\chapter{Reducing differential motion of aLIGO seismic platforms}
\label{CPSdiff}
During 2019, I spent some months working at the LIGO Hanford site (Washington, USA). This experience allowed me to be critically involved in the complicated life of a gravitational-wave interferometer. In particular, I was given the opportunity to study how to improve LIGO performances at low-frequency, focussing on the reduction of seismic motion of the platforms where the optics are located.\\
In this chapter I will demonstrate how we can modify seismic control configuration of LIGO in order to obtain different and possibly better performances for seismic motion stabilization, faster and longer locking mode and, ultimately, more gravitational waves detections. The detailed computations included in this chapter are original and partially presented to the LIGO community and stored in LIGO DCC \cite{proposal}\cite{technote1} .\\
In this chapter I will demonstrate how we can modify seismic control configuration of LIGO in order to obtain different and possibly better performance for seismic motion stabilization, faster and longer locking mode and, ultimately, more gravitational waves detections. The detailed computations included in this chapter are original and partially presented to the LIGO community and stored in LIGO DCC \cite{proposal}\cite{technote1} .\\
This work has been developed in collaboration with LIGO Hanford and LIGO Livingston laboratories, Stanford University, MIT and UoB and completed at UoB during 2020.\\
This chapter is partially including some technical notes I shared with LIGO collaboration and the contents of this study have been presented at conferences and workshops \cite{chiatalk}.\\
Essential information about the sections of LIGO involved in this study has been exposed in detail in Chapter \ref{LIGO}.
\section{Motivation: Duty cycle on LIGO}
Lock loss events are the main sources of preventing continuous observations for long periods of time: when light loses resonance in the cavities, a lock loss happens and the control systems of the optical cavities are under effort to restore stabilization. This means that during lock loss the interferometer is no longer able to be stable and the observing time is interrupted \cite{biscans}.\\
Duty cycle is one of the main topics where commissioners focus on before starting an observing run \cite{biscans}\cite{kisseltalk1}. It is needed not only to observe more gravitational waves, but also to identify noise sources and improve sensitivity \cite{biscanstalk}.\\
Duty cycle is a major focus for commissioners before starting an observing run \cite{biscans}\cite{kisseltalk1}. It is needed not only to observe more gravitational waves, but also to identify noise sources and improve sensitivity \cite{biscanstalk}.\\
Since the number of detected events over a time period N(t) is proportional to the volume of Universe under observation V, the observing time t and the rate R of astrophysical sources that can occur in a certain volume:
\begin{equation}
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@@ -44,7 +44,7 @@ Other ways to improve duty cycle is to increase the observable volume: this can
\caption[LHO duty cycle during O3b]{Example of duty cycle for Hanford Observatory, during O3b (Figure taken from \cite{kisseltalk1}) For almost 20\% of the running time the detector was not locked, which means that it was not observing. It is important to minimise this number, so more gravitational waves can be detected.}
\label{duty}
\end{figure}
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@@ -271,7 +271,6 @@ All this analysis has been performed through Matlab software.\\
\begin{figure}[h!]
\centering
\includegraphics[scale=0.3]{images/SCbode.png}
\includegraphics[scale=0.3]{images/SC.png}
\caption[Sensor correction filter]{The sensor correction filter as it is at present installed on LIGO. This filter is one of the contributors to take into account for when computing the blending filters.}
