Current FDSN Networks containing Derived for Generated Channels:

GD Network: GPS Displacement Data in SEED format (citations)

SY Network:
   S1 and S3 location IDs: Synthetic seismograms generated by the Global ShakeMovie project (Princeton University)

Derived or Generated Channel: Time series derived from observational data or entirely generated by a computer algorithm, as would be the case with synthetic seismograms.

Instrument Code
X

Orientation Code
Similar to the observable data that was modified or the observable equivalent for generated time series (synthetics).

Further Usage:
In order to document the provenance of the data, SEED header information must be available that documents the algorithms, processes, or systems that modified or generated the time series. A Channel Comment Blockette (059), providing a Uniform Resource Locator (URL), must be included. The information available at the URL must identify the processes that were applied to modify or generate the time series. This information must reference the FDSN web site.

In addition to the requirement to include a B059, it is required to put a short description of the process/instrument in the 30 character channel comment (field 7 of B052).

Addendum to the SEED Manual as of April 2010:

X & Y Instrument Codes (pdf)