Network solution as a facilitator for precise point positioning real-time kinematic (PPP-RTK)

Abstract

Precise Point Positioning (PPP) is a Global Navigation Satellite System (GNSS) processing technique for obtaining decimetre level accuracy in real-time applications within a Kalman filter. PPP is based on knowledge of the GNSS orbits and requires access to precise GNSS ephemerides and clocks. As orbital errors are long-wavelength, GNSS orbits can be predicted with high accuracy, but GNSS clocks are highly variable and not deterministic. The limiting factors of PPP are therefore GNSS clock, as well as inability to fix ambiguities to integer values. The latter is due to Uncalibrated Phase Delays (UPDs), which exist in the hardware of the receivers and satellites, and destroy the integer nature of the undifferenced ambiguities. In Network Real-Time Kinematic (NRTK) positioning, UPD errors cancel when double differencing the observations. But NRTK requires a dense network of base stations close to the user, which is not practical offshore. However, the stability of the UPDs over large distances and in time can allow a network of receivers to be employed to estimate the UPDs, which can be broadcast to the user. Various models exist to estimate UPDs, enabling a user, ambiguity fixed solution to be produced, in a method known as Precise Point Positioning Real-Time Kinematic (PPP-RTK). This study will use L1 and L2 frequencies independently in a measurement model capable of estimating the UPD and satellite clock parameters simultaneously. The estimated parameters can be transmitted to the user to acquire an ambiguity fixed position in real time. A series of tests show how factors such as the number of stations in the network, interstation distances and location of the user with respect to the network affect the accuracy of the user position. Tests show by using satellite clock and UPD corrections at a rover station it is possible to obtain position estimates with a root mean square error of 16 mm in plan and 23 mm in height.