The ORPA directory is subdivided into five subdirectories.  These directories
(ORB3600, ORB3700, ORB3800, ORB3900, ORB4000) contain full resolution data for
orbits 3600-4049. 

Data are organized into single orbit files which follow the naming convention 
ORBxxxx.RDR where xxxx is a four digit orbit number.  In addition, for each 
data file there is a label file which describes the file characteristics, 
such as number of records, record length, etc. called ORBxxxx.LBL.