Sun, as opposed to local helioseismology, which studies propagating waves in part of the Sun. We now give a brief introduction to each category.

Global helioseismology {#section[global intro]}
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Global helioseismology can itself be split into two sub-categories: Sun-as-a-star (unresolved) observations; and resolved observations. Sun-as-a-star observations are only sensitive to the lowest harmonic degrees ($0\le l\le 3$, and occasionally $l=4$ and 5). These modes are the truly global modes of the Sun as they travel right to the energy-generating core. However, by making resolved observations of the solar surface spatial filters can be applied that allow measurements of modes with degrees into the 100s or even 1000s.

The Birmingham Solar Oscillations Network has been making Sun-as-a-star line-of-sight velocity observations for over 30 years, covering cycles 21 (although with limited coverage), 22, 23, and 24. BiSON is an autonomous network of 6 ground-based observatories strategically positioned around the world so as to allow observations of the Sun to be made 24hr a day. This is important as gaps in the time domain contaminate frequency spectra, making it more difficult to accurately obtain the parameters that describe the modes, such as their frequencies. One can also make continuous observations of the Sun 