The system is known as Hyperion, and here’s how it works …
A ground-based eye-safe optical laser automatically tracks a manned or unmanned aircraft passing overhead, focusing its beam on a device on the vehicle’s underside. Called a Modulated Retro Reflector (MRR), this device modulates the reflected laser light, turning it into an optical code that transmits large amounts of encrypted data from the aircarft back to the ground crew. They in turn decode the message.
Currently the system is limited to a range of 1 km (0.6 mile), although work to improve this figure is ongoing. And yes, it might be simpler to just have a laser in the aircraft, although the equipment can be quite heavy – this is particularly an issue with small drones, in which every gram counts.
Hyperion has already been successfully tested using a drone, and may even one day be used to receive data from low-orbit satellites. It is hoped to be in commercial use within three to five years.