CN0 Estimator

The CN0 (Carrier-to-Noise density ratio) estimator provides a measure of signal quality in dB-Hz. Each TrackedSignal on a TrackedSat holds its own CN0 estimator, so a multi-signal satellite produces one CN0 value per signal.

Default Estimator

The default CN0 estimator is the Moments Method, implemented as MomentsCN0Estimator. It buffers prompt correlation values and estimates CN0 from the first and second moments of their magnitudes. The buffer holds num_prompts_for_cn0_estimation prompts (default 100, ~100 ms for L1 C/A) — the estimator returns 0.0 dB-Hz until at least one prompt has been pushed in, and the estimate sharpens as the buffer fills.

update(estimator, prompt)

You can shrink or grow the buffer by building the TrackedSat yourself and handing it to add_satellite!'s escape-hatch overload:

julia> using Tracking, GNSSSignals

julia> using Tracking: Hz

julia> track_state = TrackState(; signal = GPSL1CA());

julia> sat = TrackedSat(GPSL1CA(), 1, 50.0, 1000.0Hz;
                        num_prompts_for_cn0_estimation = 200);

julia> track_state = add_satellite!(track_state, :default, sat);

julia> get_prn(track_state, 1)
1

What the estimator returns

The estimator's output depends on what's been pushed into its buffer:

• Before any track call has completed an integration — the buffer is empty and estimate_cn0 returns 0.0 dB-Hz. This is the value seen immediately after add_satellite!.
• Driving a noise-free signal through track — the noise-moment estimate goes to zero, so the C/N₀ ratio diverges and the result is Inf dB-Hz. The Quick start shows this behavior.
• Driving a noisy signal — the estimate converges to the underlying CN0 as more prompts fill the buffer. Real-world signals typically land in 30–50 dB-Hz; values below ~25 dB-Hz are unusable for conventional tracking.

The doctest below builds a noisy L1 C/A signal at a known 45 dB-Hz CN0 and drives 25 1-ms tracking cycles through it. The estimator's output converges to the input within a few dB:

julia> using Tracking, GNSSSignals, Random

julia> using Tracking: Hz

julia> using GNSSSignals: gen_code, get_code_frequency

julia> function run_cn0_demo()
           sys = GPSL1CA()
           fs = 4e6Hz
           num_samples = 4000  # 1 ms at 4 MHz
           prn = 1
           cn0_db_hz = 45.0
           sigma = sqrt((fs/Hz) / 10^(cn0_db_hz/10) / 2)
           code_freq = get_code_frequency(sys)
           rng = MersenneTwister(0)
           track_state = TrackState(; signal = GPSL1CA())
           track_state = add_satellite!(track_state; prn, code_phase = 0.0, carrier_doppler = 0.0Hz)
           for _ in 1:25
               clean = gen_code(num_samples, sys, prn, fs, code_freq, 0.0)
               noise = sigma .* randn(rng, ComplexF64, num_samples)
               track_state = track(clean .+ noise, track_state, fs)
           end
           estimate_cn0(track_state, 1)
       end;

julia> run_cn0_demo()  # converges toward the 45 dB-Hz input as the buffer fills
49.1 dB-Hz

The 1-σ noise amplitude is derived from the target CN0 by inverting C/N₀ = signal_power / (σ² / Fs). With unit-amplitude code samples this collapses to σ² = Fs / 10^(CN0_dB/10), split evenly across the complex sample's real and imaginary parts. The seeded MersenneTwister(0) keeps the doctest deterministic; the bounds above (40–50 dB-Hz) leave room for the natural spread of the moment estimator with a 25-prompt fill of the default 100-sample buffer.

estimate_cn0(estimator, integration_time)

Custom CN0 Estimators

You can implement your own estimator by creating a subtype of AbstractCN0Estimator and implementing:

• Tracking.update(cn0_estimator::MyCN0Estimator, prompt) — return a new estimator with the latest prompt added (immutable update).
• estimate_cn0(cn0_estimator::MyCN0Estimator, integration_time) — return the CN0 estimate as a dB-Hz quantity.

To plug your custom estimator into a satellite, build a TrackedSignal with your cn0_estimator, splice it into a fresh TrackedSat, and add it via the escape-hatch overload — see the TrackedSat docstring for the full constructor surface.