[gembud] how to display - composite level III NCZ

hi
I was reading about GDRADR and some other stuff about how to create a grid files etc. but can someone summarize what exactly I need to display reflectivity factors from multiple radars at some close times. I have level III data from 8 radar sites. Some of the selected files that I would like to display the composite NCZ reflectivity data are:
KLCH_SDUS64_NCZLCH_200504300600
KLIX_SDUS64_NCZLIX_200504300602
7000KMOB_SDUS64_NCZMOB_200504300558
etc......
If I want to display the composite Z for the time around 06UTC from all 8 radar stations, what do I need to do first. I used GDRADR with the following parameter setting
#! /bin/csh
$GEMEXE/gdradr << EOF
GRDAREA = 27;-98;40;-82
PROJ    = LCC
KXKY    = 720;500
GDPFUN  = NCZ
GDFILE  = 20050430_06h_radr.gem
RADTIM  = 050430/06
RADDUR  = 5
MAXGRD  = 100000
r
exit
EOF
$GEMEXE/gpend


thanks