Data Set Overview :  Contents: A. Operations overview B. Dataset status C. Scientific use  A. Operations overview ----------------------  This dataset contains Rosetta LAP data from the Rosetta flyby of asteroid 21 Lutetia.  Operation dates: July 7 - 13, 2010 Objective: science, diagnostics Environment: solar wind at 2.7 AU Scientific potential of LAP data acquired: Good.  Description of operations:  The LAP operations are further described in the relevant operations report, IRFU-ROS-OPR-LUT in the DOCUMENT/FLIGHT_REPORTS directory of this archive.  LAP was on during the full Lutetia flyby, monitoring the solar wind density variations through the measurement of the probe-to-spacecraft potential most of the time, with some operations also in Langmuir probe sweep mode to provide information on photoemission.  From CA-2h50min to CA+3h10min, LAP was in burst mode with higher telemetry rate, sampling the probe-to-spacecraft potential at 57.8 s-1. Outside this time span, LAP had normal telemetry rate, usually sampling at 0.9 s-1. In both modes, LAP also produces shorter records at 18.75 kHz sampling typically every 32 s (burst mode) or 160 s (normal mode).  One hour after closest approach, LAP handed over probe 2 to MIP for use in the MIP LDL (Long Debye Length) mode. During this period, the only LAP data available derives from probe 1. More LDL operations were run at the end of operations.  Operations were fully nominal. The LAP data acquired are good and scientifically useful, though any user must take great care of the limitations set by the varying pointing, the tenuous plasmas, and the spacecraft-plasma interaction. In particular, the large pointing variations at closest approach make any data analysis here extremely difficult and prone to spurious effects.   B. Dataset status -----------------  This dataset contains scientifically useful data from the Rosetta RPC-LAP instrument. The data contained are primary instrument data, calibrated to physical units (volts and amperes) and with instrumental offsets removed. See the caveats in Section C below for data use.  The main differences of this version 1.0 to the version previouslyavailable on the PSA are:- Addition of geometry files (see EAICD for details). - Major documentation upgrade, particularly the EAICD. - Bug affecting some bias values corrected. - Bug affecting truncated 20-bit voltage measurements corrected.   C. Scientific use -----------------  The data can be used for scientific analysis, but this requires some familiarity with this type of data. At a later stage, a DERIVED data archive will be delivered, providing directly useful scientific quantities (e.g. plasma density, wave spectra, electron temperature).  Caveats:  a. For probe-to-spacecraft potential (Vps) time series: this is a commonly  used proxy for the plasma density in tenuous plasmas. However, because the  perturbations from the solar panels, the wake formed behind the s/c and  solar panels in e.g. the solar wind, and the photoelectron cloud around  the spacecraft are all sensitive to the probe location, Vps can be used  as a density proxy only during intervals of constant pointing. See the  paper by [EDBERGETAL2009] ('Simultaneous measurements of the Martian plasma environment by Rosetta and Mars Express', Planet. Space Sci., 57,  1085-1096, 2009. doi:10.1016/j.pss.2008.10.016) for an example of how  Vps may be used as a plasma density proxy. Perturbations from wake  and photoemission have been studied by Sjogren (report available at  http://www.space.irfu.se/exjobb/2009_alex_sjogren/). b. For the same reasons, the probe bias sweeps are also sensitive to  the spacecraft pointing. However, for all sweeps  obtained prior to the comet phase, except some acquired in Earth's  plasmasphere, the ion contribution to the data is so low that the  photoemission saturation current can be obtained at all angles for  which the probe is sunlit. Note however that the probe may be  partially shadowed by its supporting rod (the stub), and that  surface inhomogeneities may cause the photoemission to vary  also with the pointing. c. For all sweeps obtained prior to the comet phase, except some  acquired in Earth's plasmasphere, the dominating contributions to  the probe current are probe photoemission (at negative bias voltage)  and collection of photoelectrons emitted by the spacecraft and  solar panels (at positive bias voltage). Hence, probe sweep data  can be interpreted in terms of local plasma parameters only in the  Earth's plasmasphere. The main reasons for occasionally running  such sweeps in other environments, e.g. the solar wind and the  Earth's magnetosphere, are to gather data for investigation of  spacecraft-plasma-probe interactions and to monitor probe  photoemission. d. It is also necessary to only compare data acquired at the same  bias. This is why data are presented as (current, voltage) pairs. e. There sometimes is an unexplained slope in the sweep data at negative  bias, in the photoemission dominated region. This means an uncertainty  to the data, and the details of this are yet unknown. Please see  discussion in the latest version of the operations report,  IRFU-ROS-OPR-LUT_Vxx, available in the DOCUMENTS/FLIGHT_REPORTS  of this archive. f. The bias values in the files, current and voltage alike, is not  measured but constructed from the instrument command log and known  modes. Bias settings changes therefore show up in the data files  when a bias command is issued, while it can be seen in the data  that it really takes effect 2-3 seconds later. This caveat does  not apply to probe bias sweeps.   Parameters :  Primary data:  1. Probe-to-spacecraft potential time series, measured at controlled  bias current.  Available at two time resolutions:  - Sampled at 57.8 Hz, further downsampled to typically 0.9 Hz when  not in burst mode, quasi-continuous data with short gaps every  32 s.  - Sampled at 18.75 kHz, in brief snapshots repeated with  cycle which is a multiple of 32 s (typically between 32 and 160 s) 2. Probe current measured at controlled bias voltage.  Available in three varieties:  - Probe bias voltage sweeps. Bias voltage stepped, probe current measured,  duration a few seconds, repeated with a cycle which is a multiple of 32s. - Sampled at 57.8 Hz, further downsampled to typically 0.9 Hz when  not in burst mode, quasi-continuous data with short gaps every  32 s.  - Sampled at 18.75 kHz, in brief snapshots repeated with  cycle which is a multiple of 32 s (typically between 32 and 160 s).  - In addition, there are a few calibration sweeps taken with the  probe disconnected to evaluate instrument offsets. These are used  in the preparation of the CALIBRATED data but have no scientific  value on their own. Data from these are included in the EDITED data. 3. Housekeeping data. These are only included with the EDITED archive  and consist mainly of instrument digital status. They have no  direct scientific value on their own.   Processing  : Processing of the CALIBRATED data involves the following steps:  1. Time tagging. 2. Determination of instrument offsets from instrument calibration  runs (operational macro 0x104). This is done by a stand-alone  application. The resulting offsets are stored in the CALIB  directory as (bias, offset) pairs in TM units. 3. Compensation of probe current measurements for instrument offsets,  using the most recently determined of the offset calibration files  of step 2 above. 4. For each measured value (volts or amperes), the corresponding bias  value (amperes or volts, respectively) is added to the data. All  data are thus presented as (current, voltage) pairs, where the bias  is not measured but reconstructed from the instrument command history  (see Caveat f above). 5. Generation of table files and corresponding PDS label files.   Data  : All data are presented as ASCII tables with stand-alone PDS labels. The content of the data is described in each label. There is one file for every acquisition period (32 s in duration) and data type, meaning there is a large number of files.  The files names indicate the data content, as described in Section 3.1.4 of the EAICD. In brief, the convention is as follows:  File name: RPCLAPYYMMDD_AAAa_bcdefghxx.EXT  YYMMDD : year, month and day, all with two digits. AAA : Alphanumeric counter reseting at zero every new day. a : Type, T:20 bit ADC, S:16 bit ADC, H : Houskeeping b : Format, R:Edited Raw C:Calibrated D:Derived c : Instrument mode  E : E-Field (Current bias and voltage measurments)  D : Density (Voltage bias and current measurments d : Bias mode, S:Sweeping B:Constant Bias e : Sensor: 1 : Probe 1, 2 : Probe 2, or 3 : Derived from 1 and 2 f : Analog filters, 8 : 8 Khz 4 : 4 Khz g : Telemetry rate, M : Minimum N : Normal B : Burst h : For science data this character is always an S. x : For contingency, not present if not needed.  Example:  EXT is either LBL or TAB. Note that the alphanumeric counter is independently generated for EDITED and CALIBRATED archives, so EDITED and CALIBRATED data from the same time do not necessarily reside in files with similar (except for the b : R or C above) names.    Ancillary Data  :  For any scientific analysis, needed ancillary information would be spacecraft position and attitude. This can be found in the daily geometry files provided with the data.   Coordinate System : The geometry files provide positions and velocities related to the ECLIPJ2000 axes directions, centred in the sun or in the target body. The spacecraft pointing is specified by the transformation matrix between ECLIPJ2000 and the s/c coordinate axes.   Software : N/A   Media/Format : Electronic transfer

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