Comparison among IRI, GPS-IGS and ionogram-derived total electron contents Man-Lian Zhang a, * , Sandro M. Radicella b , Jian-Kui Shi a , Xiao Wang a , Shun-Zhi Wu a a Laboratory for Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, P.O. Box 8701, 100080 Beijing, PR China b Aeronomy and Radiopropagation Laboratory, The Abdus Salam International Center for Theoretical Physics, 34100 Trieste, Italy Received 3 September 2004; received in revised form 17 November 2004; accepted 4 January 2005 Abstract In this paper, we compare the ionogram-derived total electron content (ITEC) obtained at the low latitude station Hainan (geograph- ical coordinates 19.4°N, 109.0°E; Dip = 22.8°), China with that derived from GPS signals produced by the International GPS Services for Geodynamics (TECigs) and the TEC data obtained by the altimeter on TOPEX satellite when it passed over nearby Hainan. We also compare them with the results produced by the current International Reference Ionosphere model (IRI2001) TECiri. Data used for the present comparison study are both the daily hourly and monthly median values for the 12 months of the descending solar activity year 2003. Our study showed that (1) in general ITEC is in agreement with TOPEX-TEC, although there are exceptions; (2) the total electron contents produced by all techniques (TECigs, ITEC and TECiri) can reproduce quiet well the remarkable semi-annual variation behavior of the total electron content; (3) the variation trends of ITEC and TECigs are generally in good agreement, however during nighttime to pre-sunrise hours ITEC is systematically lower than TECigs, whereas during daytime hours around local noontime ITEC is usually larger than TECigs. The absolute difference between ITEC and the TECigs (|nTEC1| = |ITEC-TECigs|) of the monthly median data is usually under the limit of 10–15 TECu (1 TECu = 10 16 el/m 2 ) during the diurnal variation, except for the time period 09–16UT(17–24LT) in September and October when the value of |nTEC1| is larger than 15 TECu, fluctuating between about 15–25 TECu; (4) during nighttime hours TECiri is generally larger than TECigs, whereas during daytime hours TECiri is generally smaller than TECigs except for the months June and July when TECiri is slightly larger than TECigs; (5) the difference between ITEC and TECigs (DTEC1) and that between TECiri and TECigs (DTEC2) are almost reverse in their positive/negative signs at a given local time, however, the magnitudes of the maximum absolute values of |nTEC1| and |nTEC2| are comparable. Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Total electron content; Ionogram; GPS-TEC; IRI model 1. Introduction Total electron content (TEC) is one of the most important ionospheric parameters since the radio wave signals traveling through the ionosphere will suffer refraction and retardation that are closely related to the total electron content along the radio wave path. In many practical applications such as satellite naviga- tion and global positioning, ionospheric delay correction related to TEC must be made. Therefore, it is important to have accurate TEC specification models. The current topside electron density and TEC model such as the one used in the International Reference Ionosphere (IRI) Model (Rawer et al., 1978; Bilitza, 2001) has shortcom- ings in specifying the topside electron density profile and total electron contents. In recent years, many efforts including search for more topside electron density pro- file and TEC data have been spent on the improvement or replacement of the IRI topside electron density pro- file and TEC model. Two new sources of TEC data that can be used for ionospheric study and model develop- ment are now available. One is the total electron content derived using dual-frequency GPS observation. 0273-1177/$30 Ó 2006 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.01.113 * Corresponding author. Tel.: +86 10 62582680; fax: +86 10 62534546. E-mail address: zhangml@center.cssar.ac.cn (M.-L. Zhang). www.elsevier.com/locate/asr Advances in Space Research 37 (2006) 972–977