Near verticial incidence skywave https://en.wikipedia.org/wiki/Near_vertical_incidence_skywave https://en.wikipedia.org/wiki/Near_vertical_incidence_skywav... u

Near verticial incidence skywave https://en.wikipedia.org/wiki/Near_vertical_incidence_skywave https://en.wikipedia.org/wiki/Near_vertical_incidence_skywav... using the HF bands and HF antenna very close to the ground. This will cause your signal to basically only go up and then bounce off the ionosphere. This causes you signal to skip a ring of a ~100 miles around you forming a donut of gound locations that can receive it from the ionosphere bounce. The triangulation will not be to your location to to the ionospheric bounce. Locally it'll propagate via groundwave but that will quickly die out with the first hills and valleys. A network of ~300 mi seperated stations doing NVIS could be fairly hard to locate. To make it slightly harder you could try using ultrawide bandwidth modulations (UWB) at HF freqs but propagations differences between the freqs will make it hard. I say all of this but it depends on your threat model. Nowadays major nations have electromagnetic signals intelligence satellites even for HF up in orbit and have a line of sight to everything.