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\begin{document}
\begin{article}
\begin{opening}

\title {THE STUDENT DUST COUNTER ON THE NEW HORIZONS MISSION}

\author{M.  \surname{HOR\'ANYI},
V.  \surname{HOXIE},
D.  \surname{JAMES}$^{\#}$,
A.  \surname{POPPE}$^{\#}$,
C.  \surname{BRYANT}$^{\#}$,
B. \surname{GROGAN}$^*$,
B.  \surname{LAMPRECHT},
J.  \surname{MACK},
F.   \surname{BAGENAL},
S.  \surname{BATISTE},
N.  \surname{BUNCH}$^*$,
T.  \surname{CHANTHAWANICH}$^*$,
F.   \surname{CHRISTENSEN}$^*$,
M. \surname{COLGAN}$^*$,
T.  \surname{DUNN}$^*$,
G.  \surname{DRAKE},
A.  \surname{FERNANDEZ}$^{\#}$,
T.   \surname{FINLEY}$^{\#}$,
G.  \surname{HOLLAND}$^{\#}$,
A.  \surname{JENKINS}$^*$,
C. \surname{KRAUSS}$^{\#}$,
E. \surname{KRAUSS}$^*$,
O.  \surname{KRAUSS}$^*$,
M. \surname{LANKTON},
C.  \surname{MITCHELL}$^{\#}$,
M.  \surname{NEELAND}$^*$,
T.  \surname{REESE},
K.  \surname{RASH}$^*$,
G.  \surname{TATE},
C.  \surname{VAUDRIN}$^*$,
J.  \surname{WESTFALL}
}


\runningauthor{HOR\'ANYI ET AL.}
\runningtitle{Student Dust Counter}


\institute{Laboratory for Atmospheric and Space Physics, University of
Colorado, Boulder, CO, 80309-0392, USA
\email{mihaly.horanyi@colorado.edu}\\ $^*$ undergraduate student \\
$^{\#}$ graduate student\\}




\date{Received ; accepted }


\begin{abstract} The Student Dust Counter (SDC) experiment of the New
Horizons Mission is an impact dust detector to map the spatial and
size distribution of dust along the trajectory of the spacecraft
across the solar system. The sensors are thin, permanently polarized
 polyvinylidene fluoride (PVDF) plastic films that generate an
 electrical signal when dust particles penetrate their surface.  SDC
 is capable of detecting particles with masses $m > 10^{-12}$ g, and
 it has a total sensitive surface area of about 0.1 m$^2$, pointing
 most of the time close to the ram direction of the spacecraft. SDC is
 part of the Education and Public Outreach (EPO) effort of this
 mission. The instrument was designed, built, tested, integrated, and
 now is operated by students.

\end{abstract} \keywords{interplanetary dust, instrumentation,
education and public outreach}

\end{opening}

\section{Introduction}
\label{Introduction} 



The Student Dust Counter (SDC) is an impact dust detector on board the
New Horizons Mission. It is designed to map the spatial and size
distribution of interplanetary dust particles in order to verify the
existence of the predicted structures in our dust disk, the Zodiacal
cloud.  Five spacecraft have carried dust detectors beyond the
asteroid belt: Pioneers 10 and 11 \cite{Humes1980}, Ulysses, Galileo
\cite{Grun1993}, and Cassini \cite{Srama2004}.  SDC will provide the
first dust measurements beyond 18 AU, where the Pioneer sensors
stopped working.  After the Pluto-Charon fly-by, SDC will continue to
measure dust on into the Kuiper Belt. These observations will advance
our understanding of the origin and evolution of our own solar system,
and allow for comparative studies of planet formation in dust disks
around other stars.

SDC is part of the Education and Public Outreach (EPO) effort of the
New Horizons mission and is the first science instrument on a
planetary mission to be designed, built, tested and operated by
students.  The SDC project has an unusual history. A similar
professional dust instrument was part of a competing proposal to New
Horizons in a parallel Phase A study. After the selection of New
Horizons, motivated by the potential scientific contribution of a dust
instrument, the idea emerged to redirect some of the funds from
traditional EPO activities, and to involve a group of students to try
their hands on building space hardware. The advanced state of the rest
of the New Horizons payload, and the risk of involving unexperienced
students made this request difficult. With the strong support of the
mission PI, the NASA EPO board agreed to try the `SDC experiment'.

To minimize the risk SDC might pose to the mission, all quality
assurance inspections and the final flight assembly was done by NASA
certified personnel, and student activities were supervised by
professionals. However, the student team, consisting of up to 20
engineering and physics undergraduate and graduate students, was
responsible for the work done in all phases of this project, including
presentations at all NASA milestone reviews.  SDC was built and tested
to the same NASA engineering standards as every other flight
instrument.  Due to the long duration of the New Horizons mission,
generations of future students will continue to be involved, handing
over their skills to the groups that follow them.

After 6 months of successful operations in space the SDC instrument
was renamed, and the dedication reads: \begin{quote} {\it New
Horizons, the first mission to Pluto and the Kuiper Belt, is proud to
announce that the student instrument (SDC) aboard our spacecraft is
hereby named ``The Venetia Burney Student Dust Counter'' in honor of
Ms. Venetia Burney Phair, who at age of eleven nominated the name
Pluto for our solar system's ninth planet. May ``Venetia'' inspire a
new generation of students to explore our solar system, to make
discoveries which challenge the imagination, and to pursue learning
all through their lives.} \end{quote}

\section{Science background}
\label{Science}

The interplanetary dust particle (IDP) population reflects a complex
balance between sources, transport, and sinks which are all functions
of the dust particle sizes and their positions in the solar
system. IDPs are created primarily in asteroid-asteroid collisions,
from the disintegration of comets near the Sun, from collisions
between Kuiper Belt objects (KBOs), and the continuous meteoroid
bombardment of the moons, rings, asteroids and KBOs. Data from
near-Earth and deep space missions suggest five distinct IDP
populations \cite{Devine1993}. While this model makes predictions for
IDP populations throughout the solar system, it has not been validated
by direct measurements beyond 18 AU.

Beginning at about 3 AU from the Sun, the Pioneer 10 and 11
observations of particles larger than about 10 $\mu$m in radius showed
a constant spatial density (number of particles per unit volume) out
to 18 AU \cite{Humes1980}. At this distance, the argon and nitrogen
gas mixture in the Pioneer pressurized detector cells froze and no
further measurements could be made.

 IDPs born on the outskirts of the solar system slowly lose orbital
 energy due to Poynting-Robertson drag and migrate toward the Sun. A
 10 $\mu$m radius grain born at 50 AU reaches the inner solar system
 in about 6.5 My. However, the continuous migration can significantly
 slow down or come to a halt due to mean motion resonances with the
 planets \cite{Liou_Zook1999, Martin_Malhotra2002,
 Martin_Malhotra2003}. Grains can also get ejected from the solar
 system via close encounters with the planets \cite{Liou1996}. Orbital
 integrations find that Neptune prolongs the lifetime of the inwardly
 migrating dust grains outside its orbit, imprinting its resonance
 structure on the spatial density of dust in the Kuiper Belt.

Jupiter, on the other hand, acts as a gate-keeper and keeps the inner
part of our solar system relatively dust free by ejecting up to 80\%
of the particles that would cross its orbit. The nature of the inner
planets effect on dust distribution is largely hidden due to dust
production from active comets.

SDC measurements made as New Horizons crosses the solar system will
help to understand the generation, the transport, and the loss
processes of IDPs. The measurements of the density variations outside
the orbit of Jupiter will allow for estimating the rate of dust
production in the Kuiper Belt, the collisional history of the region,
and the mass distribution of the primordial KBO population
\cite{Stern1996}.

\section{Instrument description} \label{Instrument}
 
 The SDC instrument (Figure \ref{SDC}) consists of a set of
 polyvinylidene fluoride (PVDF) film impact sensors, carried on a
 detector support panel, which is mounted on the exterior of the New
 Horizons spacecraft.
 It is outside the spacecraft multi-layer insulating (MLI) blanket,
 facing the ram direction. Signals from the sensors are collected
 through an intra-harness that runs from the detector assembly into
 the spacecraft interior to the instrument electronics box mounted
 opposite the detector panel.

%FIGURE 1
\begin{figure}[t]
  \centering
%\includegraphics[width=0.6\textwidth]{SDC_DiagramBW.eps}
   \includegraphics[width=0.6\textwidth]{SDC_figure1.eps}
  \caption{\small{The Student Dust Counter provides \smSim0.1 m$^2$ of
  sensitive area. It was designed to measure the mass of IDPs in the
  range of $10^{-12}$ to $10^{-9}$ g. It continues registering the
  impacts of bigger dust particles without the ability to determine
  their mass. It weighs 1.6 kg and consumes 5.1 watts of average
  power.}}
  \label {SDC}
\end{figure}


The measurement requirements of the SDC instrument were established by
estimating the particle mass detection limit needed to resolve the
expected features of the dust distribution. The characteristic width
of the predicted resonance structures is on the order of 1 AU, hence a
spatial resolution of {$\approx$}0.1 AU assures adequate
sampling. With the characteristic spacecraft speed, during cruise, of
{$\approx$}13 km/s, this is equivalent to an integration period of one
week. Based on the current models \cite{Devine1993} outside of dust
structures, a detector, with a lower mass detection limit of about
10$^{-12}$ g and 0.1 m$^2$ surface area, is predicted to have on the
order of 1 IDP hit per week. SDC was designed to resolve the mass of
IDPs in the range of $10^{-12} < m < 10^{-9}$ g within factors of $<
2$, covering an approximate size range of $1 - 10 ~\mu$m in particle
radius. Bigger grains are also recorded without the ability of
resolving their mass.

\subsection {Sensor Design} 

SDCs dust impact detection is based on the use of permanently
polarized PVDF films. An impacting particle causes a depolarization
charge when it penetrates the film. PVDF sensors require no bias
voltage, they are simple, inexpensive, reliable, electrically and
thermally stable, mechanically rugged, radiation resistant, and do not
respond to energetic ions or electrons. PVDF dust detectors have been
extensively tested and calibrated in laboratory experiments
\cite{Simpson1989a, Simpson1989b, Simpson_Tuzzolino1985,
Tuzzolino1992, Tuzzolino1996} and have an excellent track record in
space experiments. PVDF sensors were flown on the VEGA 1 and 2,
Stardust, Cassini, and ARGOS missions \cite{Tuzzolino2001,
Tuzzolino2003, Srama2004}.


\subsubsection {PVDF Signal Generation} 

The magnitude of the PVDF depolarization charge depends on particle
momentum as well as whether the particle penetrates into or through
the film \cite{Simpson_Tuzzolino1985}. SDC uses 28 $\mu$m thick PVDF
films which have been shown to stop particles up to $10^{-10}$ g and
speeds up to 20 km/s \cite{Simpson_Tuzzolino1985}. For particles
stopped in the film, the number of electrons generated is given by
\cite{Simpson_Tuzzolino1985}


\be
 N_e = 3.8\times10^{17}  m[{\rm g}]^{1.3}v[{\rm km/s}]^{3.0} ,
 \label {STeqn}
\ee

where $m$ is particle mass, in grams, and $v$ is the impacting
velocity in km/s.  The speed of long-resident dust grains following
circular Kepler orbits in the outer solar system is much less than the
velocity of the spacecraft, hence we take $v$ in Eq. {\ref {STeqn}} to
be the spacecraft velocity.

For charge sensing amplifiers, the noise floor is proportional to the
detector capacitance \cite{Spieler2005} which is a function of the
detector area, the material permittivity and the thickness (C =
$\epsilon_0 \epsilon_r$ A/d). The relative permittivity of PVDF, like
many polymers, changes dramatically with temperature varying from
$\epsilon_r = 11$ at 25 \degC ~ to 2.5 at -120 \degC. For a given
thickness of PVDF, and a given operating temperature, the required
lower limit of detectable impact-induced charge determines the maximum
allowable area of a single sensor. From Eq. {\ref {STeqn}} the SDC
mass threshold of $10^{-12}$ g (sub-micron radius) requires a signal
threshold of $6\times10^6~e$. To ensure a false detection rate of less
than 1 per month, a signal-to-noise ratio (SNR) greater than 5 is
required for an RMS noise level of {$\approx$}$1\times10^5 ~e$. Given
the characteristics of the charge sensitive amplifier design used (see
section {\ref {analog_signal_chain}}) the maximum allowable
capacitance per sensor was determined to be \smSim30 nF for an
equivalent area of {$\approx$}80 cm$^2$.

To achieve a total sensitive area of 0.1 m$^2$ SDC is composed of 12
sensor patches, each 14.2 cm x 6.5 cm, mounted onto the top surface of
a support panel. In addition, there are two reference sensor patches
mounted on the back side of the detector support panel, protected from
any dust impacts. These reference sensors, identical to the top
surface sensors, are used to monitor the various background noise
levels, from mechanical vibrations or cosmic ray hits of our
electronics, for example.


\subsubsection{Sensor Construction}
\label{Sensor}

The SDC detectors (Figure \ref{frame_exp}) are based on similar
instruments from the University of Chicago.  The PVDF film used by SDC
is manufactured with 1000 $\AA$ of Al/Ni electrode material on the top
and bottom surfaces. The film is bonded between a pair of G-10
fiberglass frames and electrical contact to the PVDF is made through
signal wires running around the interior of the frame apertures and
bonded to the electrode material with conductive silver filled
epoxy. The signal wires are routed to connection tabs where a
miniature coaxial cable is attached.

%FIGURE 2
\begin{figure}[t]
  \begin{center}
% \includegraphics[width=1.0\textwidth]{detector_frame_BW.eps}
    \includegraphics[width=1.0\textwidth]{SDC_figure2.eps}
  \caption{\small{Exploded view of a single SDC PVDF detector. The
  detector is mechanically bonded with structural epoxy.  Electrical
  connection from the PVDF metalization to the signal wires is through
  silver filled epoxy. The signal wires are soldered to miniature
  coaxial cables leading to the system electronics. Three mounting
  screws tie the space charging frame and the detector to the
  underlying honeycomb support panel.}}
  \label{frame_exp}
  \end{center}
\end{figure}

\subsection{Mechanical design} \label{Mechanical} 

The detector assembly of SDC is built on a \smSim1 cm thick aluminum
composite honeycomb panel which provides mounting and support for the
dust sensors onto the exterior spacecraft deck. The panel is attached
to the spacecraft with a three point compliant mount formed by
titanium flexures. The flexures are sized to accommodate the thermal
expansion mismatch between the spacecraft deck and detector
assembly. Sensor wiring on the detector assembly is routed in a
harness channel designed to withstand dust impacts.


\subsubsection{Thermal Design} \label{Thermal}

The thermal design of the SDC detector assembly was driven largely by
the requirement that the detector panel be able to maintain passive
thermal control while in view of the sun during early mission
maneuvers.  The PVDF sensor film itself has poor thermo-optical
properties, and is prone to overheating when exposed to direct
sunlight.  PVDF will undergo permanent depolarization at temperatures
over 85 \degC ~ \cite{Simpson_Tuzzolino1985}. To maintain the PVDF
temperature below a design target of 65\degC, a high emissivity
polyimide tape is applied to the backside of the PVDF film to
radiatively couple it to the honeycomb support panel below. The panel
facesheet spreads out the heat from beneath the detectors. The top
surface is covered with silverized Teflon tape that reflects 90\% of
the incident solar energy, preventing the panel itself from
overheating.  The detector support panel area was sized analytically
to provide adequate radiating area around the 12 PVDF sensor patches.
   

\subsection{Electrical design} \label{Electrical} 

The SDC electronics are carried on two multilayer printed wiring
assemblies (PWA) housed in the SDC electronics box. Signals from the
detectors come through the Intra-harness onto the analog PWA where
they are amplified, conditioned and converted to digital data. The
digital data is collected to registers in the field programmable gate
array (FPGA) on the digital PWA (Figure \ref{block_diagram}) and from
there to the microprocessor which adds time-stamps to the data and
stores them in long term non-volatile memory. The digital PWA also
contains the power supply, system health monitoring circuitry and
interface electronics for spacecraft communications.

%FIGURE 3
\begin{figure}
 \begin{center}
 % \includegraphics[width=1.0\textwidth]{Block_DiagramBW.eps}
     \includegraphics[width=1.0\textwidth]{SDC_figure3.eps}  
  \caption{\small{SDC Electronics block diagram. Signals from each
  PVDF detector are routed to individual analog electronics
  chains. Under FPGA control the science and housekeeping data are
  collected and passed to the system microprocessor which manages long
  term storage of data into flash memory. The FPGA also acts as a
  switch between the redundant spacecraft communications lines and the
  microprocessor.}}
  \label{block_diagram}
\end{center}
\end{figure}


\subsubsection{Analog Signal Chain}
 \label{analog_signal_chain}
 
Given the large capacitance of the PVDF detectors, care was taken to
simultaneously minimize the noise floor and to maximize the
signal-to-noise ratio. Analysis showed that SNR is maximized when
detector capacitance is equal to the input capacitance of the charge
sensitive amplifier's (CSA) JFET (junction field effect transistor)
\cite{Radeka1974,Spieler2005}. To address the problems of matching
JFETs to very large capacitance detectors work had previously been
done \cite{Bertolaccini1988} to develop a process that produces high
capacitance JFETs with good transconductance.

A by-product of the specific JFET design is an increased gate volume
sensitive to energetic particle charge deposition.  A CRME96 radiation
analysis using the gate dimensions and a critical deposited charge of
$1\times10^6~e$ gives $2.3\times10^{-7}$ event s$^{-1}$ during solar
max and roughly doubles during solar min. In the nominal case the
upset rate is well below the one per month science requirement. Using
an ordinary solar flare model with mean composition, the upset rate
rises to $1.1\times10^{-5}$ event s$^{-1}$, suggesting that higher
than expected event rates should be checked against possible space
weather effects.

%FIGURE  4
\begin{figure}
  \centering
%\includegraphics[width=0.9\textwidth]{Signal_Chain.eps}
 \includegraphics[width=0.9\textwidth]{SDC_figure4.eps} 
   \caption{\small{Each of the 14 analog signal chains contains a JFET
   input charge sensitive amplifiers followed by a two stage CR-RC
   shaper circuit. The first stage shaper includes a logarithmic gain
   term supporting a $ 2\times10^5$ to $5\times10^{10}$ electron input
   range. The shaper output is captured in a peak-hold stage long
   enough to be sampled by a scanning analog to digital converter
   circuit.}}
  \label{signal_chain}
\end{figure}

The CSA output is routed to the two stage CR-RC shaper circuit (Figure
\ref{signal_chain}). Using the given CSA design values, theoretical
noise limits can be calculated \cite{Spieler2005}, allowing for the
selection of an optimal shaper peaking time $\tau =10 ~\mu$s.
Measured SNR values from prototype circuits agree with the predicted
values and show a SNR $>10$ for $10^{6}$ electron input signals.
 
To accommodate signals over the $\approx$ 4 orders of magnitude
required, the first stage of the shaper includes a logarithmic
compression.  In order to keep the circuit size small the temperature
dependence is uncompensated and must be corrected in post-processing.

Following the shaper section each analog signal chain has a peak hold
circuit that retains the shaper pulse maximum for sampling by the
analog to digital converters (ADC). The fourteen peak-hold outputs are
divided into two groups of seven with each group routed to its own
analog multiplexer (MUX) and 16 bit ADC.
 A side effect of the signal chain electronics is that low level
 signals produce a digital output number (DN) of \smSim65000 with
 increasing magnitude analog signals producing decreasing DN values.

The response curve of a representative analog chain through the ADC is
shown in Figure \ref{gain_curve}. The gain shift in the upper size
range (lower right of the plot) is from the temperature sensitivity of
the diodes in the logarithmic compression term of the first stage
shaper. This temperature dependency was characterized during
pre-launch calibration (see Section \ref{electronics_cal}) and the
temperatures driving this shift are monitored during flight by
temperature sensors on the shaper circuits. Additionally the thermal
environment of the SDC electronics box is expected to be stable over
long time scales.

\subsubsection{On-board Stimulus} \label{stimulus_test} 

To track changes in the gain of the signal chains during flight the
SDC electronics includes stimulus circuitry that can inject selectable
charge quantities into any of the signal chains at the JFET gates. The
responses to these stimuli are collected in the same manner as science
data and reported along with a measurement of the stimulus
voltage. The nominal injection levels are shown on Figure
\ref{gain_curve}.

%FIGURE 5
\begin{figure}
  \begin{center}
 % \includegraphics[width=0.7\textwidth]{gain_curve.eps}
     \includegraphics[width=0.7\textwidth]{SDC_figure5.eps}
    \caption{\small{A typical SDC charge to DN response
    curve. Temperature dependent changes in gain are characterized
    during calibration. In-flight temperature monitoring and charge
    injection (the 8 onboard stimulus levels are indicated as vertical
    dashed lines) support post-processing correction for gain curve
    shifts.}}
  \label{gain_curve}
\end{center}
\end{figure}


\subsubsection{Digital Control Electronics} 

The two analog MUX and ADC sets are controlled in parallel by the FPGA
on the digital PWA. The ADC conversion values from each channel are
stored to registers and compared against individually programmable
threshold values in the FPGA. Upon a valid event detection the FPGA
executes one additional scan of every channel, to assure the shaper
pulse has time to peak, then it stops scanning, leaving each channel's
peak value in the FPGA registers. An interrupt is sent to the
microprocessor to indicate that the science data is ready. The FPGA
provides a control register which can be set to prevent any given
channel from generating interrupts.

The system has a radiation tolerant 8032 microcontroller, 32 kbyte
anti-fuse PROM for program storage, 32 kbyte rad-hard SRAM and 4 Mbyte
non-volatile flash RAM. The data retention time of the flash memory,
rated at 10 years, is extended by providing circuitry to power it on
only when the system is accessing it. The SDC flight software resides
in \smSim 24 kbyte of the 32 kbyte PROM. The system can be configured
to load code from flash memory into SRAM and run it from there,
supporting in-flight updates if necessary.


\subsection{Software design}
\label{Software}

For most of the long journey to Pluto, the New Horizons payload will
be in ``hibernation'', with the exception of SDC, which was designed
for standalone operations.  Much of the SDC flight software is related
to managing itself during these long periods extending up to 500 days.
In addition to data collection and storage, the software continuously
executes a set of programmable `autonomy' rules that allow the
instrument to adjust for many off-nominal conditions.

When SDC is turned on it immediately starts taking science
data. Ground commands are required only to perform calibrations or
reconfigure settings. The ground can also turn channels off or on,
erase the flash memory, and request telemetry packets.  SDC provides
several data packet types including science data, housekeeping
information, calibration results, memory dumps, and message
logs. Communications, as defined by the spacecraft, is limited to one
command sent per second, and one telemetry packet received per
second. A telemetry packet can contain up to 1024 data bytes.

\subsubsection{Data Management} \label{data_management} 

Whenever a science interrupt occurs the software compares each
channel's value from the FPGA against that channel's current threshold
value. For any channel exceeding its threshold the channel number, the
measured value, and the threshold value are stored to flash memory
along with the mission elapsed time (MET).

In addition to science data, flash memory is used for storing system
housekeeping data and maintaining system critical variables, all of
which are stored in triplicate. A simple majority vote is used to
correct any corrupted values. A health check is performed on the flash
memory everyday to ensure it can be written to and erased. Flight
software maintains a table of ill-performing flash memory blocks.

\subsubsection{Autonomy Rules} \label{autonomy} 

The 4 Mbyte of flash memory is much larger than the total expected SDC
data volume. However, it is possible for elevated noise levels to be
mis-identified as science and potentially fill up the flash memory,
leaving no room for real science. A high rate of noise events can also
cause the software to hang while trying to process all the interrupts
it receives. To mitigate the risk of filling-up flash memory during
the long periods of unattended operation, the software has two sets of
autonomy rules to control the data rate from the channels.  Set A
monitors the number of interrupts received from each channel every
second. Whenever a channel generates 3 interrupts in a second (the
maximum reasonable interrupt rate) that channel is blocked at the FPGA
from generating further interrupts for some programmable period,
typically one hour. These rules accommodate transient noise periods,
like spacecraft maneuvers.

Set B monitors the number of interrupts received from each channel in
a day.  If a channel reaches the daily limit, by default 20, that
channel's threshold is changed to a previously selected, less
sensitive level. There is a table of three decreasingly sensitive
threshold values for each channel, the values of which can be changed
by ground command.  The threshold of each channel can be increased
twice, and if it remains in violation of Set B, then the channel is
blocked from generating interrupts for a longer period, typically 30
days.  After this time the channel is turned on at its original
threshold and allowed to go through the table one more time.  Upon
reaching the end of the threshold table a second time the channel is
permanently blocked from generating interrupts until enabled by ground
command. The intent of this rule set is to autonomously adjust the
instrument's sensitivity to permanent changes in the noise
environment, such as degradation of the electronics. The time-out
period of rule A and all of the rule B default values are adjustable
by ground command.
 
\subsubsection{In-Flight Calibration Functions} 
\label{noise_cal}

The flight software manages the setup and data collection of the
Stimulus Test (\ref{stimulus_test}). In addition it can perform a
Noise Floor calibration. This consists of temporarily suspending the
autonomy rule set B and changing the thresholds of all the channels to
a series of levels sensitive enough that electronics noise is
detectable. By counting the number of science interrupts obtained at
each threshold over specified times, the statistical distribution of
the noise on each channel can be calculated. The flight software
allows for up to five levels and test times. The test thresholds are
configurable from the ground, but the durations are permanently
set. At the completion of a test set, autonomy rule B is re-enabled
and the channel thresholds are reset to their initial values.


\subsection{Instrument Calibration }
\label{cal}

\subsubsection {PVDF Sensor Characterization} 
\label{PVDF_cal} 

In the summer of 2003 and 2006 a set of SDC / PVDF sensors were taken
to the dust accelerator facility at Max Planck Institute for Nuclear
Physics in Heidelberg, Germany to verify that they show similar
response to hypervelocity dust impacts (Figure \ref{dust_scope}) as
has been published for previous instruments utilizing PVDF
\cite{Simpson_Tuzzolino1985}.

%FIGURE 6
\begin{figure}
  \begin{center}
  %\includegraphics[scale = .4]{dust_scopeBW.eps}
    \includegraphics[scale = .4]{SDC_figure6.eps}
  \caption{\small{Example of output pulse from a PVDF sensor to a
  $3.1\times10^{-11}$ g iron particle impacting at 4.52 km/sec.  The
  top trace is from the accelerator charge induction tube (100 mm,
  6.1V/pC), the middle trace is the output of the shaper
  ($1.47\times10^{-5}$ mV/e) and the bottom trace is from the CSA
  ($7.3\times10^{-7}$ mV/e) (note: gains are not flight values).}}
  \label{dust_scope} \end{center} \end{figure}

More than 80 iron dust particle impacts where collected from 4
separate prototype sensors (Figure \ref{sdc_dust}).  At the lower
particle sizes accurate measurements of CSA response was
difficult. With these points excluded, the measured slope is close to
that of Simpson and Tuzzolino (Eq. \ref{STeqn}) but with an
offset. This offset could be due to differences in the thickness of
the metallic layers, their exact composition, and/or differences in
the manufacturing of the PVDF, including its level of
polarization. The fit to our calibration data (using the same notation
as in Eq.1) is 

\begin{equation}
  N_e =  5.63\times10^{17} m[{\rm g}]^{1.3}v[{\rm km/s}]^{3.0} .
  \label{SDCeqn}
\end{equation}

%FIGURE 7
\begin{figure}
  \begin{center}
  %\includegraphics[width=0.7\textwidth]{SDC_DustBW.eps}
     \includegraphics[width=0.7\textwidth]{SDC_figure7.eps}
  \caption{\small{Dust impacts on SDC prototype PVDF sensors. Plot of
  ratio N/v$^3$ vs projectile mass.  N (electrons) is the measured
  detector signal and v (km/s) is projectile velocity reported by the
  dust accelerator.}}  \label{sdc_dust} \end{center} \end{figure}

\subsubsection{Electronics Calibration} \label{electronics_cal} 

The gain curve shown in Figure \ref{gain_curve} illustrates that the
system response to sensor produced charges is highly non-linear. A
calibration function is needed to identify: a) the compression stage
gain variation with electronics box temperature; and b) the possible
change in CSA gain due to temperature driven changes in PVDF
capacitance.  The calibration equation then becomes Q(DN, T$_b$,
C$_{det}$(T$_d$)), where Q in the measured charge, DN is the reported
value, T$_b$ is the temperature of the electronics box and
C$_{det}$(T$_d$) is the capacitance of the PVDF detector as a function
of detector temperature, $T_d$.

PVDF detectors are very sensitive to acoustics and mechanical
vibrations, making them impractical to use during calibration. From
characterization of PVDF sensor capacitance over temperature, fixed
capacitor values were selected to represent the detector capacitance,
C$_{det}$, at 25, -30, and -100 \degC. A range of reference charge
pulses were generated with a standard calibrated charge injection
circuit, and the electronics box temperature was collected from SDC
telemetry points, measured at the shaper circuitry (the same
measurements that are reported in flight). The measured electronics
temperatures, T$_b$, were 50, 40, 34, and -7 \degC.


For each electronics box temperature and C$_{det}$ value, 21 different
amplitude charges were injected 100 times into each of the 14
channels. For each charge pulse injected the resulting DN value and
accompanying T$_b$ were collected from the SDC telemetry. The
pre-launch baseline calibration function for each channel,
Q$_{ch}$(DN, T$_b$), was produced by fitting that channel's data with
a $10^{th}$ order polynomial. C$_{det}$ was dropped as a parameter to
the calibration function after analysis showed that CSA gain changes
due to variations in detector capacitance were insignificant.

\section{Data analysis}

When SDC data is received on Earth, it is sent to the New Horizons
mission operations center at the Johns Hopkins University's Applied
Physics Laboratory. The SDC data, as well as several ancillary
spacecraft data packets, are also sent to SDCs home institution, LASP,
via the Ground System Equipment Operating System
(GSEOS). Simultaneously, it is also received at the Tombaugh Science
Operations Center (TSOC) of the Southwest Research Institute (SwRI) in
Boulder.

The data are sent through ``levels'' of processing software that
progressively decode more of the packets and send the decoded data to
the database.

\begin{itemize} 

\item Level 0 software extracts the data packets out of the GSEOS
files.

\item Level 1 software separates the data by packet type, such as
Science, Housekeeping, Calibration, flash and SRAM memory dumps, and
decommutates the type specific data into the Level 1 database.

\item Level 2 software generates charge values from Level 1 DN values
using {\mbox Q$_{ch}$(DN, T$_b$)} from section \ref{electronics_cal}

\item Level 3 software uses Eq. 2 to convert charge to equivalent mass

\item Level 4 software removes noise using the reference detectors and
other coincident events to produce validated IDP impact rates as
function of dust mass.  \end{itemize}

The data products from Levels 0 through 2 are produced both at TSOC,
as well as at LASP, using identical source controlled software. Data
products from levels 3 and 4 are only produced at LASP.

\section{Initial results}

\subsection{Initial Turn-on and Checkout} 

On March 2, 2006, SDC was powered-on and commanded to report back a
full set of housekeeping values. With receipt of fully nominal values,
a series of commands were sent instructing the instrument to initiate
a set of Stimulus Tests (ST) (section \ref{stimulus_test}) and Noise
Floor calibrations (NFC) (section \ref{noise_cal}). All tests ran
successfully and the results showed that SDC had survived the rigors
of launch.  Both ST and NFC were repeated several times over the next
few months to better characterize the post-launch behavior of the
instrument.

\subsection{In flight calibration}

\subsubsection{Stimulus Calibration}

ST results were compared to pre-launch baseline tests. Figure
\ref{stim_results} shows the change in stimulus responses for one of
the channels. As of the time of the test, with the exception of
channel 11, all of the stimulus test results were holding steady
within the variation expected from ground testing. Some time between
instrument delivery to the spacecraft and launch, the detector on
channel 11 began exhibiting symptoms of degraded electrical contacts
to the PVDF. As a result, channel 11 is expected to have a reduced
sensitivity.
 

%FIGURE 8
\begin{figure}
  \begin{center}
 %\includegraphics[angle=90,width=0.9\textwidth]{DN_v_Charge_Ch4_STIM.ps}
  \includegraphics[width=0.9\textwidth]{SDC_figure8.eps}  
  \caption{\small{Stimulus tests of channel 4 performed in April and
  August, 2006, and the pre-flight calibration curve.}}
  \label{stim_results}

\end{center}
\end{figure}

\subsubsection{Noise Floor Calibration} 

With the sensitivity of the PVDF detectors to acoustics and mechanical
vibrations, the in-flight NFC was the first good measure of SDC
systematic noise. For each channel the average noise rate (noise
events collected divided by collection period) versus threshold is
calculated.  The logarithm of the rates is used to fit a second degree
polynomial to characterize the noise profile of that channel. The
threshold value where the fit crosses the one hit per month rate, is
the lowest of the three thresholds for that channel to support
autonomy rule B (section \ref{autonomy}). Figure \ref{ch4_noise}
displays Noise Floor calibration data for a single channel and
illustrates the threshold correction calculation.

%FIGURE 9
\begin{figure}
  \begin{center}
%  \includegraphics[width=0.9\textwidth]{SDC_NOISE_Ch4_Apr_Aug.ps}
     \includegraphics[width=0.9\textwidth]{SDC_figure9.eps}
    \caption{\small{SDC channel 4 noise collected in April and August
    of 2006 using the Noise Floor calibration routine
  (section \ref{noise_cal}). The curves are fits to the collected
  rates at five separate thresholds (marked on the upper horizontal
  axis). The new science collection thresholds (lower vertical lines
  labeled 1, 2, and 3) are also indicated.
  }}
  \label{ch4_noise}
\end{center}
\end{figure}

A corrected set of thresholds were uploaded in late June 2006 and
subsequently used for science collection through the fall.  Two or
three more rounds of calibrations are scheduled before entering cruise
phase, during which the instrument must operate autonomously for up to
500 days.

\subsection{Dust measurements}


SDC was designed to take advantage of the quiet state of the
spacecraft during cruise phase. Various active spacecraft operations
cause mechanical shocks that are picked up by the PVDF sensors and
registered by SDC as science events. This is particularly true during
three-axis pointing and active spin mode when the spacecraft
frequently fires short bursts of the attitude thrusters.  Level four
data reduction (section 4) is used to filter out any hits that appear
within a second of any thruster firing, thereby allowing science
recovery between firings. However because the thruster induced events
are often frequent enough to violate the autonomy rule B (section
3.4.2 ), during spacecraft maneuvers many SDC detector channels switch
off for prolonged periods.  During the first six months of flight,
there are several periods, some weeks or months long, where SDC was
completely off.

 

%FIGURE 10
\begin{figure}
  \begin{center}
  %\includegraphics[width=0.9\textwidth]{hits.ps}
   \includegraphics[width=0.9\textwidth]{SDC_figure10.eps}
  \caption{\small{The number of single events registered on each of
  the SDC channels (with the exception of channel 11).
  These were recorded between 7/14/ 2006 and 8/15/2006, during a total
  operating period of about 2.5 weeks. The average number of hits and
  the $\pm 1 \sigma$ error bars are also indicated for both the front
  (continuous line) and the reference (dashed line) sensors.}}
  \label{sci_points}
\end{center}
\end{figure}


In spite of the initial difficulties of collecting science data, SDC
does routinely take science during the short quiet periods available.
As an example, Figure \ref{sci_points} shows the number of events
recorded on each channel during segmented periods between July 14 and
August 16, 2006, totaling 2.5 weeks. Events that were coincident
between channels were removed, and channel 11 was excluded in this
analysis. The average number of hits on the front sensors was
19.7$\pm$5.4, and on the reference sensors it was 18.0$\pm$4.2. The
difference of these are taken to be real dust hits of 1.73$\pm$6.9
during this period, approximately consistent with our expectations of
about 1 hit/week.  The huge error bar is a consequence of the ongoing
activities with other instruments on the spacecraft. Following the
Jupiter encounter in February 2007, all other instruments will
hibernate between scheduled yearly checkouts. SDC will stay on and
start making measurements without interference.


\section{Conclusions}
\label{conclusions}

As part of the Education and Outreach effort of this mission, SDC
provided a unique, hands-on opportunity for a group of undergraduate
and graduate students to learn about building space hardware.
Frequent `all-hands' meetings during the development phases of SDC
ensured that students and professionals working on all the subsystems
(mechanical, thermal, analog and digital electronics, software, and
operations) continuously discussed the requirements, as well as the
schedule and the budget. SDC was delivered on time and within
budget. A large fraction of the initial SDC students are pursuing
careers in space research.  During the next decade a smaller rotating
group of students will operate the instrument and analyze our data.
The SDC project was the first of its kind to be supported by EPO
funds. Based on its history to date, it proved to be an excellent
investment as part of the EPO effort of New Horizons.

In-flight tests, calibrations, and the initial science data show that
SDC is performing according to expectations.  It will continue to make
measurements to improve our understanding of the spatial distribution
of dust in the solar system.  SDC will be the first dedicated
instrument to measure dust impacts outside 18 AU, and it is expected
to continue operating for the entire duration of the mission, deep
inside the Kuiper Belt.

\acknowledgements We are grateful to A. Stern for inviting SDC on the
New Horizons Mission, and to NASA for accepting the risk of a student
built flight hardware as part of EPO.  We thank APL for accommodating
SDC as a late addition. We acknowledge significant initial help from
A. Tuzzolino at the University of Chicago.  We thank E. Gr\"un and
R. Srama for their contributions and use of the Dust Accelerator
Facility at the Max Planck Institute for Nuclear Physics in
Heidelberg, Germany.  At LASP, R. Arnold, J. Johnson, P.  Sicken, and
J. Tracy assembled the flight version of SDC, and G. Lawrence guided
the initial design of our electronics.  M.H. was supported in part by
the Alexander von Humboldt Foundation while on a sabbatical leave at
the Max-Planck-Institute for Extraterrestrial Physics in Garching,
Germany.


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\end{document}
