UVI Key Parameters

 

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UVI KPGS Notes

"Early on, it was recognized that, in order to accomplish the ISTP scientific goals, there would need to exist the ability to quickly survey, in a coordinated and ongoing fashion, the vast array of scientific data being gathered by the instrumentation. Such a timely survey should reveal scientifically interesting phenomena (events) almost immediately, and should be of great value during the scientific campaigns. Thus, the concept of Key Parameters (KPs) was born. In general, KPs are low resolution time series (on the order of 1 min) computed from each instrument.... In general, each KP data set covers one complete day."
- Mish, W. H, J. L. Green, M. G. Reph, and M. Peredo, "ISTP Science Data Systems and Products," Space Sci. Rev., v71,
pp.822-823, 1995

DISCLAIMER: UVI Key Parameters are presented as summary data only and are governed by the ISTP rules for data usage. In some cases parameters such as pointing and calibration may not be adequate to support quantitative scientific analysis. For this reason, they are not to be used for presentation, publication, or analysis. Please contact us to obtain data for these purposes.

ISTP Key Parameters are generated and provided by the ISTP Central Data Handling Facility. In addition, some ISTP Key Parameters are available online.

The UVI Key Parameter Generation Software (KPGS) is designed to produce background - corrected, calibrated images with pointing information on a regular basis. The actual output period of the KPGS is a function of data quality and availability but has a typical period of one output record every 5 minutes.

The UVI KPGS produces output only for normal instrument mode when the spacecraft is in 9.2 second science telemetry mode. Thus no Key Parameters (KPs) are produced when the spacecraft is in maneuver telemetry mode or when UVI is in star (fast) mode. In addition, no KPs are produced when the despun platform is pointed away from the earth since no auroral imaging is possible at these times.

The structure of the output file is given by the UVI KPGS Skeleton Table .
A postscript version of the UVI KPGS Design Document is also available.

A list of KPGS versions is available, briefly detailing differences between different versions of the UVI KPGS.

Key Parameters

Time Parameters
Flags
Instrument Parameters
Pointing Parameters
Image Data

Time Parameters

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Each output record is tagged by a time corresponding to the center of the image exposure time. Begin and end of exposure are given relative the center times.

Epoch
Required by ISTP Standard. Number of milliseconds since 01-Jan-0000. Doesn't mean much to people but computers love it. Many utilities can break this down to dd-mmm-yyyy hh:mm:ss.ccc.
Time_PB5
Required by ISTP Standard. Three-element vector containing year, day of year, and millisecond of day. This is for non-computer types who don't want to decode Epoch.
IMG_MINUS_MSEC
IMG_PLUS_MSEC
These define the time of the observation. Epoch and Time_PB5 are as defined by the project and refer to the _middle_ of the image. Image exposure begins at Epoch - IMG_MINUS_MSEC and ends at Epoch + IMG_PLUS_MSEC. IMG_MINUS_MSEC and IMG_PLUS_MSEC are in milliseconds.
NOMINAL_OUTPUT_PERIOD
This is the requested time between output records, in minutes. Note that the actual time between output records will vary as a function of the operating modes and the quality of the telemetry. A gap condition is declared if a period of time equal to twice NOMINAL_OUTPUT_PERIOD passes without an output record. The cause of the gap is indicated by POST_GAP_FLAG.

Flags

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KPGS_VERSION
The version number of the software that produced the Key Parameters. Software updates reflect bug fixes and performance enchancements.
POST_GAP_FLAG
Required by ISTP Standard. A gap is defined if the time between records is greater than twice the nominal output time (NOMINAL_OUTPUT_PERIOD). The following values are defined for POST_GAP_FLAG:
  • O = No Gap
  • 1 = Wrong Mode. This includes both instrument and spacecraft telemetry mode. The UVI KPGS process data for only normal instrument mode transmitted in 9.2 second telemetry mode.
  • 2 = Missing Data
  • 3 = Noisy Data. Not trapped by the UVI KPGS.
  • 4-9 = undefined
  • 10 = High voltage not enabled. No image can be produced when the high voltage is disabled.
  • 11 = Instrument gain set to 0. No image can be produced when the gain is zeroed.
  • 12 = The first minor frame containing UVI housekeeping was zero filled. The first minor frame contains critical parameters used to process the image data. The data cannot be processed if the first minor frame is corrupted.
  • 13 = Unable to sync with telemetry stream.
  • 14 = No background images were present. Image will be processed without background subtraction. This will seriously comprimise the validity of the intensity calibration and is flagged by QUALITY_FLAG.
  • 15 = Requested filter setting was not present. Once a gap has been declared the current image is output, regardless of the requested filter setting.
  • 16 = Spacecraft was at lower altitudes where image quality is limited due to noise from passage through the radiation belts.
  • 17 = The despun platform was pointed away from the earth, preventing images of the auroral region.
  • 18 = Image data lay outside requested process window. This is a debug mode and should not be seen in normal production data.
  • 19 = Unknown.

Note that long gaps may be caused by multiple events. POST_GAP_FLAG attempts to represent the most severe event contributing to the gap.

Also, since each image frame typically requires a minimum of 4 major frames (36.8 s) very short values of NOMINAL_OUTPUT_PERIOD on the order of 1 minute may always encounter a gap since the desired images will be several minutes apart. This condition is not trapped and will result in an unknown value for the post gap flag.

The utility GAPdecode from the UVI Utilities page can be used to decode the gap values.

QUALITY_FLAG
Required by ISTP Standard. QUALITY_FLAG is a bit-mapped flag in which each bit corresponds to a single quality condition. The most significant bit (minus sign) is not used. Consequently up to 31 different quality conditions can be simultaneously flagged. The flags are ordered in severity with increasing bit position. The following _hexadecimal_ values have been defined for QUALITY_FLAG:
  • 0x = No errors or quality conditions.
  • 1x = An error occurred writing an SFDU comment.
  • 2x = Image time was outside of selected processing window. This indicates a debug condition and should not be seen for production data.
  • 4x = Some level zero minor frames had fill values. This can result in single pixel errors (streaks, dropouts, spikes, etc.). If housekeeping values are lost some of the image information may be lost.
  • 8x = Some level zero minor frames had sync errors. This can result in single pixel errors (streaks, dropouts, spikes, etc.). If housekeeping values are lost some of the image information may be lost.
  • 10x = The image single frame integration period could not be determined due to bad telemetry and was assumed to be 4 major frames. This is the typical integration period but may be too long by one or two major frames following a mechanism movement.
  • 20x = The despun platform was in motion or had not settled down from a motion. The image may be blurred or may not reflect the higest spatial resolution possible with the instrument.
  • 40x = The pointing calculations have not been validated or may be unreliable for other reasons.
  • 80x = The time tags for this image may be unreliable.
  • 100x = There was an error decoding star mode data.
  • 200x = Some major frames were missing but an image could be partially reconstructed.
  • 400x = Calibration data is missing or otherwise invalid. An uncalibrated image is shown. For early mission data this may indicate that the calibration data is not yet validated and should be used with caution.
  • 800x = A background image could not be found. No background subtraction was performed. This compromises the validity of the calibration.
  • 1000x = The requested output image could not be found. An available image was output instead.

The utility QFdecode from the UVI Utilities page can be used to decode the quality flag values.

Instrument Parameters

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SYSTEM
UVI has two independent detector/electronic systems that share a common optical path.
  • +1 = primary system
  • -1 = secondary system
SEQ
UVI is capable of storing up to 50 predefined observing sequences (numbered 0 to 49). These sequences define filter, gain, and integration settings. The sequence definitions are not fixed and can be expected to change considerably in the early mission lifetime. It is anticipated that the sequence definitions will become more stable as the mission progresses.
MODE
UVI has three operating modes.
  • 1 = Normal
  • 2 = Miniframe (Star)
  • 3 = Idle

Normal mode produces one 200 x 228 image typically every 4 major frames. Star mode produces multiple miniframe images every major frame. Idle mode produces no image output.

GAIN
UVI has 16 gain settings with 16 being the most sensitive. In addition, a gain setting of 0 indicates the high voltage to the detector is off.
DOOR
The entrance aperture is covered by a door which is closed at launch and before thruster firings to prevent contamination of the internal optics and mechanisms. The door is fitted with a MgF2 window to allow observations with the door closed.
  • +1 = open
  • -1 = closed
FILTER
The UVI KPGS may output one of five intensity images (one for each filter position). This parameter specifies which filter is used.
  • 1 = shutter (not used for KPGS)
  • 2 = OI 1304
  • 3 = OI 1356
  • 4 = LBH short
  • 5 = LBH long
  • 6 = solar

Pointing parameters

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GCI_POSITION
Spacecraft position vector (km) in GCI coordinates.
ATTITUDE
The (normalized) spacecraft attitude vector in GCI coordinates. This is a vector pointing along the spacecraft spin axis and corresponds to the 'up' direction in UVI images.
GCI_SUN
Sun position vector in GCI coordinates.
GCI_LOOK_DIR
A normalized vector in GCI coordinates pointing along the center of the UVI field of view.
DSP_ANGLE
Nadir offset angle of the despun pointing platform in degrees. Positive offset is in a direction _opposite_ to the spacecraft rotation.
GEODETIC_LAT
GEODETIC_LON
For each pixel in the image data, pointing information is calculated in the form of geodetic latitude and longitude. These are written to two output images: latitude and longitude, respectively. To save space only a subset of the pixels is written to the UVI Key Parameter file. The pointing data images are sparse matrices representing every 10 pixels of the image data.

Image Data

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IMAGE_DATA
This is the requested image data. The image is background subtracted and calibrated to radiance values (photon cm^-2 cm^-1). The data is a rectangular two-dimensional array (228 rows x 200 columns). Only a circular region of the rectangular array contains valid image data. The corners of the image (non-valid image data) are filled with a fixed fill value (CORNER_FILL = -127).

KPGS Versions

v1.0
Delivered 10/14/95. Development version - not for public release.
v1.1
Delivered 2/21/96. Development version - not for public release.
v1.2
Delivered 3/27/96. Development version - not for public release.
v1.3
Delivered 4/23/96. Development version - not for public release.
v1.4
Delivered 5/20/96. Development version - not for public release.
This is the first version with the key parameter KPGS_VERSION. This is also the first version to show brightnesses in units of photon flux.
v1.5
Delivered 6/19/96. FIRST VERSION CERTIFIED FOR PUBLIC RELEASE. Includes 3 sigma noise spike removal, clipping of negative values to zero, and limits on output frames with short integration periods (
v1.6
Delivered 10/24/96. This version included the addition of a saved, generic, background image for use with operating modes that do not include background images.
v1.7
Delivered 11/5/96. This version is a bug fix to v1.6 that prevents output of partial images. THIS IS THE LOWEST ACCEPTABLE VERSION FOR DATA BETWEEN 8/1/96 and 3/30/97.
v1.8
Delivered 5/2/97. This is a bug fix to a problem that first appeared on 3/31/97 due to an unexpected bit pattern that occurred during normal door operations. THIS IS THE LOWEST ACCEPTABLE VERSION FOR DATA FROM 3/31/97.
v1.9
Delivered 12/23/98. This is a bug fix to a problem that first appeared on 11/16/98 due to a change in UVI operations that produced significant viewing periods without background images. A scalar background subtraction is now performed. THIS IS THE LOWEST ACCEPTABLE VERSION FOR DATA FROM 1/1/99.

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Questions or comments? / G. A. Germany / University of Alabama in Huntsville