Data access in GIPSY is done through a set of routines collectively known under the name `` GDS '' (GIPSY Data System). ... A set consists of one or two files: the descriptor file and the image file. ... In GIPSY there are kinds of coordinates: . ... To retrieve a descriptor item, given its name and level, the GDS reading routines start looking at the specified level and if it is not found there, higher levels are tried. ... Every GDS routine has an integer error argument, which often is optional. ...