What android broadcasts are sent by gmaps while navigating?

Question

I am developing an android application which captures the location of every turn the user made while navigating from a source to a destination. My application would launch gmaps automatically and will run in the background, until the user has completed navigation. I would like to know what broadcasts gmaps would be sending to the android system during navigation?

For example, during navigation we would be getting voice instructions such as "in 100ft turn left etc". This happens with the help of some broadcasts. I would like to utilize them.

Also, please guide me if what my thinking is wrong and is there any other easy way to find out the turns. Kindly, note that my application requires to know about the location of turns at least a 100ft before the actual turn but not after the turn.

Please note that I am not interested in using gmaps API as they have a service charge beyond certain threshold and I am trying to build a free application.

Thanks.