The DATA directory contains two subdirectories, each of which contains subdirectories of data files: LOW_RES contains low resolution data. HIGH_RES contains high resolution data. DATA -| | - HIGH_RES (contains high resolution data in ASCII files, | 1 orbit per file, in 100-orbit subdirectories) | | - LOW_RES (contains low resolution data in ASCII files, | 1 orbit per file, in 100-orbit subdirectories) Documents describing the PVO ion detector instruments and their intercalibration can be found under the DOCUMENT directory at the root level of this disk. The subdirectories of DOCUMENT, and the information which they contain, are described below. BRACE Appendix A from Brace and Kliore (Space Science Reviews, 1991) which describes some of the characteristics of the PVO ion detectors and their intercalibration. GREBOWSK Appendix from Grebowsky et al (JGR, 1993) which discusses the OIMS instrument response to superthermal ions. MILLER Miller et al (Icarus, 1984) - This paper also discusses some of the characteristics of the PVO ions detectors and their intercalibration. INST Taylor et al (IEEE Trans. Geoscience and Remote Sensing, 1980). Original instrument description and initial observations paper. DATA Dataset documentation provided by the OIMS principal investigator with the data at the time it was submitted to NSSDC. MISSION Mission and Spacecraft information for both the Orbiter and probes derived from the Mission and Host catalog files. The hypertext version includes many GIF images of useful tables and figures published in Fimmel et al, 1995 (NASA SP-518, Pioneering Venus: A Planet Unveiled). The following are descriptions of the datasets on the disk. Further descriptions of the datasets, instruments, spacecraft and mission can be found in the CATALOG directory at the root level of the disk. PV-OIMS HIGH RESOLUTION DATASET The dataset has fixed length records. Each record represents one complete OIMS experiment data cycle (normally 6.3 seconds but this time can vary with the bit rate). Within each record there are 55 samples of mass and 55 samples of densities associated with those masses. The sequence of masses within a record normally will follow the OIMS "explore" mode, but this sequence will vary when the experiment occasionally is in the "explore-adapt" mode. To compute the time associated with a particular mass and density, the dwell time should be used, along with the time of day. The dwell time is the time interval (milliseconds) between samples. Thus, for the "nth" sample, the time (in seconds) would be computed as follows: TIME = UT + (n-1) * DWELL/1000. (where UT is time of day) The user should be aware that some of the data values have been coded. For example, a density of 9.9999e+04 indicates that a density could not be computed for that point. (This can be due to several reasons, such as telemetry interference or occurrence of a gap in data.) Also, a negative value of density represents a "flagged" data point and should be used with caution. Other flag values are described by the MISSING_CONSTANT lines in the format files (*.FMT) accompanying the data. PV-OIMS LOW RESOLUTION DATASET This dataset was provided by the principal investigators to the National Space Science Data Center. The data are from the first 2997 orbits of the Pioneer Venus Orbiter. (Not all of these orbits are included on the present disk volume.) Each data record includes the year and day of year; time (second of day); orbit number; time tag (from -1800 to +1800 in increments of 12); and the number densities (number per cc) of 12 individual positive ion species (O, O2, NO CO2, C, N, H, He, atomic oxygen 18, O2, CO, and N2). The values are derived by averaging of all measurements within the 12s of each UADS sample time. Possible error sources are described in the documentation. The values -9999.999 in the seconds-after-periapsis column, and -999999 in the density column, signify flagged (invalid or missing) data values.