECU, andfor cycle 33 the modelis cut off at 40 TECU. It is this saturationwhich causesthe poor linear fit. This characteristic of the Bent model does not
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distance ranges andinthreelatitude regions forcycle18.Correlation coefficients, linearfit parameters, and scattering errorsareshownin eachplot.
bodewell for its predictivepowerduringmoreactive ionospheric conditions,suchasduringmagneticstorms and near solar maximum. Generally, due to the
theTOPEXmeasurements. All of thefittingparameters for the two cyclesandbothtechniques are listedin
saturation, the Bent model TEC values are lower than
geomagnetic latituderegions'northernhemispheric,
Table 2. Statistical results are shown in three
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Table 2. CorrelationsBetween the Global IonosphericMapping, the Bent Model, and TOPEX Ionospheric TotalElectronContentMeasurements North Equator South R Slope,degError R Slope,deg Error R Slope,deg Error Cycle 18 0-1000
km
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0.98
45.0
2.62
0.91
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45.8 39.0
7.39 10.32
0.97 0.81
47.5 38.7
3.18 6.55
0.88
42.6 34.2
11.02 10.25
0.87 0.80
43.2 35.0
6.01 6.05
km
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0.83
Cycle 33 o- 1000 km
GIM Bent 1000-2000 GIM Bent
0.97 0.89
48.5 39.0
2.01 2.84
0.96 0.90
45.8 41.7
3.42 5.04
0.93 0.92
43.5 33.4
1.85 1.45
0.95 0.90
48.5 36.5
2.37 2.42
0.94 0.94
47.2 40.7
4.94 3.98
0.80 0.86
40.7 29.7
3.27 1.69
km
equatorial,and southernhemispheric. There are higher correlation coefficients (R) between GIM and TOPEX than between Bent and TOPEX
in TEC measurements.
The average slope of the linear fit for GIM measurementsis 44.5ø. This is very closeto the "ideal" 45.0ø. The averageslopefor the Bent model is 38.5ø. For 0-1000 km, GIM errors (in TECU unit) are almost always lower than thosefor Bent, while for 1000-2000 km, the two are morecomparable. 4.
Discussion
As we statedbefore,oneof the advantages of the GIM techniqueis that a global TEC specificationmay be generatedhourly or more often, while TOPEX will requireabout60 daysof data to generatesucha global map to cover all local times. However, along the TOPEX track, the altimeter data do provide highresolution measurementsof the ionospheric TEC. Because the Bent model comes from average values basedon past measurementsover severalyears, it is generally a good long-term average but has less sensitivity to small spatial and temporal variations. Owing to the global distributionof GPS stationsand continuousoperation, GIM measurementsare better suitedfor the accuratestudyof global and large-scale ionosphericstructuresthan both TOPEX data and the Bent model.
We find that the relatively large RMS betweenGIM and TOPEX in cycle 18, shownin Figure 3, is due to a few large outliers.Theseoutliersshowthe existenceof somebig differences(>30 TECU) betweenthe GIM and TOPEX in TEC measurements. After carefully examining these data, we found that these large differencescomemainly from two typesof data. One represents someartificialover-shoot spikesgenerated by GIM fitting techniquesrelative to the smoothlyvarying TOPEX measurements. The othersare smoothprofiles measuredby GIM, while TOPEX detectssomenarrow
transientTEC peaks.The former discrepancymay be reduced by improving GIM fitting techniques, in particular,by regularizingthemapswith simulateddata. However,
not much can be done about the latter
discrepancy at thisstagesimplybecausewe do not have enough GPS stations to resolve such small spatial variations.(As the globalnetworkkeepsgrowing,this problem will diminish.) There are several sources of errors which lower the
accuracy of GIM measurements[Klobuchar et al., 1995]. One is the instrumental biases from the GPS
system [Wilson and Mannucci, 1993], which include both the satellite and receiver biases [Sardon et al., 1994]. Multipathof signalpropagationat low elevation anglescan also causeerrorsin determiningLOS TEC values.Another error comesfrom mappingthe TEC values to vertical under an assumptionof a shell
ionosphere with constant peakheight.If theionosphere has a horizontal gradient, mapping to vertical will producelarge errorsbecausethe correctIPP locationis poorlydeterminedby thefixed shell-height assumption. Such errorsare probablylargestdue to the Appleton anomaly at low latitude. The total errors are estimated to have an upperlimit of about5.0 TECU.
The statisticalresultsshowthat the GIM technique hasreducedperformancein the equatorialand southern hemisphericregions.Within 1000 km of the nearest station,only the low-latitudeaccuracyis significantly worse,probablybecauseof theinadequacies of the shell model in the presenceof large latitudinalgradientsor transient variations in ionosphericstructurewhich cannot be effectively followed. At larger distances (1000-2000 km), even the southernlatitudesare worse than the north. Assuming comparableionospheric structurein the middle latitudes (north or south), this may be due to the small number of GPS stations locatedin thisarea,particularlybackin 1993(cycles18
and 33). Although we have computedthe statistics
HO ET AL.: ACCURACY STUDY OF IONOSPHERICMEASUREMENTS
1511
3. GIM is quiteaccurate upto 2000-kmdistances (~1o takingthe nearestreceiverinto account,it is possible that improvedresultsare obtainedwhen the nearest elevationangle). Beyond2000 km, GIM accuracy,on receiveris partof a clusterthatproduces simultaneous average,is reducedto the Bent model levels. This that the GIM wasunableto trackionosphere overlappingmeasurements of a region. In the north, suggests the sameminimumdistancemay nevertheless include changesin regionswith sparseor no data.As a result, severalreceivers withincomparable distances, addingto the data-drivenmodel (GIM) is more accuratein the wheretherearemorestations. the strengthof the solution. For sparselypopulated northernhemisphere 4. Note that in the equatorialregion,the data-driven latitude bands, the nearestreceiver may be the only receiverthat contributes significantly,andthe reduced modelalsoperformsbetterthan the Bent model.The Bent model predictionsare, on average,lower (~5.0 datastrengthleadsto lessaccuracy. In 1993,onlyfour GPS stationswereavailablein the TECU) thanTOPEX valuesandoftenshowa saturation low-latituderegion. On the basisof the resultsof at large TEC values. During ionosphericdisturbed showslargerdifferences from Figures 3 and 5, direct measurementsfrom GPS periods,GIM sometimes available within 1500 km (7ø elevation angle) would
TOPEX
provide fairly precise mapping. Globally, if the distancebetweenany two global networkstationsis
the ionosphere. 5. Througha spatialandtemporalinterpolation, TEC valuesmay be filled in thoseregionsbeyond2000 km from eachGPS station.A globalionospheric TEC map can be madeevery 15 min (or more frequently).This
reducedto 3000 km (27ø in latitudeor longitudeabout
theequator),GIM will haveverygoodcorrelation with
values due to untracked transient variations of
the TOPEX measurements everywhere. We needto furthercharacterize theGIM performance GIM technique will provide a powerful tool for features. during ionosphericor geomagneticstorm periods. specifyingtheglobalionospheric In conclusion, the Bentmodelgenerallywill calibrate During theseperiods,the ionospherebecomesmore delaywith 20-50%errors.The largesterrors irregular.In the database,we haveexaminedcycle 18, ionospheric which includes three small storms. A preliminary occurnear the equatorduring active ionosphere.The
survey reveals that there are a number of outliers data-driven(GIM) approachis clearly superior,with are not verticalTEC errorsgenerallybetween3 and 10 TECU need to or at the 10-20% error level (when comparedwith the maximum daytime signal strength).These errors are identify whethereach outlier comesfrom real GIM observationsor is due to interpolationerror or an error expectedto decreaseas more GPS stationscome onin the TOPEX data. In the near future we will use the line, especiallyin theequatorialregions,andasthedatanear real-time GPS data to nowcastthe ionospheric drivenGIM algorithmimproveswith time. stormsby usingglobalTEC onsetsignatures. Acknowledgments. We thank M. Reyesfor his help in data analyses.The researchconductedat the Jet 5. Summary Propulsion Laboratory, California Institute of For the first time, we have performed a Technology, was performed under contract to the comprehensive statisticalstudyof the performance of a NationalAeronautics andSpaceAdministration. GPS data-driven model with compared to a References climatological model. We have compared the agreementsof the GIM and the Bent model with TOPEX altimiterdata (usedas a sourceof groundtruth Bent, R. B., S. K. Llewellyn, G. Nesterczuk,and P. E. for ionosphericTEC measurements). From this study Schmid, The developmentof a highly successful the followingconclusions havebeendrawn: worldwideempiricalionosphericmodel,in Effectof 1. The data-driven(GIM) techniquehas muchbetter the Ionosphere on Space Systems and performancethan the predictionsof the historicaldata Communications,edited by J. Goodman,pp.13-28, Natl.Tech.Inf. Serv.,Springfield,Va., 1976. (Bent) model under most conditions. The average between GIM TEC measurements that consistent with TOPEX observables. We
difference between GIM
and TOPEX
in TEC values is
very small (less than 1.5 TECU) within a 2000-km range.The RMS graduallyincreaseswith an increasing distancefrom the GPS stationandapproaches the Bent results (while the Bent model shows no such distance
dependence with a constantlargeRMS). 2. The data-driven(GIM) techniquehassignificantly higher accuracythan the Bent model in specifyingthe ionosphere.Within a 1000-kmdistancerange(elevation angle > 14ø),GIM has a goodcorrelation(R > 0.93) to TOPEX in TEC measurements.The averageslope is 44.5ø (nearthe ideal 45ø).The Bentmodelshowsa poor
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C. M. Ho, U. J. Lindqwister,A. J. Mannucci,B. D. Wilson, and D. N. Yuan, Jet PropulsionLaboratory, 4800 Oak Grove Drive, Pasadena, CA 91109-8099.
(email:
[email protected])
(ReceivedOctober1, 1996;revisedJanuary27, 1997; acceptedFebruary11, 1997.)
