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Your GNSS receiver is a powerhouse of physics and math. Understanding how it works is the first step toward better field performance.
Explore the three factors that dictate precision:
Geometry: Satellite positions and DOP.
Signals: Ionospheric modelling and data.
Corrections: The power of RTK and PPP.
1. The Role of Geometry
Precision begins with geometry, measured in GNSS as Dilution of Precision (DOP). Simply put, a lower DOP equals higher precision. Your receiver calculates its position based on its distance from various satellites; if those satellites are clustered together, the calculation is weak.
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The Multi-Constellation Advantage: In the past, surveyors had to wait for optimal satellite alignment. Today, by tracking GPS, GLONASS, Galileo, and BeiDou simultaneously, receivers utilise dozens of satellites. This ensures optimal geometry and low DOP, eliminating the need for mission planning even in obstructed areas like urban canyons.
2. Signal Processing & Interference
Satellites broadcast unique codes on frequencies like L1, L2, and L5. Your receiver uses this data for two critical functions:
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Ranging: To calculate distance, the receiver generates a replica of the satellite’s code, aligning it with the incoming signal. It uses both the code (pseudorange) and the carrier wave (carrier phase ranging) to compensate for clock synchronisation errors.
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Ionospheric Modelling: The Earth’s ionosphere is the largest source of signal error. However, because interference affects frequencies differently, a multi-frequency receiver can compare signals to mathematically model and remove this bias.
3. The Necessity of Corrections
Even with perfect geometry, a standalone rover cannot achieve survey-grade accuracy. Bridging the gap from meters to centimetres requires external corrections via two main methods:
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RTK (Real-Time Kinematic): A local base station at a known location calculates errors and transmits corrections to the rover. This offers high precision but requires both units to track the same satellites.
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PPP (Precise Point Positioning): This method eliminates the local base station. Instead, an external service streams modelled corrections (for orbits, clocks, and atmosphere) via satellite or internet. Success here depends on the service’s ability to support all the constellations your receiver tracks.
Conclusion
Real-world accuracy is the sum of these parts: constellation availability, signal integrity, and correction data.
