Introduction
When you look at a GNSS receiver's status screen, you'll see numbers next to each satellite, often 30, 40, 50. These are Signal-to-Noise Ratio (SNR) values, and they tell you a lot about signal quality.
What Is SNR?
Signal-to-Noise Ratio measures the strength of the satellite signal relative to background noise. It's typically expressed in dB-Hz (decibel-Hertz).
Simple terms: Higher numbers = cleaner, more reliable signal.
| SNR (dB-Hz) | Quality | What It Means |
|---|---|---|
| 45–55 | Excellent | Strong, clean signal |
| 35–44 | Good | Usable for positioning |
| 25–34 | Fair | May have errors, likely low elevation |
| <25 | Poor | Weak, likely blocked or noisy |
What Affects SNR?
Satellite Elevation
- High overhead: 45–55 dB-Hz
- Near horizon: 30–40 dB-Hz (longer path through atmosphere)
Environment
- Open sky: 40–50+ dB-Hz
- Light tree cover: 30–40 dB-Hz
- Heavy tree cover: 20–30 dB-Hz
- Indoors: ~0 dB-Hz (no signal)
Antenna Quality
- Good antenna = higher SNR
- Smartphone antenna = limited
- Survey-grade antenna = excellent
Using SNR to Troubleshoot
Problem: Position is jumping around
Check SNR: If all satellites are 25–35 dB-Hz, you're in a marginal environment. Move to better sky view.
Problem: Can't get a fix
Check SNR: If all satellites are <25 dB-Hz, signals are too weak. You're likely indoors or under heavy cover.
Problem: One direction consistently has low SNR
Check surroundings: Something is blocking that part of the sky (building, tree line).
SNR and Modern Signals
| Signal | Typical SNR | Note |
|---|---|---|
| L1 C/A | 35–50 dB-Hz | Legacy signal |
| L5 | 30–45 dB-Hz | Higher power, better penetration |
| L2C | 33–48 dB-Hz | Modern civilian signal |
Vital Points
- SNR measures signal strength relative to noise
- Higher SNR = better, more reliable positioning
- SNR varies with elevation, environment, and antenna
- Use SNR to diagnose problems, low SNR across all satellites means poor environment
- Different signals have different typical SNR values