Conventions

A harmonic constant is only meaningful together with its convention. Every OTC constant set points to a declared convention record, and a converter never guesses one.

Phase reference

• OTC publishes Greenwich phase lags g in degrees, with time in UTC. This is NOAA's phase_GMT convention.
• A source that gives phases in a local time zone is converted to Greenwich by its adapter, and the conversion is recorded. Example: Kartverket's API gives phases with utcoffset="+01:00".
• The convention record states the reference as greenwich_utc, or local with the offset, for the source data before conversion.

Astronomical arguments and nodal corrections

• The reference predictor uses the XTide/NOAA (Schureman) conventions. The equilibrium arguments V0+u and the node factors f are applied at prediction time, per year, from the predictor's tables.
• Constants fitted by OTC are therefore free of nodal modulation: the fit uses the same f and u that the predictor applies again. Applying them twice, or not at all, is the error behind the OO1 behaviour described on the Why page.
• The convention record names the V0 model (for example schureman_tcd) and the nodal handling (f_u_at_prediction with formula ids, or none).
• Fitted constants are tied to the predictor's tables by construction. The build records the tables' SHA-256, and a change to the tables triggers a full rebuild.

Constituent definitions

OTC uses one canonical name per constituent and records the source's own name next to it. Names that differ between producers are mapped explicitly:

Constituents that need care

SA (solar annual)

NOAA and the reference predictor use V = h. The Doodson Sa (056 554) is h − p1, which differs by about 77°. TICON-3 SA phases showed this offset (+76° measured); TICON-4 SA phases follow V = h.

M1

Several incompatible definitions exist (Doodson 155 555, 155 655 and the Schureman composite). OTC uses the Schureman composite, as NOAA does. TICON-4 M1 is about −105° from NOAA's M1, and is not used.

OO1

Must be fitted with the same nodal corrections that are applied at prediction time. TICON-4 OO1 on short records behaves as if no effective nodal correction was applied.

EPS2, MKS2

NOAA does not publish them, so they cannot be cross-checked against NOAA.

Units

Amplitudes in metres, phases in degrees, speeds in degrees per hour, times in UTC. Field names are stable within a major format_version; see Data and downloads.

The convention table

The full constituent table (Doodson numbers, speeds, node-factor formula ids and the IHO standard name where one exists) is published with the first release.