The ORPA directory is subdivided into twelve subdirectories.  These directories
(ORB4050, ORB4100, ORB4200, ORB4300, ORB4400, ORB4500, ORB4600, ORB4700, 
ORB4800, ORB4900, ORB5000, and ORB_MISC) contain full resolution data for
orbits 4050-5055, as well as some miscellaneous orbits. 

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.