Instrument Overview
===================
The Ion Composition Analyzer (ICA) is part of the Rosetta Plasma
Consortiumn(RPC). ICA is designed to measure the three-dimensional
distribution function of positive ions in order to study the
interaction between the solar wind and cometary particles. The
instrument has a mass resolution high enough to resolve the major
species such as protons, helium, oxygen, molecular ions, and heavy
ions characteristic of dusty plasma regions. ICA consists of an
electrostatic acceptance angle filter, an electrostatic energy filter,
and a magnetic momentum filter. Particles are detected using large
diameter (100 mm) microchannel plates and a two-dimensional anode
system. ICA has its own processor for data reduction / compression and
formatting. The energy range of the instrument is from 25 eV to 40 keV
and an angular field-of-view of 360 deg times 90 deg is achieved
through electrostatic deflection of incomingparticles.
The ICA instrument is based on the design of three earlier versions
of this type of instrument. Those are the TICS instrument flown on the
Swedish-German research satellite Freja which was operated between
1992-1996, the IMIS instrument which was part of the ASPERA-C
experiment on the ill-fated Mars-96 mission, and the IMI instrument
on the Japanese Nozomi mission to Mars. The instrument also has
heritage from the ASPERA experiment flown on the Soviet spacecraft
Phobos-2 to Mars. Furthermore an almost identical mass resolving
ion spectrometer has been flown on the Mars Express mission, the IMA
sensor of the ASPERA-3 instrument. One more copy, only slightly
modified, is part of the ASPERA-4 instrument on Venus Express.
 
RPC-ICA  Characteristics
============================
Quantity                Range
Energy: Range           25 eV to 40 keV
        Resolution      Delta E/E$=0.07
Scan:   32 (solar wind) 96 (otherwise)
Angle:  Range (FOV)     90 x 360 degrees
        Resolution      5.0 x 22.5 degrees (16 elevation steps x 16
                        sectors)
 
Temporal
resolution: 2D distribution 4 s (12 s full energy range)
            3D distribution 64 s (192 s full energy range)
Geometric factor
per 22.5 degree sector  6 10^{-4} cm^2 sr
per 360 degree sector   1 10^{-2} cm^2 sr
 
 
RPC-ICA Electronics
===================
 
The ICA electronics can be divided into two main groups, high voltage
power supplies for biasing of the various electrostatic filters and
the MCP assembly, and digital electronics to handle the instrument
operations. The high voltage supplies are laid out on two round boards
in the cylindrical sensor part of the instrument. The high voltages
are achieved by transforming an input voltage of 25 V to
approximately 500 V, and subsequent chains of diode-capacitor voltage
multipliers. ICA has two separate high
voltage power supplies. One supply is dedicated to keeping the front
side of the MCP biased to approximately -3 kV, while the other one
supplies all other high voltages as fixed raw voltages. These raw
voltages are then regulated by the use of high voltage opto-couplers.
The electrostatic entrance filter can be stepped between +/-2.5 kV
to an accuracy of 1.2 V using a 12-bit D/A-converter and the
electrostatic energy filter is stepped between 0 and -4.5 kV also
using a 12-bit D/A-converter and a high voltage opto-coupler. A high
accuracy at low voltage settings is achieved by fixing the -4.5 kV
supply at a voltage of about -100 V, and using a -11.25 V supply
with a 12-bit D/A-converter connected to the outer ESA hemisphere
to step the lower energy range. The post-acceleration voltage is
also taken from the raw -4.5 kV voltage and set to the desired
voltage using an opto-coupler. The digital electronics performs
the following main functions:
a) Reading data from the double-buffered sensor memory to the CPU and
processing the data.
b) Feeding the IEEE 1355 serial interface to the PIU with processed
and formatted data.
c) Receiving commands on the serial interface from the PIU.
d) Controlling the high voltage power supply settings and monitoring
voltages and temperatures.
The digital electronics are built around an MA37150 processor. The
on-board software runs from a 1 Mbit large bit-error corrected RAM.
Flight software is stored both in 256 kbit PROM and in 4 Mbit EEPROM
to enable patching of the software. An 8 Mbit memory is used both as
working area for the CPU, and as a buffer for formatted data to be
sent to the PIU. Data is transmitted to and from the PIU via a IEEE
1355 serial interface.
 
ICA On-board data reduction
============================
Every sample period (approx. 125 ms), the ICA sensor produces a
mass-angle-count matrix of the size 32 x 16 x 16 bits, for a total of
8192 bits per sample or 64 kbits per second. To reduce this large
amount of data to the bit rates allocated to ICA (5, 100 and 1000
bits/s), the on-board CPU performs both data reduction and data
compression. The basic measurement cycle consists of performing 16
energy sweeps at different settings of the acceptance angle filter,
in order to step through the full field of view of the instrument.
The energy sweep contains either 32 or 96 steps. A full 96 energy step
measurement cycle is completed in 192s.
The data is first reduced from a 16-bit representation to 8-bits
using a logarithmic compression. Directions shadowed by the spacecraft
are masked out to avoid sending invalid data. Options are available to
mask out certain mass-channels which show large noise levels as well
as certain acceptance angles which cannot be reached for certain
energy levels due to finite resolution of the digital-to-analog
converters controlling the energy-acceptance filter, see CALIB
directory. Depending on the selected mode, the data is then reduced
by different methods.
 
Basically, the following two methods are used:
a) The 32 mass channels are grouped together based on calibration
data, and the sum of the grouped mass channels (representing a certain
ion such as H+ or a certain ion mass range) is transmitted instead of
individual mass channels.
This will be referred to as mass-lookup tables and is used when the
number of transmitted mass channels is less than 8. For 8 and 16 mass
channels a simple integration of neighboring mass channels is used.
b) Summation of angular bins, both the 16 stepped elevation angle and
the 16 sector directions.
 
The reduced data is finally compressed with a loss-less compression
algorithm based on the Rice method. This compression reduces the data
with approximately a factor of 5. Due to the data compression, the
output data packets have variable length, and are buffered until
enough compressed and time-stamped packets are available to fill a
fixed length packet transmitted to the PIU.
The buffer is also used to allow for higher than average production
of data for some time. If data is coming in to the buffer faster than
it is being removed by the TM stream an overflow will occur and data
will be lost. The instrument can automatically adjust the reduction
so that it fits the current efficiency of the compression. As an
adjustable high and low ''watermark'' in the buffer is exceeded,
the data reduction is changed accordingly.
 
Mode description
=================
ICA has in principle only one scientific mode. It measures 32 mass
channels from 16 different sectors for 96 different energy intervals.
The different science modes differ only in how much they reduce the
data in order to fit a certain telemetry rate. The science modes are:
ID       Name
MSPO     Minimum spectra only
MSIS     Minimum, selected ion species
MEXM     Minimum, energy mass matrix
NRM      Normal mode, intermediate resolution
HAR      High angular resolution
EXM      High mass resolution
For the end user, the used mode will be evident from the resolution
of the data. Within each of the NRM, HAR and EXM modes there exist 8
sub-modes with different degree of binning of the data. HAR and EXM
are intended for burst mode and differ in their strategy for reduction
of the data. EXM keeps high mass resolution as far as possible,
whereas HAR reduces mass resolution and preserves spatial resolution
if the available telemetry is not enough to keep the current
resolution of the data. NRM is intended for Normal mode telemetry
and the minimum modes are intended for minimum telemetry rate.
 
ICA feld-of-view
=================
The field-of-view is described in the EAICD document found in the
DOCUMENT directory of this data set. A figure can also be found in the
GEOMETRY directory, named ICALOCATION.JPG. ICA is located on the
spacecraft +Z side, looking out over the edge on the + Y side. Its
primary field-of view is the spacecraft x-z plane. The elevation scan
can bring in particles from approximately +/- 45 degrees from this
plane. For some angles of arrival the spacecraft will obstruct the
field-of-view. This happens for sectors 10-15 and sector 0, for
elevation index values 0 to 7.
 
Output data
===========
The output is energy spectrograms for certain mass channels and
directions. The mass channels corresponds to physical locations on
the detector plate, and calibration results must be used to determine
which mass channels corresponds to a certain ion species or range of
ion species. The data is obtained from 16 sectors looking in different
directions and 16 different elevation angles out of the detector
plane.
 
The RPC ICA-CREW
================
PI:           Rickard Lundin
Co-PI:        Hans Nilsson
TM:           Kjell Lundin
 
Instrument:   Olle Norberg, R. Lundin, S. Barabash, Kjell Lundin,
Hans Borg
Development:  P. Riihelae
On-board S/W: Hans Borg
EGSE S/W:     Hans Borg
Archive S/W:  Jan Karlsson
Co-Is:
S. Barabash
A. Fedorov
J.-A. Sauvaud
H. Koskinen
E. Kallio
J. L. Burch
 
References
H. Nilsson, R. Lundin, K. Lundin, S. Barabash, H. Borg, O. Norberg,
A. Fedorov, J.-A. Sauvaud, H. Koskinen, E. Kallio, P. Riihelae and
J. L. Burch RPC-ICA:  The  Ion Composition Analyzer of the Rosetta
Plasma Consortium, Space Science Reviews,
doi:10.1007/s11214-006-9031-z, 2006