Feature: Navigation
Figure 5: Consecutive LiDar scans
GPS correction methods Understanding the sources of error is important when choosing a GPS signal correction method. T e real-time kinematic positioning (RTK), precise point positioning (PPP) and state space representation (SSR) methods all have their pros and cons, suitable for diff erent applications; see Table 1.
RTK RTK, or real time kinematics, correction is the classic, “gold standard”, for high- precision GPS signal correction. It involves establishing a base station near the target area (typically within 30-50km) that provides a reference signal to the receiver, returning precise positioning calculations; see Figure 2. Although highly accurate, the main
drawback of classical RTK solutions is the need for a dense infrastructure of base stations, to enable signal correction at a global scale. T e method is most useful for
autonomous vehicles and consumer navigation in urban areas, but less so for applications in remote areas.
PPP PPP uses a diff erent approach; it leverages a limited number of highly precise and accurate stations to correct signals. T e algorithm behind PPP splits the
correction responsibility between PPP stations and GPS receivers. First, the PPP stations model the various known error sources within the GPS system, such as ephemeris inaccuracies, clock discrepancies and group delays. T is information is then sent to GPS receivers to perform further calculations on local conditions and minimise the error. GPS receivers combine the signal data
they have collected over time with the known error sources provided by the PPP stations, to measure both universal and localised errors (such as ionospheric and tropospheric eff ects), ultimately calculating how much signals must be corrected. Whilst this method is highly accurate, the scarcity of PPP stations results in slower signal correction time. For example, there are only ten PPP stations in the US, causing signal correction to take up to 25 minutes. PPP is most suited for use by heavy
equipment and applications that can wait for their positioning information before starting. It is less suitable for consumer GPS, autonomous vehicles and applications that need instant positioning information.
SSR SSR is the cutting edge of GPS signal correction technology. Not only does it provide the ephemeris, clock and code bias discrepancy data that PPP off ers, it also delivers information about more localised
ionospheric and tropospheric interferences. However, many GPS receivers do not
know what to do with all of this data, or how to convert it into meaningful positions. To accommodate this, SSR data can be converted into a virtual base station (VBS), to simulate an RTK base station for legacy receivers. T is innovative approach enables the use of SSR data even with conventional GPS receivers, making high- precision positioning more accessible. SSR is ideally suited for highly technical
teams who can build a VBS in areas without dense infrastructure of base stations, but less so for applications using generic receivers.
Choosing the right solution Diff erent methods for correcting GPS signals off er varying levels of accuracy and suitability for specifi c applications. Nevertheless, all correction methods must be scaleable, effi cient and as accurate as possible. Point One Navigation manages a
network of over 1,440 global base stations, including over 100 in the US, with at least four stations in each of the top 50 US metro areas. T is network continues to expand, delivering centimeter-level GPS signal corrections – even in areas without cellular coverage. Using this robust network, Point One Navigation has developed a GPS correction
www.electronicsworld.co.uk July/August 2024 53
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