Multi-Path GNSS Receiver Dramatically Improves Accuracy in Complex Environments

NTT and FURUNO ELECTRIC Co., Ltd. have developed a receiver for GPS and other global navigation satellite systems (GNSS) that dramatically improves time-synchronization accuracy in areas with severe reception conditions such as among buildings and in mountainous areas.

By integrating a new satellite signal selection algorithm developed by NTT into a time synchronization GNSS receiver (GF-88) from FURUNO, in addition to signals from satellites in line-of-sight locations, it has become possible to use multi-path signals (reflected or diffracted from buildings and other structures), which previously inhibited accuracy of time synchronization. In a real multi-path reception test environment, time error was reduced to approximately 1/5 of earlier values. This is a remarkable result in that it promises to enable time synchronization accuracy close to that obtained in open-sky reception environments with no obstructions, even in environments previously considered poor and unsuitable for accurate time synchronization, such as among buildings or in mountainous areas.

Accurate time synchronization with Coordinated Universal Time (UTC), using Global Navigation Satellite Systems such as GPS, is used in a wide range of fields for synchronizing mobile base stations, financial trading, earthquake measurements, and other purposes. Particularly in recent years, mobile base stations based on TD-LTE and other TDD methods are spreading globally and require transmitted signals to be synchronized accurately, down to the microsecond, to avoid interference of signals between cells.

However, in non-ideal reception environments such as among buildings or mountains, with structures surrounding the antennas the open area, where signals from navigation satellites can be received directly, is limited and reception of multi-path signals reflected from or diffracted by surrounding structures can significantly degrade the accuracy of time synchronization. Measures to improve accuracy of time synchronization have been studied in the past, including placing antennas in locations where they can receive signals directly from greater numbers of visible satellites, and also filtering based on the elevation of the signal direction or a signal strength threshold to exclude signals from non-line-of-sight satellites that cannot be seen directly from the receiver location. Even so, these earlier methods were not always able to ensure adequate accuracy, and also had issues with reliability.

As such, NTT has developed a new GNSS receiver algorithm for selecting suitable navigation satellite signals and conducted measurements to verify the effectiveness of this algorithm.

To compute the four parameters for position and time from navigation satellite signals, signals from at least four satellites must be received, but it is not always possible to receive four or more signals from visible satellites in so-called “urban-canyon” environments obstructed by surrounding buildings. In this study, they have conceived a new satellite selection algorithm in which satellite signals suitable for improving the time accuracy are selected in an iterative selection process based on the estimated reception location and the signal arrival times.

This algorithm first selects the signals from visible navigation satellites, and if there are fewer than four, it also selects the minimal number of signals with the smallest propagation delay from non-visible satellites. Thus, in poor reception environments, accuracy is improved by aggressively pruning unsuitable satellite signals. This could be called a “select few” satellite selection method. NTT developed a prototype, with this algorithm incorporated into the GF-87 time-synchronization GNSS receiver from FURUNO, and evaluated its performance in a test environment with multi-path reception. They were able to demonstrate accurate time synchronization with time error of approximately one fifth that of earlier devices.

Since this method is able to select the best satellite signals regardless of the number of visible satellites in the environment, antennas can be installed with less attention to the location, and significant increases in accuracy of time synchronization can be expected in a wider range of reception environments.

FURUNO plans to begin sales of the new GF-88 series time synchronization GNSS receivers incorporating this new technology in April of 2019, and to deploy it widely in fields such as 4G/5G mobile base stations, financial trading, power grids, and data centers.

As with earlier systems, the GF-88 will support GPS, GLONASS and the Quasi-Zenith Satellite System (QZSS), and also Galileo satellites. It also supports simultaneous reception from four QZSS satellites including a geostationary satellite Michibiki-3. They expect that supporting multi-GNSS will increase the total number of navigation satellites that can be used. Thus, more satellites will be visible in environments with restricted open sky, greatly increasing the effectiveness of our technology to exclude un-needed signals from non-visible satellites, and realize highly-accurate time synchronization. The GF-88 series also includes a model that implements high holdover performance using a precision crystal oscillator, so that it can maintain high time precision even when the satellite signal cannot be received temporarily due to jamming or other interfering signals. This can be applied to applications requiring high reliability, such as 5G mobile base stations, or applications in finance or power grids.

NTT is also continuing research to further expand application areas for GNSS.