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The GPS position displayed in d3kOS comes from a GPS antenna on your NMEA 2000 network. Understanding what affects GPS accuracy helps you interpret the position data and troubleshoot problems.
Typical accuracy. A modern GPS receiver using the L1 frequency achieves approximately 3 to 5 metre accuracy under open sky with good satellite geometry. In practice, on a boat on open water, 5 to 10 metre accuracy is typical and adequate for coastal navigation.
HDOP — horizontal dilution of precision. HDOP is a measure of satellite geometry. It appears in d3kOS on the GPS status panel. HDOP of 1.0 is ideal (satellites spread evenly around the sky). HDOP below 2.0 is good for navigation. HDOP above 5.0 means position accuracy is degraded — tall cliffs, bridge spans, or a building blocking part of the sky are typical causes.
WAAS / SBAS. North American GPS antennas can receive WAAS correction signals from geostationary satellites, improving accuracy to approximately 1 to 3 metres. WAAS lock is indicated in d3kOS as a “D” (differential) status beside the GPS fix. European waters use EGNOS; Asia-Pacific uses MSAS.
What does not affect GPS accuracy. Weather (clouds, rain, fog) does not affect GPS signals — they pass through the atmosphere. Radio interference from VHF transmissions is also not a factor. The main issues on boats are antenna placement (obstructions above the antenna) and multipath reflections from masts and superstructure.
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