WSPR uses High Frequency (HF) ionospheric propagation to send signals around the world. Typical frequencies are in the range 3 MHz to 28 MHz. Transmission power levels are up to 5 W continuous and the duration of the transmission is 110.6 seconds.
The US military (Air Force and Navy) has invested heavily in research into HF ionospheric propagation since 1956, when a definitive set of experiments showed that the ionosphere is generally sufficiently stable for a HF ionospheric radar to succeed for over-the-horizon aircraft detection. As a result, the US military has developed a number of Over-The-Horizon-Radar (OTHR) systems over the years, that have been in operation since 1961. These OTHR systems have been deployed in Chesapeake Bay, California, Virginia, Maine, Aleutian Islands, England and Australia.
The US military tested Round-The-World propagation successfully in 1978 and tested Precision Targeting of Aircraft successfully in 1985. Typical frequencies are in the range 3 MHz to 30 MHz. Transmission power levels are up to 25 kW with a pulse width of 100 µs.
WSPR can be used as an OTHR and has two key advantages, firstly in the large number and geographic spread of the transmitters and receivers and secondly in the long integration time of 110.6 seconds in the receivers.
A short technical note can be downloaded here