Caterpillar Model 42 Elevating grader, just curious about something

Cat is running a commercial on TV where the Model 42 Elevating Grader is featured. I would like to know what the round disk on the front is used for. I've seem the same feature on other models too. It looks like some kind of antenna or aiming device. It is directly opposite the steering wheel. Here is a link to the commercial on youtube. http://ift.tt/1vECcEI

If the link is missing, search for BUILT FOR IT™ on YouTube. Thanks for your time.

Caterpillar Model 42 Elevating grader, just curious about something