## Data Set Information |---------------------------------------------------------------------------- | Data Set Title | Cassini RPWS LP Continuous Current Data Collection | Data Producer Name | Dr. Jan-Erik Wahlund | Data Set DOI | https://doi.org/10.17189/08gb-td90 | Start Time | 1999-01-01T01:15:59.893 | Stop Time | 2017-09-15T10:31:47.790 | Original PDS3 ID | CO-SS/S-RPWS-3-LPCNTCUR-V1.0 (https://doi.org/10.17189/1519610) | Original Release Date | 2015-04-15 #### Data Set Citation Information Wahlund, J.-E., Kurth, W.S., and Granroth, L.J., Cassini RPWS LP Continuous Current Data Collection, NASA Planetary Data System, 2015. https://doi.org/10.17189/08gb-td90. # Cassini RPWS Langmuir Probe Continuous Current Data Collection Description Cassini RPWS Langmuir Probe continuous current data are calibrated measurements acquired at up to 20 Hz rate at fixed bias voltage during the entire Cassini mission. The data in this collection were originally released in 2015 as a PDS3 data set (DATA_SET_ID = "CO-SS/S-RPWS-3-LPCNTCUR-V1.0"). The PDS labels in this version have been migrated to comply with the PDS4 standard. The data are unchanged. ## Data Set Overview The Cassini Radio and Plasma Wave Science (RPWS) Langmuir Probe continuous current data set comprises calibrated probe current measurements acquired at up to 20 Hz rate at fixed bias voltage during the entire Cassini mission. This particular data set includes spacecraft event time, bias voltage, and measured current for each sampled data point. Data are presented in tables organized so as to have fixed-length records for ease in data handling. These can be used to infer high temporal resolution density fluctuations between standard Langmuir Probe sweeps. If related to densities determined during a sweep prior to or following the 20 Hz data, or using other electron density estimates, the relative currents can be assigned absolute densities. ### Parameters This data set comprises spacecraft event time, bias voltage, and measured current for each sampled data point that was acquired by the RPWS. ### Processing Data in this data set were processed by the use of a number of software programs which assemble segmented mini-packets in the raw telemetry packets into complete sets then de-compress the data that were compressed by one of a number of compression algorithms by the RPWS flight software onboard. The conversion from telemetry data to physical measured electric current units is made through lookup tables created by two 3rd order polynomial fits to the ground tests calibration data. The ground test calibration data are presented in Chapter 9 of the RPWSCAL document in the DOCUMENT directory. These data are calibrated using the best calibration tables and algorithms available at the time the data were archived. The continuous current data themselves consist either of 21.5s long time series of currents at the indicated bias voltage consisting of 432 samples each (20Hz), or of 624s long time series of currents consisting of 38 samples (survey, 61mHz). The time series are usually made every 24s (20Hz) each during targeted moon flybys or during inner magnetosphere crossings. In the outer magnetosphere the rate is lower with time series starting each 680s (survey). Prior to and during SOI the bias voltage was set based on the most recent sweep to use an optimum setting. Afterwards the bias was set to +11.5V, except during deep (<1200km from C/A) Titan flybys where a fixed bias of +4V was used. Some of the time series data show data spikes of external interference origin. These have not been corrected for, but can easily be filtered away by the user. There are several literature references regarding how to utilize the continuous current data specific for the RPWS Langmuir probe analysis. The Langmuir probe spherical sensor is 5 cm in diameter, and coated with (baked) TiN. ### Data The RPWS Langmuir Probe continuous current data set is given as daily ASCII tables of three columns. The first column is spacecraft event time (SCET), the second column is the set bias potential in volts, and the third column is the measured current in amps. #### Ancillary Data Ancillary data archived separately in the Cassini RPWS Ancillary bundle (urn:nasa:pds:cassini-rpws-ancillary). These data include a series of files that describe the modes of the RPWS as a function of time and provide a time-ordered listing of Instrument Expanded Block (IEB) trigger commands (the mode by which the RPWS is reconfigured). Also a detailed description of each of the modes (or IEBs) is provided. Other data which are ancillary to this data set and which are archived separately from this collection are the Navigation and Ancillary Information Facility's SPICE kernels describing the position and attitude of Cassini and various solar system bodies as a function of time. #### Coordinate System This data set consists of calibrated current time series measured by the RPWS Langmuir probe. These data are presented as detected by the sensor and are not rotated into any other coordinate system. If desired, the SPICE kernels can be used with the SPICE toolkit to convert from the spacecraft frame to virtually any frame which may be of use in analyzing these data. For some applications, the data may be considered point measurements at Cassini and may be entirely adequate with no coordinate transformations at all. However, for analysis purposes using this data set, the photo-electron cloud around the spacecraft, the photo-electron emission from the Langmuir probe itself, as well as possible ram plasma wake effects will indeed depend on the orientation of the spacecraft versus the solar and plasma ram flow directions. ## Confidence Level Overview This data set contains calibrated current measurements acquired by the Cassini RPWS Langmuir probe at 20 Hz rate for various moon flybys and inner magnetospheric crossings, and slower survey rate otherwise. Every effort has been made to ensure that all data returned to JPL from the spacecraft are included and that the calibration is accurate. The noise level of the Langmuir probe is estimated to be close to 100 pA derived from the sampled current data for negative bias voltage in thin plasma regions (\<0.1 cm\*\*-3). Calibration and other systematic errors are smaller than the noise level. In micro-meter sized dust regions the individual dust hits can be seen as spikes in the voltage data. These have not been corrected for since it contains potential science results. Furthermore, some of the current data show artificial data spikes of external interference origin. These have not been corrected either, but can easily be filtered away by the user. Wake effects may also affect the quality of the data if the probe is in the wake of the spacecraft. The measurement characteristics of this data set has otherwise a very good confidence level. ### Review The RPWS Langmuir probe continuous current data were reviewed internally by the Cassini RPWS team prior to release to the PDS. The data set has also completed PDS peer review and is certified. This version, in which the PDS metadata have been migrated to PDS4, has undergone internal PDS standards review at the PDS PPI Node. The data are unchanged from the from the previously reviewed version. ### Data Coverage and Quality All data in the stated interval are included, to the best of our knowledge and attempts to determine completeness. In general, the RPWS/LP instrument started to operate just after launch in 1997, thereafter with intermittent operation periods near Earth and Jupiter flybys until three months before Saturn Orbit Insertion (SOI). The RPWS/LP was thereafter operated more or less continuously from SOI and forward, and gave higher resolution data during each moon flyby. Data gaps are due to spacecraft anomaly resolution or simply to downlink gaps, some of which were imposed by limitations on DSN station availability. The continuous current data are only acquired in particular modes of the Langmuir probe, dependent on available telemetry and are typically scheduled during icy satellite and Titan flybys. ### Limitations Occasional RPWS Langmuir probe current measurements have interference in individual samples. The RPWS HFR receiver produced these interference signals in the LPR in certain negative bias voltage ranges. These samples can be filtered away before analysis and should not pose a problem for the user analysis that deliver densities. Thermal electron output data will usually not be available, due to spacecraft photoelectron current contamination, for number densities below 1 cm\*\*-3 and/or for temperatures above 10eV. ## References - Gurnett D.A., W.S. Kurth, D.L. Kirchner, G.B. Hospodarsky, T.F. Averkamp, P. Zarka, A. Lecacheux, R. Manning, A. Roux, P. Canu, N. Cornilleau-Wehrlin, P. Galopeau, A. Meyer, R. Bostrom, G. Gustafsson, J.-E. Wahlund, L. Aahlen, H.O. Rucker, H.P. Ladreiter, W. Macher, L.J. C. Woolliscroft, H. Alleyne, M.L. Kaiser, M.D. Desch, W.M. Farrell, C.C. Harvey, P. Louarn, P.J. Kellogg, K. Goetz, and A. Pedersen, The Cassini Radio and Plasma Wave Science Investigation, Space Sci. Rev., 114, 395-463, December 2004.