  The Sagnac speed meter interferometer topology can potentially provide
enhanced sensitivity to gravitational waves in the audio-band compared to
equivalent Michelson interferometers. A challenge with the Sagnac speed meter
interferometer arises from the intrinsic lack of sensitivity at low frequencies
where the velocity-proportional signal is smaller than the noise associated
with the sensing of the signal. Using as an example the on-going
proof-of-concept Sagnac speed meter experiment in Glasgow, we quantify the
problem and present a solution involving the extraction of a small
displacement-proportional signal. This displacement signal can be combined with
the existing velocity signal to enhance low frequency sensitivity, and we
derive optimal filters to accomplish this for different signal strengths. We
show that the extraction of the displacement signal for low frequency control
purposes can be performed without reducing significantly the quantum
non-demolition character of this type of interferometer.
