Clearer signals for better global positioning
Multipath interference is common in environments such as forests or at the horizon where reflections from the Earth's surface are problematic. Although modern global positioning system (GPS) receivers have high accuracy in LOS conditions, multipath interference remains an important source of error in the global navigation satellite system (GNSS). It impairs the algorithms for time delay estimation and this is the key to positioning services. An EU-funded consortium has successfully developed a multipath and spoofing (purposefully generated interference) signal mitigation receiver concept within the scope of the DISAP (Displacement synthetic aperture antenna advanced technology demonstrator) project. Synthetic aperture techniques in which data from multiple sensors – or a sensor moving among multiple locations – is combined have been widely employed in radar systems. DISAP scientists adapted this concept for GNSS receivers, modifying one project partner's existing software receiver. The novel concept will significantly improve the accuracy and reliability of GNSS tracking and eliminate the scenarios in which GNSS navigation even becomes impossible. Two prototypes were produced and validated, a rotating antenna with radius of 50 cm and a vertical antenna with maximum vertical displacement of 1.4 m. The former is designed for surveying in forests or buildings. The latter targets GNSS reference station use where precise low-elevation measurements are required. This includes local GNSS stations at airports that broadcast GNSS correction data to approaching aircrafts for safe approach and landing. All signal processing is done with a personal computer in real time and both systems support standard satellite signals (GPS L1, L2P, L2C, L5 and Galileo E1, E5a). The antennas were tested using a dedicated multipath test bed on a roof top with a good field of view and performance was compared to a static antenna. The DISAP antennas were shown to suppress 70-80 % of the multipath. In addition, both antennas were also able to identify and mitigate spoofing signals. DISAP receiver and antennae concepts are operational and follow-up projects are expected to bring the mature technology to market. Significantly improved global positioning can be expected in a number of difficult and safety-critical applications.
Keywords
Multipath interference, GPS, GNSS, synthetic aperture, signal processing, spoofing signals
