GNSS receivers determine their position by receiving radio signals from satellites. Their performance can vary based on several external factors:
- Number of satellites in view: How many satellites the receiver can "see" and use for positioning.
- Quality of satellite signals: How well the signals are received from those satellites.
- Presence of interference: Anything that might block or distort the satellite signals, like tall buildings or bad weather.
To indicate how accurate their position fix is, GNSS receivers typically surface a solution type, which is a number representing the quality of the position they've calculated:
- 0 - Fix Not Available - No position fix, GNSS receiver is unable to determine a valid position.
- 1 - SPS (Standard Positioning Service) - Basic GNSS position fix using only satellite signals, accuracy typically several meters.
- 2 - DGPS (Differential GNSS) - Uses correction data from a reference station, improving accuracy to within 0.5–1 meter.
- 4 - RTK Fixed (Real-Time Kinematic, Integer Fix) - High-precision fix with resolved integer ambiguities, accuracy within centimeters.
- 5 - RTK Float - Partial ambiguity resolution, resulting in decimeter-level accuracy.
- 6 - Dead Reckoning - Uses inertial or other sensor data when GNSS signals are unavailable, accuracy varies depending on duration and sensor precision.
The number of satellites in view affects the accuracy and reliability of the position fix. More satellites mean the receiver has more information to calculate a precise location. These satellites can come from different global systems, or constellations, such as:
- GPS (USA)
- GLONASS (Russia)
- Galileo (European Union)
- BeiDou (China)
Using multiple constellations increases the chances of receiving strong signals and improves positioning accuracy. This is especially useful in challenging environments like city centers with tall buildings ("urban canyons") or areas with dense trees, where it's harder for the receiver to "see" satellites.