The ORPA directory is subdivided into five subdirectories.  These directories
(ORB3100, ORB3200, ORB3300, ORB3400, ORB3500) contain full resolution data for
orbits 3100-3599. 

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.