The location of the point C
on the in situ mean sea surface must be carefully chosen
as it links the two different types of measurements (satellite
and in situ data). Whatever the case, were it a direct comparison
(case of the track 1, equivalent to the absolute
methods) or an indirect comparison (regional method),
the altimeter SSH must be of very good quality at this
point, which means that it should not be chosen too close
to the coast, because of the land contamination effects in
the altimeter observations. Concretely, a series of about
20 locations for point C is determined along the track of
each considered mission and a mean bias is evaluated by
averaging over them, which reinforces the stability of the
bias estimates. The position of the most coastal of all the
points C is determined by the variability of the bias estimates,
which dramatically increases as the distance to the
coast diminishes, due to lower-quality altimeter data.