Improving SIRGAS Ionospheric Model

BRUNINI CLAUDIO; AZPILICUETA FRANCISCO; GENDE MAURICIO; CAMILIÓN EMILIO; GULARTE ERIKA

Marne la Vallée

Simposio; IAG Symposium REFAG2010; 2012

IAG

Not long ago, the IAG Sub-Commission 1.3b called SIRGAS (Sistema de Referencia Geoce´ntrico para las Ame´ricas), i.e. the Regional Reference Frame for Central and South America, operates a service for computing regional ionospheric maps based on the GNSS observations provided by its Continuously Operating Network (SIRGAS-CON) (Brunini et al. 2011a). Since 2008, a continuous time series of maps describing the vertical Total Electron Content (vTEC) distribution for the SIRGAS region, with time resolution of 1 h, is available at the SIRGAS web page (www.sirgas.org). As other vTEC maps computed within the geodetic community (e.g.: Herna´ndez-Pajarez et al. 2009), SIRGAS maps were originally based on the so called thin layer ionospheric model (Brunini et al. 2011b). According to this model, the vertical structure of the ionosphere (from about 50 to 1,000 km above the Earth´s surface), is aproximated with a spherical shell of infinitesimal thickness with equivalent vTEC (located somewhere between 350 and 450 km height). Within this approximation, the satellite-to-receiver slant Total Electron Content (sTEC) is converted into an equiv lent vTEC on the shell, by means of a geometrical mapping function that only depends on the satellite elevation and the height of the shell. Spatial and temporal variations of the equivalent vTEC are represented on the shell by means of different kinds of 3-D (latitude, longitude and time) mathematical functions (e.g.: spherical harmonics expansion). The parameters of these functions are estimated from the GNSS observations, along with the inter frequency biases (IFB) 32 that account for the frequency-dependent delays produced by the GNSS satellite and receiver hardware and firmware. The ionospheric model used by SIRGAS, known as La Plata Ionospheric Model (LPIM) (Azpilicueta et al. 2005), has continuously evolved from the initial ?thin layer with equivalent vTEC? simplification, to the present formulation in which the E, F1, F2 and top-side ionospheric layers are considered, and their vertical electron density (ED) distributions are approximated with empirical functions in similar way than the International Reference Ionosphere (IRI) (Bilitza and Reinisch 2008) or NeQuick (Nava et al. 2008) models.