Positioning - GPS & GLONASSThere are two positioning satellite constellations in operation with worldwide coverage: the US Global Positioning System (GPS) and the Russian Global Navigation Satellite System (GLONASS). Either network is capable of sub-decimeter accuracies when differentially corrected for ionospheric effects.
Our rovers utilize and correlate both systems providing highly precise geographic positioning and redundancy in low signal areas. Nearby base stations supply differential correction data that broadcast in real-time via satellite to our receivers. Differentials measure the positional drift due to transmission delays in the ionosphere. Streaming differential telemetry eliminates post-survey processing and ensures immediate accuracy.
The positional receivers are synchronized with our geophysical instruments. Geophysical measurements are automatically paired and recorded with real-world coordinates. Data acquisition is precise, efficient, and readily uploadable for instantaneous in-field map creation.
The surveyor maximizes precision by minimizing the Positional Dilution of Precision (PDOP) and sets limits for data accuracy in the receiver. PDOP is a statistical estimation of positional accuracy based on satellite geometry and quantity. In low signal areas, however, precision must be sacrificed to collect data. The surveyor then temporarily sets the PDOP limit higher than normal to complete the survey.
For geometric reasons, satellites at low angles to the horizon decrease positioning accuracies. The elevation mask is therefore set to ignore low angle satellites to increase precision.
All of our maps are projected in Universal Transverse Mercator (UTM). This allows coordinates and distances to be expressed in meters, which are easier to interpret than the units of degrees used in the traditional latitude/longitude system.
The earliest coordinate systems assumed that the earth was a sphere. Refinements to these systems in the nineteenth century corrected lateral distortions in distance by integrating an ellipsoidal representation of the earth, called a geodetic datum. There are various geodetic data, both local and global. The North American Datum 1983 (NAD83) is the most commonly used datum for accurate projections in North America. The World Geodetic System 1984 (WGS84) is the standard with respect to global applications.
Elevation data are attainable but not always practical. Three-dimensional triangulation requires, at minimum, four reference points compared to two-dimensional positioning, which requires three. Therefore vertical positioning is approximately half as accurate as horizontal positioning. In areas where skyline obstructions limit satellite telemetry, elevation data sometimes must be forgone to acquire suitable horizontal coordinates.
Elevation data are helpful when topography is necessary for geotechnical interpretation of site conditions. Topographical information can be included as an overlay on geophysical survey maps, but LIDAR offers superior precision in areas of poor satellite signal.