NSSDCA ID: 1978-051A-06
Mission Name: Pioneer Venus OrbiterThe Cloud Photopolarimeter (OCPP) used a simplified version of the Imaging Photopolarimeter (IPP) flown on Pioneers 10 and 11 to provide low-resolution, four-color maps of the Venusian cloud cover with a high-resolution imaging capability near apocenter. The principal objective of this investigation was to determine the properties of the clouds and haze, including the vertical and horizontal distribution of the particles, cloud particle size and refractive index, the cloud-top height, and the number density of particles.
The instrument consists of a 3.7 cm Cassegrain telescope with a 15.7 cm focal length, backed by a 16 position filter wheel. The filter wheel holds diagonal reflectors and various filters that feed to one of three UV enhanced silicon photodiode detectors that send the signals to four different channels. Each detector represents one mode of the OCPP. The imaging mode has a 365-nm filter, a 0.4 x 0.45 mrad aperture field of view, and a single detector that feeds into the imaging channel. It can provide scan lines containing 1016 8-bit measurements of intensity (radiance), with a resolution of 30 km at the subspacecraft point. Approximately 3.5 hours are required to image the full planetary disk, or five full images in the roughly 18 hours of the orbit centered on apoapsis. Smaller fractions of the disk can also be imaged at higher cadence. The channel is sampled every 0.488 ms (0.610 ms low sample rate), absolute radiometric accuracy is 5%. This technique gives high photometric precision to study vertical atmospheric structure, individual features in the clouds, and accurarte tracking of cloud-level winds.
The photopolarimetry mode uses four passband filters centered at 270 nm (far ultraviolet), 365 nm (ultraviolet), 550 nm (visible), and 935 nm (near infrared), with three half-wave retarder positions on the filter wheel for each of these. The field of view is 6.5 x 8 mrad, giving a resolution of 250 - 500 km at the subsatellite point. The output from the filter wheel passes through a Wollaston prism that send two orthogonally polarized beam to two detectors, which feed their measurements to two photopolarimetry channels. The two signals are sampled simultaneously every 9.52 ms (28.6 ms low sample rate), with 11 bit a/d conversion, and an absolute radiometric accuracy of 5%. This mode measures the polarization of scattered sunlight, and the variation in polarization with phase angle and zenith angle. Physical properties of the aerosols and cloud particles can be determined from the polarization as a function of phase angle and wavelength, and the pressure at the cloud tops and vertical distribution of aerosols can be studied using the polarization as a function of zenith angle and wavelength.
The limb-scan mode uses a 690 nm filter with a 0.3 x 0.35 mrad field of view aperture and a single detector that feeds into the limb-scan channel. This mode is used when the orbiter is near periapsis and involves scans of visible light across the limb of the planet when the spacraft is near periapsis, giving a resolution of about 1 km for latitudes within 45 degrees of periapsis. The signal channel is sampled every 0.310 ms (0.397 ms low sample rate) with 8 bit conversion, absolute radiometric accuracy is 5%. These scans are used to determine the vertical structure of high-altitude haze layers.
There is a calibration lamp for periodic calibration checks. The instrument can view 0 to 145 degrees from the spacecraft spin axis, the angle can be set by ground command. It uses the 5 rpm spin of the spacecraft to scan the field of view across the planet, and the spacecraft orbital motion provides translation of the scan lines for two-dimensional images. It uses a first in/first out 8192 bit memory (3072 bit section for photopolarimetry mode). The data format is 56 bits of status information followed by 8136 bits of data. The instrument is 22 x 28 x 46 cm in size with a mass of 5 kg, and uses 5.4 W power.
Mass: 5 kg
Power (avg): 5.4 W
Questions and comments about this experiment can be directed to: Dr. Edwin V. Bell, II
Name | Role | Original Affiliation | |
---|---|---|---|
Dr. David L. Coffeen | Co-Investigator | NASA Goddard Institute for Space Studies | |
Dr. Andrew A. Lacis | Co-Investigator | NASA Goddard Institute for Space Studies | alacis@giss.nasa.gov |
Dr. Peter H. Stone | Co-Investigator | Massachusetts Institute of Technology | phstone@mit.edu |
Dr. K. Kawabata | Co-Investigator | NASA Goddard Institute for Space Studies |