An old axiom says that in order to know where you are going, you first have to know where you are. To
that, add that you should know which way you are facing. Makers of wireless handsets, proving the old
axiom true, have already installed Global Positioning System (GPS) receivers, and are now poised to
flood the market with phones containing tiny electronic compasses that allow the gadget to sense exactly
what direction it’s facing.
Complete the sentence according to the text: “ A (an) _____ will let you know where you are, whereas to know which direction you are looking you need a (an) ______.