A GNSS receiver is a complex system of physics and mathematics working in concert. Understanding how your receiver achieves its accuracy is key to getting the most out of it in the field.
This process can be broken down into three fundamental concepts: the geometry of the satellites, the information within their signals and the power of external corrections.
Scatter plot showing circle error probable for two GNSS receivers using Trimble RTX corrections.
All about geometry
The foundation of a good position is good geometry. In the world of GNSS, this is measured as dilution of precision (DOP). A lower DOP value means better precision.
How it works: Your receiver's position is calculated based on its distance from multiple satellites. The geometric arrangement of those satellites in the sky directly impacts the certainty of that calculation. A poor arrangement, where satellites are clustered together, leads to a high DOP and less reliable results.
Why more constellations matter: In the early days of GPS-only receivers, surveyors often had to plan their work around times of day when satellite geometry was good. Today, by tracking multiple constellations (GPS, GLONASS, Galileo BeiDou), receivers have access to dozens of satellites at any given time. This drastically improves the geometry, lowers the DOP and almost entirely eliminates the need for mission planning, even in obstructed environments like urban canyons.
Listening to the signals
Each satellite broadcasts a unique code on multiple frequencies (like L1, L2 and L5). Your receiver performs two key tasks with this information:
Calculating range: The clocks on the satellite and in your receiver are never perfectly synced. Your receiver calculates the range to a satellite by generating a replica of its unique code and constantly adjusting its timing to match the incoming signal. This process uses both the code itself (code pseudorange) and the signal's carrier wave (carrier phase ranging) for a precise estimate.
Fighting interference: The single biggest source of error for a GNSS signal is the Earth's ionosphere, which can slow the signal down. Because this interference affects different frequencies differently, a receiver tracking multiple signals can compare them, model the ionospheric bias and mathematically remove a significant portion of the error.
The power of corrections
Even with perfect geometry and signal processing, a standalone rover isn't accurate enough for survey-grade work. To get from meters to centimeters, it needs corrections. There are two primary methods for this in real-time:
Differential real-time kinematic (RTK): This classic method uses a base station at a known location. The base calculates the error for each satellite signal it sees and transmits this correction data to the rover. The rover applies these corrections to its own measurements for a precise position. The limitation is that the base and rover must be tracking the same satellites for the correction to work.
Precise point positioning (PPP): In this workflow, the rover uses an external service that provides modeled corrections for satellite orbits, clocks and the atmosphere. These corrections are delivered via satellite or the internet. This removes the need for a local base station, but performance depends entirely on the correction service supporting all the constellations and signals your advanced receiver is capable of tracking.
Ultimately, the accuracy you see on your controller is the result of this entire system working together. Peak performance is only possible when you have availability across all three areas: constellations, signals and corrections.
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