  An assessment of variance in ocean current signal and noise shared by in situ
observations (drifters) and a large gridded analysis (GlobCurrent) is sought as
a function of day of the year for 1993-2015 and across a broad spectrum of
current speed. Regardless of the division of collocations, it is difficult to
claim that any synoptic assessment can be based on independent observations.
Instead, a measurement model that departs from ordinary linear regression by
accommodating error correlation is proposed. The interpretation of independence
is explored by applying Fuller's (1987) concept of equation and measurement
error to a division of error into shared (correlated) and unshared
(uncorrelated) components, respectively. The resulting division of variance in
the new model favours noise. Ocean current shared (equation) error is of
comparable magnitude to unshared (measurement) error and the latter is, for
GlobCurrent and drifters respectively, comparable to ordinary and reverse
linear regression. Although signal variance appears to be small, its utility as
a measure of agreement between two variates is highlighted.
  Sparse collocations that sample a dense grid permit a first order
autoregressive form of measurement model to be considered, including
parameterizations of analysis-in situ error cross-correlation and analysis
temporal error autocorrelation. The former (cross-correlation) is an equation
error term that accommodates error shared by both GlobCurrent and drifters. The
latter (autocorrelation) facilitates an identification and retrieval of all
model parameters. Solutions are sought using a prescribed calibration between
GlobCurrent and drifters (by variance matching). Because the true current
variance of GlobCurrent and drifters is small, signal to noise ratio is near
zero at best. This is particularly evident for moderate current speed and
meridional current component.
