GNSS accuracy can be affected by the atmosphere, satellite orbit and clock data, antenna setup, local signal reflections, and radio interference. Some issues cause random position movement, while others create a more consistent offset. Looking at the symptom pattern helps identify the most likely cause.
Ionosphere
Strong ionospheric disturbances from geomagnetic storms or high solar activity degrade GNSS signal propagation and ionospheric modelling. When disturbance levels are high (high Kp index), RTK engines, including those using PointPerfect Flex corrections, may struggle to reliably resolve ambiguities and hold RTK-Fixed. The receiver can then remain in RTK-Float even under very good open-sky conditions. The ionospheric impact varies by time of day, solar activity, latitude.
Symptoms customers notice
- Slower convergence / unstable accuracy
- More frequent loss of “fixed” in RTK-like modes
- Stuck in “float” mode
- Worse performance during geomagnetic storms / at equatorial or high-latitude regions
Mitigations
- Dual frequency: Dual-frequency receivers can cancel much of the ionospheric delay by comparing two bands, which usually improves accuracy and reliability, especially in challenging conditions.
- Use all supported constellations
- Repeat your tests during a quiet ionospheric period and compare performance
Troposphere
The troposphere is the lowest layer of the atmosphere (where weather happens). GNSS signals slow down slightly as they pass through it because the air has density and water vapor. That slowdown looks like the signal travelled a longer distance than it really did, which becomes a range error and shows up as a position error.
Symptoms
- Vertical error sensitivity: Often impacts vertical accuracy more than horizontal.
- Low bias that correlates with weather changes
- Gets worse when the receiver relies heavily on low-elevation satellites
Mitigations
- Using a correction service that sufficiently modelling the tropospheric errors, PointPerfect model is supposed to model and remove the tropospheric error.
- Use more satellites at higher elevations (avoid obstructed sky view)
Multipath
Signals reflect off buildings, water, vehicles, metal surfaces, causing delayed replicas.
Symptoms
- Position scatter near structures
- Errors that repeat in certain locations
- Often worse in urban canyons and near reflective surfaces
- Can look like a consistent offset that changes when moving a few meters
Mitigations
- Move antenna away from reflective objects
- Use a ground plane / better antenna placement
- Raise antenna height
- Choose open-sky location
- Use receivers/antennas with strong multipath mitigation
Note: If the observed issue is “works fine in open field but not near buildings” ? prioritize checking for multipath issue and sky-view/antenna placement optimization.
Satellite orbit (ephemeris) and clock errors
To compute your position, the receiver needs two things from each satellite:
- Where the satellite was when it transmitted the signal (orbit / ephemeris)
- Exactly what time it was on the satellite (satellite clock) Every satellite sends out this data. If there are small errors in the orbit or clock information, the receiver calculates an inaccurate "range" to that satellite, which leads to a position error.
Symptoms
- Typically, the impact of orbits error is minor.
- Can contribute to region-wide bias when combined with other errors
Mitigations
- Using corrections (such as PointPerfect) reduces these errors.
- Multi-constellation helps averaging/robustness
Antenna phase center + installation effects
- The antenna’s Phase Center Offset (PCO) is the constant distance between the antenna’s physical reference point and its electrical measurement point, the location where measurements are referenced.
- Poor mounting can introduce bias (tilt, nearby metal, cable issues).
Antenna phase centre and installation effects are typically small, systematic biases:
- Horizontal: usually a few mm to ~2 cm
- Vertical: typically ~1–5 cm
Symptoms
- The solution looks good, but a consistent centimetre-level offset remains (often in height), especially when returning to the same point.
- Vertical bias is common (height errors show up more because vertical is generally weaker than horizontal in GNSS geometry)
- Differences in solution between two setups using different antenna models
Mitigations
- Place the antenna with open sky, away from reflective surfaces; mount high and rigid.
- Use the recommended ground plane (or none, if that’s the recommendation).
RF interference / jamming / spoofing
- RF interference: Unwanted radio energy near GNSS frequencies that raises the “noise floor” or overloads the receiver front-end. Typical causes: nearby transmitters, faulty electronics, DC/DC converters, LTE/5G boosters, radar, bad USB chargers, industrial machinery.
- Jamming: A specific kind of interference where someone uses a device to intentionally transmit noise or tones to block GNSS signals (or a strong accidental source acts like it).
- Spoofing: Fake GNSS-like signals that try to trick the receiver into computing a wrong position/time.
Symptoms
- Sudden drop in satellites used
- C/N0 (signal strength) drops across bands
- Frequent cycle slips; fix instability
- Works in one location but fails in another consistently
Mitigations
- Change location: Move the antenna and receiver away from suspected noise sources (vehicles, power systems, radio transmitters, high-current wiring). Even a few meters can help.
- Improve antenna placement: Put the antenna in a clear-sky, elevated spot and away from reflective metal and electronics; keep the RF cable run tidy and away from noisy power cables.
- Use a recommended GNSS antenna and a proper ground plane to reduce out-of-band energy and reflections.
- F9 (latest firmware) and X20 provide RF interference indicators and security signal message for jamming/spoofing status via UBX monitoring messages (UBX-SEC-SIG).
Summary
GNSS errors can come from atmospheric effects, satellite orbit and clock data, multipath, antenna installation, and RF interference. Random movement usually points to signal quality, antenna placement, multipath, or interference. A stable offset usually points to reference frame, projection, transformation, or antenna offset issues.
For high precision applications, start with the basics: clear sky view, correct antenna installation, valid correction data, suitable receiver configuration, and a clean RF environment.