A tethered aircraft or balloon carrying a communications base station for rapid deployment in emergency situations. Electrical power is delivered from a generator on the ground using a pulsed electrical supply system in which each power pulse is delivered over a cable and acknowledged, and pulses only continue to be delivered whilst such acknowledgements are received by the ground station. This reduces the risks associated with delivering electrical power over an aerial tether, and avoids the need for an earth (ground) connection, reducing the risk from lightning.