| DESCRIPTION |
Instrument Overview
===================
The Ion and Electron Sensor (IES) is one of the 5 instrument
members of Rosetta Plasma Consortium (RPC). The objective of the
instrument is to measure the flux of ions and electrons as a
function of energy and direction in the solar wind, and in the
environment during the flybys of Earth, Mars, the two asteroids,
and comet 67p/Churyumov-Gerasimenko.
Scientific Objectives
=====================
The scientific objectives of the instrument are to measure and
understand the dynamics, structure, and evolution of the plasma
properties in these environments. Of particular interest is the
interaction of the solar wind with the comet, especially the
formation of pickup ion structures and their relation to the local
magnetic field.
Calibration
===========
The IES instrument was calibrated on the ground in the ion
calibration facility at the Southwest Research Institute,
San Antonio, TX, USA. In addition cross-calibrations are performed
in flight with the RPC-LAP and RPC-ICA instruments.
Operational Considerations
==========================
RPC-IES should be operated continuosly as long as power and
telemetry rate permit. When possible, all other RPC instruments
should be operated while RPC-IES is in operation in order to allow
joint analysis of the resulting data. RPC-IES high voltages should
be turned off 5 min before until 5 min after thruster firings to
prevent arcing.
RPC-IES Instrument Description
=========
The Rosetta Ion and Electron Sensor (RPC-IES) is comprised of a
double toroidal top-hat electrostatic analyzer (ESA), one analyzer
for electrons, the other for ions, arranged back-to-back. The
common entrance aperture has a 360 degree field of view in the
symmetry (denoted here by azimuth) plane. Electrostatic angular
deflection optics give a scanned field of view of +/- 45 degrees
normal to the sensor symmetry axis (denoted here by the elevation
angle).
The instrument objective is to obtain ion and electron
distribution functions over the energy range from 4.32 eV/e to
17.67 keV/e, with a basic time resolution of 128 s. This geometry
allows IES to analyze both electrons and positive ions with a
single entrance aperture, simultaneously. The IES top hat
analyzers have toroidal geometry with a smaller radius of
curvature in the deflection plane than in the orthogonal plane.
This toroidal feature results in a flat deflection plate geometry
at the poles of the analyzers and has the advantage that the focal
point is located outside the analyzers rather than within them, as
is the case with spherical top hat analyzers. The IES field of
view (FOV) thus encompasses a total solid angle of 2.8 Pi
steradians.
Ions and electrons approaching the IES first encounter a
toroidal-shaped grounded grid encircling the instrument. Once
inside the grid the electric field produced by bipolar electrodes
deflects ions and electrons with a range of energies and incident
directions into a field-free entrance aperture containing serrated
walls to minimize scattering of ultraviolet light and stray
charged particles into the instrument. The particles then enter
the top hat region and the electric field produced by the flat
electrostatic analyzer segments of the ion and electron analyzers.
Particles with an energy accepted by the ESA and within a narrow
4% energy pass band will pass through the analyzers and be focused
onto either the electron or ion microchannel plates (MCPs), which
produce charge pulses on 16 discrete anodes for each, which define
the azimuth acceptance angles. For electrons the anodes are of
equal width so the azimuth resolution is 22.5 degrees. (It was
discovered after launch that electron channel 11 was noisy so it
was decided not to download the data from that channel. Hence
only fill data appear for that channel.) For ions the 16 anodes
are divided unequally in size, with 9 of them (each 5 degrees
wide) oriented in the direction expected to view the solar wind
most of the time (anodes 3 to 11). The remaining 7 anodes are
each 45 degrees wide. For both electrons and ions the nominal
resolution in the elevation direction is 5 degrees. This
resolution would provide 18 measurement bins over the 90 degrees
full elevation FOV. However, in order to simplify the instrument
electronics, the FOV has been divided into 16 (=24) bins. This
results in a small gap in coverage between bins.
The selected energy will correspond to a particular 5 degree
elevation entrance angle, depending on the ratio of voltages on
the angle deflectors and the ESAs. Note that the use of the terms
'azimuth' and 'elevation' angle for IES differs from the
conventional terminology of 'polar' and 'azimuth'.
Viewing Maps (from the RPC-IES IK data)
======================================
IES/Electron Sector Layout
The IES/Electron Sector's 360.0 x 90.0 degrees aperture is split
into 256 sectors -- 16 in-plane (azimuthal) by 16 cross-plane
(polar). In the in-plane direction the sectors are numbered from 0
to 15, in the cross-plane direction the sectors are also numbered
0 to 15. All 16 in-plane sectors are 22.5 degrees wide with their
center view directions 22.5 degrees apart. In the cross-plane
direction all sectors have the same size of 5 degrees with their
center view directions 6 degrees apart. In-plane the 'view
direction' of each sector is looking inwards, from the sector
position through the center of the aperture.
This diagram illustrates in-plane (azimuthal) IES/Electron sector
layout:
^ +Yies
| A# indicate the azimuthal
V12 | V11 sector # position in
V13 ....|.... V10 the sensor assembly.
.' A4 | A3 `.
V14.' A5 | A2`. V9 V# indicate the # sector
. A6 | A1. view direction.
V15. | . V8
.A7 | A0. For example, for
. o--------------> +Xies Sector 2 the view
.A8 / +Zies A15. direction is the vector
V0 . / . V7 emanating from the
.A9 / A14. aperture center through
V1 .A10 / A13. V6 the point designated
`. / A11 A12 .' by V2.
V2 `......... ' V5
/ V3 V4
V
Azimuthal sector '3'
view direction
This diagram illustrates cross-plane (polar) IES/Electron Sensor
sector layout:
+Yies Polar Sector '3'
^ ^ view dir. P# indicate the polar
V8 | V7 / sector # position in
.----|----. V3 the sensor assembly.
V15 ,-' P8|P7 `/. V0
.' | P3 `.
`.P15 | / P0.' V# indicate the # sector
`. | / .' view direction.
`. | / .'
`. | / .' For example, for polar
`. |/.' Sector 3 the view
<-------------`o'+Xies direction is the vector
+Zies .' `. emanating from the
.' `. aperture center through
.' `. the point designated
.' `. by V3.
.'P15 P0`.
`. .'
V15 `-. P8 P7 ,-' V0
`---------'
V8 V7
IES/Ion Sector Layout
The IES/Ion Sector's 360.0 x 90.0 degrees aperture is split into
256 sectors -- 16 in-plane (azimuthal) by 16 cross-plane (polar).
In the in-plane direction the sectors are numbered from 0 to 15,
in the cross-plane direction the sectors are numbered 0 to 15.
Not all 16 in-plane sectors have the same size: seven of them --
sectors 0-2 and 12-15 -- are 45 degrees wide while the other
nine -- sectors 3-11 -- are 5 degrees wide. In the cross-plane
direction all sectors have the same size of 5 degrees with their
center view directions 6 degrees apart. In-plane the 'view
direction' of each sector is looking inwards, from the sector
position through the center of the aperture.
This diagram illustrates in-plane (azimuthal) IES/Ion sector
layout:
+Yies
^
| A# indicate the azimuthal
| sector # position in
V15 ....|.... V0 the sensor assembly.
.' A7 A11 `.
.' A3 | A12 `. V# indicate the # sector
V14. | . V1 view direction.
. A2 | A13 .
. | . +Xies For example, for
. o--------------> Sector 12 the view
. / +Zies . direction is the vector
. A1 / A14 . emanating from the
V13. / . V2 aperture center through
. / . the point designated
`.A0 A15.' by V12.
V12 ......... ' V3
/ V11 V7
V
Azimuthal sector '12'
view direction
This diagram illustrates cross-plane (polar) IES/Ion Sensor sector
layout:
+Yies Polar Sector '3'
^ ^ view dir. P# indicate the polar
V8 | V7 / sector # position in
.----|----. V3 the sensor assembly.
V15 ,-' P8|P7 `/. V0
.' | P3 `.
`.P15 | / P0.' V# indicate the # sector
`. | / .' view direction.
`. | / .'
`. | / .' For example, for polar
`. |/.' Sector 3 the view
<-------------`o'+Xies direction is the vector
+Zies .' `. emanating from the
.' `. aperture center through
.' `. the point designated
.' `. by V3.
.'P15 P0`.
`. .'
V15 `-. P8 P7 ,-' V0
`---------'
V8 V7
Electronics
===========
Pulses from the segmented anode are amplified by charge-sensitive
preamplifiers (CSPs) and recorded in the 16 x 24 bit ion and
electron counters. The data are buffered before being sent to the
output serial register for transmission to the RPC Plasma
Interface Unit (PIU) as serial telemetry packets. The stepping
sequences of the angle and energy deflection voltages of the
instrument are fixed in memory.
The IES instrument contains a single micro-controller (RTX20X10)
which communicates with the RPC-PIU over the IEEE 1355 bus,
transmits the collected science data, and monitors the instrument
status. The flight software is written in the C and Forth
programming languages. The PIU stores and re-transmits to the
spacecraft the data stream that the instrument produces. No
special data handling is required. Commands and command sequences
for IES are formulated by the IES team, sent to Imperial College,
where they are translated to the proper format and sent to the
project ground system. They are eventually uploaded to the
spacecraft and are stored for later execution. In some cases the
command may be executed immediately. The sequence provides for
selection of one or more operational modes (see below) or in some
cases repeated cycling through a series of modes.
High Voltages
=======
The voltages for the ESAs, deflector (DEF), and MCPs are derived
from a single supply of 8514 volts. The ESA and DEF voltages are
stepped according to LUTs. The ESA sweeps between 0.407 V and
1667 V in 128 approximately logarithmically spaced steps. The DEF
steps between -6667 and +6667 V, alternating negative to positive
and positive to negative between the 2 deflector plates. The MCP
potentials can be set to a potential between +/- 2500 V and
+/-3500 V, the positive level for the electrons and negative for
the ions. Except for a short test on May 1-2, 2010, the levels
have been at 2500 V based on measurements on the ground before
flight.
Data Binning
=======
Since IES produces more data than can be transmitted within its
telemetry allocation, in order to compress the data they are
binned in a lossy fashion by use of look up tables (LUTs) stored
onboard in the instrument. (See the specific documentation on the
details of the different data modes.) For example, the counts from
adjacent energy steps can be combined and the sums transmitted to
ground, or the counts from a range of energy and/or angle bins or
any combination of these, are transmitted, depending on the
scientific objective for a particular observation period. Hence
the appearance of the data structure will depend on the mode for
that particular observation.
Flyback
=======
Cycling of ESA voltages is completed using 128 steps (0 to 127)
which include 4 steps (124 to 127) during 'flyback' (FB), the
transition from the highest voltage of 1667 V (step 123) to 0 V
(step 127). While the actual transition time does not require all
4 steps, i.e. the 0 V level may be attained in 2 steps or less,
science data readings during the first three steps of the flyback
should be considered unreliable. When flyback steps are averaged,
they are averaged only with other flyback steps. Step 127 may be
considered as 0V for background measurements if not averaged.
Location
========
The IES instrument is located on the +Z deck of the Rosetta
spacecraft in the corner formed by the +X and +Y spacecraft
surfaces. The instrument is oriented such that the symmetry axis
of the ESAs (i. e. through their poles) is at 45 degrees to the +Z
as well as the +X and +Y spacecraft surfaces. Hence the azimuth
plane of the FOV (and hence the elevation 0 degrees) is also
tilted upward 45 degrees. This arrangement was selected in order
to minimize interference to the FOV by any spacecraft or other
instrument structures but will still allow viewing the solar wind
when the spacecraft is oriented favorably (i.e. when the angle
between the spacecraft +Z axis and the sun is between 70 degrees
and 100 degrees).
Operational Modes
=================
When powered on, the IES instrument runs its boot PROM code. The
boot PROM, by default, checks all RAM, EEPROM and PROM resources
within the IES instrument and reports their status in
housekeeping. Whether the PAUSE-PROM or RESUME-PROM mode is
entered depends on what was planned for the first operation.
PAUSE-PROM prevents the PROM from going automatically into the
EEPROM code so that telecommanding or maintenance mode
telecommands can be executed. The boot PROM can execute the entire
suite of maintenance mode commands but only a limited set of
telecommands. In order to program the EEPROM using the activate
patch function in maintenance mode, IES must be running from the
boot PROM. This is because the boot PROM code runs using a lower
clock frequency which is amenable to the EEPROM write timing. The
EEPROM code is run at a faster clock frequency to accommodate all
the tasks that must be executed during science data acquisition.
Housekeeping is generated every 32 seconds. Maintenance and event
messages are possible from this mode.
RESUME-PROM is a waiting mode to allow telecommands or maintenance
mode commands to be received by IES before automatically going to
the EEPROM code. Housekeeping is generated every 32 seconds. Event
messages are possible from this mode. LVSCI-EEPROM is the first
EEPROM mode entered and allows IES commands to be executed. This
is the mode used for low-voltage stimulation operation.
Housekeeping is generated every 32 seconds. Science and event
messages are possible from this mode.
HVSCI-EEPROM is entered if an IES-INSTR-PROG-MODE HVSCI
telecommand is received. Here, high-voltage telecommands can be
executed to turn on the HV supplies and manipulate their settings.
All plasma science data are acquired in this mode. Housekeeping is
generated every 32 seconds. Science and event messages are
possible from this mode.
LVENG-EEPROM is used for executing maintenance commands and memory
manipulation telecommands. Note that the EPROM cannot be written
in this mode due to the timing constraints mentioned in the
PAUSEPROM description. Housekeeping is generated every 32 seconds.
Maintenance and event messages are possible from this mode.
A science data collection mode is determined by the LUT(s)
specified in the definition of the mode and determined by the
command sequence sent to the instrument. See the 3 tables in the
Appendix to the Rosetta-RPC-IES Planetary Science Archive
Interface Control Document, Document No. 10991-IES-EAICD-01 for
details of the data collapsing for each science mode. These tables
give the definitions currently stored in IES memory and show how
the full 128 energy by 16 azimuth by 16 elevation bins are
selected and or combined to fit either the normal or burst
telemetry cases.
Acquisition Timing
==================
IES uses cycles of varying durations to acquire ION and ELC data
across the energy and angular ranges to allow tailoring of
energy/angular resolution and time resolution. A cycle requires
128, 256, 512 or 1024 seconds to complete sweeping across the
energy range using 128 steps. At each energy step, the deflection
voltages are swept using 16 steps (elevations). Each step requires
a rise and settle time of 30ms during which no acquisition takes
place followed by integration time or acquisition window. The
following table lists the cycle durations, and the associated
acquisition times.
Cycle Duration (seconds): 128
Duration to complete each energy step which inclues 16 elevation
steps (seconds): 1
Acquisition/integration time at each elevation/deflection step
(milliseconds): 32.5
Cycle Duration (seconds): 256
Duration to complete each energy step which inclues 16 elevation
steps (seconds): 2
Acquisition/integration time at each elevation/deflection step
(milliseconds): 95
Cycle Duration (seconds): 512
Duration to complete each energy step which inclues 16 elevation
steps (seconds): 4
Acquisition/integration time at each elevation/deflection step
(milliseconds): 220
Cycle Duration (seconds): 1024
Duration to complete each energy step which inclues 16 elevation
steps (seconds): 8
Acquisition/integration time at each elevation/deflection step
(milliseconds): 470
Measured Parameters
===================
Energy: Range 4.32 eV to 17.67 KeV
Resolution 4%
Angle: Range (FOV) 90 deg x 360 deg (2.8 Pi sr)
Resolution (electrons) 5 deg (elev) x 22.5 deg (azim)
(16 azimuthal x 16 elevation)
Resolution (ions) 5 deg (elev) x 45 deg (azim)
for 7 sectors
5 deg (elev) x 5 deg (azim)
for 9 sectors
(16 azimuthal x 16 elevation)
Temporal resolution:
3D distribution 128s
downlink rates Normal: 5 bps
Burst: 250 bps
Geometric factor:
total (ions) 5 x 10e-4 cm^2 sr eV/eV counts/ion
per 45 deg sector 5 x 10e-5 cm^2 sr eV/eV counts/ion
per 5 deg sector 7 x 10e-6 cm^2 sr eV/eV counts/ion
total (electrons) 5 x 10e-5 cm^2 sr eV/eV counts/electron
per sector (electrons) 5 x 10e-6 cm^2 sr eV/eV counts/electron
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