  The Time-of-Arrival (ToA) of coronal mass ejections (CME) at Earth is a key
parameter due to the space weather phenomena associated with the CME arrival,
such as intense geomagnetic storms. Despite the incremental use of new
instrumentation and the development of novel methodologies, ToA estimated
errors remain above 10 hours on average. Here, we investigate the prediction of
the ToA of CMEs using observations from heliospheric imagers, i.e., from
heliocentric distances higher than those covered by the existent coronagraphs.
In order to perform this work we analyse 14 CMEs observed by the heliospheric
imagers HI-1 onboard the twin STEREO spacecraft to determine their front
location and speed. The kinematic parameters are derived with a new technique
based on the Elliptical Conversion (ElCon) method, which uses simultaneous
observations from the two viewpoints from STEREO. Outside the field of view of
the instruments, we assume that the dynamics of the CME evolution is controlled
by aerodynamic drag, i.e., a force resulting from the interaction with
particles from the background solar wind. To model the drag force we use a
physical model that allows us to derive its parameters without the need to rely
on drag coefficients derived empirically. We found a CME ToA mean error of
$1.6\pm8.0$ hours ToA and a mean absolute error of $6.9\pm3.9$ hours for a set
of 14 events. The results suggest that observations from HI-1 lead to estimates
with similar errors to observations from coronagraphs.
