This paper proposes a single frequency based ionosphere gradient
detector. Since ionospheric delays can strongly change locally and rapidly
due to ionospheric disturbances, the large differences in ionospheric delays between the receiver stations may arise. Therefore, it is necessary for the improvement of the positioning accuracy, availability and reliability to develop the detection method in such cases. In our proposed method,Kalman filter is applied to single difference (SD) code pseudorange and SD carrier phase measurements to estimate SD ionospheric delays, SD carrier phase ambiguities and SD receiver clock error.Further, we apply SD ionosphere-free combinations to determine absolute ionospheric delays in order to distinguish ionospheric delays from carrier phase ambiguities. The estimating performance of our proposed method is evaluated with ionospheric anomalous observable data.