Altti Jokinen

Integrity monitoring of fixed ambiguity Precise Point Positioning (PPP) solutions

Traditional positioning methods, such as conventional Real Time Kinematic (cRTK) rely upon local reference networks to enable users to achieve high-accuracy positioning. The need for such relatively dense networks has significant cost implications. Precise Point Positioning (PPP) on the other hand is a positioning method capable of centimeter-level positioning without the need for such local networks, hence providing significant cost benefits especially in remote areas. This paper presents the state-of-the-art PPP method using both GPS and GLONASS measurements to estimate the float position solution before attempting to resolve GPS integer ambiguities. Integrity monitoring is carried out using the Imperial College Carrier-phase Receiver Autonomous Integrity Monitoring method. A new method to detect and exclude GPS base-satellite failures is developed. A base-satellite is a satellite whose measurements are differenced from other satellite’s measurements when using between-satellite-differenced measurements to estimate position. The failure detection and exclusion methods are tested using static GNSS data recorded by International GNSS Service stations both in static and dynamic processing modes. The results show that failure detection can be achieved in all cases tested and failure exclusion can be achieved for static cases. In the kinematic processing cases, failure exclusion is more difficult because the higher noise in the measurement residuals increases the difficulty to distinguish between failures associated with the base-satellite and other satellites.

Fixed ambiguity Precise Point Positioning (PPP) with FDE RAIM

Precise Point Positioning (PPP) has been one of the major research interests in the Global Navigation Satellite System (GNSS) research field in recent years. PPP is a promising method because it can provide centimeter level positioning accuracy by using only one GNSS receiver, without using local reference networks. It is clear that this can provide cost savings compared to the traditional Real Time Kinematic (RTK) method, particularly if high accuracy positioning is required in remote areas.

Integer ambiguity validation in high accuracy GNSS positioning

Abstract Carrier phase observations are required for high-accuracy positioning with Global Navigation Satellite Systems. This requires that the correct number of whole carrier cycles in each observation (integer ambiguity) is determined. The existing methods have been shown to perform differently depending on the observables. Subsequently, the ratio test used for ambiguity validation was developed further including combining it with the integer aperture concept. The key challenges in using the ratio test are the existence of biases in float solutions and stochastic dependence between the two elements of the ratio. The current methods either make assumptions of independence and nonexistence of biases or use simulations together with the bias-free assumption. We propose a new method taking into account both challenges which result in an unknown distribution of the ratio test statistic. A doubly non-central F distribution (DNCF) is proposed for the determination of threshold. The cumulative distribution function (CDF) of DNCF over-bounds the CDF of ratio test statistic distribution in case there is a bias in the float solution and a correlation between the two elements of the ratio. The Precise Point Positioning (PPP) method with products from CNES and measurement data from 10 NOAA stations are used to verify the proposed method. The test results show that the proposed method improves the performance of ambiguity resolution achieving a lower rate of wrong fixing than current state of the art.

Receiver Autonomous Integrity Monitoring for Fixed Ambiguity Precise Point Positioning

There are still many challenges in Precise Point Positioning (PPP) including formulation of mathematical models, fast resolution of integer ambiguities, ambiguity validation and integrity monitoring. Research to date has focused on error modelling and ambiguity resolution. The ambiguity validation and integrity monitoring is still to be investigated in detail. Early research on PPP integrity has addressed the transferability of the Carrier phase based Receiver Autonomous Integrity Monitoring (CRAIM) algorithms developed for conventional Real Time Kinematic positioning (cRTK). However, there are significant differences between cRTK and PPP in the characteristics of the corresponding residual errors. For example, the satellite clock errors are removed in cRTK; while there are still satellites clock errors remaining in PPP after the application of correction products. The magnitude of these residual satellite clock errors depends on the quality of the products used. The residual errors in PPP are expected to be bigger than those in cRTK. These errors have significant negative impacts on ambiguity validation and integrity monitoring. This paper addresses these challenges.A Doubly Non-Central F distribution (DNCF) is justified for the use with popular ratio test for ambiguity validation. The residual errors in the PPP are characterised for the two key processes in RAIM, failure detection and derivation of protection levels. The correction products used for tests were from Centre National d’Etudes Spatiales (CNES). The GNSS measurement data used were from the American National Oceanic and Atmospheric Administration (NOAA). This selection is to ensure that data from same stations used to test the method are not part of the data sets for the generation of correction products. A dataset from 2 NOAA stations was used for testing. Test results show that the PPP algorithm with the DNCF based ambiguity validation can reach sub-decimetre accuracy. The protection levels calculated shown to over-bound the position errors all the time. The relative lower protection levels give the potential for the proposed method to be used in critical high accuracy applications.

New methods for dual constellation single receiver positioning and integrity monitoring

Navigation system integrity monitoring is crucial for mission (e.g. safety) critical applications. Receiver autonomous integrity monitoring (RAIM) based on consistency checking of redundant measurements is widely used for many applications. However, there are many challenges to the use of RAIM associated with multiple constellations and applications with very stringent requirements. This paper discusses two positioning techniques and corresponding integrity monitoring methods. The first is the use of single frequency pseudorange-based dual constellations. It employs a new cross constellation single difference scheme to benefit from the similarities while addressing the differences between the constellations. The second technique uses dual frequency carrier phase measurements from GLONASS and the global positioning system for precise point positioning. The results show significant improvements both in positioning accuracy and integrity monitoring as a result of the use of two constellations. The dual constellation positioning and integrity monitoring algorithms have the potential to be extended to multiple constellations.