Hi there,
I think I can design the narrow bean RF proximity sensor for you. I am a Radio Physics and Electronics engineer with a Masters degree. I have 11 years of experience working with various types of Txs and Rxs. Currently I teach electronics to undergraduate students in a college in Kolkata, India. from what I understand, your design requirement is rather simple. I think you just need to pick the right hardware components that can withstand the extreme weather conditions. For the job, I will need to buy the components to figure what works best under the conditions you mentioned. Please let me know if I could help.
Best,
Bidyut R.