The direct estimation in the track 130 and the mean
regional bias are both very close to the absolute bias
obtained by the OCA team in the same parameters configuration.
The difference in the number of available cycles is
due, on the one hand, to the fact that the GIM ionosphere
correction is available only from cycle 40 in the ESA GDR
products used by the OCA, whereas the CTOH distributes
the correction from the cycle 10. On the other hand, the
data editing strategies of the two teams are different, the
OCA being stricter on the altimeter data selection. When
computing the mean biases over exactly the same 16 cycles,
the NOVELTIS absolute bias increases to 46.2 cm with a
smaller variability (2.7 cm) and the OCA bias decreases
to 43.6 cm with a variability of 2.5 cm. This is coherent
with the observations of Bonnefond et al. (submitted for
publication) in Ajaccio, where they noticed that the Envisat
bias is dramatically impacted by the land contamination in
the altimeter footprint, due to the configuration of the site
and the presence of small islands. In particular, they notice
a very large decrease in the bias (about 3.0 cm) between
13 km and 7 km from the coast. The Envisat biases computed
with the regional method in Ajaccio are limited to
about 13 km off the Corsica coast, so they do not integrate
this decrease. This could consequently explain the 2.6 cm
difference observed between the two absolute estimates
when considering exactly the same cycles. Nevertheless,
the difference between the estimates of the two teams must
be carefully analysed as the decrease of the bias when considering
altimeter data close to the coast may not be the
only explanation. Indeed, once again, the corrections are
not exactly interpolated in the same way in each method
and other discrepancies may exist that need to be further
investigated.