Current methods

This section discusses the current high accuracy positioning methods in use. The positioning methods are selected based on accuracy, integrity, continuity, time criticality and availability criteria, specified in Section 1.2.1.

The current high accuracy (centimetre-level) GNSS positioning methods are summarised in Table 2.4: cRTK, PPP, network RTK (Takac and Lienhart, 2008), and PPP with local ionosphere corrections (Chen et al., 2011). The principles of the cRTK and PPP methods are described in Section 1.1.2. The principles of network RTK and PPP with local ionospheric corrections

13 methods are described below. The PPP category in this section refers to both float and fixed ambiguity PPP methods.

In the case of network RTK, corrections for errors such as atmospheric errors are calculated based on measurements from local GNSS reference networks (Takac and Lienhart, 2008).

The benefit of employing network RTK compared to cRTK is improvement in positioning accuracy and the possibility of accommodating longer distances from the nearest reference station. The network enables better modelling of the ionospheric and tropospheric errors.

(Takac and Lienhart, 2008).

In the case of PPP with a local ionosphere correction, PPP is employed, together with local ionospheric delay corrections (Chen et al., 2011). The benefit of employing the corrections is to reduce the convergence time of PPP. However, generating sufficiently accurate corrections requires a density of reference stations similar to network RTK (Collins et al., 2012).

Based on the analysis in Table 2.4, close to immediate positioning convergence can be achieved with cRTK, network RTK and PPP with local ionospheric corrections. However, all of these methods require a local reference network and cannot be used globally. On the other hand, PPP can be used globally without local reference networks. However, the position solution convergence time is typically 20 minutes or more (Grinter and Roberts, 2011). Thus, PPP is not suitable for applications which require immediate centimetre level positioning accuracy

The integrity monitoring methodology for centimetre level positioning accuracy for mission critical applications is still under research. Research on the integrity of cRTK has been done in the literature, for example, in Feng et al. (2009) and Langel et al. (2012). For example, it is shown in Schuster et al. (2012) that cRTK integrity monitoring can be applied to kinematic applications, when the length of cRTK baseline was short (between 500 and 8000 m). On the other hand, integrity monitoring for the other methods, is more difficult because anomalies in the correction products need to be taken into account.

It is also difficult to define continuity specifically for other methods such as cRTK, because integrity monitoring is not sufficiently established for these methods. However, if continuity

14 is defined as the ability to achieve centimetre-level positioning accuracy after initial convergence, cRTK, network RTK and PPP with local ionospheric correction methods can achieve higher continuity than the PPP method, because PPP may require significant time for re-convergence (Geng, 2009).

Method Time criticality Coverage area Integrity Reference cRTK Real-time with

Network RTK Real-time with close to

Table 2.4 The current centimetre level accuracy GNSS positioning methods

Each of these methods can be mapped to the applications identified in Section 2.1. In the case of surveying, the positioning method is selected based on the required accuracy and operating environment. For static surveying, any of cRTK, network RTK, or PPP methods can be used. To obtain millimetre level static positioning accuracy, long datasets are required.

For example, with a baseline length of 30 km and cRTK, at least 2 hours of data is required to obtain from 5 to 10 millimetre level accuracy (RICS, 2010).

For real-time or kinematic surveying, currently the cRTK or network RTK methods are typically used (RICS, 2010). In addition, some non-GNSS surveying technologies are used in

15 difficult environments, where the quality or availability of GNSS signals is not sufficient (RICS, 2010).

If centimetre level positioning is required for agricultural applications, the cRTK method is commonly used (GSA, 2012). PPP can also be used for agricultural applications (Chen et al., 2011). However, the primary challenge is the long convergence time of PPP solutions. To overcome this issue, local reference networks can be used with PPP to reduce convergence times (Chen et al., 2011).

In the case of tsunami warning systems, cRTK is used, for example, in Kato et al. (2005).

Real-time PPP can also be used for tsunami and earthquake warning and monitoring systems (Blewitt et al., 2009).

The determination of the orbits of LEO satellites is conventionally done using radar based tracking technologies (Montenbruck et al., 2005). Employing PPP to determine satellite orbits based on dual-frequency measurements provided by a GNSS receiver installed to the satellite is demonstrated in Montenbruck et al. (2005) and Laurichesse et al. (2009a). The cRTK method is not a suitable method for the determination of satellite orbits, because there are no GNSS reference networks available in space.

The cRTK method is currently used or at least using it is under research in the case of most aviation, military and intelligent transport applications, which require centimetre level positioning accuracy. PPP is currently not suitable for these applications, because the solution convergence time is unacceptably long and integrity is not sufficient. In general, centimetre-level GNSS positioning for life or safety-critical applications is still under research, this includes integrity monitoring for PPP and cRTK

All of the methods shown in Table 2.4 can be used for static or low dynamics maritime applications, if immediate position solution convergence is not required. For example, PPP is applied to maritime applications in Geng et al. (2010c). In the case of marine applications requiring immediate position solution convergence, cRTK is currently the only suitable method. Similar methods can be used for oil and gas applications as is used for maritime and surveying applications.

16 Based on the analysis on the requirements of the applications and current centimetre-level positioning methods, an ideal positioning method would provide immediate real-time centimetre level positioning with high integrity, continuity and availability anywhere on Earth. However, it is not possible to fulfil all of these requirements on the basis of any current technology alone. Therefore, different technologies must be used for different applications.

To make PPP more practical for any of the applications discussed, improvements are required in terms of convergence time, integrity, continuity and availability. In terms of accuracy, centimetre level accuracy already achieved after the PPP convergence period is sufficient for most applications.

The primary focus of this thesis is improving PPP compared to the current methods.

Therefore, an extensive literature review is carried out on the current PPP methods in Chapter 4 and on the current PPP error correction methods and products in Chapter 3. The PPP method review also includes the current fixed ambiguity PPP methods. In addition, Section 4.1 provides a review of the cRTK method in order to provide a reference for comparison with the current PPP methods

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