RPCs are either supplied with the data products (as in the case of QuickBird *.RPB files and IKONOS *_RPC.txt files) or they are generated from the sensor information in the original file header information (as in the case for ASTER and SPOT 5 data). If you have RPC information from another source, you can use ENVI's User Defined RPC reader to create the RPC file. Also, beginning in ENVI 4.3, you can generate RPCs for satellite pushbroom sensors and digital/scanned aerial photos by selecting GCPs through ENVI's Build RPCs tool.