| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set consists of all of the calibrated data from the
Cassini Plasma Spectrometer (CAPS) on-board the Cassini spacecraft
during the entire Cassini mission. The following calibrated data
products (calculated from their corresponding uncalibrated data stets)
are produced:
Electron Spectrometer data product (ELS),
Ion Beam Spectrometer data product (IBS),
Ion Mass Spectrometer ion data product (ION),
Ion Mass Spectrometer singles data product (SNG),
Ion Mass Spectrometer 'time of flight' linear electric field data
product (TOFLEF),
Ion Mass Spectrometer 'time of flight' straight through data
product (TOFST).
These data were acquired in a mix of CAPS operating modes beginning with
the first instrument checkout in January 1999 and continuing throughout
the Cassini Tour, through the end of prime mission a subsequent
extended mission. The data set covers the time period from
1999-004T03:07:47 UT until 2012-154T06:11:45. Sampling rates were
variable and depended upon the downlink capabilities and other
activities on-board.
For times when CAPS is not producing data due to being turned off or
due to communication issues, the data set will not contain data.
This effect can be sensor/data type specific.
For times when CAPS is producing data that is not suitable for science
(for instance, mcp calibration tests, parts of the turn on sequence
when sensor is on but the mcp voltages have yet to be initiated) the
data is replaced with fill values. If there is a record of all fill
that you think is scientifically interesting you should use the
uncalibrated data instead and confer with the CAPS PI.
Likewise, if the energy table is unknown, or actuator angle is unknown
for the whole period of the record, then the dimensions of the data may
all be fill values too. Another possibility is any dead time correction
or cross talk correction that gave negative counts resulted in all data
at that energy being set to fill.
Parameters
==========
Data Sampling
-------------
Data acquistion strategies varied throught the mission. During the
cruise phase, the instrument was only capable of 2 distinct rates. In
addition, the spacecraft telemetry modes were not fully developed
which lead to peculiar recording strategies as well. As the mission
progressed, the flight software for both the CAPS instrument and the
Cassini spacecraft matured. During the Jupiter period, the spacecraft
implemented data policing, where an instrument was assigned a data
allocation and the spacecraft would not record any more data from an
instrument who had reached their negotiated limit. This strategy was
useful during the planning process, but careful control had to be
exercised to not run over the allocation. Data gaps in the CAPS data
can come from a variety of sources: instrument was off, specific
sensors were off, ground communication problems, CAPS over-allocation
issues, the instrument was in sleep mode for Probe activities, or
there was a problem with commanding on-board the spacecraft.
During the cruise phase, the average data rate for CAPS was low. If
at all possible, the instrument was set to cycle between our lowest
rate in order to get contiguous data) and a higher rate (for higher
resolution data). The combined high & low rate were designed to give
us the average low rate data. This scheme did not change as we
entered Approach Science and Tour, but the average data rate was
increased. In addition to running in survey mode, we also acquired
data in higher resolution modes around Magnetospheric boundaries,
Titan flybys, icy satellite flybys, ring & satellite campaigns, and
distant torus observations. In addition, any data volume allocation
that was not allocated to other instruments was distributed to those
who could take advantage of the increase in data volume (and hence
data resolution).
At the maximum CAPS telemetry rate of 16 kbits/s all data products
coming out of the science and calibration modes can be accommodated
without the need for compression. The exception is the compressed
extraction of ions by the SAM algorithm, and a semi-logarithmic
collapse of all data words. The collapse replaces 16-bit data words
stored in the DPU with 8-bit data words to be returned to Earth. For
small data numbers, the 8-bit values are equal to the 16-bit values,
but for higher values the scale is logarithmic. A similar 32- to
16-bit compression is used for TOF data. As a result of this
compression, the uncertainty in the higher data numbers is
roughly +/- 0.015N rather than the statistical +/- N^(1/2). No
attempt was made to carry out more exotic on-board compression
routines such as moment calculations or image-like compressions, with
the exception of a run-length compression of the sparse, IBS data.
The contents of the CAPS data products at 16 kbits/s are distributed
among the three sensors, the ACT and housekeeping channels as
indicated below. Data products are organized along A-cycle (32.0 s)
boundaries. Acquisition and formation of B-cycle data products is
more complex than the A-cycle process: The CPU2 extracts TOF data in
the form of 512 channels each of ST and LEF data. In the default
mode, adjacent energy steps are sampled to produce
2RES x 32E x 512TOF = 32,768 words. In the standard CAPS telemetry
mode of 16 kbits/s, each word of B-cycle data is summed over 8
A-cycles, whereas for some lower rate modes it is summed over 16
A-cycles (i.e., the B-cycle is 8 A-cycles long for the 16 kbits/s mode
and 16 A-cycles long for these other modes). In the
very early CAPS data, there was even a mode where it is summed over 32
A-cycles. Most CAPS data products are generated at lower data rates
by collapsing (summing) the 16 kbps data over adjacent energy,
elevation and/or azimuth bins. In addition, snapshots (uncollapsed
subsets of the 16 kbps data) may be included. The subset of data
included in the snapshot can be determined on the spacecraft, so that
the snapshot contains the peak of the velocity distribution. Leaving
out certain products entirely produces the smallest possible datasets.
The modes used by the CAPS instrument were revised prior to the
Cassini Jupiter encounter to include 0.25, 0.5, and 1 kbps rates, in
addition to the original 2 and 16 kbps modes. These modes were further
revised before reaching Saturn, to add 4 and 8 kbps modes and
incorporate experience from analysis of the Jupiter data. Other data
products that can be included as options (at the expense of sensor
data) are memory readout of control tables for diagnostic purposes.
Sequential event data that are used to verify IMS operations can also
be included.
Telemetry products in 16 kbits/s mode
--------------------------------------
Data channels
-----------------------
Product EQ EL AZ MQ LOG TOF
---------------------------------------
ELS 63 8 16 . . .
IBS 255 3 16 . . .
IMS ION 63 8 8 7 . .
IMS TDC LOG 63 . 8 . 4 .
IMS TDC SNG 63 8 8 . . .
IMS TOF LEF 32 . . . . 512
IMS TOF ST 32 . . . . 512
ACTUATOR 32 position samples
HOUSEKEEPING 164 bytes of data
---------------------------------------
EQ=energy/charge, EL=elevation,
AZ=azimuth, MQ=mass/charge, LOG=logical,
and TOF=time-of-flight
Data are collected as raw counts. For the ELS sensor, the 63 energy
steps are swept through every 2 seconds. For the IBS sensor, the 225
energy steps are swept through every 2 seconds, or 127 energy steps
are swept through every 1 second. For the IMS sensor,
the 63 energy steps are swept through every 4 seconds. To learn more
about the energy range covered by each of the sensors and any other
details regarding data acquisition, please see [YOUNGETAL2004].
Processing
==========
The uncalibrated counts are collected, collapsed, and compressed on board
the spacecraft. The data is then downlinked to the Jet Propulsion Query
Servers where it is queried from the server (in standard data unit
format, SFDU) and ingested into the CAPS oracle database and stored as
CCSDS packets. From this point, the data is collected into blocks of
CCSDS packets which comprise an A-cycle (32 seconds) of data. These
data are then decompressed into their separate sensor pieces and stored
as raw, decompressed counts in tables in the database. From this point,
archive files are generated with the raw, decompressed counts, into
uncalibrated data set files.
These calibrated level 3 files are produced solely from those
uncalibrated (level 2) files, plus the addition of the yearly PDS
archived SPICE kernels for Cassini position/orientation. These are often
slightly improved from the SPICE kernels used in the uncalibrated level 2
ANC files, as such position/orientation values are not identical, however
the difference is minute, usually equivalent to a few milliseconds.
While spacecraft position/orientation is found via SPICE, CAPS' field of
view from the spacecraft body is provided by the uncalibrated ACT files.
From the level 2 CO_CAPS_UNCALIBRATED_DS.CAT file:
> For the actuator data product (ACT), in December 2004 the position
> monitor became unreliable. Based on our understanding of the
> position at the time the monitor failed, the actuator file was
> updated with an estimated value. This estimation continued until
> TBD time, at which point the actuator was moved and began reporting
> reliable readings again. While trying to move the actuator to +90
> degrees on March 21, 2005, we again encountered a time period where
> the position was adjusted to account for the monitor reading
> unreliably. On May 26, 2005 at 10:00:00, the actuator position
> monitor was modified in the flight software to be the estimated
> position instead of the hardware position. This data is also
> corrected before being placed into the archive file to account for
> any drift in the estimated position.
Utilizing the ACT and SPICE data, the spacecraft field of view in theta
and phi is provided for every level 3 record, along with transformation
information to get to J2000 co-ordinates or Saturn-centered
radial-theta-phi.
Data
====
The data are stored in multiple data files and have been organized by
product type. Each file contains a maximum of 6 hours of data, with only
4 files being generated per day per product. These are the exact same
periods used for the level 2 uncalibrated files - where each uncalibrated
file becomes a calibrated files here. Format of the data files can be
found in the CAPS instrument archive specification [FURMANETAL2005].
The format can also be found in the .FMT files co-located with the data
files.
Coordinate Systems
==================
The data are provided in the spacecraft frame. Each record contains
transformation matrices to J2000, and from J2000 to Saturn centered RTP.
By using the first then the later one can go from the spacecraft frame to
Saturn Centered RTP. Velocity data is also provided.
Ancillary Data
==============
The idea is that all ancillary data required is given with each data
record. This includes mcp or cem voltages so that one can uses these
at such future times that calibrations are refined. Energy tables and
field of view of the sensors are provided as different dimensions of the
data.
References
========
[FURMANETAL2005] CAPS standard data products and archive volume software
interface specification, Version 1.9, JPL SIS ID: IO-AR-017, Southwest
Research Institute, San Antonio, TX 78250, 2005.
[YOUNGETAL2004] Cassini Plasma Spectrometer Investigation, Space Sci.
Rev. 114, 1-112, 2004.
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