GNSS has proven to be a valuable positioning technology, but it has inherent weaknesses that hinder its effectiveness in certain scenarios. Fundamentally, GNSS needs to be able to ‘see’ the sky, so it can receive the direct signal from the satellites. In a real-world environment, accessing a direct signal is not always possible, and it cannot be relied upon as the only solution to provide a device with accurate location at all times.
Multi-band GNSS can be utilised to mitigate the detrimental effects of multipath. The technology uses multiple frequency bands, typically combining generic L1 (GPS L1, Galileo E1, GLONASS G1 and BDS B1) and modernised L5 signals (GPS L5, Galileo E5a and BDS B2a). The rationale behind this approach lies in the distinct characteristics of different frequency bands. Although other bands such as L2 exist, this article will not discuss them because receivers that support them typically have a higher cost of ownership that is not compatible with mass-market devices.
The GNSS positioning technologies that can improve accuracy in outdoor devices
The L1 band is susceptible to multipath interference, whereas the L5 band exhibits superior multipath mitigation capabilities. By integrating both signals, the multi-band GNSS system can effectively discriminate between direct and reflected signals, enhancing the accuracy of positioning results. In addition to the advantages offered by the L5 signal characteristics, the implementation of multi-band signals can also help alleviate the adverse impact of ionospheric disturbances on positioning accuracy.
Real-time kinetic (RTK) positioning is a technique designed to counteract signal errors in GNSS positioning. It utilises a nearby reference station with known coordinates or a network of reference stations (also known as network RTK) to provide correction data in real-time via a carrier (cellular, broadcast radio, or satellite). The basic principle behind RTK is that it uses the carrier-phase differential technique to compensate for common errors from the satellites and atmosphere using the correction data. This approach significantly improves the GNSS accuracy to centimetre or decimetre level in open or semi-open environments.
Dead reckoning (DR) is a technique that provides continuous positioning even in the absence of GNSS signals. It relies on internal sensors (such as accelerometers and gyroscopes) and external sensors (such as odometers or speed pulses), to estimate a vehicle’s movement based on its initial position and subsequent changes in velocity, orientation, and position.
The underlying principle of DR is that even in situations where GNSS signals are weakened or unusable due to reflections or blockages, the vehicle’s motion can still be tracked by integrating data from these sensors over time. While DR does not provide absolute positioning, it can bridge GNSS signal gaps and offer reliable positioning estimates, making it valuable in scenarios where signal reflections or brief signal interruptions are common.
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