In this paper, variability of trend over Turkey is investigated statistically by Sliding Window Statistical Analysis (SWSA) method. First and second moment of the variability of ionospheric trend is calculated by SWSA method in a period of several years. Variance bounds are also obtained. This method is applied to Total Electron Content (TEC) estimates which are obtained from Turkish National Permanent GPS Network (TNPGN-Active) between 2009 and 2012. The Wide Sense Stationarity (WSS) period of ionospheric trend over Turkey is determined for the first time.