One IONEX vertical-TEC grid-node sample.
The epoch is a split Julian date with a time-scale tag. Latitude and longitude are in degrees, vertical TEC and optional RMS are in TECU.
Epoch as %{time_scale:, jd_whole:, jd_fraction:}.
%{time_scale:, jd_whole:, jd_fraction:}
@type epoch() :: %{time_scale: String.t(), jd_whole: float(), jd_fraction: float()}
@type t() :: %Sidereon.GNSS.Ionosphere.TecSample{ epoch: epoch(), lat_deg: float(), lon_deg: float(), rms_tecu: float() | nil, vtec_tecu: float() }