The SensorML document states that the value of the uniqueID can be a URN but doesn't have to be:
BeginQuote:
The value of the gml:identifier can be a URI (e.g. URL or URN) or a series of string
characters that uniquely identify the object. An example of a globally unique identifier
for a weather station is given below:
urn:icd:stations:FR876638a7s8f9d55
EndQuote
The RelaxNG needs to be corrected for this as it currently is requiring a URN.
The SensorML document states that the value of the uniqueID can be a URN but doesn't have to be:
BeginQuote: The value of the gml:identifier can be a URI (e.g. URL or URN) or a series of string characters that uniquely identify the object. An example of a globally unique identifier for a weather station is given below: