A Multiple Lock Detector for the Signal Abnormality Detection in the GPS Receiver

M.H. Jin, Y.S. Choi, H.H. Choi, S.J. Lee, C. Park

Abstract:	The factors leading to performance degradation of the GPS receiver are diverse. Among them, such as the signal blockage and the signal jamming has become a topic. The signal blockage means a temporal disconnection of GPS signal. It occurs when the GPS receivers pass through an obstacle or located in the complex radio environment. This abnormality increases the signal tracking error and causes the cycle-slip. Also the signal jamming and spoofing are electronic countermeasure (ECM) for transmits jamming and spoofing signal to the target receiver. These signals cause the increase of measurement error in the GPS receiver. Therefore, the appropriate countermeasure is required to prevent the adverse effect caused by the abnormalities of the GPS signal. In the existing research, the techniques for detecting and compensating the signal abnormality has been proposed by monitoring the change rate of the carrier phase measurement. However, if the change rate is too small and cycle-slip does not occur, it is difficult to detect the signal abnormality. In this case, we can use the lock indicator which is derived from the correlation value. This parameter is more sensitive to the signal abnormality than the change rate of the carrier measurement. If the lock indicator is used, the signal abnormality can be detected even when the change rate is too small. In general, the GPS receivers compare the lock detector and the tracking threshold to determine whether the signal is tracked in the GPS receivers. The output of lock detector, called the lock indicator is obtained by a moving average of the correlator output and it has close relationship with forgetting factor. Therefore, if the signal abnormality occurs, it is possible to change the tracking state depending on the forgetting factor. A large forgetting factor makes a lock indicator sensitive to noise. The GPS receivers determine that tracking lock loss in the small noisy environment despite the signal tracking is possible. However, with a small forgetting factor, GPS receivers keep tracking during a long signal blockage. Therefore, we can consider a multiple lock detector for the signal abnormality detection that has different forgetting factors. The forgetting factors can be different according to the GPS receiver design parameter. To apply in the various GPS receiver, the method of determining the forgetting factors based on the theory is required. This paper proposed a multiple lock detector for the detection of the signal abnormality and the decision method of forgetting factors. The forgetting factors are determined by proposed method and the signal abnormality is detected by proposed lock detector using this parameter. Also the proposed lock detector makes the GPS receiver can tracks the GPS signal efficiently. Finally, the usefulness of proposed method is verified by software-based simulation.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	1577 - 1582
Cite this article:	Jin, M.H., Choi, Y.S., Choi, H.H., Lee, S.J., Park, C., "A Multiple Lock Detector for the Signal Abnormality Detection in the GPS Receiver," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1577-1582.
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