How do I change the default wifi network that Ubuntu connects to on startup? There is an unsecured public wifi network in my range, as well as my own home WPA secured wifi that I want to use by default. However, on startup Ubuntu always connects to the unsecured one. Sometimes I forget to reconnect to the other one, which is very dangerous.

I am running a default setup of Ubuntu Destkop 11.10 with Gnome 3. I have looked under network settings but I can't find a 'default' option.