Since the project may be temporary, and involves the government then they may arrange permission to send data via shortwave transmitter. Data can be in the form of morse code, created by turning the oscillator on and off.
The shortwave allocation is from 1.7 to 30 MHz. You'll want to research which band is best for your geography, latitude, weather, and radio traffic. A ham operator can advise you, and he may even be aware of radio stations, repeaters, satellites you might purchase time to use for your operation, etc.
Try to locate the antenna at the top of a mountain. Or if that's impossible then lift the antenna high via helium balloon, to give you greater transmitting range. Hope that your signal bounces off the ionosphere. (The only way you can have line-of-sight is by raising your antennas a mile high.)
Consider that electric power can come from batteries, or wind, water, solar, etc.