Arduino and Decatur Si-2 Radar
That appears to use normal RS232 voltage levels, so you would be better off with an older version of an arduino-style board having the 9 pin serial connector and serial level shifter, instead of the USB chip and USB connector that they've had for the past couple of years.

You can still do serial communications with a modern usb-type arduino, but as the atmega chip itself only supports logic level communication, you need a level shifter such as a max232 to talk at rs232 voltage levels.  The level shifter also inverts the sense of the signal to meet the standard.

In terms of software, usually your best bet is to start with a pc terminal program talking to the device by hand, and once you have it sorted out, write your arduino software to do what you were doing manually.