Impact of the GNSS Time Offsets on Positioning Reliability

With the development of GPS, GLONASS, Galileo, Compass, QZSS and the IRNSS, there has been growing interest in the development of system independent receivers. However, one of the problems encountered in system independent receivers is in the different time systems employed by each of the satellite navigation systems. To overcome this problem it has become a standard practice to solve for the time differences within the receiver’s navigation solution via a combination of receiver clock corrections and/or time offsets. While this technique overcomes the problem of the different time systems, it is at the cost of a satellite from each additional time system. Despite this, the numerous studies that combine multiple satellite navigation systems this way have still found that there are significant benefits in improved accuracy, integrity, continuity and availability. To enhance interoperability though satellite navigation system providers are intending to measure and transmit the time offsets to other time systems. The subsequent use of these time offsets will provide a more accurate navigation solution than without them. However, the problem with using the time offsets is that they pose an additional integrity risk because they are also potential sources of faults. However, with the use of the time offsets for multiple constellation solution, a proper Receiver Autonomous Integrity Monitoring method has not been developed. Thus, mathematical models to account for the time differences with and without the time offsets are presented in this paper. Furthermore, the model that incorporates the time offset allows the application of Receiver Autonomous Integrity Monitoring to detect the presence of any faults within the time offsets. The reliability of the linear models is then compared using GPS and GLONASS geometry in terms of the Minimal Detectable Biases, Protection Levels and the correlation coefficients. The results of this analysis indicate that a more reliable solution can be obtained with the time offsets because they are additional measurements.