Abstract
The number of satellites visible to a Global Navigation Satellite System (GNSS) receiver is important for high accuracy surveys. To aid with this, there are software packages capable of predicting GNSS visibility at any location of the globe at any time of day. These prediction packages operate by using regularly updated almanacs containing positional data for all navigation satellites; however, one issue that restricts their use is that most packages assume that there are no obstructions on the horizon. In an attempt to improve this, certain planning packages are now capable of modelling simple obstructions whereby portions of the horizon visible from one location can be blocked out, thereby simulating buildings or other vertical structures. While this is useful for static surveys, it is not applicable for dynamic surveys when the GNSS receiver is in motion. This problem has been tackled in the past by using detailed, high-accuracy building models and designing novel methods for modelling satellite positions using GNSS almanacs, which is a time-consuming and costly approach. The solution proposed in this paper is to use a GIS to combine existing, freely available GNSS prediction software to predict pseudo satellite locations, incorporate a 2.5D model of the buildings in an area created with national mapping agency 2D vector mapping and low density elevation data to minimise the need for a full survey, thereby providing savings in terms of cost and time. Following this, the ESRI ArcMap viewshed tool was used to ascertain what areas exhibit poor GNSS visibility due to obstructions over a wide area, and an accuracy assessment of the procedure was made.
Highlights
Satellite visibility is of the greatest importance for high accuracy surveying
There is, a strong motivation for surveyors to ensure that an adequate number of satellites are visible at the required location, as it may provide savings in terms of financial cost, where having to resurvey an area if the accuracy requirements set down in the survey specification are not met or time when a low number of visible satellites will cause the receiver to lose the Global Navigation Satellite System (GNSS) signal resulting in delays as the surveyor waits for signal to be reacquired
One restriction is that the objects must be in the same coordinate system; the novel methodology of calculating pseudo satellite positions proposed in this paper has enabled its application
Summary
Satellite visibility is of the greatest importance for high accuracy surveying. four satellites are the minimum number mathematically required for a 3D positional fix [1], most receivers require a minimum of five satellites to operate in Real Time Kinematic (RTK) mode. Predicting how many satellites will be visible to the receiver at a particular location is relatively easy for surveys in unobstructed areas thanks to existing desktop GNSS planning packages like Trimble Planning [2] or similar online GNSS planning tools [3]. These tools are capable of predicting satellite visibility at any location of the globe, at any time of day. This paper will present a method to minimise the time spent creating models for GNSS survey planning
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