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VEGA1/VEGA2 VENUS DATA (PDS)

NSSDCA ID: PSPA-00648

Availability: Archived at NSSDC, accessible from elsewhere

Description

This description was generated automatically using input from the Planetary Data System.

Data Set Overview ================= The balloon data set comprises time-ordered telemetry readings (internally referred to as 'level 2', entailing some interpolation and reconstruction see CRISP1990; KREMNEV1986) from the balloons released into the Venus atmosphere by VEGA 1 and VEGA 2. A 'level 3' product for each balloon is also developed, integrating these measurements with a trajectory estimated from Very Long Baseline Interferometry (VLBI - see PRESTON1986; SAGDEEV1992) measurements of the balloon position on Venus' disk measured by radio telescopes, and from a model of the balloon altitude/pressure history see CRISP1990 and DORRINGTON2013. The lander data set is a pressure and temperature profile obtained from the METEO instrument (see LINKIN1986B) on the VEGA-2 lander during its descent in the Venus atmosphere. It represents the most complete profile obtained by any Venus probe to date (the precision and altitude resolution of the Venera landers is not well-documented, and the results have not been available in tabular form; the Pioneer Venus data are reasonably well-documented and of fairly high quality, but no direct temperature measurements below 12km were obtained because of sensor failures on all four probes at that altitude). The START-TIME and STOP-TIME are parachute opening/telemetry start, and landing times, Moscow time of reception at the ground station. Parameters ========== The 'level 2' dataset (archived as VG1BL_EDR.DAT, VG2BL_EDR.DAT) comprises time, temperature, pressure, vertical wind (relative to the balloon) and cloud backscatter (nephelometer) data, as physical units together with data quality flags. The 'level 3' dataset (archived as VG1BL_RDR.DAT, VG2BL_RDR.DAT) adds the latitude, longitude from radio tracking, and the inferred zonal and meridional wind speeds. The vertical speed of the balloon is derived from the balloon measurements (W_b), as distinct from the motion of the air relative to the balloon (W_a). The lander dataset comprises time, temperature, pressure and derived altitude. Processing and Data Quality =========================== The implementation of the Vega balloon experiments was challenged by severe resource demands, and data from the Vega balloons was sent directly to Earth via a radio link whose power was limited by the lithium battery carried in the balloon gondola. The transmitter was operated for short periods (75s) with intervals in between (see KREMNEV1986), and the data were heavily compressed - specifically only the least significant 6 bits of each 12-bit measurement word were transmitted typically, with the most significant 6 bits (not expected to change rapidly) telemetered much less often. Since the physical quantities changed more rapidly than expected, the MSBs had to be sometimes estimated. Linear interpolation was used to fill the data drop-outs (e.g. data with quality set to 0). The points with data quality = 0 are not used in subsequent processing, but made the files easier to plot on the primitive computers at that time. As an example of the ambiguity introduced by the compression scheme, consider the sequence of units and tens data 4, 7, 2, 6, 7, 9, 3, 30, 6, 9, 2, 4, 6, 7, 1, 70, 8, 2, 1, 7, 9, 1, 3, 20,... Are those numbers: 14, 17, 22, 26, 27, 29, 33, 30, 36, 39, 44, 56, 67, 71, 70 .... ? Or was it some other permutation? 24, 27, 32, 26, 27, 29, 33, 30, 36, 49, 54, 66, 67, 71, 70 .... Additionally, there is a small glitch in the pointer used for the wrap-around output data buffer that lost track of where (which variable) was being actually readout on each downlink. This was determined by Lee Elson by matching data patterns. Dave Crisp then reconstructed the rest of the dataset. Fortunately, the gain of the temperature sensor was set such that it rarely changed by more than 1 MSB. The quantity, Tn, is the 'calibration' temperature recorded on the nephelometer board. This variable, which was critical for interpreting the nephelometer data, was not sent back often, but it was one of the only variables that was sent back as a full 12-bit word. Although not very useful scientifically, this variable served primarily as a sanity check for the balloon model. The outputs from the anemometers also suffered some challenges. There was some of static friction (stiction) near zero RPM and a finite response time. Examples of results from the Balloon model are included in the files Crisp_BalloonModel_VEGA1.dat and Crisp_BalloonModel_VEGA2.dat in the /extras directory. Those model data were used in the level 3 products. The relevant pressure, temperature quantities for the lander were selected from one of four temperature sensors and one of three pressure sensors, with various mounting characteristics. The data were calibrated for dynamic effects. The lander altitude is a derived quantity.

These data are available on-line from the Planetary Data System (PDS) at:

https://pds-atmospheres.nmsu.edu/PDS/data/vega_5001/

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. Ralph D. LorenzGeneral ContactJohns Hopkins Universityralph.lorenz@jhuapl.edu
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