| DESCRIPTION |
CASSE
======
Instrument Overview
===================
The Comet Acoustic Surface Sounding Experiment (CASSE) should
study the cometary surface layer by applying active and passive
methods.
To this end, CASSE uses accelerometers (ACC) and custom-made Piezo
stacks (called transmitter = TRM) mounted in the soles of Philae's
feet that can record and generate vibrations in the audible
frequency range. The temperature of each sensor is monitored by a
thermistor (PT 1000) for calibration purposes.
By command, any combination of receivers and transmitters can be
selected. The sounding frequency can be varied from 10 Hz to
10000 Hz for a duration up to 2000 s. The sampling frequency can
be varied from 80 Hz to 100000 Hz for a recording duration also
up to 2000 s.
The nominal gain can be varied in 16 irregular steps from 1 to
168 by combining four amplifier stages. Thus, CASSE can either
excite elastic waves that are transmitted through the cometary
matter beneath Philae and recorded by CASSE sensors or detect such
waves generated by other artificial or natural sources. The use of
triaxial accelerometers allows a polarization analysis and thus the
distinction of different wave types.
The CASSE electronics is controlled by the common SESAME
flight software.
Scientific Objective
====================
The scientific objective of CASSE is to study lateral and vertical
elastic and structural properties of the cometary surface layer
over the foot-to-foot distance of about two meter. The quality of
these investigations depends on the elastic properties of the
surface material and its mechanical contact to the lander soles.
The primary goal of the CASSE investigation is to determine elastic
parameters like Young s modulus and the Poisson ratio as well as
their daily and seasonal variations by analyzing the registered
signal profiles of (repeated) active soundings or by recording
artificial signals like the Philae touchdown and the MUPUS-Pen
hammering.
Additional goals are:
- The monitoring of thermally or impact-caused cometary
activity and the localization of activity spots;
- The determination of the macro-structure of the cometary
surface, such as the expected layering or embedded
inhomogeneities, by using refraction and / or reflection
seismograms;
- The study of emitted particles impacting on the soles during the
orbit phase as well as during the descent to the cometary
surface.
Electronics
===========
The CASSE sensors are controlled by an electronics board (PCB) as
part of the SESAME central electronics. The general design of the
CASSE PCB is that of a triggerable 12-channel transient recorder
(9 accelerometer and 3 transmitter channels) with a sampling rate
of up to 100 kHz that will be distributed evenly on the channels
selected. The electronics also includes a signal generator with a
variable output frequency for the three transmitters. The received
signals coming from the input multiplexer are amplified with a
selectable gain ranging from 1 to 168 (0 to 44.5 dB), converted by
an 7-bit custom-made analog-to-digital converter (ADC) plus sign
bit and recorded in a 128 kByte ring buffer. The transfer function
of the ADC is quasi-logarithmic, using piecewise linear approximation.
A recorded signal is read from the CASSE memory by the SESAME flight
software and further processed or handed over to the Philae Command
and Data Management System (CDMS) for downlink.
Location
========
The three CASSE triaxial accelerometers (Bruel and Kjaer, type 4506
W002) are mounted in the left sole of each Philae s feet (looking
from Philae s center) above the electrode mesh of the SESAME
Permittivity Probe (PP) instrument. The other sole of each foot
contains a custom-made Piezo stack (producer FHG-IfZP) that can
generate and record vibrations.
The ACC are connected by 4-wire cables separated from the
transmitters (using 9-wire cables) to the CASSE electronics in
order to reduce cross talk between transmitting and receiving
lines. But this principle is broken in three common connectors
between the CASSE electronics and the sensors.
The thermistors (PT 1000) are mounted directly on each sensor
using the a 2-wire connection with a common ground in the 9-wire
cables of the transmitters.
Subsystems
==========
None
Operational Considerations
==========================
SESAME-CASSE is sensitive to external vibrations. During the cruise
phase CASSE recorded e.g. the vibrations caused by the dither
actuators of the orbiter gyros as well as those generated by the
Philae flywheel. Any mechanical activity that is not needed for
the proper CASSE measurement was avoided during CASSE operations.
In addition, CASSE also detected CONSERT soundings as fake
acceleration signals. Thus parallel operation of CASSE and CONSERT
were reduced to a minimum and avoided during the touchdown
measurements at Agilkia.
Operational Modes
=================
CASSE can be operated in 5 modes:
1) Health Check: This mode executes a programmed sequence with no
tele-command parameters.
A ping (a vibration with 1000 Hz and a duration of 5 ms) is
sequentially generated by the transmitters of each foot and
simultaneously recorded by the accelerometer on the same foot
(x, y, z axes; total listening duration per foot: 40 ms, sampling
rate per channel: 16 kHz). The amplifier gain is set to 53.79.
The measurements are repeated once. As this mode uses the closest
possible distance between sounder and receiver for CASSE, the
health of transmitter, accelerometer and CASSE electronics
can be checked by this operation. The quality of the Health Check
measurements is checked by the repetition.
2) Listening Mode: The simplest operating mode of CASSE is the
Listening mode. Any combination and number of accelerometer
channels and transmitters can be operated as receivers. Various
parameters like sampling frequency and duration and gain can be
set by executing a CASSE jobcard (a tele-command used for
setting the CASSE instrument parameters) before executing the
Listening mode. The common operation of accelerometers and
transmitters as receivers should be avoided, as in this case
large DC offsets are generated by the CASSE electronics.
3) Trigger Mode: The Trigger mode is a special case of the
Listening mode as one or more receiver channels are designated
as trigger channels. All incoming data in the receiver channels
are stored in the CASSE RAM in FIFO mode until the signal
exceeds predefined positive or negative trigger levels or the
Trigger time-out duration is reached. The trigger levels and
gain can be set either by command or automatically by executing
at least two Listenings before the Trigger operation. When a
trigger event is detected, additional data are stored in the
CASSE RAM until the end of the recording duration.
These data, including a time period before the trigger event
(called 'trigger delay'), are then transferred to the SESAME
Common Data Processing Unit.
4) Sounding Mode: In the Sounding mode at least one transmitter
is excited and the signal via the Philae landing gear and / or
the cometary surface can be registered by any combination
of CASSE receivers. By using a CASSE jobcard, the sounding
frequency and duration can be set. It should be noted that
the CASSE electronics only allows a limited number of
combinations of sounding and receiving frequencies within
the operating ranges of transmitters and accelerometers.
5) Stacking Mode: The Stacking mode is a special case of the
Sounding mode, as the soundings can be repeated and up to
127 linearized time series can be stacked in order to improve
the signal-to-noise ratio.
In any operation mode of the SESAME flight software version FM-3
statistics data (minimum, maximum and mean of each recorded time
series (in ADC units)) can be stored and transferred to ground.
This option is quite useful for the Stacking mode, as only the
statistics data can tell, whether any problematic time series
(flat lines, background noise variation) were stacked on board.
At least flat line time series, which are sometimes produced by
the CASSE electronics and result in all three statistics parameters
having the same value, can be removed on ground.
Calibration
===========
The nominal CASSE calibration comprises the following steps:
a1) All modes but Stacking mode:
Linearize the sample ADC value by inverting the
quasi-logarithmic ADC curve and convert the result to
voltage before ADC.
a2) Only Stacking mode:
Divide the sample, which is actually the sum of linearized
samples from several measurements, by the number of
measurements and perform voltage conversion. In this step
any flat line time series (see above) can be removed by
subtracting the flat line value(s) from the sum and reducing
the number of measurements accordingly.
b) Convert the voltage before ADC to the output voltage of the
sensors by dividing the ADC voltage by the nominal amplifier
gain.
c) Divide the output voltage by the sensor sensitivity to get the
acceleration in [m s^-2].
The accelerometers are individually calibrated by the producer
Bruel and Kjaer and have a typical sensitivity
10 mV/(m s**-2) +-5 percent in the range from 10 Hz to
about 5 kHz (depending on axis). No sensitivity data are
available for the transmitters, whose signals are pre-amplified
before entering the CASSE amplifier. A few measurements during
the Rosetta cruise phase indicate that an external signal in
ADC units recorded by a transmitter is about twice as large as
that recorded by an accelerometer.
Measured Parameters
===================
All CASSE operation modes record time series of acceleration values.
Most of these data are transferred to ground as un-calibrated
quasi-logarithmic ADC values. Only the Stacking mode delivers
linearized summed up time series. Additionally, several time stamps
(e.g. start and end of recording) are transmitted, in order to
determine the correct absolute time series start on ground. At least
at the beginning of each measurement, the temperatures of all CASSE
sensors (also those not used in this operation) are collected.
DIM
====
Instrument Overview
===================
DIM applies the principle of piezoelectricity to detect and analyze
impacting cometary dust particles. An impact evokes a decaying
electric signal (burst), which is a mixture of several frequencies,
at the output of the sensor. At the beginning of this transient
signal a nearly perfect half-sine wave can be observed, which lasts
for the impact duration.
The peak voltage is observed when the impact deformation reaches
its maximum (elastic impacts are supposed). During the second
quarter of the sine wave, the deformation caused by the particle
decreases until the grain leaves the sensor.
The piezoelectric sensors of DIM are mounted on a cube with about
7 cm side length. Three sides of the cube are covered with sensors,
the other three sides are either closed by aluminum plates or left
open for harness access. Each of the active DIM sides is divided
into three equally sized segments that carry rectangular
piezoelectric sensors made of PNZT7700 (Pb, Ni, Zi, Ti).
The size of each segment is 50x16x1 mm**3. Adding all nine active
segments leads to about 70 cm**2 total sensor area.
The DIM electronics is controlled by the common SESAME flight
software.
Scientific Objectives
=====================
When the ice on a cometary surface is heated by solar radiation,
the gas molecules released by the ice sublimation drag grains
composed of refractory (dust) and volatile (ices) matter from the
cometary surface. Due to the combined action of gas drag and
gravitational forces, grains are either ejected into space becoming
part of the interplanetary dust or are drawn back by gravity onto
the cometary surface. The goal of the DIM instrument is to improve
our knowledge about these particulate constituents of comets.
DIM obtains quantitative data on:
- Directional statistics of impacting particles;
- Velocity and mass distribution of back-fallen particles and for
particles on escape trajectories from the nucleus.
These data are to collected over an extended time period in order to
find possible correlations with the cometary diurnal and orbital
phases. The analysis of these data should help to:
- Improve our models of the distribution and the flux of
near-surface dust and small particles as a function of their
size and velocity;
- Understand cometary activity with its underlying processes;
- Explain the formation of cometary mantles
Calibration
===========
Each particle impact onto one of the piezoelectric sensor plates
generates an electric pulse that is registered with the instrument
electronics. The measured signals are analysed by Hertz' theory of
contact mechanics. The output signal can be approximated by a damped
sine wave. The amplitude and the width of the first half-sine pulse
are registered by the instrument electronics and used to derive
particle properties like the reduced modulus and the mass. To
calibrate the instrument signals, a large number of drop
experiments with particles made of different materials
were performed.
Operational Considerations
==========================
SESAME-DIM turned out to be very sensitive to electrical
disturbances by the MPPTs of the solar energy generators on board
Philae. Therefore, several of the measurements obtained during
Philae s descent to the surface of comet 67P (Agilkia) were
disturbed by a large number of false signals. Only very few single
false signals were recorded at the final landing site Abydos where
DIM was operated at night when the solar arrays were not
illuminated. False signals are characterised by the instrument
software as true dust impacts but for which we know by other
means that they cannot be due to dust impacts. The instrument
software is not able to distinguish false signals and dust impacts.
Electronics
===========
The DIM instrument is controlled by an electronics board as part
of the SESAME electronics and receives its +5V digital and +/-5V
analog power from the SESAME power supply board. The analog and
the digital ground are separated. The signals coming from a given
PZT sensor face are amplified by a logarithmic amplifier (U_out)
which is then sent to a peak detector (U_Peak). The DIM electronics
can detect an event if the amplified signal voltage U_out exceeds
an adaptive threshold value U_thr. The detection threshold U_thr is
the sum of an adjustable margin and a signal average.
The margin can be increased in steps of 10 dB in the range from 10
to 70 dB. Each step changes the threshold voltage by approximately
0.3 V.
The signal average is determined by the DIM electronics with a time
constant of approximately 1 s. It varies slowly with impact
properties and frequency, aiming at covering a wide range of event
voltage levels.
According to Hertz' theory the initial part of the dust impact
signal can be approximated by the first half of a sine wave. An
impact should thus show up as a voltage, crossing the threshold
voltage upwards, followed by a second threshold crossing downwards.
The period between the two threshold crossings defines the impact
duration Tc. If the amplified sensor signal crosses the detection
threshold too early (less than 1 ms after the single event
measurement was initialized), the event is classified as a
false event, else it is accepted as the beginning of a potential
real impact. In BCT2 mode (see Section Operational Modes), the
measurement is stopped after 1.6 ms. The event is ruled out as a
long event, if no second threshold crossing was detected within
500 microseconds (in case of the BC mode, this limit is 79
microseconds) during the measurement duration.
A true dust impact is thus characterised by its occurrence later
than 1 ms after the initialization of the measurement and a duration
of less than 500 microseconds for the BCT2 and 79 microseconds for
the BC mode, respectively. Regardless of the event type (dust impact,
false signal, false or long event), an adjustable dead time, the
so-called sensor signal decay time is included after the end of each
single event measurement.
Apart from the signal decay time, several waiting and latency
periods are added to the total duration of a single impact
measurement. All latency periods add up to a total instrument dead
time of approximately 10 ms.
Location
========
The DIM cube is mounted on the top side of Philae, and the three
active sensor sides point in the +X, +Y and +Z directions in the
Philae coordinate system.
Operational Modes
=================
Before any DIM measurement is started, a few operations have to be
performed to guarantee that DIM is properly working.
After DIM is switched on:
(A) A power check is performed to verify that the supply
voltages are within predefined limits. Then
(B) Electronic noise is measured on the DIM amplifier (with
the DIM sensor being disconnected): Starting from a very
low value, the detection threshold (so-called margin) is
increased in steps of 10 dB until no false
event is detected anymore. The rates of false events
typically encountered in flight were such that the margin
was set to 30 dB or 40 dB. This means that a true dust
impact must have an amplitude of at least 0.1 mV or
0.25 mV, respectively, to overcome the threshold of the
amplifier. In a next step
(C) A DIM sensor test checks if all three sensor sides are
operational. An electrical pulse (approximately 5 V for
10 microseconds) is applied to each sensor side, and the
response is registered in the same way a dust impact would
be measured. Finally
(D) The electronics performs a DIM calibration to re-calibrate
the transfer characteristic of the logarithmic amplifier
and to check the time measuring circuit: Two test pulses
are applied to the logarithmic amplifier. Pulse height and
duration are 1 mV, 8 microseconds for low-level and 100 mV,
20 microseconds for high-level, respectively. The results
of the calibration procedure were supposed to be used to
re-calibrate the amplifier transfer function in the data
evaluation on Earth. However, it turned out after launch
that the high-level value is in saturation so that this
re-calibration procedure could not be applied.
After successful execution of the above listed steps, DIM is ready
for measuring dust impacts. Two measurement modes were used to
measure dust at the comet:
(1) Single events on one sensor side can be registered in the
so-called Burst Continuous (BC) Mode. The measured peak
amplitude U_m and the impact duration T_c are stored in a
compressed way: They are scaled to a logarithmic scale,
and the counts for impacts with a particular logarithmic
U_m, T_c combination are stored in memory cells of different
sizes, depending on the expected frequency of such events.
Each BC measurement starts with a 10 s instrument warm-up
period which is not included in the measuring time.
(2) In addition to the BC mode (which delivers the logarithmic
U_m, T_c matrix), the instrument can be operated in a
so-called Burst Continuous Test2 mode (BCT2). Similar to the
BC mode, the BCT2 mode delivers U_m and T_c for each
individual impact. Here the raw uncompressed data are
transmitted instead of the compressed logarithmic values, and
in addition the event time when the impact or false signal
was registered. The number of data sets for impacts,
which can be stored and transmitted to Earth in BCT2 mode, is
limited to 350.
Measuring values for the respective sensor side of the signal average
are transmitted to Earth if sampling of average values is commanded.
Subsystems
==========
None
Measured Parameters
===================
The electrical pulse generated by a particle hitting the DIM sensor
is to first order a damped sine wave. The signal curve is analysed
by the instrument electronics, and two parameters - impact duration
T_c and the peak voltage U_m - are derived from the signal curve and
transmitted to Earth.
A) Impact Duration
The impact duration is derived from the number of counts of a
20 MHz clock.
So-called false and long events are identified. These
events are defined according to the measured impact duration.
They are counted with an onboard counter and only their total
numbers are transmitted to Earth. The impact
durations are transmitted for true dust impacts, but not for
false and long events.
B) Peak Voltage
The instrument provides the peak voltage of a single dust impact
after logarithmic amplification. The transfer characteristics of
the logarithmic DIM amplifier can vary with time. It is regularly
checked by a calibration procedure.
C) Event Time
In addition to the impact duration and the peak voltage of a
signal, the time in UTC when the event occurred is also stored
for up to 350 events transmitted in Burst Continuous Test2 mode
(not in Burst Continuous mode).
PP
==
Instrument Overview
===================
PP is based on a quadrupole configuration of sensors attached to
different parts of the Lander which are capacitively coupled to
the comet surface.
Metal meshes included in the soles of the +Y and -Y feet serve as
receiving electrodes, connected via preamplifiers inside the soles
to the SESAME electronics.
Three transmitter electrodes are integrated with the +X foot sole,
the MUPUS PEN and the APXS detector, respectively. Any combination
of transmitter electrodes can be selected by software. A digitally
generated electrical sine wave of a freely programmable frequency
is applied to the transmitter electrodes causing an AC-current to
flow through the medium and cause a varying potential in the near
environment of the Lander. Current, potential and phase difference
between them are measured to determine the electrical properties
of the medium underneath the Lander.
The PP electronics is controlled by the common SESAME flight
software.
Scientific Objectives
=====================
Characterize the complex permittivity of the nucleus surface
material, i.e. the electrical conductivity and dielectric
polarizability, and their frequency and time dependences, down
to a depth commensurate with the size of the lander. Assess the
sublimation rate of the volatile deposits, as functions of
temperature, illumination, and solar distance.
Secondary objective: Measure the plasma density, and the
electromagnetic and electrostatic waves generated by the
interaction between the nucleus environment and the solar wind,
and thus monitor the outgassing activity
of the comet.
Calibration
===========
Following calibration functions are applied to the SESAME-PP data
at SONC level:
1) Translation of control parameters into timing information
for time series data
2) Translation of measured data into physical units mA, V,
phase angle, spectral power and voltage densities for
plasma data.
Using laboratory simulations and reference measurements the measured
data are converted and the instrument team's laboratory to
conductivity and permittivity values of the material in the vicinity
of the electrodes.
Operational Considerations
==========================
SESAME-PP is very sensitive to any kind of electrical disturbances.
During active or passive (permittivity or plasma) measurements
instruments generating radio waves in the frequency range 5 Hz to
40 kHz like CONSERT and the flywheel should not be operated if
possible.
Detectors with PP-electrode attachments - APXS and MUPUS-PEN -
must not be powered while the respective attached PP-electrode is
used for PP active measurements.
Electronics
===========
The PP instrument is controlled by an electronics board as part
of the SESAME electronics and receives its +5V digital power,
+/-5V and +/-12V analog power from the SESAME power supply board.
For active measurements the SESAME software calculates the optimal
combination of a sine wave vector and a read-out frequency to
generate a frequency close to the commanded one. The data
vector is stored in the beginning of PP's on-board memory, read out
at the defined frequency, converted into voltages by an 8-bit
Digital-to-Analog Converter, then amplified to a voltage between
+10 V and -10 V.
Location
========
The PP sensor system consists of 2 receivers, implemented as
insulated wire meshes into the sole material of both soles at the
+Y and -Y foot, 1 transmitter electrode mesh integrated into the
+X foot soles, 1 transmitter built as ring disk elektrode attached
to the outside of the APXS detector lid and 1 transmitter electrode
implemented as insulated flexboard mesh attached to the MUPUS PEN
deployment mechanism joint at the PEN. The receiver electrodes are
directly connected to preamplifiers attached to the inside of one
of the sole lids of the respective foot with a low impedance coaxial
connection to the electronics board in the warm compartment while
the transmitter electrodes are connected directly via Triax cables
to the electronics.
The Triax's central shield acts as guard to reduce stray capacitance
effects.
Operational Modes
=================
PP can be operated in 6 modes:
1) Healthcheck: All 8 analog channels are measured providing a
conclusive summary of the instrument's status: ADC performance,
transmitter current monitoring for all three electrodes,
receiver preamplifiers, differential amplifier, the related
power switches and the plasma monitor
2) Passive mode: The potential difference between the two receiver
electrodes is sampled with usually 40 kHz. The time series is
analyzed with an on-board wavelet algorithm generating up to 10
logarithmically spaced spectral power bins below half the sampling
frequency. Sampling frequency, data vecor length and number of
bins can be modified by changing the related parameters in the PP
configuration table.
3) Passive test mode: The measurement of the time vector is the same
as in passive mode. Additionally to the power spectrum the complete
original time vector is included in the telemetry for detailed
post-processing on ground.
4) Active mode: A set of 20 frequencies at 3 different amplitudes
each is generated by the PP electronics, the current from the
wave generator through cables and the comet material, the induced
potential variation inside the comet medium and the phase shift
between both signals are analyzed on board and included in the
telemetry. The number of used frequencies and their values are
defined in the configuration table and can be modified by command.
The used transmitter electrode combination is defined via a
command parameter
5) Active test mode: A single frequency defined by a command
parameter is generated, the current, induced potential difference
and their phase difference are measured and included in the
telemetry. Transmitter electrode combination and amplitude of
the transmitted wave are defined as additional command parameters.
Besides the results of the on-board analysis the complete current
and potential vectors are included in the telemetry.
Special usage: By defining as current monitor instead of one of
the transmitter electrodes one of the receiver channels, the
direct response of that receiver channel as a function of time
can be recorded. Together with the differential signal both
receiver responses can be monitored separately for gain
compensation of the pre-amplifiers or stray capacitance
measurements.
6) Plasma wave monitor: The plasma environment in the vicinity of
the detector wire attached to the DIM sensor cube can be
monitored by integrating the received charges. Depending on the
amount of received charges the timer linked to an integrator
with predefined limits will be stopped earlier or later. The
timer value is provided as measurement result. The clock driving
the timer can be adjusted by command parameter between 5 MHz and
312.5 kHz to increase the sensitivity. If no sufficient number
of charges are received, the largest possible 16-bit value is
transmitted.
Subsystems
==========
None
Measured Parameters
===================
Passive mode: scaled spectral power in logarithmically scaled
bins in internal units.
Passive test mode: additionally a time vector of potential
differences in internal units
Active mode: For each used frequency the current amplitude,
potential difference amplitude and phase difference.
The given values are averaged across all measured
sine waves.
Active test mode: additionally a time vector of current and of
potential difference values.
Plasma wave monitor: timer scaling factor and integration time
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