Help needed for zip wire project

Hello. I have gotten involved with a project which was easy to start with - just determine who wins a race along two parallel zip wires using a couple of sensors and display the winner via a LED display.
I've now been asked to provide the speed of the person on the zip wire, I can easily work this out from the time it takes the person to descend the zip wire as I know it's length. What I want help with is getting a start signal from the top of the zip wire to the bottom. I thought of using a radio link, but these zips are 500m long in a forest. I need a reliable signal, so I thought about using the zip cable it's self to send the start signal down, but I'm not sure how best to do this. I've thought about linking the zip two cables together and measuring the resistance with an UNO and then breaking this link to generate my start signal, I guess this would work but I've become concerned about the buildup of static and electrical interference in the zip cables as they are strung over a large distance in the outdoors.
Can anyone offer me some advice? Can I send a signal down just one zip cable from one UNO to another? How do I protect against static and electrical interference?

There are RF modules that have quite long range with an appropriate antenna, such as


If you have two metal wires, not touching and not in contact with the ground, then you should be able to use them for communication.

There certainly is a concern about interference. Zener diodes of appropriate rating (just higher than your supply voltage) at each end and opto-isolators would seem to be in order, with a separate (battery) supply just for this link. And you might want to send a code at a low baudrate rather than a simple on/ off.

Modules like CrossRoads linked to work very well! There are several other 900MHz modules or off the shelf radios that would suffice for this setup. You can try the antennas that they come with, or if you are in a really dense forest, get specific directional antennas. Although, if you have zip lines coming through, then I bet you could get a line of sight signal.

Is there power or internet at closed network at both ends of the zip line? Maybe you could do a network communication. Use GPS receivers on both ends to “timestamp” the start and stop and then do your calculation.