Sonar para Pixhawk


I 'm
Felipe Viveiros from Brazil
I study computer engineering at PUC Campinas University
In my
course competition assignment ,
'm build- a quad with pixhawk and the drone fly auto (no radio control)
the destination chosen by the User from a smart phone android,
with an application developed by me .
I need to choose a sonar, the sonar will be necessary if you have any obstacle
in the way,
my doubt is, what the best sonar ?
which has cost-benefit relationship ?
I will implement the algorithm in the best way.
I accept suggestions from other forum for developers
tanks my friends