  The number of Sensor Observation Service (SOS) instances available online has
been increasing in the last few years. The SOS specification standardises
interfaces and data formats for exchanging sensor-related in-formation between
information providers and consumers. SOS in conjunction with other
specifications in the Sensor Web Enablement initiative, at-tempts to realise
the Sensor Web vision, a worldwide system where sensor networks of any kind are
interconnected. In this paper we present an empirical study of actual instances
of servers implementing SOS. The study focuses mostly in which parts of the
specification are more frequently included in real implementations, and how
exchanged messages follows the structure defined by XML Schema files. Our
findings can be of practical use when implementing servers and clients based on
the SOS specification, as they can be optimized for common scenarios.
