Data Set Information
|
DATA_SET_NAME |
VG2 NEPTUNE IRIS 3 RDR V1.0
|
DATA_SET_ID |
VG2-N-IRIS-3-RDR-V1.0
|
NSSDC_DATA_SET_ID |
PSPA-00427
|
DATA_SET_TERSE_DESCRIPTION |
VG2 NEPTUNE IRIS RDR
|
DATA_SET_DESCRIPTION |
Data Set Overview
=================
The data set contains measurements from both the infrared
interferometer spectrometer and the broadband reflected solar
radiometer and ancillary data. The data set is ordered by time
as measured by the Flight Data System Count (FDSC). This
represents the data frame number, modulo 60. Also included is
pointing and other information on the geometry associated with
a given data record.
Each record of the data set contains a header, radiometer
observations, and interferometer observations.
The interferometer data consists of calibrated thermal emission
spectra expressed as spectral radiances in
Watt/cm**2/sr/cm**-1. The wavenumber corresponding to each
spectral radiance value is not included in the data set; the
beginning wavenumber and the constant wave number increment are
given, permitting easy calculation of the appropriate
wavenumber for each radiance. The calibrated radiances have
been obtained from the directly measured interferograms of the
planetary body, along with deep space calibration observations.
The interferograms are first symmetrized to correct for the
fact that they are not sampled at zero path difference and to
also remove the asymmetry due to residual dispersion in the
beamsplitter and compensator. The symmetrized interferograms
are then apodized using a Hamming function and are
cosine-transformed. The responsivity obtained from the deep
space measurements and knowledge of the instrument temperature
are then used to obtain calibrated radiances.
The radiometer data include a measurement integrated over the
45.6 sec required to take one interferogram and measurements
sampled 8 times during the data frame. The latter include both
high and low gain measurements. A steady target yields
identical signals for the 8 samples each of high and normal
gain data, and for the integrated data, within the resolution
of each measurement. The integrating radiometer provides the
best resolution (by a factor of ~3); however, for an accurate
integrated measurement the target must remain stable in the
field of view for an entire frame, which seldom happens.
Radiometer data are presented as Watts at the detector (wad),
which is the integral across the instrument passband of the
wavenumber-dependent power received at the detector (wad(nu)).
For a target filling the instrument field of view:
wad(nu) = Signal at instrument
* instrument factor
= Target object illumination
* target object reflectance
* instrument grasp
* instrument filter function
For the IRIS measurements, the target is illuminated by the
sun; the target object reflectance is described by the
bidirectional reflectance function; the instrument grasp is the
telescope area-solid angle product, A * omega, multiplied by an
obscuration factor, g; and the instrument filter function, t,
is the wavelength dependent instrument passband.
Integrating over the passband gives:
wad = I * (A * omega * g) * t
where I is the spectrally integrated radiance from the target,
and t is the spectrally integrated radiance as attenuated by
the instrument, normalized by the entering flux; as such, it is
independent of the normalization of the target BDRF, and can be
calculated using a normalized reflectance spectrum. The
spectral geometric albedo, as determined from groundbased
measurements was used [HANELETAL1981A].
The flux measured by the IRIS instrument is therefore:
I = WAD / (A * omega * g * t)
where:
A * omega * g = 0.02535
t (Neptune) = 0.137 +/- 0.003
t (Triton) = 0.164 +/- 0.005
t (target plate) = 0.156 +/- 0.002
For a detailed description of the data set contents, see
[PEARLETAL1992]. Scientific results of IRIS observations of
the Neptunian system are contained in [CONRATHETAL1989],
[BEZARDETAL1991], [CONRATHETAL1991A], [CONRATHETAL1991B], and
[PEARL&CONRATH1991].
Processing Level Id : 3
Software Flag : Y
Parameters
==========
Integrated Visible Radiance
---------------------------
The broadband, reflected solar radiometer signal integrated
over the 45.6 seconds that IRIS data are taken within the
48 second data frame. The quantity given is power at the
detector in Watts.
Sampling Parameter Name : TIME
Data Set Parameter Name : INTEGRATED_VISIBLE_RADIANCE
Sampling Parameter Resolution : 45.600000
Minimum Sampling Parameter : 48.000000
Maximum Sampling Parameter : 48.000000
Sampling Parameter Interval : 48.000000
Minimum Available Sampling Int : 48.000000
Data Set Parameter Unit : WATT
Sampling Parameter Unit : SECOND
Sampled Visible Radiance
------------------------
Series of 8 radiometer samples taken during a 48 second data
frame with the high gain channel. The quantity given is
power at the detector in Watts (-1.0 indicates data off the
planet).
Sampling Parameter Name : TIME
Data Set Parameter Name : SAMPLED_VISIBLE_RADIANCE
Sampling Parameter Resolution : 2.400000
Minimum Sampling Parameter : 6.000000
Maximum Sampling Parameter : 6.000000
Sampling Parameter Interval : 6.000000
Minimum Available Sampling Int : 6.000000
Data Set Parameter Unit : WATT
Sampling Parameter Unit : SECOND
Thermal Radiance
----------------
Radiance (W cm-2 ster-1) within a 4.3 cm-1 spectral interval.
Sampling Parameter Name : TIME
Data Set Parameter Name : SAMPLED_VISIBLE_RADIANCE
Sampling Parameter Resolution : 2.400000
Minimum Sampling Parameter : 6.000000
Maximum Sampling Parameter : 6.000000
Sampling Parameter Interval : 6.000000
Minimum Available Sampling Int : 6.000000
Data Set Parameter Unit : WATT/CM**2/SR/CM**-1
Sampling Parameter Unit : SECOND
Source Instrument Parameters
============================
N/A
Measurement Information
=======================
N/A
Processing
==========
Processing History
------------------
Source Data Set ID : NSSDC 77-076A-03D
Software : UNK
Product Data Set ID : VG2-N-IRIS-3-RDR-V1.0
|
DATA_SET_RELEASE_DATE |
3000-01-01T00:00:00.000Z
|
START_TIME |
1989-08-23T05:18:33.000Z
|
STOP_TIME |
1989-08-26T09:28:32.000Z
|
MISSION_NAME |
COMET SL9/JUPITER COLLISION
VOYAGER
|
MISSION_START_DATE |
1993-01-01T12:00:00.000Z
1972-07-01T12:00:00.000Z
|
MISSION_STOP_DATE |
1996-01-01T12:00:00.000Z
N/A (ongoing)
|
TARGET_NAME |
NEPTUNE
|
TARGET_TYPE |
PLANET
|
INSTRUMENT_HOST_ID |
VG2
|
INSTRUMENT_NAME |
INFRARED INTERFEROMETER SPECTROMETER AND RADIOMETER
|
INSTRUMENT_ID |
IRIS
|
INSTRUMENT_TYPE |
INFRARED SPECTROMETER
|
NODE_NAME |
Planetary Atmospheres
|
ARCHIVE_STATUS |
ARCHIVED
|
CONFIDENCE_LEVEL_NOTE |
Overview
========
In evaluating the confidence level of a given data record,
several factors should be taken into account. These include
the interferometer data quality, instrument temperature drift,
field of view smear, and pointing accuracy.
The interferometer data quality is indicated in the data sets
by the parameter REJECT_CODE. The various values of this
parameter have the following meaning:
0 = good
1 = too many spikes in interferogram
2 = missing data in interferogram
3 = zero peak in interferogram
4 = no interferogram data,
but radiometer data are available
5 = interferogram symmetrization problem
Only code 0 indicates the interferogram is usable; however, the
radiometer data may be available in the other cases.
When spectral data are selected for analysis, care must be
taken to insure that excessive motion of the field of view on
the target body has not occurred during the time the
interferogram is taken. Smearing can be checked for by noting
the differences in latitude/longitude of the Q5 points (center
of field of view) at line counts 350 and 750. An additional
check can sometimes be made by noting variations in the sampled
radiometer data during a frame. Since the total infrared
energy and the very low resolution components of the spectral
data are recorded in a brief time surrounding the interferogram
peak, even when smear occurs this information can be associated
with the pointing information at the 24 second point, as can
radiometer sample #3.
A change in detector temperature can cause a shift in the
absolute calibration of the measured infrared radiance. The
detector temperature, IR_DET_TEMP, should be within 0.1 or 0.2K
of 200K. If it is not, the spectrum can be corrected using
I(corrected) = I(uncorrected) - B(IR_DET_TEMP) + B(200) where
B(T) is the Planck radiance at temperature T.
The Noise Equivalent Spectral Radiance (NESR) provides a
measure of the random errors of the spectra, expressed in
radiometric units. It is defined as the radiance corresponding
to a signal to noise ratio of unity and represents the
one-sigma uncertainty in an individual spectrum. It is
calculated from the standard deviation of measurements taken
while the instrument is viewing deep space. Values of the NESR
at selected wave numbers are given in the following table. A
detailed listing of NESR versus wave number can be found in the
NESR files included with each data set volume.
IRIS NESR (1.E-8 W/cm**2/sr/cm**-1)
Wavenumber (cm**-1)
200 400 600 800 1000 1500 2000
V1 Jupiter 3.05 0.43 0.56 0.75 0.65 1.03 1.89
V1 Saturn 2.62 0.56 0.55 0.73 0.66 1.07 2.32
V2 Jupiter 2.84 0.60 0.80 2.02 2.50 1.79 3.57
V2 Saturn 2.93 0.49 1.07 3.54 1.84 2.94 (1)
V2 Uranus 3.84 0.70 2.63 2.29 (2)
V2 Neptune 3.34 0.88 3.48 1.79 (3)
(1) Noisy
(2) Spectra truncated at 799 cm**-1
(3) Spectra truncated at 898 cm**-1
Radiometer calibration consists of a verification of instrument
stability by repeated determinations of t(target plate), based
on observations of a diffusely scattering target plate mounted
on the spacecraft. The calibration conversion to Watts at the
detector takes into account the detector response and
electrical gains. Observations of the target plate were
carried out before and after each encounter with the exception
of after the Voyager 2 Saturn encounter when jamming of the
instrument scan platform caused the maneuver to be aborted.
The spacecraft was oriented so that an onboard, diffusely
scattering target plate was illuminated by the sun at 30
degrees from the surface normal of the plate. The plate was
then viewed by IRIS, to provide a check on the stability of the
radiometer calibration, and by the other scan platform-mounted
instruments. Each observational sequence proceeded as follows:
view deep space, view target, view deep space (occasionally an
interval is included when the IRIS field of view is only
partially on the target; this results in data with an
intermediate signal level in the data set). The difference in
signals between target and space observations is then
multiplied by the square of the spacecraft-sun distance to
provide a normalized calibration signal (the calibration
factor). The principal sources of uncertainty are possible
system nonlinearities, signal variability associated with
instrument response to changing orientation relative to the
sun, signal variability associated with sudden acquisition or
loss of the bright target, and quantization. At Jupiter and
Saturn, overall uncertainty is dominated by possible system
nonlinearities (estimated as =< 0.5%). Otherwise, fluctuations
in the deep space signal due to excursions in scan platform
pointing immediately before and after viewing the target are
significant; these arise because the instrument has a transient
response to time-varying illumination by the sun. Transients
due to abrupt acquisition of the target damp out in the first
few frames of the target observation. Files containing the
target plate observations are included with each data set
volume. The data format follows that of the standard Voyager
IRIS records. However, all of the spectral data have been set
to zero. Bad or missing integrated radiometer and normal gain
radiometer data have also been set to zero; however, bad or
missing normal gain radiometer data have been set to a constant
negative value (corresponding to -1 DN, since the normal gain
radiometer offset for Voyager 1 IRIS is equal to 0 DN). Tables
summarizing the radiometer calibrations are contained in the
calibration section of the Voyager 1 and 2 IRIS instrument
catalog templates. The pointing information provided in the
SEDRs is derived from knowledge of the spacecraft position
(determined from trajectory analysis), the spacecraft
orientation (as indicated by sun and star sensor data and by
displacements within the limit cycle), and the articulation of
the scan platform on which the instruments are mounted. When
adequate data are not available (due to downlink loss, for
example), predicted values for pointing information are used.
The quoted 3-sigma pointing uncertainty is 0.15 degrees (to be
compared with the 0.25 degree diameter of the IRIS field of
view). In addition, there are sometimes systematic errors in
the SEDR pointing values for entire data sequences or links
that take the form of approximately constant offsets in the
given field of view locations on the picture body. When high
accuracy in pointing knowledge is required, it is best to refer
directly to images obtained simultaneously with the IRIS data,
using the pointing changes between the line count 350 and 750
Q5 values to correct for spatial drift between the times of
interferogram peaks and the shuttering of images. In
correlating images and IRIS observations, note that images are
not read out during the frames in which they are shuttered.
When a single image is made during a frame, its assigned FDSC
is one decimal count (modulo 60) greater than the FDSC of that
frame. When both cameras are shuttered simultaneously (in the
imaging modes BOTSIM and BSIMAN), the narrow angle image of the
pair is read out first (with FDSC augmented by 0.01), and the
wide angle image is read out second (with FDSC augmented by
0.02). IRIS FDSCs are never augmented.
When attempting to correlate IRIS data with that from other
Voyager instruments, it may be necessary to take into account
the relative offsets of the centers of the fields of view of
the various instruments. Offsets relative to the center of the
ISS Narrow Angle camera field of view are given in the tables
below. Elevation is positive to the right within the imaging
field of view and crosselevation is positive downward. The
offsets are expressed both in degrees and in Narrow Angle
pixels.
Voyager 1:
Instrument Elevation Cross-Elevation
IRIS +0.020 deg +0.024 deg
(+37.7 pixels) (+45.3 pixels)
ISS(WA) +0.0315 deg +0.0247 deg
(+59.4 pixels) (+46.6 pixels)
UVS +0.010 deg -0.030 deg
(+18.9 pixels) (-56.6 pixels)
Voyager 2:
Instrument Elevation Cross-Elevation
IRIS +0.016 deg -0.009 deg
(+30.2 pixels) (-17.0 pixels)
ISS(WA) -0.0308 deg -0.0068 deg
(-58.1 pixels) (-12.8 pixels)
UVS 0.0 deg +0.08 deg
(0.0 pixels) (+150.9 pixels)
PPS -0.06 deg +0.003 deg
(-113.2 pixels) (+5.7 pixels)
One difference should be noted between the IRIS data sets
described here and those previously deposited in the National
Space Science Data Center (NSSDC data set I.D. Numbers:
77-084A-03A; 77-084A-03B; 77-076A-03A; 77-076A-03B;
77-076A-03C; 77-076A-03D). The spectral radiances in the
earlier data sets were listed with a wave number spacing of
1.39051 cm**-1. This represents a significant over sampling of
the data for which the apodized spectral resolution is 4.3
cm**-1. In order to make the present data set more compact,
the sampling interval for the spectral radiances was reduced to
2.15 cm**-1. This was accomplished by Fourier transforming the
original data, resampling at larger intervals, and transforming
back to the spectral space. It should be noted that the full
intrinsic information content of the data has been preserved
with this procedure, and the two forms of presentation of the
data are entirely equivalent insofar as the information content
is concerned.
|
CITATION_DESCRIPTION |
Citation TBD
|
ABSTRACT_TEXT |
The data set contains measurements from both the infrared
interferometer spectrometer and the broadband reflected solar
radiometer and ancillary data. The data set is ordered by time as
measured by the Flight Data System Count (FDSC). This represents
the data frame number, modulo 60. Also included is pointing and
other information on the geometry associated with a given data
record.
|
PRODUCER_FULL_NAME |
BARNEY J. CONRATH
|
SEARCH/ACCESS DATA |
Atmospheres Online Archives
|
|