  We present a statistical analysis of coronal mass ejections (CMEs) imaged by
the Heliospheric Imager (HI) instruments aboard NASAs twin-spacecraft STEREO
mission between April 2007 and August 2017 for STEREO-A and between April 2007
and September 2014 for STEREO-B. The analysis exploits a catalogue that was
generated within the FP7 HELCATS project. Here, we focus on the observational
characteristics of CMEs imaged in the heliosphere by the inner (HI-1) cameras.
More specifically, in this paper we present distributions of the basic
observational parameters - namely occurrence frequency, central position angle
(PA) and PA span - derived from nearly 2000 detections of CMEs in the
heliosphere by HI-1 on STEREO-A or STEREO-B from the minimum between Solar
Cycles 23 and 24 to the maximum of Cycle 24; STEREO-A analysis includes a
further 158 CME detections from the descending phase of Cycle 24, by which time
communication with STEREO-B had been lost. We compare heliospheric CME
characteristics with properties of CMEs observed at coronal altitudes, and with
sunspot number. As expected, heliospheric CME rates correlate with sunspot
number, and are not inconsistent with coronal rates once instrumental
factors/differences in cataloguing philosophy are considered. As well as being
more abundant, heliospheric CMEs, like their coronal counterparts, tend to be
wider during solar maximum. Our results confirm previous coronagraph analyses
suggesting that CME launch sites don't simply migrate to higher latitudes with
increasing solar activity. At solar minimum, CMEs tend to be launched from
equatorial latitudes while, at maximum, CMEs appear to be launched over a much
wider latitude range; this has implications for understanding the CME/solar
source association. Our analysis provides some supporting evidence for the
systematic dragging of CMEs to lower latitude as they propagate outwards.
