Wireless (separate) ultrasonic receiver and transmitter

I have removed the ping transducer.

I want to use it outdoors so maybe IR wont work properly?

I am new to RF so not sure how to do that, but I think I need to implement an interrupt as soon as a message is received it starts the ping on the receiver because my sketch is going to get bigger which will make things worse

But how constant is the travel speed of RF that might also be why I am not getting constant readings.