jgg     2013 33 (4):  22-25    ISSN: 0412-1961  CN: 21-1139/TG PCA METHOD IN DETECTION OF IONOSPHERIC TEC ANOMALIES  BEFORE EARTHQUAKE School of Geodesy and Geomatics, Wuhan University, Wuhan 430079 Received: 2012-10-23  Revised: null  Online: 2013-08-16 Supporting info