Abstract—In GPS receivers, carrier phase measurement can be used to improve the receiver’s position accuracy. In order to maintain the accuracy, cycle-slip must be detected and repaired instantaneously and accurately. This paper first analyzes the implementation cycle-slip detection and repair method of GPS data based on Doppler measurement. Then introduce a simplified oscillator model. Based on the oscillator model, a modified method is proposed, which avoids the influence of the local oscillator bias. Data tests show that the root mean square error of the time-difference measurement residual based on the proposed method is small enough for detecting and repairing the cycle-slip instantaneously. Index Terms—Cycle-slip, Doppler measurement, GPS data, instantaneous. I. INTRODUCTION GPS is rapidly replacing most of the traditional surveying techniques and is widely used in daily life. This is due to the great flexible conditions of using this system, like no time limitation, un-necessity of inter-visibility and un-limitation for the separation between surveyed points, free of charge etc. With the continuous progress of GPS Modernization, the economy, high precision and high reliability of the GPS receiver has become more and more popular. In order to obtain accurate positioning, carrier phase measurement is usually used in positioning. In addition to the initial integer ambiguity of the carrier phase measurement, cycle-slip is still a big challenge compared with the pseudo-range measurements. Cycle-slip is discontinuity of an integer number of cycles in the measured carrier phase resulting from a temporary loss-of-lock in the carrier tracking loop of a GPS receiver. The causes of the cycle-slip are listed as below [1]: 1) Cycle-slip is caused by obstructions of the satellite signal due to trees, buildings, bridges, mountains, etc. 2) Cycle-slip is a low signal-to-noise ratio (SNR) or alternatively carrier-to-noise-power-density ratio (C/N0) due to bad ionospheric conditions, multipath, high receiver dynamics, or low satellite elevation angle. 3) Cycle-slip is a failure in the receiver software which leads to incorrect signal processing. The occurrence of cycle-slip affects not only the current measurement, but also the following epochs. It seriously degrades the positioning accuracy. In order to attain constant Manuscript received January 6, 2012; revised February 9, 2011. The paper is supported by the Fundamental Research Funds for the Central Universities. Z. Ren, L. Li, J. Zhong and M. Zhao are with the Department of Information Science and Electronic Engineering of Zhejiang University, China(e-mail: longgo2006@gmail.com; zhongjie@zju.edu.cn; eriklee@zju.edu.cn, mjzhao@zju.edu.cn). high-precision poisoning result, cycle-slip must be detected and repaired or handled with carrier phase measurements at the data processing stage. Currently, many methods are used to detect and repair cycle-slip, such as polynomial fitting, high-order difference method, combination method of pseudo-range and carrier phase, ionosphere residual method and so on[2]. But these methods have their own disadvantages: Polynomial fitting can be used for single or dual frequency measurements in post-processing, but it can’t be used in real-time cycle slip detection. High-order difference method can’t detect small cycle-slip, which is fit for post-processing. The ionosphere residual method must be used in dual-frequency receivers and cannot indicate on which channel the cycle-slip takes place. The combination method of pseudorange and carrier phase depends on the precision of pseudorange completely which can’t identify small cycle-slip. Doppler measurement is the instantaneous change rate of carrier phase. It is a very robust measurement. Therefore, Doppler measurement is an alternative way to detect and repair cycle-slip. However, in practice, the oscillator is a non-ideal clock source. The deviation in oscillator may result in Doppler measurement error. The instantaneous clock deviation estimation is not an easy work. The oscillator’s error of the receiver will be appeared in the Doppler-aided cycle-slip detection and repair method (DCDRM). Based on the relationship between Doppler measurements and carrier phase measurements, this paper proposes a new method called modified Doppler-aided cycle-slip detection and repair method (Modified DCDRM), which avoids using the corrected Doppler measurements and actual integration time to detect and repair the cycle-slip. II. INSTANTANEOUS CYCLE-SLIP DETECTION TECHNOLOGY A. Doppler-Aided Cycle-Slip Detection and Repair Method The carrier phase measurement equation can be written as[3]: ( ) ( ) u s u s N r I T c t t N Φ Φ Φ Φ δ δ ε λ λ = - + - + = + - + + (1) where Φ is the measured carrier phase; Φ u is carrier phase generated by receiver; Φ s is carrier phase arriving from satellite; λ is the carrier wavelength; r is the geometry range from receiver to GPS satellite; Instantaneous Cycle-Slip Detection and Repair of GPS Data Based on Doppler Measurement Zhoufeng Ren, Liyan Li, Jie Zhong, and Minjian Zhao International Journal of Information and Electronics Engineering, Vol. 2, No. 2, March 2012 96