Gravitational-wave bands conventionally divide based on detector technology [108*, 35]: Future extremely-low-frequency
(
10–18 to 
10–15 Hz) search programs will be based on mapping the intensity and polarization of the cosmic
microwave background; very-low frequency observations (
10–9 to 
10–6 Hz) mostly use pulsar timing observations;
low-frequency (
10–6 to 
10–1 Hz) observations currently use Doppler tracking of spacecraft (in the 2020’s a laser
interferometer in space); high-frequency (
1 to 
104 Hz) observations involve ground-based laser interferometers or
resonant bar detectors. For general reviews see [91, 108*, 36].
