Valid signals are either in the range of +3 to +15 volts or the range -3 to -15 volts; the range between -3 to +3 volts is not a valid RS-232 level
Yes a TTL to serial adapter is needed for interfacing with an RS232 serial port. The signal voltages on RS232 can cause damage to the GPS module.
I wonder if this tutorial, that I bookmarked earlier for myself, would help. It talks about using the GPIO serial port. Definitely avoids the need for level shifting!
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