The International GPS Service (IGS) Ionosphere Working Group

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    SCIENCE DIRPCT.

    doi: lO.l016/SO273- 1 177(03)00029-2

    Pergamon

    www.elsevier.com/locaIe/asr

    THE INTERNATIONAL GPS SERVICE (IGS) IONOSPHERE WORKING GROUP

    J. Feltens

    Electronic Data Systems (EDS) at Navigation Support O&e, European Space Agency (ESA), European Space Operations Centre (ESOC), Robert-Bosch-Sk 5,

    D-64293 Darmstadt, Germany

    ABSTRACT

    This article is presented on behalf of the International GPS Service (KS) Ionosphere Working Group (Iono_WG) and describes the Working Groups activities. Detailed information about the IGS in general can be found on the IGS Central Bureau Web page: http://igscb.jpl.nasa.gov. The Iono_WG commenced working in June 1998. The working groups main task is the routine provision of ionosphere Total Electron Content (TEC) maps with a 2-hours time resolution and of daily sets of Global Positioning System (GPS) satellite and receiver hardware differential code bias (DCB) values. The computation of these TEC maps and DCB sets is based on the routine evaluation of GPS dual-frequency tracking data recorded with the global IGS tracking network. Currently five so called IGS Ionosphere Associate Analy- sis Centers (IAACs) produce with their models and software routinely TEC maps and DCB sets and pro- vide their ionosphere products to IGS Ionosphere Associate Combination Center (IACC) at ESA/ESOC. Once per week the IACC compares then these ionosphere products with a dedicated comparison algo- rithm. This algorithm is currently being upgraded to be able to compute a combined IGS ionosphere prod- uct from the individual IAACs inputs (at the publication date of this paper the upgrade was completed). This new product shall soon be made available to external users through the Crustal Dynamics Data Infor- mation Center (CDDIS), NASA Goddard Space Flight Center, Greenbelt, MD, U.S.A. Beyond the routine provision of ionosphere products, the Iono_WG intends to support the ionosphere community also with other activities, e.g. by using the IGS global tracking network and capabilities to run high-rate data cam- paigns during events of special relevance for the ionosphere. In such campaigns, dual frequency GPS re- ceivers record, depending on a receivers capability, with 1 second or with 3 second sampling rate. A first such campaign was organized during the total solar eclipse on 11 August 1999, a second campaign was run recently under the name HIRAC/SolarMax over 7 days from 23 - 29 April 2001. In the medium- and long-term, the working group intends to develop more sophisticated algorithms for deducing mappings of ionospheric parameters from GPS measurements and to realize near-real-time availability of IGS iono- sphere products. It is the intent of this paper to give an overview over the lono_WG activities and to point out the importance of GPS for routine ionosphere monitoring. 0 2003 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

    INTRODUCTION The Working Group started its routine activities in June 1998. Several IAACs provide twelve global

    TEC maps per day with a 2-hours time resolution and a daily set of GPS satellite DCBs in IONEX format files. IONEX stands for IONosphere map Exchange format and is a specific format in which the IAACs provide their TEC maps and DCB values (Schaer et al., 1998). The routine provision of daily ground station DCBs is under preparation (at the publication date of this paper the ground station DCBs were

    A& Space Rcs. Vol. 31, No. 3. pp. 635-644. 2003 0 2003 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273- 1177103 $30.00 + 0.00

  • included). The principle of deducing the TEC from dual-frequency GPS-measurements works, in short, as follows: The ionosphere is a dispersive medium, i.e. the amount of delay an electromagnetic signal suffers when travelling through the ionosphere, is proportional to the signals frequency. The GPS satellites broadcast their navigation signals on two frequencies, thus allowing in principle to measure ranges (with code or carrier) on these two frequencies, i.e. giving pairs of measured ranges (one on each frequency). Per pair each of the two observed ranges is delayed by the ionosphere to an amount proportional to the TEC experienced in the ionosphere along the signal path and, to the first order, inverse proportional to the square of the signal frequency. The two ranges of one pair can now be combined so that the ionospheric delay is eliminated to an order of 980/o, on the other hand they can also be combined in such a way that the TEC can be derived from them and then used as TEC observables for the deduction of ionospheric TEC maps. This is, in a few words, the principle of TEC deduction from GPS measurements. Corrections have to be applied to these observed TEC values, e.g. for the integer ambiguity inherent in the GPS carrier measurements, and, since the electronic pathes of the GPS signals are different for the two frequencies in the GPS satellites as well as in the receiver electronics, the so called GPS satellite and receiver DCBs have to be accounted for in the TEC observables modeling. A nice reference describing in detail the method of deriving TEC values and DCBs from GPS measurements is e.g. Newby (1992).

    Currently five IAACs contribute with ionosphere products: ??CODE, Center for Orbit Determination in Europe, Astronomical Institute, University of Beme,

    Switzerland. ??ESOC, European Space Operations Centre of ESA, Navigation Support Office, Darmstadt,

    Germany. ??JPL, Jet Propulsion Laboratory, M/S 138-308, Pasadena, California, U.S.A. ??NRCan, National Resources Canada, Geodetic Survey of Canada Geomatics Canada, Ottawa,

    Ontario, Canada. ??UPC, Polytechnical University of Catalonia, Group of Astronomy & GEomatics, gAGE/UPC,

    Barcelona, Spain. The mathematical approach used by the different IAACs to establish their TEC maps are quite differ-

    ent. Details about the individual IAACs modeling can be found in e.g. Schaer (1999), Feltens (1998), Mannucci et al. (1998), Gao et al. (1994), Hemandez-Pajares M. et al. (1999).

    The members of the Iono_WG are (due to a number of 41, the members are listed here without affiliation): Gerhard Beutler, Dieter Bilitza, Ljiljana R. Cander, Mihail Codrescu, Anthea J. Coster, Robert E., Jr. (Rob) Daniell, Patricia H. Doherty, John Dow, Mariangel Fedrizzi, Joachim Feltens, Mariusz Figurski. Werner Gurtner, Manuel Hemandez-Pajares, Pierre Heroux, Ildy Horvath, Norbert Jakowski, Ame Jungstand, Ivan Jelinek Kantor, John A. (Jack) Klobuchar, Attila Komjathy, Jan Kouba, Richard B. Langley, Reinhard Leitinger, Tony Mannucci, Angelyn W. Moore, Steven Musman, Ruth Neilan, Ola 0vsteda1, Kohichiro Oyama, Eurico Rodrigues de Paula, Christoph Reigber, Esther Sardon-Perez, Stefan Schaer, Lawrence Sparks, Peter Stewart, Rene Warnant, Robert Weber, Brian D. Wilson, Xiaoqing Pi, Yunbin Yuan, Rene Zandbergen.

    COMPARISONS Once per week the ionosphere products from the different IAACs are compared at the IACC at E&4/

    ESOC. A dedicated computer program was established at ESOC from scratch to do that task. In principle this algorithm computes for each of the twelve reference epochs an unweighted mean TEC map from the different IAAC TEC maps. Based on the residuals of the different IAAC TEC maps with respect to that mean TEC map a weight can be derived for each IAAC. Based on the weights thus obtained, a weighted mean TEC map is computed in a second step. By inspecting the residuals of the individual

  • GPS Ionosphere Working Group 631

    IAAC TEC maps with respect to that weighted mean TEC map per reference epoch, statistics about the agreement between the different IAACs TEC maps are established.

    Fig. 1. The IGS global weighted mean TEC maps for 28 March 2000 in units of 1016 m-* [TECU] for UT=Ol:OO (top panel), 0390, and 0590; these are based on the IONEX file for 28 March 2000 containing the COMBINED IGS TEC MAPS and DCBs.

  • 638

    TEC hi@ (hoi ht= 450.0 km at 2000 03/X 11sJOzOO w-loctu%.~~~~lcdTEc~md~

    Fig. 1 cont. The IGS global weighted mean TEC maps for 28 March 2000 in units of 1016 rns2 [TECU] for UT=O7:00 (top panel), 0930, and 11:OO; these are based on the IONEX file for 28 March 2000 con- taining the COMBINED IGS TEC MAPS and DCBs.

  • GPS Ionosphere Working Group 639

    Fig. 1 cont. The IGS global weighted mean TEC maps for 28 March 2000 in units of 1016 rns2 [TECU] for UT=13:OO (top panel), 15:00, and 1790; these are based on the IONEX file for 28 March 2000 con- taining the COMBINED IGS TEC MAPS and DCBs.

  • 640

    t= 450.0 km at 2000 g/alzOg d_ lh. LNErl Id

    Fig. 1 cont. The IGS global weighted mean TEC maps for 28 March 2000 in units of lOI me2 [TECU] for UT=19:00 (top panel), 21:00, and 23:OO; these are based on the IONEX file for 28 March 2000 con- taining the COMBINED IGS TEC MAPS and DCBs.

  • GPS Ionosphere Working Group 641

    The weighted mean TEC map for each reference epoch, obtained quasi as by-product when doing the comparison in this way, could be considered as something like a combination of the input IAAC EC maps. The same holds for the comparison of DCBs, which is done basically in the same way.

    However, the IAACs use very different approaches to establish their TEC maps, resulting in very different temporal and spatial resolutions. These circumstances reflect also in the comparison results; the weighting scheme in the comparison algorithm must be improved. Software upgrades for an improved weighting scheme are currently under work (at the publication date of this paper these upgrades were completed). The next chapter will tell more about the new weighting scheme. As an example of results obtained with the current (old) weighting scheme Figure 1 shows the sequence of IGS weighted mean TEC maps of 28 March 2000, a day during a period in the current solar maximum, when the TEC level was very high.

    The other important subject of comparisons are the DCBs. When directly comparing the DCB-series of the different IAACs, one can see an overall agreement in the order of about 0.3 ns (0. I ns correspond to an ionospheric range delay in the order of 3 cm). According to S. Schaer, private communication (2000), mean IAAC satellite DCB series show an agreement of about 0.1 ns, while the day-by-day varia- tions are significantly higher. For his analysis Schaer took three months (GPS weeks 1065 - 1077) of daily satellite DCB sets from the IAACs IONEX files and computed from these daily values IAAC-specific mean DCB sets. The five mean DCB sets thus obtained were then compared with respect to each other and also with respect to an overall mean set (AU). Table 1 is an extract of Schaers presentation and shows the obtained root-mean-square (rms) errors (in nanoseconds ns). When interpreting these numbers one has to keep in mind that some IAACs estimate their DCB sets together with their TEE maps, while others make separate program runs for this. Some IAACs introduce constraints in their DCBs estimation, while others do not.

    Table 1. Agreement of the distinct IAACs satellite DCB sets in [ns] (courtesy S. Schaer)

    NRcan Esoc J?% All

    CODE 0.122 0.106 0.110 0.370 0.094

    NRcan 0.109 0.144 0.371 0.104

    0.118 0.373 0.095

    0.393 0.117

    0.296

    VALIDATIONS NRCan and UPC have proposed two self-consistency methods. Both are in principle based on the

    analysis of residuals resulting from the comparison of directly from GPS-observables derived TFC values with corresponding TEC values interpolated from the IAACs TEC maps, in order to assess the quality of the distinct IAACs TJZC maps. This is done with GPS data collected at ground stations equally distributed in a global geographic grid. Table 2 lists these globally distributed ground stations.

  • Table 2. Globally distributed ground stations used for validations with GPS-observables.

    Thule, cp = 76.5

    I

    Ny-Alesund, cp = 78.9

    I

    TM, cp = 71.6 OteenlatuWenmark i = -68.8 Svalbard, Norway ii = 11.9 Russian Federation ii = 128.9 I

    Algonquin Park, cp = 46.0 I

    VillqFanca, cp = 40.4 I

    Usuda, cp =36.1 Canada E. = -78.1 Spain E, = -4.0 Japan h = 138.4 I

    Kourou, cp = 5.3 hZalindi, cp = -3.0 @ezonCity, cp = 14.6 i%x?nch Guyana h = -52.8 Kenya h = 40.2 Phillipines h = 121.1

    Santiago, cp = -33.2 Sutherland, cp = -32.4 Perth, cp = -31.8 Chile ?L = -70.7 South @ica h = 20.8 Australia ii = 115.9

    o*Iiiggins, Antarctica

    cp = -63.3 East Ongle Island. cp = -69.0 CaseY* cp = -66.3 ?b = -57.9 Antarctica h = 39.6 Antarctica )c= 110.5

    cp is the approximate latitude, h is the approximate longitude of the station.

    The new geographic-dependent weighting scheme for the comparison/combination algorithm is based on the output of these two methods. A detailed description on how these two methods work can be found in Feltens (2000). The weighted mean TEC maps coming out with this new weighting scheme will then be the official combined IGS ionosphere product.

    Beyond that, the Iono_WG intends to perform other kinds of validation, which are in short: 1) JPL proposed to make validations by comparing IAACs model vertical TEC values with TEC values derived from the Ocean Topography Experiment (TOPEX) Satellite altimeter data (validations of this type with Jason satellite and Envisat satellite altimeter data are under preparation). This type of validation shall soon be attached to the weekly comparison program runs (at the publication date of this paper the TOPEX vali- dations were attached to the weekly comparison runs). Figure 2 shows in the form of histograms the results of an analysis made at UPC with TOPEX data for each of the five IAACs for the year 2000; the Number of TOPEX observations are shown on the ordinate and the differences {TEC(TOPEX) minus TEC(GPS)} in TEC-units [TECU] (1 [TECU] = 1*1016 electrons/m*) are shown on the abscissa. When interpreting these histograms it should be taken into account that, in these plots, a negative bias means that the estimated TEC with GPS (h c 20200 km) is greater than the TOPEX TEC (h c 1300 km). Then a positive bias means directly a mean underestimation of the TEC with GPS at least equal to the bias (the TOPEX accuracy seems to be 2 TECU).

    2) The Deutsche Forschungsanstalt fur Luft.- und Raumfahrt e.V. (DLR) Femerkundungsstation Neustrelitz has proposed to make a validation with ionosondes data: Ionosondes data provide information about the critical frequencies of the different ionosphere layers. For the proposed type of validation the critical frequencyfoF, of the Frlayer shall only be used. The electron density N,F2 is proportional to the square of the critical frequency NmF2 - crd;i)*. By using TEC values derived from the different IAAC TEC-models, equivalent slab thickness values 7EC/NrnF2 could be computed. The equivalent slab thickness is a quite sensible indicator for the GPS-derived TEC-values, especially at low TEC-levels, having typically values of 200 - 400 km over daytime. Systematic studies could find out typical daily and seasonal varia...

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