Converting GPS Serial Output to TTL levels

Not realizing the output voltage of the GPS we are using, I wired it directly up to our new Arduino. After three hours of it not working, it occurred to me that the voltage output may be at RS232 levels (plus or minus 3 to 15 volts) instead of TTL (0 through 5 volts) levels. A quick check on the oscilloscope confirmed that the Garmin unit we are using outputs at plus and minus 6 volts. I ordered a sample of an ADM232 which converts between RS232 levels and TTL levels. Temporarily, we are using an OPA2350 rail to rail op amp to limit the output from 0-5 volts. This solution works, and we are now able to read NMEA data through the Arduino.
Here is the schematic of the circuit that we are using:

NMEA 0183

NMEA 0183 (or NMEA for short) is a combined electrical and data specification for communication between marine electronic devices such as echo sounder, sonars, anemometer (wind speed and direction), gyrocompass, autopilot, GPS receivers and many other types of instruments. It has been defined by, and is controlled by, the U.S.-based National Marine Electronics Association.


Link to the wikipedia article.
Arduino NMEA device driver library.

Possible Contol System Components

Arduino USB Board

• Used as the central microcontroller of the boat
• Open source hardware
• Open source software
• Very large community support
• Wide range of accessories
• Low cost ($30)
• Link to page at sparkfun

ArduPilot - Arduino Compatible UAV Controller w/ ATMega328

• UAV controller for use in unmanned cars, planes, and boats
• Plugs directly into the Arduino microcontroller
• Comes with an unmanned vehicle operating system that is FOSS
• Has an socket for a GPS receiver
• Low cost ($25)
• Link to page at sparkfun

20 Channel EM-406A SiRF III Receiver with Antenna

• GPS receiver used with ArduPilot
• Antenna included
• Small form factor
• 1 Hz update rate
  
• 5 Volts
• Very well documented
• Link to page at spakfun

Official Ardupilot FTDI Cable

• Needed to load firmware, etc... on the ArduPilot board
• $20
• Not worth it to make our own, I looked into it
• Lab checkout doesn't have one either