Laser interferometric Gravitational Wave telescopes are limited in sensitivity by seismic noise in the low frequency region (below a 10 Hz). The aim of this research is to improve performance of an advanced displacement sensor and an ultra-low noise inertial sensor, that can help measuring and correcting for seismically induced motion of the GW detetctor’s test masses. In a Rasnik, light from a back-illuminated encoded ChessField mask is projected onto a pixel image sensor, and the relative position is tracked between mask and sensor. In past experiments, Rasnik was modelled to have resolution of below ∼0.5 nm in X and Y, while RMS resolutions of ∼25 nm were measured [1], a longstanding discrepancy central to this research. It was found that, analyzing spectral information of Rasnik response data, Rasnik did in fact reach its sub-nm modelled noise floor. A resolution floor of 퐴፧ = 270 pm/√Hz was measured, shown to be limited by SNR of the pixel content and white noise aliasing. What is more, with a different setup, an optical magnification trick was shown to result in RMS resolution gain. For the forced-feedback accelerometer with interferometric readout, Heijningen [2] measured displacement sensitivities of up to 8 fm/√Hz above 30 Hz, while the instrument was modelled to be shot noise limited at 3 fm/√Hz - another discrepancy this research attempted to explain. A significant reduction of the thermal noise of the instrument’s mechanics was achieved by re-designing the feedback actuator. After that, the accelerometer’s performance was re-evaluated on a ultra-low vibration platform. A resolution of 50 fm/√Hz was measured, a factor 30 above the modelled shot noise. Sub-optimal relative intensity fluctuations in the laser, as well as laser frequency noise, were found to be responsible.