Cycle-slip detection and repair in integrated navigation systems

In order to use pure carrier-phase measurements, the initial ambiguities of the measurements have to be solved. Once obtained, these ambiguity solutions can be lost by shadowing, short-time loss of lock on one or several satellites and other effects, commonly referred to as cycle-slips. Restarting the ambiguity resolution algorithm is a time- and computation-intensive task and should be avoided as long as possible. In order to compensate for such short interruptions, an algorithm would be desirable which detects cycle-slips and is able to correct their magnitude for the individual measurements. In this paper an algorithm is presented which detects, using the redundancy inherent in the GPS-measurements, any cycle-slips. The residuals between the Doppler measurement and a calculated Doppler based on a least-squares solution show significant changes whenever cycle-slips occur. Determining in which channel the cycle slip occurred and calculating its size is possible using external information, for instance an INS-system. Instead of using the least-squares solution, the short-term stability of the INS-system is utilized to generate the calculated Doppler. Results are presented for such an integrated system showing the effectiveness in correcting even multiple simultaneous cycle-slips down to the loss of one cycle. The effects of decreasing satellite redundancy on the effectiveness of the algorithm and the influence of the information supplied by the INS system on the quality of the cycle-slip repair are investigated. Since GPS and INS sensors work in a closed loop for correction of the cycle-slips, error detection at this stage is essential for reliable operation of the integrated navigation system