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\newcommand{\Ref}[1]{Ref.~\cite{#1}}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%TITLE
\title{KAGRA Actuator Noise\\ Modeling Report}
\author{Yuta Michimura}
\date{\today}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%MAIN
\begin{document}
\maketitle

\section{Introduction}
This report is to summarize the results of actuator noise modeling for the KAGRA suspensions. The modeling was done by using MATLAB Simulink based NoiseBudget script made by Chris Wipf~\cite{NBref}.

The main script and the model for the actuator noise modeling are as follows:
\begin{itemize}
  \item \url{https://granite.phys.s.u-tokyo.ac.jp/svn/LCGT/trunk/kagranoisebudget/Suspensions/run_SAS_NB.m}
  \item \url{https://granite.phys.s.u-tokyo.ac.jp/svn/LCGT/trunk/kagranoisebudget/Suspensions/SAS.slx}
\end{itemize}

You will also need {\tt findNbSVNroot.m}, {\tt myzpk.m}, {\tt plotdobe.m}, and {\tt plotspectrum.m} in the same directroy to run the script.

The main purpose of this modeling is to check if the actuator noise meet the displacement noise requirement set by MIF group, and to check if the feedback signals to the actuators does not saturate DACs. Small actuation efficiency gives less displacement noise, but it requires more feedback voltage.
Although this script works similarly for all suspensions, here I plot the example plots mainly for BS (Type-B suspension) in the next section. Results for each suspension is summarized in the following sections.

\section{Model}
The Simulink model is shown together with the transfer functions and noises used for the simulation.

\subsection{Simulink model}
The actuator noise Simulink model is shown in \zu{SAS}. We had to use some tricks to simulate out-of-loop stability and feedback signal with Simulink NoiseBudget blocks, {\tt NbNoiseCal} and {\tt NbNoiseSink}. FlexTf is used for suspension transfer functions (light purple blocks) to use frequency response data ({\tt frd}). Seismic noise from vertical coupling is also included in the model.

\begin{figure}[t]
  \begin{center}
    \includegraphics[bb=0 0 612 792,height=140mm]{../results/SAS.pdf}
  \end{center}
  \caption{Actuator noise Simulink model.} \label{SAS}
\end{figure}

\clearpage
\subsection{Summary of KAGRA Suspensions}
KAGRA suspension configurations are summarized graphically in \Ref{SASSummary}. For longitudinal degrees of freedom, we basically have actuators for IP (inverted pendulum), IM (intermediate mass), and TM (test mass). For Type-A suspension, we also have actuators for MN (Marionette).

\hyou{TableAct} is the summary of the actuation for each suspension. Actuation efficiencies for Type-B/Bp coils in N/A are from \Ref{OSEMcalc}, those for Type-A coils are from \Ref{Miyamoto,Miyamoto2}. Magnets used for Type-A/B/Bp suspensions are SmCo.

Actuation efficiency for a Type-C TM coil are estimated from the measurement done in June 2015~\cite{IMCmeas}. The measurement for MCe gives $6.9 \times 10^{-6} \unit{m/V}$ at DC, and this gives $1.1 \times 10^{-4} \unit{N/V}$ assuming IMC mirror mass to be $0.47 \unit{kg}$ and the resonant frequency to be $0.94 \unit{Hz}$. The V-I conversion of coil driver for IMC mirrors is $20 \unit{mA/V}$ ($50 \unit{\Omega}$)~\cite{TAMAcoildriver}, so this means the actuation efficiency for a Type-C TM coil is $1.4 \times 10^{-3} \unit{N/A}$ (this is consistent with Y. Fujii's coil-magnet coupling measurement~\cite{CoilFujii}). Note that magnets for IMC are NdFeB.

Summary of the actuator design can also be found in the wiki below:
\begin{itemize}
  \item \url{http://gwwiki.icrr.u-tokyo.ac.jp/JGWwiki/KAGRA/Subgroups/VIS/ActuatorDesign}
\end{itemize}

\begin{landscape}
\begin{table}
  \begin{center}
    \caption{KAGRA suspension actuator parameters. The TM/IM masses and wire lengths for Type-B/Bp suspensions come from \Ref{TypeBDesign}. Those for Type-A suspensions come from \Ref{TypeADesign,Miyamoto,Miyamoto2}. Those for Type-C suspensions come from private communication with K. Arai, R. Takahashi, and T. Saito. TM actuation efficiency for Type-C comes from \Ref{IMCmeas}.} \label{TableAct}
    \begin{tabular}{lccccc}
      \hline \hline
	Type	& Type-A	& Type-B (BS)	& Type-B (SR)	& Type-Bp	& Type-C \\
	\hline
	Applicable mirrors & ITM,ETM & BS & SRM,SR2,SR3 & PRM,PR2,PR3 & MCi,MCo,MCe \\
	Mirror diameter & $\phi=220 \unit{mm}$ & $\phi=370 \unit{mm}$ & $\phi=250 \unit{mm}$ & $\phi=250 \unit{mm}$ & $\phi=95.95 \unit{mm}$ \\
	Mirror thickness & 150 mm & 80 mm & 100 mm & 100 mm & 29.5 mm \\
	Mirror substrate & Sapphire & Fused Silica & Fused Silica & Fused Silica & Fused Silica \\
	Mirror mass & $22.7\unit{kg}$ & $18.9\unit{kg}$ & $10.8\unit{kg}$ & $10.8\unit{kg}$ & $0.47\unit{kg}$ \\
	Intermediate Mass mass & $53.2 \unit{kg}$ & $36.5\unit{kg}$ & $15.6\unit{kg}$ & $15.6\unit{kg}$ & $0.71\unit{kg}$ \\
	Wire length between TM and IM & 0.35 m & 0.5 m & 0.5 m & 0.5 m & 0.25 m \\
	Wire length between IM and MN/BF & 0.24 m & 0.5 m & 0.5 m & 0.5 m & 0.25 m \\
	\hline
        TM magnet size [mm] & $\phi 2 \times 2 \rm{t}$ & $\phi 2 \times 3 \rm{t}$ & $\phi 2 \times 5 \rm{t}$ & $\phi 6 \times 3 \rm{t}$ & $\phi 1 \times 5 \rm{t}$ \\
	TM actuation per coil [N/A] & 0.0015	& 0.0138	& 0.0225	& 0.129	& $1.4 \times 10^{-3}$ \\
	\# of TM coils for long. & 4	& 4	& 4	& 4 	& 4 \\
	\hline
        IM magnet size [mm] & $\phi 2 \times 2 \rm{t}$ & $\phi 10 \times 10 \rm{t}$ & $\phi 10 \times 10 \rm{t}$ & $\phi 10 \times 10 \rm{t}$ & N.A. \\
	IM actuation per coil [N/A] & 0.015	& 1.25	& 1.26	& 1.26	& N.A. \\
	\# of IM coils for long. & 2 	& 1	& 1	& 1 	& 0 \\
	\hline
	MN magnet size [mm] & $\phi 5 \times 13 \rm{t}$ & N.A. & N.A. & N.A. & N.A. \\
	MN actuation per coil [N/A] & 0.45	& N.A.	& N.A.	& N.A.	& N.A. \\
	\# of MN coils for long. & 2	& 0	& 0	& 0 	& 0 \\
      \hline \hline
    \end{tabular}
  \end{center}
\end{table}
\end{landscape}

\subsection{Suspension transfer functions}
For the suspension transfer functions, the ones simulated with SUMCON developed by T. Sekiguchi~\cite{sumcon} were used.

The example suspension transfer functions from actuation on IM/TM (from respective recoil masses) to TM displacement are shown below.

\figSusp{BS}

The exmaple seismic noise supression ratio are shown below. The vertical one is also plotted. The vertical to longitudinal coupling was assumed to be 1\% in the modeling, for all the suspensions.

\figSeis{BS}

\subsection{Seismic noise}
The Kamioka seismic noise used in the modeling is the high noise model in \Ref{KamiokaSeis}, and is plotted below.

\figTemplate{../results/SeismicNoise}{Kamioka seismic noise.}

\subsection{Coil drivers}
We have two types of coil drivers, the high power one and the low power one. They are basically the copies of \LIGOlink{D0902747} and \LIGOlink{D070481}, respectively, but have different dewhitening filters compared with LIGO ones. The schematics of the high power coil driver and the low power coil driver are in \Ref{hpcd} and \Ref{lpcd}, respectively, and the resistances are $80 \unit{\Omega}$ for the high power one, and $7.8 \times 10^3 \unit{\Omega}$ at DC for the low power one. The high power one and the low power one both have switchable three-stage dewhitening filters with pole @ 1 Hz and zero @ 10 Hz (gain of 1 at DC). In the simulation, all the dewhitening filters are turned on. The low power one has a voltage gain of 1.33.

The low power ones are used for both IM and TM coils for Type-B suspensions, and the high power ones are used for both IM and TM coils for Type-Bp suspensions. V-I conversion factor for the coil drivers when all the dewhitening filters are turned off are plotted in \zu{../results/BSCoilDriver} and \zu{../results/PRMCoilDriver}. The resistance of the coil is not included here, but it is included in the model (as $13 \unit{\Omega}$ for Type-B/Bp suspensions).

\figTemplate{../results/BSCoilDriver}{Inverse of V-I conversion factors for the low power coil drivers used for Type-B.}

\figTemplate{../results/PRMCoilDriver}{Inverse of V-I conversion factors for the high power coil drivers used for Type-Bp.}

The transfer functions of the whitening and the dewhitening filters are plotted below.

\figTemplate{../results/BSWhitening}{Whitening and dewhitening filters for IM and TM.}

Noises of coil drivers used in the model are plotted below, as input equivalent noise to the V-I conversion stage. The spectra come from \LIGOlink{T080014} (high power) and \LIGOlink{T0900233} (low power).

\figTemplate{../results/BSCoilDriverNoise}{Input equivalent low power coil driver noise spectra.}

\figTemplate{../results/PRMCoilDriverNoise}{Input equivalent high power coil driver noise spectra.}

\subsection{DAC}
The DAC used for KAGRA is 16 bit and has the range of $\pm 10 \unit{V}$. So, the least square bit for the DAC is $20\unit{V} / 2^{16}\unit{cnts} = 0.305\unit{mV/cnts}$. The DAC noise is plotted below.

\figTemplate{../results/DACNoise}{DAC noise.}

\subsection{Magnetic noise coupling}
Magnetic noise calculation method is summarized in \Ref{Magnoise2}. This study shows the force noise from the coupling between the magnetic field gradient and overall magnetic moment of magnets is dominant. So, I included this noise for this modeling.

The magnetic field gradient noise is plotted below. The magnetic field gradient noise is estimated by dividing measured magnetic field noise in \Ref{MagneticMeasurement} noise by 1 m. Since this noise is measured when there are no equipments installed. Virgo experience show that the magneic noise will be 50 times larger. Detailed measurement inside the cryostat is on going by H. Tanaka.

Note that the spectrum above 70 Hz and below 0.13 Hz is extrapolated.

\figTemplate{../results/MagneticGradientNoise}{Magnetic field gradient noise.}

\subsection{Quantum noise}
The quantum noise incluing the radiation pressure noise and the shot noise is simulated by Optickle by Y. Aso \cite{MIFDesign}. The simulated quantum noise used in this model is plotted below. DARM is fedback to ETMs, MICH is fedback to BS, PRCL is fedback to PRM, and SRCL is fedback to SRM.

\figTemplate{../results/DisplacementEquivalentQuantumNoisesBRSE}{Displacement equivalent quantum noise for BRSE.}

\clearpage
\section{Results for BS}
Resulting plots for BS actuator noise modeling are shown. BS is suspended by a Type-B suspension, but differs from other Type-B's since the mirror mass is heavier.

\subsection{Openloop transfer function}
The openloop transfer funtion is shown below.

\figOLTF{BS}

\subsection{Noise budget}
The displacement noise budget and the actuator noise budget are shown below. The lines labeled "Requirement" show the BS displacement noise requirement in \Ref{MIFDesign}, and the safety factor of 10 is included. Quantum noise contribution will be subtracted using feedforward technique~\cite{MIFDesign}.

As you can see, the seismic noise and the actuator noise meet the requirement above 10 Hz. The magnet size for BS TM was originally 6 mm dia, 3 mm thick, but was changed to 2 mm dia, 3 mm thick to reduce the actuator noise and magnetic noise.

\figDispNoise{BS}
\figActuatorNoise{BS}

\subsection{Feedback signal saturation check}
The spectra of feedback signals for IM and TM are shown in the figures below. The blue lines labeled "DAC limit" shows the DAC range ($2^{16}$).

As you can see, the RMS's of the feedback signals do not exceed the DAC limit.

\figFB{BSIM}
\figFB{BSTM}

\clearpage
\section{Results for SRM}
Resulting plots for SRM actuator noise modeling are shown. SRM is suspended by a Type-B suspension. Although displacement noise requirements for SRM and BS is similar, SRM is more severe to the actuator noise since SRM is lighter than BS.

\subsection{Noise budget}
The displacement noise budget and the actuator noise budget are shown below.

As you can see, the seismic noise and the actuator noise meet the requirement above 10 Hz. Quantum noise contribution will be subtracted using feedforward technique~\cite{MIFDesign}. The magnet size for SRM TM was originally 6 mm dia, 3 mm thick, but was changed to 2 mm dia, 5 mm thick to reduce the actuator noise and magnetic noise.
\figDispNoise{SRM}
\figActuatorNoise{SRM}

\subsection{Feedback signal saturation check}
The spectra of feedback signals for IM and TM are shown in the figures below.

As you can see, RMS of the feed back signals do not exceed the DAC limit.
\figFB{SRMIM}
\figFB{SRMTM}

\clearpage
\section{Results for PRM}
Resulting plots for PRM actuator noise modeling are shown. PRM is suspended by a Type-Bp suspension. Type-Bp suspension is basically Type-B, but upper stage (Standard Filter) is fixed. So, actuation transfer functions are the same as SRM ones, but seismic suppression ratios are different. To compensate this, high power coil drivers are used instead of low power ones.

\subsection{Noise budget}
The displacement noise budget and the actuator noise budget are shown below.

As you can see, the seismic noise and the actuator noise meet the requirement above 10 Hz.
\figDispNoise{PRM}
\figActuatorNoise{PRM}

\subsection{Feedback signal saturation check}
The spectra of feedback signals for IM and TM are shown in the figures below.

As you can see, RMS of the feed back signals do not exceed the DAC limit.
\figFB{PRMIM}
\figFB{PRMTM}

\clearpage
\section{Results for IMC suspensions}
Resulting plots for actuator noise modeling for IMC suspensions are shown. IMC mirrors are suspended by Type-C suspension, which is a modified version of the old TAMA PO type suspension. Type-C suspension is a double pendulum fixed on a three-stage stack. There are no actuators for IM. Whitening and dewhitening filters are not used for coils for IMC suspensions. Coil driver for IMC suspensions are different from Type-B/Bp ones; we use TAMA coil drivers described in \Ref{TAMAcoildriver}. V-I conversion is a flat $50\unit{\Omega}$. We will replace the IMC coil drivers to the high power coil drivers in the future.

For calculating the vertical motion of IMC mirrors, only the isolation ratio from the double pendulum is used in this modeling. The vertical motion should be smaller than the model since we also have isolation from stacks. The vertical to longitudinal coupling was also assumed to be 1\%.

\subsection{Noise budget}
The displacement noise budget and the actuator noise budget are shown below. The displacement noise requirement for the IMC suspensions comes from the frequency noise requirement after the frequency stabilization servo using IMC length.

As you can see, the seismic noise and the actuator noise well meet the requirement above 10 Hz. Quantum noise is calculated assuming input power to IMC to be 100 W.
\figDispNoise{IMC}
\figActuatorNoise{IMC}

\subsection{Feedback signal saturation check}
The spectra of feedback signals for IM and TM are shown in the figures below.

As you can see, RMS of the feed back signal does not saturate the DAC limit.
\figFB{IMCTM}

\clearpage
\section{Results for ITM/ETM}
Resulting plots for ITM/ETM actuator noise modeling are shown. ITM/ETM is suspended by a Type-A suspension. Low power coil drivers are used for TM, IM, and MN stages. However, for IM and MN stages, modified version is used. R65 and R95 of the driver are modified to be 700$\unit{\Omega}$ instead of 3.9 $\unit{k \Omega}$.

\subsection{Noise budget}
The displacement noise budget and the actuator noise budget are shown below.

As you can see, the seismic noise and the actuator noise meet the requirement above 10 Hz. Quantum noise used here is calculated assuming green locking with input power of 100 mW. This is to check if suspension actuator ranges are sufficient for the green lock. Actually, AOM is used as an actuator for the green lock, but we might also use suspensions for transitioning from the green lock to the IR lock.

\figDispNoise{ETM}
\figActuatorNoise{ETM}

\subsection{Feedback signal saturation check}
The spectra of feedback signals for MN, IM and TM are shown in the figures below.

As you can see, RMS of the feed back signals do not exceed the DAC limit. The magnet size for ETM MN was originally 2 mm dia, 2 mm thick, but was changed to 5 mm dia, 13 mm thick since it saturated the MN DAC. We can further reduce the feedback by implementing damping servo on upper stages.
\figFB{ETMMN}
\figFB{ETMIM}
\figFB{ETMTM}

\clearpage
\section{Saturation on lock acquisition}
We also have to check if the DAC and the coil drivers do not saturate on lock acquisition. The maximum current the low power coil drivers can produce is 10 mA (AD8671)~\cite{lpcd}, and that for the coil drivers for IMC is 440 mA (EL2099)~\cite{TAMAcoildriver}.

The force we need to stop the TM mirror can be roughly estimated by
\begin{equation}
  F = \frac{m v}{\Delta t},
\end{equation}
where $m$ and $v$ are the mass and the velocity of the mirror, respectively. $\Delta t$ is the time it takes to pass the linewidth of the error signal $d$, and $\Delta t = d / v$. So,
\begin{equation}
  F = \frac{m v^2}{d}.
\end{equation}

Thus, the velocity requirement for the mirror can be calculated as

\begin{equation}
  v_{\rm req} = \sqrt{\frac{F_{\rm max} d}{m}}.
\end{equation}

The estimation for the force and corresponding current to each coil, voltage to each coil driver (DAC output) are show in \hyou{TableLockAcq}.

\begin{table}[H]
  \begin{center}
    \caption{The velocity requirements for suspensions from lock acquisition.} \label{TableLockAcq}
    \begin{tabular}{lccccc}
      \hline \hline
	Mirrors & ITM,ETM & BS & SR & PR & IMC \\
      \hline
	Type	          & A	& B (BS)& B (SR)& Bp	& C \\
        Mass [kg]         & 22.7& 18.9	& 10.8	& 10.8	& 0.47 \\
        Linewidth [nm]    & 9 (green)	& 532 & 14 & 9 & 1 \\
        Maximum force [N] & 7.7 uN	& 71 uN & 115 uN &  65 mN & 1.1 mN \\
        Velocity req. [um/sec] & N.A.	& 1.4	& 0.38	& 7.3	& 1.5 \\
      \hline \hline
    \end{tabular}
  \end{center}
\end{table}

TAMA BS prototype experiment and Kamioka seismic measurement indicates 0.2 um/sec for Type-B suspensions. The estimated mirror velocity for Type-Bp suspension by A. Shoda is 4.4 um/sec. The estimated mirror velocity for Type-C suspension using the Matlab code from R. Takahashi is 2.3 um/sec.

From this study, we can say that;

\begin{itemize}
  \item We can reduce BS TM actuation efficiency even more if we want to reduce magnetic noise.
  \item It is desirable to increase number of turns or use bigger magnets for IMC.
\end{itemize}

Note that common mode rejection between mirrors are not included in this calculation.

For Type-A suspensions, lock acquisition is done using the frequency actuator (AOM) of the arm length stabilization system, so different discussion is needed.

\section{Summary}
\begin{itemize}
  \item Smaller magnets for BS TM and SRM TM made actuator noise small enough.
  \item High power coil drivers for PRM IM and TM was OK for actuator noise.
  \item We can reduce BS TM actuation efficiency even more if we want to reduce magnetic noise.
  \item It is desirable to increase number of turns or use bigger magnets for IMC.
  \item Larger magnets for ITM/ETM MN are sufficient for locking.
  \item Frequency noise should also be included.
\end{itemize}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%BIBLIOGRAPHY
\begin{thebibliography}{99}
\bibitem{NBref} The source code is available from \\
\url{https://svn.ligo.caltech.edu/svn/aligonoisebudget}. \\
Some instructions are given at \\
\url{https://awiki.ligo-wa.caltech.edu/aLIGO/NoiseBudget}.

\bibitem{SASSummary} Yuta Michimura: Summary of Suspension Configurations, JGW-D1503415. \\
\url{http://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/DocDB/ShowDocument?docid=3415}

\bibitem{TypeBDesign} Riccardo DeSalvo: Recycler and Beam Splitter suspension structure, JGW-T1100571. \\
\url{http://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/DocDB/ShowDocument?docid=571}

\bibitem{TypeADesign} Takanori Sekiguchi: Type-A SAS Mechanical Model Parameters, JGW-T1302090. \\
\url{http://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/DocDB/ShowDocument?docid=2090}

\bibitem{Miyamoto} Takahiro Miyamoto: Actuator design of KAGRA cryogenic payload, JGW-G1605938. \\
\url{http://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=5938}

\bibitem{Miyamoto2} Takahiro Miyamoto: status report 20170425, JGW-G1706476. \\
\url{https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=6476}

\bibitem{OSEMcalc} Mark Barton: OSEM Coil/Magnet/Flag Calculation, JGW-T1503239, Table 1. \\
\url{http://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/DocDB/ShowDocument?docid=3239}

\bibitem{IMCmeas} Yuta Michimura {\it et al.}: MCe�̕��i�A�N�`���G�[�^�`�B�֐��̑���(6/25-26) on IOO blog. \\
\url{http://gwclio.icrr.u-tokyo.ac.jp/lcgtsubgroup/inoutoptics/2015/06/mce625-26.html}

\bibitem{TAMAcoildriver} Gerhard Heinzel: TAMA coil driver modification (2000). \\
\url{http://tamago.mtk.nao.ac.jp/tama/ifo/general_lib/circuits/000414_coil_driver/coildrv.pdf}

\bibitem{CoilFujii} Yoshinori Fujii: coil-magnet coupling measurement, JGW-T1503605. \\
\url{http://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=3605}

\bibitem{hpcd} Masahiro Kamiizumi: High Power Coil Driver Board, JGW-D1503503. \\
\url{http://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=3503}

\bibitem{lpcd} Masahiro Kamiizumi: Low Power Coil Driver Board, JGW-D1503507. \\
\url{http://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=3507}

\bibitem{sumcon} The SUMCON is available from the KAGRA SVN; \\
\url{https://granite.phys.s.u-tokyo.ac.jp/svn/LCGT/trunk/VIS/sumcon}

\bibitem{KamiokaSeis} Takanori Sekiguchi: Seismic Spectrum in Kamioka Mine, JGW-T1402971. \\
\url{http://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/DocDB/ShowDocument?docid=2971}

\bibitem{MIFDesign} Yoichi Aso, Yuta Michimura, Kentaro Somiya: KAGRA Main Interferometer Design Document, JGW-T1200913, Figure 4.1 and 4.2. \\
\url{http://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/DocDB/ShowDocument?docid=913}

\bibitem{MagNoise} Kenji Ono: Evaluation of BS and RM noise arisen from the AC component of geomagnetism field, JGW-T1503469. \\
\url{http://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/DocDB/ShowDocument?docid=3469}

\bibitem{MagNoise2} Tomofumi Shimoda: Magnetic noise calculation for BS (and ETM/ITM, PRM/SRM), JGW-T1504459.\\
\url{http://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/DocDB/ShowDocument?docid=4459}

\bibitem{MagneticMeasurement} Sho Atsuta: Measurement of Schumann Resonance, JGW-G1605544.
\url{http://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/DocDB/ShowDocument?docid=5544}

\end{thebibliography}

\end{document}