  We consider a two-ray ground reflection scenario with unknown distance
between transmitter and receiver. By utilizing two frequencies in parallel, we
can mitigate possible destructive interference and ensure ultra-reliability
with only very limited knowledge at the transmitter. In order to achieve this
ultra-reliability, we optimize the frequency spacing such that the worst-case
receive power is maximized. Additionally, we provide an algorithm to calculate
the optimal frequency spacing. Besides the receive power, we also analyze the
achievable rate and outage probability. It is shown that the frequency
diversity scheme achieves a significant improvement in terms of reliability
over using a single frequency. In particular, we demonstrate the effectiveness
of the proposed approach by a numerical simulation of an unmanned aerial
vehicle (UAV) flying above flat terrain.
