| DATA_SET_DESCRIPTION |
Data Set Overview
=================
Contents:
A. Operations overview
B. Dataset status
C. Scientific use
A. Operations overview
----------------------
LAP operations are described in the operations report, IRFU-ROS-OPR-ESC1
in the DOCUMENT/FLIGHT_REPORTS directory of this archive.
B. Dataset status
-----------------
This dataset contains scientifically useful data from the Rosetta
RPC-LAP instrument. The data contained are primary instrument data,
in instrument units (TM) and NOT calibrated to physical units
(volts and amperes) and with instrumental offsets NOT removed.
See the caveats in Section C below for data use.
A possible timing issue warned for in a previous delivery (the pre-lander
phase, PRL) turned out to be spurious (due to an error in timing checks).
There are thus no known timing issues.
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 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 Edberg et al ('Simultaneous measurements of the Martian plasma
environment by Rosetta and Mars Express', Planet. Space Sci., 57,
1085-1096, 2008. 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. It is also necessary to only compare data acquired at the same
bias. This is why data are presented as (current, voltage) pairs.
d. The bias values in the files, current and voltage alike, are not
measured but constructed from the instrument command log and known
modes. A bias change therefore shows up in the data files
when the 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, where timing is kept internally
by the instrument.
e. After hibernation, LAP probe 2 (P2) showed strong signs of contamination.
Clear signs of this include that current to P2 is always lower than
to P1 at similar bias voltage, and sweeps in both directions (available
from macro 204) showing hysteresis effects. To avoid problems, only use
probe 2 current measurements obtained at fixed negative bias potential,
as available in e.g. macros 0x525, 0x624 and 0x805. The PI discourages
any other use of P2 data in this archive except for diagnostics.
f. The LAP macros mainly used changed during the period covered in
this archive.
(i) November 2014
The macros used for Langmuir mode operations in this period were
mainly 0x515 and 0x604, with macros 0x803 and occasionally 0x807
form LDL operations with MIP. Of these, only 0x807 have bias sweeps
covering most of the instrument hardware range (plus minus 31 V),
needed for the high negative spacecraft potential encountered. The
fixed bias voltage of 0x803 and 0x807 (+10 V) turned out not to be
ideal for the same reason, and the variations of s/c potential in
response to density fluctuations in the plasma can for these macros
sometimes be so strong that there even is anticorrelation of probe
current with plasma density. The problem is less for macros 0x515
and 0x604 (+20 V) but caution is needed.
(ii) December 2014 -- March 2015
In early December, macros based on what we have learned from the
September-November data could be uploaded to the instrument. Macros
0x525, 0x624 and 0x805 cover most of the available bias range in
the sweeps, avoid fixed positive bias potential on probe 2 to
minimise contamination problems, and have one of the probes at
fixed negative potential to follow the ion current, which is less
dependent on s/c potential variations. These three macros provide
the bulk of the data in this archive.
(iii) All period
Some operations of LAP in electric field (voltage measurement) mode
were inserted when illumination so allowed (both probes illuminated),
typically meaning some hours a couple of times per month. These
operations used macros 0x503 and 0x504 depending on available data
volume, and operated the probes at negative bias current. Probe 2
data may here need some caution, due to low photoemission and other
effects of contamination. If attempts are made to derive electric
field data from these measurements, greatest care has to be taken
to avoid common mode problems, and a keen awareness of the limitations
of the instrument for this purpose (asymmetric booms, booms short
compared to solar panels) must be maintained.
g. There is obvious interference from the MIP instrument when using its
LDL mode (macros 803, 805 and 807 in this dataset). This affects
mainly the data sampled at kHz frequencies at fixed bias and in
sweeps. For the fix bias snapshots at 18.75 kHz sampling frequency,
this mostly affects the first samples in a record, but can in longer
records sometimes be seen also further into the record. In sweeps in
macro 807, MIP interference can be detected as spikes where one or
twp samples deviate from their neighbours. Some MIP interference may
occur also outside of LDL operations, as may of course interference
from other sources.
h. Until the end of operations at the comet, the first priority of the
LAP team will have to be the science operations of the instrument, to
make sure there will be useful data to archive. The archived data
set is complete, and it has not been possible to scan every part of
it for every possible problem before delivery, so there may be numerous
data quality issues in addition to this list. While the data clearly
have a great science potential, analysis has to be done with great
care. Anybody using the present dataset is strongly encouraged to
contact a LAP CoI for cooperation.
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 a
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 32 s.
- Sampled at 57.8 Hz, further downsampled to typically 0.45 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 and no user or this archive
should have any interest in them.
Processing
==========
Processing of the CALIBRATED data involves the following steps:
1. Time tagging.
2. Determination of instrument offsets from instrument calibration
runs (operational macro 104). 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
=================
Please see EAICD for description of coordinate systems in the geometry
files. Other data are independent of coordinates.
Software
========
N/A
Media/Format
============
Electronic transfer
|
.png)
.png)
