GPS validation

Validation is a pure function of the GPS table and the parameter set. It never deletes rows: it sets a bit mask of quality flags on every sample and derives a per-sample validity flag from it. Downstream stages use only valid samples and carry the flags through to the profiles.

Flags

bit	name	rule	invalidates
1	NO_FIX	fix quality < 3 (i.e. no 3-D fix), or latitude and longitude both exactly 0	yes
2	HIGH_DOP	dilution of precision > 3.0; confidence is halved above 2.0	yes
4	DUPLICATE_STATIONARY	identical position to the previous sample while speed < 0.5 m/s: the receiver is holding, treated as stationary	no
8	POSITION_JUMP	identical position while moving (≥ 3 m/s), or a step from the last good sample larger than $\max(2\,v\,\Delta t + 3\,\mathrm{m},\; 60\,\mathrm{m/s}\cdot\Delta t)$: twice the distance covered in one sample interval at the current speed, plus a 3 m noise allowance, with a 60 m/s floor speed	yes
16	SPEED_SPIKE	reported speed > 80 m/s; or a speed step whose acceleration exceeds 15 m/s² and reverses within 3 samples; or a Hampel outlier (window of 7 samples, threshold 3 × 1.4826 MAD, and at least 2 m/s away)	yes
32	ACCEL_IMPLAUSIBLE	sustained |dv/dt| > 12 m/s² for longer than 0.5 s: confidence lowered to 0.3	no
64	TIME_NONMONOTONIC	sample time not increasing	yes
128	INTERPOLATED	speed filled by linear interpolation inside a gap ≤ 0.5 s	no
256	PRELOCK	every sample before the first 2 s run of fixed, low-DOP samples	yes
512	EDGE	first and last 0.5 s of each contiguous run (filter warm-up); excluded from peak statistics	no
1024	PROJ_LOST	reserved for samples whose track projection was lost; the projection stage reports this per lap (as a count of lost samples) rather than in the GPS flags	no

A sample is valid when none of the invalidating bits is set.

Session-level checks

• Valid fraction = valid samples / all samples. Below 0.8 the session is blocked ("not enough valid GPS").
• Longest valid run must be at least 60 s or nothing can be analysed.
• Gaps: a gap is any interval longer than 3 sample periods; gaps > 1.0 s split the data into separate runs for lap detection.
• Effective rate = (valid samples − 1) / (time span of valid samples), reported in Hz.
• Speed consistency (independent witness): over 2 s windows the position-derived speed and the receiver's reported speed must agree within 1.5 m/s in at least 95 % of moving windows; otherwise a warning is raised and confidence is downgraded. Calibrated during development on the 18 Hz HERO9 recording.

Projection to metres

Positions are converted to a local east-north plane about the track origin: $x = (\lambda - \lambda_0)\cos\varphi_0 \, R_e \, \pi/180$, $y = (\varphi - \varphi_0) R_e \pi/180$ with $R_e = 6\,371\,008.8$ m. Track extents are under 1 km, so the flat-earth error is below 1 mm.

What was verified on real data

• HERO9 (18 Hz, GPS5): 7 speed spikes with |dv/dt| > 15 m/s² in one heat, all caught; 958 duplicate positions, all while stationary.
• HERO9 chaptered file: the first 18 samples have fix = 0 and DOP 99.99 (PRELOCK).
• Valid fraction on the reference recordings 99.8–100 %; effective rates 10.0 Hz (HERO11) and 17.7 Hz (HERO9).

Tests: every rule is triggered and not triggered in unit tests; synthetic recordings carry an injected spike, pre-lock zeros, duplicates, a jump and a gap; the flag counts on both reference recordings are pinned by regression tests.