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Extending goGPS for Snapshot Positioning
Abstract and Figures
In order to estimate the coordinates of a GNSS device, navigation satellites transmit a precise reference clock and their orbit parameters. However, if the received signal is very weak or too short, traditional positioning algorithms cannot be applied. A technique known as Coarse Time Navigation or Snapshot Positioning addresses this problem by providing coarse time and ephemeris by means other than satellite transmission. The goGPS library is an open source library in MATLAB and Java for precise positioning by various stand-alone and relative techniques. This paper presents how Coarse Time Navigation can be added to goGPS. Problems that are unique to this technique will be discussed, as well as some solutions.
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... Such behavior normally only occurs in case of very short acquisition time. In these cases, the receiver is not able to receive the Time-of-Week (TOW) parameter from the satellite and thus to resolve the pseudorange ambiguity (see for instance [18]) PRN02 PRN13 PRN31 Figure 7 Pseudoranges for the three GPS satellite measurement that present ambiguity. ...
Global Navigation Satellite System (GNSS) have been routinely used in the last decades to estimate water vapor content in the lower part of the atmosphere. Water vapor measurements have been successfully integrated in Numerical Weather Prediction (NWP) models, showing a positive impact on the forecast of rain events. Since few years, it is possible to obtain raw measurements from the GNSS receivers embedded in smart device such as smartphones and tablets. Due to the ubiquity of such devices, retrieving information about the troposphere from them would be of great interest. In fact, this would widen the possibility to monitor the water vapor field at high resolution. This contribution evaluates the quality of the Zenith Total Delay (ZTD) derived from a Nexus 9 tablet and a Xiaomi MI8 smartphone for a long static session. Such quantity can be linked to the water vapor content of the atmosphere using local meteorological data. The results are evaluated against a geodetic grade dual-frequency receiver showing a bias of around 3 mm with a standard deviation of 5 mm.
goGPS is a free and open source satellite positioning software package aiming to provide a collaborative platform for research and teaching purposes. It was first published in 2009 and since then several related projects are on-going. Its objective is the investigation of strategies for enhancing the accuracy of low-cost single-frequency GPS receivers, mainly by relative positioning with respect to a base station and by a tailored extended Kalman filter working directly on code and phase observations. In this paper, the positioning algorithms implemented in goGPS are presented, emphasizing the modularity of the software design; two specific strategies to support the navigation with low-cost receivers are also proposed and discussed, namely an empirical observation weighting function calibrated on the receiver signal-to-noise ratio and the inclusion of height information from a digital terrain model as an additional observation in the Kalman filter. The former is crucial when working with high-sensitivity receivers, while the latter can significantly improve the positioning in the vertical direction. The overall goGPS positioning accuracy is assessed by comparison with a dual-frequency receiver and with the positioning computed by a standard low-cost receiver. The benefits of the calibrated weighting function and the digital terrain model are investigated by an experiment in a dense urban environment. It comes out that the use of goGPS and low-cost receivers leads to results comparable with those obtained by higher level receivers; goGPS has good performances also in a dense urban environment, where its additional features play an important role.
Providing enhanced satellite positioning as a web service can be an effective way to enable low-level GPS receivers to perform surveys with a good accuracy and to reduce hardware cost, by removing computation capability and embedded proprietary software. goGPS is an open source application for achieving sub-meter accuracy with low-cost GPS receivers by exploiting real-time kinematic positioning, Kalman filtering, aid from a digital terrain model, and in general by integrating GPS data with other sources of information. Since goGPS directly processes raw GPS observations, it provides a means to substitute black-box processing components (e.g., GPS chipsets) with open source positioning software. goGPS can work either in real-time or post-processing, by acquiring raw GPS data in input and providing positioning (i.e., coordinates) in output. Though originally developed in MATLAB, goGPS was recently ported to Java in order to have the possibility to provide it as a web service, thus allowing a wider user base to develop and use it. Since real-time GPS positioning heavily relies on fast matrix computation, a careful selection of Java matrix libraries was carried out in order to obtain optimal performances. An Open Geospatial Consortium standard Web Processing Service (WPS) implementation of goGPS by means of ZOO WPS framework was developed and tested in order to let lightweight clients just acquire raw GPS data, send them to a server for processing, and receive back the accurate positioning.
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