Calian
Precise Positioning for Railway Vehicles
Achieving Reliable Train Location with GNSS Solutions
This article first appeared in the Railway-News magazine, Issue 3 2022.
Government agencies in North America (e.g. US Federal Railroad Administration), Europe (e.g. European Union Agency for Railways) and other jurisdictions are looking to increase productivity and improve rail safety by applying new GNSS precise-positioning technology.
Mass-market GNSS receivers are built into cell phones, cameras and first-generation automotive navigation systems. These GNSS receivers use satellite PRN code signal timing to determine the difference between transmit and receive time and then derive the satellite’s range to the GNSS antenna. Latitude, longitude, height (of the antenna) and time can all be estimated with range measurements from four satellites. With a clear sky view, this technique, known as code tracking, can provide horizontal positioning within a 2–3m diameter to a 95% confidence level.
But rail applications commonly require sub-10cm precision, only achievable with multi-constellation, multi-frequency receivers in realtime kinematic (RTK) systems or commercial, wide-area corrections services. Real-time RTK systems rely upon GNSS signal carrier phase measurements to achieve the accuracy needed for positive train control (PTC).
An RTK system consists of a base reference receiver at a known location, a rover (the train), and a communication system to transmit base station coordinates and GNSS observables to the rover. The RTK technique provides excellent performance (rapid convergence and accuracy) when the baseline (base to rover) is short (≤30km). In this case, the base and rover receivers largely have common signal paths so that differencing techniques can be used to cancel many common errors, thereby providing for estimation of a distance vector between the known base station location and the unknown rover location.
An analogy would be to imagine the distance vector between base and rover as a measuring tape with imprecision of 1mm at the base station (VeraPhase / VeraChoke) and either 2mm (VeroStar) or 8mm (TW3972) at the rover (train).
