#### Description:

This attribute identifies the spectral system in which the SourceVel attribute value (the source velocity) is supplied and returned. It can be one of the following:

- `"` VRAD`"` or `"` VRADIO`"` : Radio velocity (km/s)

- `"` VOPT`"` or `"` VOPTICAL`"` : Optical velocity (km/s)

- `"` ZOPT`"` or `"` REDSHIFT`"` : Redshift (dimensionless)

- `"` BETA`"` : Beta factor (dimensionless)

- `"` VELO`"` or `"` VREL`"` : Apparent radial (`"` relativistic`"` ) velocity (km/s)

When setting a new value for the SourceVel attribute, the source velocity should be supplied in the spectral system indicated by this attribute. Likewise, when getting the value of the SourceVel attribute, the velocity will be returned in this spectral system.

If the value of SourceSys is changed, the value stored for SourceVel will be converted from the old to the new spectral systems.

The default value is `"` VELO`"` (apparent radial velocity).

Type:

  String.

#### Applicability

##### SpecFrame

All SpecFrames have this attribute.
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