Best way to capture cargo pod landing?

[Omnifarious]

What is the best way to capture a cargo pod landing/crashing?

I want to have a special item that, if landed in a cargo pod when there is no cargo landing pad,, will automatically build a few items on the surface near where the cargo pod landed.

I was thinking I could somehow detect the cargo pod landing, and then do whatever it is I need to do when that happens.