Actually, only one sign in the movie says that, and it was written by an activist shown briefly, so the actual spelling is "prawn".   

 "Command Module" 

In the plot summary, it says that Christopher had the command module under his house and flew to the mothership in it. However, in the movie it is implied that it's not a command module but some sort of ship, and the only reason Christopher needed it was to get to the mothership. If nobody disagrees, I will change it to say that.