Acta Geodaetica et Geophysica Hungarica 33. (1998)

1. szám - NNSS - Leitinger R.–Spalla P.–Ciraolo L.: Latitude dependent mean ionospheric height – A new approach to the TEC evaluation from NNSS data

Acta Geod. Geoph. Hung., Vol. 33(1), pp. 61-73 (1998) LATITUDE DEPENDENT MEAN IONOSPHERIC HEIGHT - A NEW APPROACH TO THE TEC EVALUATION FROM NNSS DATA R Leitinger1, P Spalla2, L Ciraolo2 The evaluation procedures for NNSS Differential Doppler data normally use a fixed ionospheric height (usually of 400 km; considerably larger heights have to be used in the equatorial anomaly region). An iterative procedure allows the use of latitude dependent mean ionospheric height, hi. If a reasonable model can be used from which to derive the latitude dependence of the F layer peak height, hm, the formula hi = hm + 50 km leads to a reasonable model for hi = hi (</>). We demonstrate that such an approach leads to improvements in the latitude dependence of vertical electron content in cases of comparatively large station to station differences of TEC. The most important region in which the iterative proce­dure should be applied is the low latitude region, especially in the vicinity of a peak of the equatorial anomaly. Keywords: ionospheric height; latitude dependence; NNSS; TEC 1. Introduction Vertical electron content (TEC) is an important indicator for the overall ioniza­tion status of the ionosphere. At least over Europe we have only navigation satel­lites as data sources; the modern “Global Navigation Satellite Systems” (GNSS, presently the US system GPS and its Russian equivalent GLONASS) and the re­maining active satellites of the former US Navy Navigation Satellite System (NNSS). GPS and GLONASS have satellites in orbit heights around 20000 km and orbit in­clinations of 55°(GPS) and 65°(GLONASS). NNSS has satellites in nearly polar and nearly circular orbits in heights around 1100 km. We discuss the concept of “mean ionospheric height” and a latitude dependence for this quantity using the NNSS geometry but remark that GPS/GLONASS data reduction too needs this concept. 2. Slant and vertical electron content The NNSS satellites transmit two carrier signals phase coherently (transmitted frequencies f\ = p fr, /2 = q fr, fr■ “reference frequency”, p = 3 and q = 8 are integer numbers; nominally fr = 50 MHz). The observed quantity is the carrier phase difference Ф = Ф\/р — Ф2/4 (in a sloppy terminology it is usually called “Differential Doppler”). In a (sufficiently 'Institut für Meteorologie und Geophysik, Universität Graz, Halbärthgasse 1, A-8010 Graz, Austria, e-mail: leitinger@bkfug.kfunigraz.ac.at 2IROE Firenze 1217-8977/98/$ 5.00 ©1998 Akadémiai Kiadó, Budapest

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