Short answer
The ratio of received signal power to noise power spectral density at the receiver. Expressed in dB-Hz (because the ratio has units of Hz). A clean rooftop GPS L1 receiver should see 45–50 dB-Hz on strong satellites; below 25 dB-Hz the receiver loses track.
Detailed explanation
C/N₀ is the fundamental quantity describing GNSS signal quality at the receiver. It's carrier power C (in dBm or dBW) divided by noise spectral density N₀ (in dBm/Hz), so the ratio has units of Hz — written dB-Hz to keep the dimensions straight.
Don't confuse C/N₀ with SNR (signal-to-noise ratio, in dB). SNR depends on the bandwidth you're measuring over; C/N₀ does not. This makes C/N₀ the more fundamental quantity for GNSS — it's the same number whether your receiver uses a 2 MHz or a 20 MHz front-end bandwidth.
Typical numbers for GPS L1 C/A with a 3 dBi antenna and a clean sky view: 45–50 dB-Hz on strong satellites near zenith, 35–45 dB-Hz on weaker ones at low elevation, 25–35 dB-Hz in partial shadow, below 25 dB-Hz the receiver can't maintain carrier-phase tracking and reverts to code-only or loses fix.
C/N₀ is invariant down the cable / amplifier chain — both signal and noise grow equally with gain, so the ratio is the same at the antenna output, after the LNA, or at the receiver. A common mistake is to compute it after adding gain — see our link-budget calculator for a worked example.
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Open the toolRelated terms
LNA
An amplifier with very low added noise (typical NF 1–2.5 dB at L-band) placed as close to the antenna element as possible. Sets the cascaded noise figure of the entire receiver chain and lets the antenna drive long cable runs without C/N₀ loss.
TTFF
The elapsed time from receiver power-on (or warm/cold start) to the first valid position fix. Cold start (no almanac, no recent position, no time) is typically 30–60 seconds; warm start (with valid almanac and approximate position) is 5–15 seconds; hot start (with current ephemeris) is <1 second.
Multipath
Distortion of a GNSS signal caused by reflections off nearby surfaces (buildings, ground, vehicles) arriving at the antenna nanoseconds after the direct path. Adds pseudorange noise of several metres in unmitigated urban environments — the dominant error source for many high-precision applications.