dropout

An outage. The receiver goes silent, and silence is the one thing a line chart cannot show by drawing a line — so the gap is shaded instead. Jamming looks exactly like this, which is why absence is treated as an anomaly in its own right rather than as missing data.

last agreed 0s before the end

glonass_cn0_spread: OKgps_cn0_spread: OKgps_position_error: OKgps_time_offset: OKntp_time_offset: OKroughtime_time_offset: OK

What each chart shows

Every formula below is the one the code runs, with its source location. Thresholds are not repeated here — they come from the daemon's own published settings.

`gps_time_offset`

Variable: gps_time_offset — seconds
Source: The time field of each $GPRMC sentence, against the receiving host's own clock.
Description: How far GPS-reported UTC sits from local time. A time spoof drags this away while the independent references stay put — which is the disagreement the monitor exists to notice.
Formula: gps_time_offset = gps_utc − local_unix_time (collectors/gps.py:54)

`glonass_cn0_spread`

Variable: <constellation>_cn0_spread — dB-Hz
Source: The per-satellite carrier-to-noise values in each $GPGSV cycle, one channel per constellation.
Description: Spread of signal strength across the satellites in view. A real sky fades with elevation, so healthy spread is wide; a simulator transmitting every channel at equal power collapses it toward zero.
Formula: cn0_spread = pstdev(per-satellite C/N₀) (collectors/cn0.py:106)

`gps_cn0_spread`

Variable: <constellation>_cn0_spread — dB-Hz
Source: The per-satellite carrier-to-noise values in each $GPGSV cycle, one channel per constellation.
Description: Spread of signal strength across the satellites in view. A real sky fades with elevation, so healthy spread is wide; a simulator transmitting every channel at equal power collapses it toward zero.
Formula: cn0_spread = pstdev(per-satellite C/N₀) (collectors/cn0.py:106)

`gps_position_error`

Variable: gps_position_error — metres
Source: The latitude/longitude of each $GPRMC fix, against the site's surveyed antenna position.
Description: Distance between where the receiver says it is and where it actually is. The antenna does not move, so anything but noise here is the fix being wrong.
Formula: gps_position_error = haversine_m(fix, surveyed_site) (collectors/gps.py:55, geo.py:10)

`ntp_time_offset`

Variable: ntp_time_offset — seconds
Source: An SNTP exchange with one or more network time servers (RFC 4330). With two or more, a median consensus corroborated by a quorum.
Description: Network time minus local time — a reference that does not come from the sky. A GPS spoofer would also have to own the network to move this.
Formula: offset = ((t2 − t1) + (t3 − t4)) / 2 (collectors/ntp.py:153)

`roughtime_time_offset`

Variable: roughtime_time_offset — seconds
Source: A Roughtime server's signed response, verified with Ed25519 and a Merkle inclusion proof for our nonce.
Description: Cryptographically verified time minus local time. An attacker cannot forge this without the server's private key, which is what separates it from plain NTP.
Formula: roughtime_time_offset = verified_midpoint − local_unix_time (collectors/roughtime.py)

How every channel is judged

The same three detectors run on every channel above. Thresholds are per channel and are the ones the daemon actually applied — the dotted bands on each chart are drawn from them.

Detector	Rule	Source
step	alarm when abs(x − target) > threshold	discrepancy/step.py:31
cusum	S⁺ = max(0, S⁺ + (x − target − k)); S⁻ = max(0, S⁻ + (target − x − k)); alarm when max(S⁺, S⁻) > h, then reset	discrepancy/cusum.py:55-58
staleness	alarm when now − last_sample.timestamp > max_sample_age_s	discrepancy/engine.py:126