This attribute (together with SourceSys, SourceVRF, RefRA and RefDec)
defines the "Source" standard of rest (see attribute StdOfRest). This is
a rest frame which is moving towards the position given by RefRA and
RefDec at a velocity given by SourceVel. A positive value means
the source is moving away from the observer. When a new value is
assigned to this attribute, the supplied value is assumed to refer
to the spectral system specified by the SourceSys attribute. For
instance, the SourceVel value may be supplied as a radio velocity, a
redshift, a beta factor, etc. Similarly, when the current value of
the SourceVel attribute is obtained, the returned value will refer
to the spectral system specified by the SourceSys value. If the
SourceSys value is changed, any value previously stored for the SourceVel
attribute will be changed automatically from the old spectral system
to the new spectral system.
When setting a value for SourceVel, the value should be supplied in the
rest frame specified by the SourceVRF attribute. Likewise, when getting
the value of SourceVel, it will be returned in the rest frame specified
by the SourceVRF attribute.
The default SourceVel value is zero.