DATA SET OVERVIEW :  The Inbound Monochrome Image Data Record (IMIDR) is a set of rectified monochromatic images of Venus acquired by the Pioneer Venus Orbiter Ultraviolet Spectrometer (PVOUVS). Each image is accompanied by a pseudo-image of several geometric parameters such as latitude and illumination angle, so that the parameters associated with a given pixel in the image can be found in the corresponding pixels in the pseudo-images. The image size is 128 x 128 pixels. The pixel size is usually 125 x 125 km, but for some images larger pixel sizes are used. If a given pixel is sampled more than once during the acquisition of an image, the appropriate signals and geometric variables are averaged; if a pixel is not sampled, the image and pseudo-images contain zeroes.  The IMIDR provides a mission-long set of images at several far-UV and near-UV wavelengths associated with day and night airglow emissions and with reflected sunlight. All Venus phases are sampled, as are all levels of solar activity during 1978-1992.  Descriptions of the scientific utility and past research uses of this data set can be found in the following references: STEWART_ETAL_1979, STEWART_ETAL_1980, PHILLIPS_ETAL_1986B, ESPOSITO_ETAL_1988, BOUGHER_ETAL_1990, ALEXANDER_1992, and ALEXANDER_ETAL_1993.   PARAMETERS :  The PVOUVS recorded the field-of-view-averaged brightness as the spacecraft rotated on its spin axis and moved along its orbit. The brightness is reported in the images as counts / 4 msec. The length of the slit is typically several image pixels, and smearing in the north-south direction is present (it is noticeable at the southern limb). The width of the slit is usually less than one pixel.  The geometric variables associated with each image pixel depend on the sun-Venus-spacecraft geometry and on the spacecraft spin axis orientation.   PROCESSING :  The PVOUVS science data stream was screened for obvious noise spikes. The instrument configuration was at times uncertain due to problems with uplink record-keeping and with on-board command execution. When the uncertainties could not be resolved using the OUVS command plans and the downlinked science and instrument status data, the science data were discarded.  The rectification is performed as follows. The OUVS line-of-sight is offset from the spacecraft spin axis s by an angle a. Each sample is characterized by the spacecraft position vector P and the line-of-sight unit vector l. An image plane is defined whose normal z lies in the P,s plane and makes an angle 180-a with s. Two unit vectors y and x are defined in the image plane: y lies in the P,s plane and points in the northern hemisphere, and x makes up a right-handed set with y and z. The X and Y coordinates of the sample in the image plane are proportional to l.x and l.y, and its distance from the image origin is the length of the perpendicular from the planet center to the line-of-sight. Note that the unit vector y is fixed throughout the image acquisition, but x and z rotate slowly about y as the spacecraft moves along its inclined orbit.  Errors in the reported spin axis orientation s and spin rate w and in the data timing produced distorted rectifications. w could be corrected using an OUVS internal spin rate monitor. Where necessary and possible, adjustments were made to s and to the data timing to produce an undistorted rectification.   DATA :  The IMIDR consists of a set of structures, one per image, that contain an image and eight supporting pseudo-images, as follows :  II.IMG - the PVOUVS signal, expressed as counts per 4 msec II.BIN - the equivalent number of 4-msec samples in each pixel II.RP - the perpendicular distance from planet center to line-of-sight (km) II.SLAT - the colatitude measured from Venus' north orbit pole (deg) II.SLON - the hour angle measured retrograde from Venus' antisolar 	 meridian (deg) II.LZA - the local zenith angle between the Venus-centered 	 spacecraft position vector and the line-of-sight (deg) II.SZA - the illumination angle (deg) II.EMA - the emission angle (deg) II.SCA - the scattering angle (deg)  Geometric parameters are evaluated at the intersection of the line-of-sight with a Venus-centered sphere of radius 6200 km (ie, altitude 150 km), or, if the line-of-sight does not intersect this sphere, at the point of closest approach to the planet's center.   ANCILLARY DATA :  SEDR tapes supplied by the Pioneer Project were used for all ancillary data concerning Venus' orbit around the Sun, PVO's orbit around Venus, and PVO's orientation, spin rate, and spin phase. All timing information came from the EDR tapes supplied by the Pioneer Project. Information about the instrument configuration came from the EDR tapes. Where possible, the data from these various sources and the science data from the instrument were cross-checked for consistency, and decisions about validity were made. Instrument calibration information was obtained from pre-flight laboratory calibrations [STEWART_1980] and from several in-flight calibration operations using the stars Spica and Hadar.   COORDINATE SYSTEM :  The coordinate system used for the images is described in the Processing section above. The SLAT and SLON pseudo-images use a system of Venus colatitude measured from Venus' orbital north pole and hour angle measured retrograde from the plane containing Venus' orbit pole and the Sun-Venus vector.   SOFTWARE :  No software is provided with this dataset.   MEDIA/FORMAT :  The PV OUVS IMIDR will be available on compact disc write-once (CD-WO) media and on-line through the Planetary Atmospheres Node Home Page (http://atmos.nmsu.edu/). Organization and formats will be according to PDS and ISO 9660 level 1 standards.

CONFIDENCE LEVEL OVERVIEW :  The major confidence issues associated with the IMIDR have to do with the instrument calibration and configuration, specifically the grating position (ie wavelength) at which the image was acquired. The uncertainty in sensitivity is estimated to be +-30%. The instrument grating position is secure when the telemetry format included the grating position monitor; when it did not, it is judged that a combination of the command planning and an assessment of image appearance yielded a high confidence level.   DATA COVERAGE AND QUALITY :  Command planning for OUVS for most of the PVO mission called for 4 monochromatic images per week on the dayside and 7 per week on the nightside, for a total of about 4000. The actual number is substantially less, for one or more of these reasons: - Venus superior conjunctions - loss of telemetry due to spacecraft or DSN problems - intermittent failure of the OUVS grating control unit - inability to replace EDR/SEDR data lost on the ground The image set, despite these outages, cover the entire mission and provide adequate sampling in Venus phase, and solar activity (both solar rotation and solar cycle).  There are many cases where only partial images were obtained. Most of these are included in IMIDR because of their contributions to studies of long-term variations.  Image quality is affected by the following factors:  - Instrument dark counts. Normally the dark count rate was dominated by cosmic ray events in the detectors, and was about 1/sec in both channels. If the front face of the sunshade was sunlit, an extra 1.5 counts/sec appeared in the F-channel. On a small number of orbits, dark counts due to the aftereffects of solar flares either contributed noticeably to or dominated the dark count rates.  - Signal level. In all of the airglow images the effects of counting statistics are evident. Images in reflected sunlight are generally free of this effect.  - Instrument telemetry rate. The OUVS readout frequency depended on the number of words assigned to OUVS in the telemetry format and on the downlink telemetry rate. The frequency with which OUVS swaths across the planet were downlinked typically varied from about 25 to about 300 per hour, depending mostly on the Venus-Earth range.  - Instrument integration period. Images were acquired with integration periods of 8, 16, or 32 msec. The image rectification algorithm assigned all the counts in a sample to the image pixel appropriate to the center of the sampling period. Integration periods of 16 and especially 32 msec led to an undersampling of the grid of image pixels and a loss of spatial resolution in the east-west dimension.  - Instrument slit length. The length of the OUVS field of view (.024 rad) led to noticeable image smearing in the north-south dimension, more obvious at the southern limb which was viewed from ranges of 20000-50000 km.   LIMITATIONS : The IMIDR is not a complete record of the appearance of Venus at all UV wavelengths. Only selected wavelengths were studied, and many gaps in the data set exist.   ERRATA : The algorithm used to generate the IMIDR products described herein contained an error. The error rejected valid data words but introduced no bias. The OUVS instrument generated 256-word data blocks. The algorithm assigned each word to the appropriate pixel in the rectified image, and accumulated the total counts and the number of words assigned to each pixel. The final rectified image was then the ratio of the array containing accumulated counts to the array containing the number of assigned words, ie the average counts. When more than one word within a given data block was assigned to a particular pixel, however, the algorithm discarded all but the last such word. Thus, the average counts were obtained from a smaller set of words than were actually available. No systematic errors in the IMIDR products result from this problem, except that the values in the .BIN array reflect the unintended rejection of valid data words.