The Sensor Observation Service (SOS) extension for ArcGIS Server is implemented as a so-called Server Object Extension (SOE). It allows querying of observations, procedures/sensors, as well as features observed by sensors. The interface is compliant with the SOS 2.0 standard from OGC.
The SensorML response is not valid as gml:ids have to be unique within one document. The id "FeatureOfInterest-1" is used for every feature.
example:
http://ags.dev.52north.org:6080/arcgis/rest/services/EEA/AirQualitySos/MapServer/exts/52nArcGisSos/DescribeSensor?service=SOS&version=2.0.0&request=DescribeSensor&procedure=Network_GBXXXX&procedureDescriptionFormat=http%3A%2F%2Fwww.opengis.net%2FsensorML%2F1.0.1&f=xml