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We can observe the changes of Total Electron Content, TEC, in ionosphere byanalyzing the data from GNSS satellites. There are many reports about TECanomalies after earthquakes, i.e. large earthquakes often disturb theionosphere. Up to now, preseismic TEC anomalies have been reported in severalpapers. However, they are not so clear as coseismic TEC anomalies, and theiranalysis methods have some problems for practical earthquake prediction. Onefactor making it difficult to detect TEC anomalies is large noises in TEC data.Non-negligible TEC disturbances are caused by many natural mechanisms. Toovercome this difficulty, we propose correlation analyses between one GNSSstation and GNSS stations surrounding it. First, we model TEC time series overa few hours using polynomial functions of time. Second, we calculate predictionerrors as the departure of the TEC time series from the models over time scaleof a few minutes, and define it as the TEC anomaly. Third, we calculate thecorrelation between anomaly of one GNSS station and those at the surroundingstations. Although such a correlation method has long been used for radiocommunications, in particular for spread spectrum communications and very longbaseline interferometry (VLBI) to increase SNR (signal to noise ratio), it hasnot been widely applied for TEC analyses. As a result of our method, wedemonstrate that the correlation analysis let us detect TEC anomaly one hourbefore 2011 Tohoku-oki earthquake. In this paper, we examined the 2011Tohoku-oki case extensively.

Correlation Analysis for Total Electron Content Anomalies on 11th March, 2011