MacGPSDO

The instrument as a calibre

Every moving part is driven by something the receiver reports. The escape wheel steps on the 1PPS edge. The balance swings once a second with its amplitude set by the sawtooth. Barrel winding shows satellites in the fix, and the σy hand reads the current stability decade.

The chapter ring around it is the last 24 hours: signal strength on the inner band, discipline on the outer, midnight at the top. Numbers sit outside the dial, where numbers belong.

Allan deviation

Four estimators run over the logged phase series: ADEV, MDEV, HDEV, TDEV. Slope guides at τ⁻¹ and τ⁻½ sit behind the curves, dot size follows sample count, and the cursor snaps to a computed τ.

A difference counts only when its own timestamps are spaced correctly, so a dropout removes just the differences crossing it. HDEV cancels linear frequency drift, which ADEV reads as noise.

The series is the receiver's clock against GNSS time. Short τ measures receiver noise. Disciplining shows past about 100 s. σy(1 s) here reads 1.0e-8, σy(100 s) 5.0e-10.

Scroll back through the log

Drag pans, scroll zooms about the pointer, double-click returns to live. Nine quantities are selectable: clock bias and drift, sawtooth, time accuracy, signal, satellites, geometry, front-end gain, interference. Each keeps its own vertical scale.

SQL decimation returns roughly one point per pixel column, so a thirty-day window costs what five minutes does. Bucket contents are averaged. Sawtooth carries min and max. Lock states take the worst value in the bucket, so a one-second dropout inside a five-minute bucket still appears.

What the antenna cannot see

Signal strength accumulates in 5°×5° cells. After a few hours the antenna pattern emerges, each cell compared against the median of its elevation ring so low-angle roll-off stays out of the result.

Blocked sky yields no samples, which looks the same as sky nothing has crossed. Prediction resolves it. Cells expected at least three times with nothing received are marked blocked.

Interference and antenna faults

Front-end gain, broadband noise, jamming state, antenna line. A rising interference indicator with falling C/N₀ points at interference. An AGC jump with a dead antenna line points at the cable or the bias-T. Satellite count alone separates neither.

MacGPSDO enables the receiver's interference monitor (CFG-ITFM) at connect, which is what gives its jamming state meaning. Left disabled, the raw indicator free-runs.

Time, sun and moon

UTC in large type, with local time, the receiver's own UTC, and the offset of the Mac's clock beneath it. GPS week, 10-bit week, rollover count, time of week, leap seconds with any announced change counted down, MJD, day of year, sidereal time.

Sun position matches pyephem to 0.01°. Moon matches to 0.3°, once topocentric parallax is applied.

Grid references

Maidenhead to ten characters, H3 cell, Plus Code, Geohash, MGRS. Click any row to copy it. Every encoder is checked against its reference implementation: H3 against Uber's library across resolutions 0-15, MGRS against pyproj, Plus Codes against the OLC test vectors.

A running survey averages the fix and reports scatter, which measures multipath better than the receiver's own accuracy figure.