A GNSS receiver does not directly measure position. It measures signal properties from multiple satellites and uses those measurements to calculate a position solution.
The main GNSS observables are:
- Pseudorange, also called code measurement. This is a range estimate based on the satellite signal code. It is useful for acquiring satellites and calculating an initial position, but it is relatively noisier than carrier phase measurements.
- Carrier phase: This is a very precise measurement of the signal carrier wave. It is used for high-precision positioning methods such as RTK, PPP-RTK and PPP. Carrier phase measurements are precise, but they contain an unknown integer cycle ambiguity that must be resolved for the best accuracy.
- Doppler: This measures the change in signal frequency caused by relative motion between the satellite and receiver. Doppler is mainly used for velocity estimation and signal tracking.
GNSS receivers can report many status values. These values do not all describe the same thing. Some describe satellite geometry, some describe signal quality, some describe correction timing, and some describe the final position solution.
Understanding the difference helps identify whether a positioning issue is caused by sky view, antenna performance, interference, correction delivery, or receiver configuration.
DOP: satellite geometry
DOP means Dilution of Precision. It describes how satellite positions in the sky affect the position solution.
DOP does not measure signal strength. It measures satellite geometry. Satellites spread widely across the sky usually provide better geometry. Satellites clustered in one area of the sky usually provide weaker geometry.
Common DOP values include:
- HDOP: horizontal position geometry
- VDOP: vertical position geometry
- PDOP: 3D position geometry
- GDOP: position and receiver clock geometry
Lower DOP is better. High DOP usually means poor satellite geometry, partial sky view, or obstruction. Strong signal levels do not guarantee good DOP.
If DOP is high, improve sky view and enable additional GNSS constellations when supported.
C/N0: signal quality
C/N0 is a GNSS signal quality metric. It is usually reported in dB Hz.
Higher C/N0 usually means the receiver is tracking a stronger and cleaner signal. Lower C/N0 can indicate obstruction, poor antenna placement, antenna or cable issues, interference, or weak signals.
A receiver can see many satellites and still perform poorly if the tracked signals are weak or noisy. Low C/N0 can cause unstable tracking, cycle slips, loss of fix, or reduced RTK reliability.
Sudden C/N0 drops across many satellites may indicate obstruction or RF interference. Low C/N0 on only some satellites may be related to satellite elevation, local reflections, or partial sky view.
C/N0 should not be used alone to judge accuracy. Check it together with DOP, satellite count, fix type, correction state, and multipath risk.
Satellite count: visible, tracked, and used satellites
Satellite count can refer to different things:
- Satellites in view: satellites that the receiver can see or report
- Satellites tracked: satellites the receiver is actively tracking
- Satellites used: satellites included in the position solution
A higher satellite count is usually helpful, but it does not guarantee accuracy. The satellites also need good geometry, good signal quality, and suitable measurements for the positioning method ( such as standalone, DGNSS or RTK) being used. Importantly, satellites included in the solution are not necessarily those for which corrections are successfully applied.
Satellites spread across the sky are usually more useful than satellites clustered in one area. Low-elevation satellites may be more affected by obstruction, atmosphere, and multipath.
Fix type and correction state
GNSS receivers may report position status in different ways. Fix type describes the receiver position state. The correction state describes whether correction data or high-precision methods are being applied. Common fix states include:
- No Fix or Searching: the receiver does not yet have a valid position solution.
- 2D Fix: The receiver has calculated horizontal position, but altitude is constrained or not fully solved.
- 3D Fix: the receiver has calculated latitude, longitude, and altitude.
- Standalone: the receiver is calculating position from GNSS satellite measurements only, without correction data. A standalone solution can be 2D or 3D.
- DGNSS or SBAS: the receiver has applied differential or satellite-based augmentation corrections to improve a standard GNSS position solution. This is typically better than standalone, but it is not the same as RTK.
- RTK Float: The receiver is using high precision corrections and carrier phase measurements, but the carrier phase ambiguities are not fully resolved.
- RTK Fixed: The receiver is using high precision corrections and carrier phase measurements, and the carrier phase ambiguities are resolved. This is the highest confidence carrier phase state and can provide centimeter-level accuracy under suitable conditions.
Always check fix type before judging accuracy. A receiver can output positions normally while still being in Standalone, DGNSS, or RTK Float.
Update (Navigation) rate
Update rate describes how often the receiver outputs a position. It is usually expressed in Hz.
For example, 1 Hz means one position output per second, 5 Hz means five position outputs per second, and 10 Hz means ten position outputs per second.
A higher update rate does not mean better accuracy. A receiver can output data at 10 Hz while the position is still Standalone, RTK Float, noisy, or affected by weak signal conditions.
For troubleshooting, separate these two questions:
- Is the receiver outputting data at the expected rate?
- Is the receiver reaching the expected fix quality and accuracy?
Correction age and latency
Correction age describes how old correction data is when the receiver uses it. It may also be called age of corrections, RTCM age, correction latency, or time since last correction.
High-accuracy positioning depends on timely and consistent correction data. If corrections arrive late, arrive irregularly, or stop, the receiver may take longer to converge, fall back to a lower quality state, or show unstable accuracy.
A receiver may receive correction data but still not apply it if the stream, format, region, or receiver configuration is not suitable.
Recommended checks:
- Confirm the receiver is receiving correction data
- Check the correction age or age of corrections
- Check for packet loss, reconnects, buffering, or unstable internet
- Confirm that the correction stream and format are supported by the receiver
- Confirm that the correction data is being decoded and applied, not only received
For PointPerfect Flex specific latency guidance, see: What latency or age of corrections does PointPerfect Flex require?
Quick interpretation guide
- If DOP is high, check sky view and satellite geometry.
- If C/N0 is low, check antenna placement, obstruction, RF interference, cable quality, and antenna suitability.
- If satellite count is low, check sky view, constellation settings, antenna placement, and receiver configuration.
- If RTK Fixed is unstable, check correction age, correction input, multipath, sky view, ionospheric conditions, and RF interference.
- If the issue appears only in one location, check local obstruction, multipath, antenna placement, and interference.
Summary
GNSS quality metrics should be interpreted together. DOP describes satellite geometry. C/N0 describes signal quality. Satellite count describes satellite availability. Fix type and correction state describe the receiver solution. Update rate describes output frequency. Correction age describes whether correction data is timely enough for high-accuracy positioning.
A good GNSS solution usually requires clear sky view, strong signal tracking, good satellite geometry, valid correction data, low correction age, and a stable fix state.