PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "2015-12-15, H.-H. Fischer, M. Knapmeyer, K. Seidensticker (DLR) initial release " RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = RL INSTRUMENT_ID ="SESAME" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "SURFACE ELECTRIC SOUNDING AND ACOUSTIC MONITORING EXPERIMENT" INSTRUMENT_TYPE ={"ACOUSTIC SENSOR", "DUST IMPACT DETECTOR", "PERMITTIVITY PROBE"} INSTRUMENT_DESC = " 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 " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID ="SEIDENSTICKERETA2007" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END