Maybe you could create a file in /etc/udev/rules.d/99-navitgps.rules with the following informations:
SUBSYSTEM=="usb", ATTRS{idVendor}=="1915", ATTRS{idProduct}=="7777", MODE="0664", GROUP="plugdev"
where idVendor and idProduct come from the following command:
udevadm info -a -n /dev/ttyACM0