  The GOLF instrument on board SoHO has been in operation for almost 25 years
but aging of the instrument has now strongly affected its performance,
especially in the low-frequency p-mode region. At the end of the SoHO mission,
the ground-based network BiSON will remain the only facility able to perform
Sun-integrated helioseismic observations. Therefore, we want to assess the
helioseismic performances of an \'echelle spectrograph like SONG. Indeed, the
high precision of such an instrument and the quality of the data acquired for
asteroseismic purpose calls for an evaluation of the instrument ability to
perform global radial-velocity measurements of the solar disk. Data acquired
during the Solar-SONG 2018 observation campaign at the Teide Observatory are
used to study mid- and low-frequency p modes. A Solar-SONG time series of
30-day duration is reduced with a combination of the traditional IDL iSONG
pipeline and a new Python pipeline described in this paper. A mode fitting
method built around a Bayesian approach is then performed on the Solar-SONG and
contemporaneous GOLF, BiSON, and HMI data. For this contemporaneous time
series, Solar-SONG is able to characterise p modes at a lower frequency than
BiSON and GOLF (1750{\mu}Hz against 1946 and 2157 {\mu}Hz respectively), while
for HMI it is possible to characterise a mode at 1686 {\mu}Hz. The decrease of
GOLF sensitivity is then evaluated through the evolution of its low-frequency
p-mode characterisation abilities over the years. [abridged]
