\documentclass[a4paper,12pt]{article}

\title{                    SEPT Housing
}
\author{                 Stephan B\"ottcher
\and                      J\"org Falenski
\and                   Reinhold M\"uller-Mellin
\and                        Eckbert Rode
}
\date{          Christian-Albrechts-Universit\"at zu Kiel
\\[10pt]
              Final DRAFT: \today,
              Version: \rcs $Revision: 1.20 $,
                       \rcs $Date: 2002/11/18 12:47:08 $
}
\def\rcs$#1: #2 #3${#2}

\usepackage{graphics}

\usepackage[latin1]{inputenc}

\newcommand\units{\,\mathrm}

\usepackage[NowIKnow]{IDontKnow}

\newcommand\tablecaption{%
  \belowcaptionskip=\abovecaptionskip
  \abovecaptionskip=0.5\abovecaptionskip
  \caption}

\usepackage{hyperref}

\begin{document}

\maketitle

\begin{abstract}
  This note describes the mechanical housings of the \emph{Solar
    Electron Proton Telescope} (SEPT) for IMPACT on the NASA space
  probes STEREO, for the purpose of a structural analysis to verify
  environmental specifications required for launch.
\end{abstract}

\vspace{2cm}

This document is available electronically at:
\begin{center}
  \href{http://www.ieap.uni-kiel.de/space/project/stereo/sept/notes/#housing}
       {http://www.ieap.uni-kiel.de/
        space/project/stereo/sept/notes/texts/housing.pdf}
\end{center}

\clearpage
\tableofcontents
\listoffigures
\listoftables
\clearpage

\section{Description of the Housing}

The SEPT consists of two identical modules which are mounted at
different locations on the spacecraft.  One module (SEPT-E) detects
particles in the ecliptic in the direction of the solar magnetic
field, the other (SEPT-NS) perpendicular to the ecliptic.

Each module is composed of two parts: the housing of the sensor on the
top, and the \emph{electronics box} (E-box) on the bottom.  The sensor
housing is mounted on top of the E-box with four screws (M4).  The
sensor is electrically and thermally connected to the E-box.

The E-box is mounted on the spacecraft with four screws size ANSI\,8.
The E-box mounting provides electrical and thermal insulation from the
spacecraft.  While SEPT-E is mounted directly to the spacecraft
platform, SEPT-NS will be mounted on a bracket, as shown if
Fig.~\ref{fig:bracket}.  The bracket weighs $270\units{g}$.  The
mechanical analysis for SEPT-NS shall include this bracket.

Fig.~\ref{fig:sept-closed} shows the instrument from the front and
from the side.  Fig.~\ref{fig:stereo-ab} on
page~\pageref{fig:stereo-ab} indicates the mounting positions of the
SEPT modules on the spacecraft.  See Table~\ref{tab:weight-breakdown}
on page~\pageref{tab:weight-breakdown} for a weight breakdown of the
SEPT modules.

\begin{figure}[p]
  \begin{center}
    \mbox{%
      \resizebox{!}{9cm}{
        \includegraphics*{sept-closed-frontview}
      }
    }

    \mbox{%
      \resizebox{!}{9cm}{
        \includegraphics*{sept-closed-sideview}}
    }
  \end{center}
  \caption{SEPT Housing overview.}
  \label{fig:sept-closed}
\end{figure}

\subsection{Materials}

The housing is made of aluminum (AA\,7075, DIN\,3.4365), with a few
exceptions:
\begin{itemize}
\item The sensor doors are made from titanium, for its low thermal
  conductance.
\item The pinpuller rods are made of bronze, to avoid friction in the
  door mechanism. 
\item All screws, washers, etc., are stainless steel.
\end{itemize}
The material thickness is $1\units{mm}$ except where noted otherwise.


\begin{figure}[p]
  \begin{center}
    \mbox{%
      \resizebox{!}{4cm}{
        \includegraphics*{ebox-body-frontview}}%
      \hskip 0.5cm
      \resizebox{!}{4cm}{
        \includegraphics*{ebox-body-sideview}}
    }%
  \end{center}
  \caption{The body of the electronics box.}
  \label{fig:eboxbody}
\end{figure}

\begin{figure}[p]
  \begin{center}
    \mbox{%
      \resizebox{!}{4cm}{
        \includegraphics*{ebox-frontview}}%
      \hskip 0.5cm
      \resizebox{!}{4cm}{
        \includegraphics*{ebox-sideview}}
    }%
  \end{center}
  \caption{The electronics box with covers.}
  \label{fig:eboxclosed}
\end{figure}

\begin{figure}[p]
  \begin{center}
    \mbox{%
      \resizebox{!}{4cm}{
        \includegraphics*{ebox-bottom-topview}}%
      \hskip 0.5cm
      \resizebox{!}{4cm}{
        \includegraphics*{ebox-topview}}
    }%
  \end{center}
  \caption{The electronics box with bottom (left) and top (right) covers.}
  \label{fig:eboxcovers}
\end{figure}

\subsection{Electronics Box}

The E-box is a rectangular box with removable top and bottom covers.
See Figs.~\ref{fig:eboxbody} to~\ref{fig:eboxcovers}.

The body of the box is, like all other parts of the housing, machined
from a solid block of aluminum (no welding).

The top (bottom) covers are are fixed with ten (twelve) stainless
steel screws (M2) from the sides through holes in the body, into
threads which are cut into extension on the covers.

Two \emph{Printed Circuit Boards} (PCB) with an EMI shield in between
are mounted inside the E-box.  The PCBs and the EMI shield are
assembled outside the box, inserted from the bottom, and screwed with
six screws (M3) onto threaded holes that are part of the E-box body.
The connectors for the electronics are pushed from the inside through
their respective holes in the E-box body, and fixed with screws.

\begin{figure}[ht]
  \begin{center}
    \mbox{%
      \resizebox{!}{7cm}{
        \includegraphics*{sensor-topview}}%
    }%
  \end{center}
  \caption{Sensor top view.  Silicon detectors (cyan), magnets
    (magenta), foil (yellow), foil carrier ring (brown), detector
    housing (green), Delrin (gray).}
  \label{fig:sensor-top}
\end{figure}

\begin{figure}[ht]
  \begin{center}
    \mbox{%
      \resizebox{!}{5cm}{
        \includegraphics*{magnet-frontview}}%
      \hskip 1cm
      \resizebox{!}{5cm}{
        \includegraphics*{magnet-sideview}}
    }%
  \end{center}
  \caption{Magnet assembly.  Magnets and yokes (magenta),  frame
    and magnet support (dark blue).}
  \label{fig:magnetassy}
\end{figure}

\subsection{Sensor Housing}

The sensor (Fig.~\ref{fig:sensor-top}) has four apertures for
incident particles, looking in two opposite directions.  Each pair of
opposite views share a pair of silicon detectors.  The particles
either have to pass through a thin Parylene foil or through a magnetic
field.  The foil stops slow protons, and the magnetic field filters
electrons.


\subsubsection{Magnet Assembly}

The sensor includes four NdFeB Magnets, size $16 \times 16 \times 8
\units{mm³}$.  The magnets are glued to two yokes, made of
Vacoflux\,50.  The magnet-yoke assemblies are held apart by a magnet
support, which also serves to enclose the particle's path to the
detectors.  The complete magnet assembly is inserted into the
\emph{sensor frame}, which is $3\units{mm}$ thick, surrounding the
magnet assembly, and which extends with $0.8\units{mm}$ thick sheets
at both sides.  See Fig.~\ref{fig:magnetassy}.

The mass of the magnets is $15.6\units{g}$ each (density
$7.63\units{g/cm³}$), and each yoke's mass is $31.2\units{g}$ (density
$8.12\units{g/cm³}$).

\begin{figure}[ht]
  \begin{center}
    \mbox{%
      \resizebox{!}{4cm}{
        \includegraphics*{feet-sideview}}%
      \hskip 0.5cm
      \resizebox{!}{4cm}{
        \includegraphics*{feet-frontview}}%
      \hskip 0.5cm
      \resizebox{!}{4cm}{
        \includegraphics*{feetc-sideview}}
    }
  \end{center}
  \caption{Sensor side support.  The side supports slide from below between
    the extensions of the sensor frame, and the inner support wings of
    the side supports slide into the slits of the frame and its
    covers.}
  \label{fig:feetassy}
\end{figure}
\begin{figure}[ht]
  \begin{center}
    \mbox{%
      \resizebox{!}{5cm}{
        \includegraphics*{feet-topview}}%
    }
  \end{center}
  \caption{Sensor side supports and collimator top view.}
  \label{fig:feetappassy}
\end{figure}

\subsubsection{Sensor Side Support}

The magnet assembly will be covered on both sides by frame covers,
with round holes for the particle paths.  Two \emph{side supports}
will slide between the extensions of the sensor frame.  See
Figs.~\ref{fig:feetassy} and~\ref{fig:feetappassy}.  The side
support's inner enforcement wings slide into slits of the sensor
frame and cover.  The outer support wings also serve to mount
pinpullers and a connector.

\begin{figure}[ht]
  \begin{center}
    \mbox{%
      \resizebox{!}{5cm}{
        \includegraphics*{app-sideview}}%
      \hskip 0.5cm
      \resizebox{!}{5cm}{
        \includegraphics*{app-frontview}}
    }
  \end{center}
  \caption{Collimator assembly.  The positions of the screws are
      indicated on the front view.  The grayed screw positions are for
      the reverse side.  Some positions are shared.}
  \label{fig:appassy}
\end{figure}


\subsubsection{Collimators}

The collimators of the telescope will be mounted on top of the frame
covers.  Two of these collimator structures also serve as the housing
for the detectors and foil assemblies.  The hinges for the sensor
doors and their release mechanism are part of the collimator
structure.  See Fig.~\ref{fig:appassy}.


\subsubsection{Screws}

The sensor is held together by stainless steel screws (M2).  The right
side of Fig.~\ref{fig:appassy} shows the positions of the screws:

Five screws on in the center on each side hold the collimators and
frame covers and screw into threads in the magnet support.

Four screws on each side hold the collimators and frame covers and
screw into threads cut into the frame.

Four long screws go through the collimators and frame covers, all the
way through the side supports and are tightened with nuts on the other
side.  Four more screws go likewise all the way through, but not
through the collimator flanges.

Twenty short screws and nuts, five on the top and bottom of each side,
hold together the frame and frame covers.

Four screws and nuts, one each on the left and right of each side,
fix a collimator, frame cover, frame, and side support.


\subsubsection{Detector Assembly}

The silicon detector pairs are assembled inside separate housings.
The foils are mounted in aluminum rings.  A foil carrier ring and a
detector housing is mounted inside a collimator.  The detector housing
is surrounded by Delrin rings, for electrical isolation.  See
Fig.~\ref{fig:sensor-top}.

\begin{figure}[p]
  \begin{center}
    \mbox{%
      \resizebox{!}{7cm}{
        \includegraphics*{coverclosed-sideview}}%
      \hskip 0.5cm
      \resizebox{!}{7cm}{
        \includegraphics*{coveropen-sideview}}%
    }
  \end{center}
  \caption{Sensor doors side view.  Left: doors in closed position,
    right: doors in open position.}
  \label{fig:doors-side}
%\end{figure}
%\begin{figure}[p]
  \begin{center}
    \mbox{%
      \resizebox{!}{5cm}{
        \includegraphics*{coveropen-frontview}}
    }
  \end{center}
  \caption{Sensor doors front view, with the doors opened.  The
    pinpuller rod is drawn in both the idle (dark blue) and
    activated (pink) position.}
  \label{fig:doors-front}
\end{figure}

\subsubsection{The Sensor Doors}

The four sensor openings are covered with doors, which are to be
opened only after the spacecraft reached a stable orbit, to prevent
direct sunlight to shine on the sensitive detectors and magnets during
maneuvers.  These doors are not reclosable.  The opening mechanism is
activated with two pinpullers, opening two doors each.  The doors are
not sealed.

Fig.~\ref{fig:doors-side} shows a side view of the doors in closed and
open position, and Fig.~\ref{fig:doors-front} shows the front view.
The pinpuller, when activated, pulls a rod by a few millimeters and
releases the door locks.  The doors open by the force of springs.

The left door opens when the rod slips out of its lock, but the right
door opens when an extension of the rod slips out of an extended hole,
so that the rod can slip through an opening in the lock.  The rod is
guided by holes that are part of the collimator structures.  These
guides are designed such that the (extension of) the rod does not slip
out even when the pinpuller is fully activated.

The doors are made of titanium, while the rod is made of bronze.
The hinges and guides are part of the collimators which are made of
aluminum.  

\subsection{Connectors and Miscellaneous Attachments}

Various objects are attached to the housing.

\begin{figure}[t]
  \begin{center}
    \mbox{%
      \resizebox{!}{5cm}{
        \includegraphics*{pinpuller-topview}}%
    }
  \end{center}
  \caption{Pinpuller mounting top view.  Also shown is the
    connector for the supply of the pinpuller activation current.}
  \label{fig:pinpuller-top}
%\end{figure}
%\begin{figure}[t]
  \begin{center}
    \mbox{%
      \resizebox{!}{5cm}{
        \includegraphics*{pinpuller-sideview}}%
    }
  \end{center}
  \caption{Pinpuller mounting side view.  Only the near half
    of the sensor is shown.  The sensor doors are drawn both opened
    and closed.}
  \label{fig:pinpuller-side}
\end{figure}

\subsubsection{Pinpullers}

The sensor doors are activated by two pinpullers from TiNi Aerospace,
Inc., Model P5-403-10S, which are attached with two screws to outer
wings of the side supports.  These pinpullers weigh
$15\units{g}$ each.

The axis of the pinpuller comes with an internal thread (M2) where the
bronze rod is screwed into.  The rod diameter is $2\units{mm}$.  The
extension of the shaft that locks the right door is $8\units{mm}$ long
and the extended diameter is $4\units{mm}$.  See
Figs.~\ref{fig:pinpuller-top} and~\ref{fig:pinpuller-side}.

The pinpullers are activated electrically from the spacecraft.  The
connection is made via a 9-pin subminiature D-type connector which is
screwed to an outer wing of a side support.  Such a connector weighs
$6\units{g}$. 

\begin{figure}[t]
  \begin{center}
    \mbox{%
      \resizebox{!}{5cm}{
        \includegraphics*{purge-frontview}}%
      \hskip 0.5cm
      \resizebox{!}{5cm}{
        \includegraphics*{purge-sideview}}%
    }
  \end{center}
  \caption{Purge nipple and thermistor (green).  The purge nipple is
    the upper left and the thermistor is the lower right part in
    green.}
  \label{fig:purge}
\end{figure}

\subsubsection{Purge Nipple}

To purge the sensor with nitrogen gas before launch there is a
\emph{purge nipple} screwed into the sensor frame from the side.  The
side support has a hole at that position.  The design of the purge
nipple is not yet final, because a flow limiter needs to be
incorporated.  See Fig.~\ref{fig:purge}.

\begin{figure}[p]
  \begin{center}
    \mbox{%
      \resizebox{!}{7cm}{
        \includegraphics*{cable-frontview}}%
    }
  \end{center}
  \caption{Cables and connectors front view.  The detector signal
    cables are drawn in light green.  The connector on the right
    between the support wings of the E-box supplies the thermal
    hardware.  The MDM connector on the front is the link to SEP
    central.  To its left is the grounding stud.}
  \label{fig:cables-front}
%\end{figure}
%\begin{figure}[p]
  \begin{center}
    \mbox{%
      \resizebox{!}{7cm}{
        \includegraphics*{cable-sideview}}%
    }
  \end{center}
  \caption{Cables and connectors side view.  The detector signal
    cables are shown with connectors.  Also shown on the lower right
    is the connector for the thermistor.  In the center, on the E-box
    sidewall is shown the thermostats for the survival heater, and the
    connector for the thermal hardware.  At the lower left are the SEP
    connector and the grounding stud.}
  \label{fig:cables-side}
\end{figure}


\subsubsection{Detector Signals}

The detector signals are routed through twelve coax cables type
RG\,178, six for each detector pair, which leave the detector housing
to the side, over an angle of $135^\circ$.  The cables attach to the
E-box via SSMC type connectors.  Such a connector pair weighs $2.5
\units{g}$.  

The cable will need to be fixed to the E-box cover somehow.  This part
of the design is not yet finished.

Figs.~\ref{fig:cables-front} and~\ref{fig:cables-side} show the
positions of these cables and various other connectors.

\subsubsection{SEP Connection}

The electronics upstream connection of the E-box to \emph{SEP central}
goes through a 31-pin MDM connector.

\subsubsection{Thermistor}

A thermistor is required to measure the temperature of the sensor,
close to the silicon detectors.  The thermistor is glued into an
stainless steel cylinder, which is attached to a side support from the
inner side below the sensor.  The connection is made with a coax cable
from the inner end of the thermistor to a connector on the back side
of E-box, opposite to the SEP connector.  Cable and connector are the
same type as for the detector signals.  See Fig.~\ref{fig:purge}.

\subsubsection{Thermal Hardware}

Survival and operational heaters will be attached on the outside to
the bottom of the E-box.  Two thermostats for the survival heater will
be glued to the sidewall of the E-box.  The electrical connections to the
spacecraft will use a 9-pin MDM connector which is attached to the
E-box near the thermostats.  See Figs.~\ref{fig:cables-front}
and~\ref{fig:cables-side}.

\subsubsection{Ground Stud}

A stainless steel grounding stud is screwed to the front of the E-box
next to the SEP connector.  See Figs.~\ref{fig:cables-front}
and~\ref{fig:cables-side}.

\begin{figure}[tp]
  \begin{center}
    \mbox{%
      \resizebox{0.49\hsize}{!}{%
        \includegraphics{b-rear-iso-close_102802-ann}}%
      \hspace{0.01\hsize}%
      \resizebox{0.49\hsize}{!}{%
        \includegraphics{a-rear-iso-close1_102802-ann}}%
    }
  \end{center}
  \caption{STEREO-B (behind, left) and STEREO-A (ahead, right).}
  \label{fig:stereo-ab}
\end{figure}

\begin{figure}[p]
  \resizebox{\hsize}{!}{%
    \includegraphics{2053429-n-s-brkt-11-08-02-ar}%
  }
  \caption{SEPT-NS mounting bracket design drawing.}
  \label{fig:bracket}
\end{figure}

\section{Vibration Levels}

These are quoted from the environmental specifications for instruments
on the STEREO spacecraft.

SEPT-E is mounted to the spacecraft Y-axis panel, SEPT-NS is mounted
to the spacecraft Z-axis pannel.  The SEPT mounting plane contains the
thrust axis, the spacecraft X-axis (Fig.~\ref{fig:stereo-ab}).

\subsection{Instrument Design Load Factors}

\begin{quote}\itshape
The instrument shall be designed to the limit loads (maximum expected
loads) multiplied by the appropriate factor of safety.  The design
loads shall be applied separately in three orthogonal axes to the
instrument center of gravity.
\end{quote}
See Tables~\ref{tab:load-factor} and~\ref{tab:safety-factors} for the
factors.

\subsubsection{Margin of Safety for Instruments}
\begin{quote}\itshape
  Instrument strength analysis must show a positive margin of safety
  (MS). Margins of safety for yield strength (MSY) and ultimate
  strength (MSU) are defined as follows:
\end{quote}
\begin{eqnarray*}
  \mathrm{MSY} = \frac{\mbox{Material Yield Strength}}
                      {1.3\times\mbox{Applied Stress}} -1 &>& 0 \\[5pt]
  \mathrm{MSU} = \frac{\mbox{Material Ultimate Strength}}
                      {1.4\times\mbox{Applied Stress}} -1 &>& 0 \\
\end{eqnarray*}

\subsection{Instrument Stiffness}

\begin{quote}\itshape
  Instruments shall be designed such that the primary vibration modes
  are above $50\units{Hz}$.
\end{quote}

\subsection{Instrument Sine Sweep Vibration Tests}

\begin{quote}\itshape
  Instruments shall be subjected to the following sinusoidal vibration
  levels.  These shall be applied in each of three orthogonal axes.
\end{quote}
See Table~\ref{tab:sinevib} for the levels.  

\subsection{Random Vibration Levels}

\begin{quote}\itshape
  All instruments shall be subjected to the following random vibration
  levels.  These shall be applied to each of three orthogonal axes,
  one of which is parallel to the thrust axis. 
\end{quote}
See Table~\ref{tab:randomvib} for the levels.



\begin{table}[p]
  \tablecaption{SEPT weight breakdown}
  \label{tab:weight-breakdown}
  \newcommand\tabT[2]{{\textbf{#1}} & $#2\units{g}$}
  \newcommand\tabM[2]{{\qquad #1}& $#2\units{g}\qquad$}
  \newcommand\tabL[2]{{\qquad\qquad #1}& $#2\units{g}\qquad\qquad$}
  \begin{center}\begin{tabular}{lr}
    \tabT{E-Box}                  {240.1} \\
[5pt]
    \tabM {housing}            {100.1} \\
    \tabL  {box}             { 60.5} \\
    \tabL  {covers}          { 35.6} \\
    \tabL  {screws}          {  4.0} \\
[5pt]
    \tabM {electronics}        {132.0} \\
    \tabL  {PCBs}            { 88.0} \\
    \tabL  {EMI shield}      {  8.0} \\
    \tabL  {connectors}      { 13.0} \\
    \tabL  {internal cables} { 18.0} \\
    \tabL  {screws}          {  5.0} \\
[5pt]
    \tabM {thermal hardware}   {  8.0} \\
[10pt]
    \tabT{sensor}                 {425.4} \\
[5pt]
    \tabM {housing}            {193.0} \\
    \tabL  {frame}           { 33.5} \\
    \tabL  {frame covers}    { 24.8} \\
    \tabL  {magnet support}  { 20.0} \\
    \tabL  {side supports}   { 50.8} \\
    \tabL  {collimators}     { 30.5} \\
    \tabL  {doors}           { 17.2} \\
    \tabL  {screws}          { 16.2} \\
[5pt]
    \tabM {magnets}            {130.0} \\
[5pt]
    \tabM {detector system}    { 48.6} \\
[5pt]
    \tabM {pinpuller and rod}  { 37.6} \\
[5pt]
    \tabM {miscellaneous}       { 16.2} \\
[15pt]
    \tabT{SEPT total weight}      {665.5}
  \end{tabular}\end{center}
\end{table}


\begin{table}[p]
  \tablecaption{Quasi static Load Factors}
  \label{tab:load-factor}
  \begin{center}
    \begin{tabular}{cc}
      \textbf{Component Weight} & \textbf{Limit Load} \\
          $< 4.5\units{kg}$     & $30\units{g}$
    \end{tabular}
  \end{center}

  \tablecaption{Factors of Safety for Instruments}
  \label{tab:safety-factors}
  \begin{center}
    \begin{tabular}{ll}
      \multicolumn{2}{c}{\textbf{Design}} 
\\[3pt]
      Material Yield    & ${}\ge 1.3 \times \mbox{limit load factors}$ \\
      Material Ultimate & ${}\ge 1.4 \times \mbox{limit load factors}$ \\
      Buckling          & ${} > 2.0 \times \mbox{limit load factors}$ \\
      Composites        & ${} > 2.0 \times \mbox{limit load factors}$ 
\\[5pt]
      \multicolumn{2}{c}{\textbf{Vibration Test}} 
\\[3pt]
      Sine Vibration    & $1.4 \times \mbox{max expected level}$ \\
      Random Vibration  & $\mbox{max expected level} + 3\units{dB}$ \\
    \end{tabular}
  \end{center}
\end{table}

\begin{table}[p]
  \tablecaption[Sine Sweep Levels]{Sine Sweep Levels for Instruments
    Weighing Less than or Equal to $45\units{kg}$.
    $\mbox{Rate}=4\units{octaves/min}$.  Above $50\units{Hz}$,
    component response may be limited to peak input level.}
    \label{tab:sinevib} 
  \begin{center}
    \begin{tabular}{cc}
      \multicolumn{2}{c}{\textbf{Thrust Axis (X)}}
      \\ Frequency & Acceleration 
      \\  $5$  to $7.4\units{Hz}$ & $0.5\units{in}$ \rlap{(double amplitude)}
      \\  $7.4$ to $23\units{Hz}$ & $1.4\units{g}$
      \\ $25$   to $27\units{Hz}$ & $16.0\units{g}$
      \\ $29$   to $100\units{Hz}$ & $1.4\units{g}$
      \\[10pt]
      \multicolumn{2}{c}{\textbf{Lateral Axes (Y,Z)}}
      \\ Frequency & Acceleration 
      \\  $5$  to $6.3\units{Hz}$ & $0.5\units{in}$ \rlap{(double amplitude)}
      \\  $6.3$ to $19\units{Hz}$ & $1.0\units{g}$
      \\  $21$ to $23\units{Hz}$ & $12.0\units{g}$
      \\  $25$ to $100\units{Hz}$ & $1.0\units{g}$
    \end{tabular}
  \end{center}
%\end{table}

%\begin{table}[t]
  \tablecaption[Random Vibration Levels]{Random Vibration Levels for
    Instruments Mounted Directly to Spacecraft Panel and Weighing Less
    than or Equal to $22.7\units{kg}$}
  \label{tab:randomvib}
  \begin{center}
    \begin{tabular}{cc}      
      \multicolumn{2}{c}{\textbf{Perpendicular to Mounting Panel}}
      \\ Frequency & PSD Level
      \\ $20\units{Hz}$ & $0.0063\units{g^2/Hz}$
      \\ $20$ to $80\units{Hz}$ & $+6\units{dB/oct}$
      \\ $80$ to $800\units{Hz}$ & $0.1\units{g^2/Hz}$
      \\ $800$ to $2000\units{Hz}$ & $-9\units{dB/oct}$
      \\ $2000\units{Hz}$ & $0.0065\units{g^2/Hz}$
      \\[5pt] \multicolumn{2}{c}{$\mbox{Overall Amplitude}=10.4\units{g\ rms}$}
      \\ \multicolumn{2}{c}{$\mbox{Duration} = 60\ \textrm{seconds}$}
      \\[10pt]
      \multicolumn{2}{c}{\textbf{Parallel to Mounting Panel}}
      \\ Frequency & PSD Level
      \\ $20\units{Hz}$ & $0.0031\units{g^2/Hz}$
      \\ $20$ to $80\units{Hz}$ & $+6\units{dB/oct}$
      \\ $80$ to $800\units{Hz}$ & $0.05\units{g^2/Hz}$
      \\ $800$ to $2000\units{Hz}$ & $-9\units{dB/oct}$
      \\ $2000\units{Hz}$ & $0.0032\units{g^2/Hz}$
      \\[5pt] \multicolumn{2}{c}{$\mbox{Overall Amplitude}=7.4\units{g\ rms}$}
      \\ \multicolumn{2}{c}{$\mbox{Duration} = 60\ \textrm{seconds}$}
    \end{tabular}
  \end{center}
\end{table}

\end{document}
