BiSON - Integrated-light observations of the Sun's Oscillations in Doppler velocity since 1975

The Birmingham Solar Oscillations Network (BiSON) project observes the Doppler velocity of the Sun in integrated light using the potassium D1 line, from a global network of six fully or partially automated observing stations. These observations are used for low-degree helioseismology, detecting acoustic modes of angular degree 0 to 4. Since the deployment of the network in the early 1990s, BiSON has collected observations at 40-second cadence with a duty cycle of 70 to 80 per cent. Together with sparser observations stretching back to the 1970s, this gives a unique record of the Sun's low-degree "five-minute" oscillations - acoustic modes that are one of very few probes into the deep solar interior -- over more than four solar cycles. This enables us to measure the solar rotation to a precision of a few nanohertz and to study the changes in the frequency, amplitude, and damping of the acoustic modes in response to solar activity levels. Such studies reveal not only the correlation of frequency and the anticorrelation of mode amplitude with activity, but also the subtle differences between the response in different cycles. For example, the decrease in mode amplitude at the maxima of Cycles 23 and 24 is very similar even though the cycles are of different strengths. Here we present a brief overview of the network, some of the recent and historic results, and prospects for the future, including plans to modernize the network using a new generation of compact, fibre-fed instruments.