Viking 1 Orbiter Experiments
Orbiter Imaging
The Viking visual imaging subsystem (VIS) consisted of twin high-resolution, slow-scan television framing cameras mounted on the scan platform of each orbiter with the optical axes offset by 1.38 deg. Each of the two identical cameras on each orbiter had a 475-mm focal length telescope; a 37-mm diameter vidicon, the central section of which was scanned in a raster format of 1056 lines by 1182 samples; and six color filters to restrict the spectral bandpass of an image to limited portions of the near-visual response characteristics. Each field of view was 1.54 deg x 1.69 deg with each picture element (pixel) subtending 25 microradians. The slight offset of the optical axes and the alternate shuttering mode of operation (the interval between frames being 4.48 s) provided overlapping, wide-swath coverage of the surface. Individual images are identified by picture number (PICNO), which is a unique identifier of the scene. Elements of the PICNO are as follows: the first three digits denote the revolution (REV) during which the image was shuttered; the letter A is Viking Orbiter 1, B is Viking Orbiter 2; and the last two digits are the frame number. Operation of this experiment was terminated on August 7, 1980.
Infrared Thermal Mapper (IRTM)
The purpose of the IRTM experiment was to measure the temperatures of the atmosphere and areas on the surface of Mars. The amount of sunlight reflected by the planet was also measured. The IRTM was a multichannel radiometer mounted on the orbiter's scan platform. Four small telescopes, each with seven infrared detectors, were aimed parallel to the visual imaging optical axis, and made observations every 1.12 s. The instrument was capable of measuring differences of 1 C throughout a temperature range of -130 deg C to +57 deg C. The field of view was circular, 5 milliradians in diameter. Operation of this experiment was terminated on August 7, 1980.
Mars Atmospheric Water Detector (MAWD)
The MAWD used an infrared grating spectrometer mounted on the orbiter scan platform that was boresighted with the television cameras and the IRTM. The instrument measured solar infrared radiation reflected from the surface through the atmosphere to the spacecraft. Spectral intervals were selected coincident with the wavelength of water-vapor absorption lines in the 1.4-micrometer band. The quantity of water vapor along the line of sight was measured from 1 to 100 micrometers of precipitable water with an accuracy of 5 % or better. The instantaneous field of view of the instrument was 2 x 17 milliradians, and a stepping mirror rotated the line of sight through 15 positions to provide a roughly rectangular field of view of 17 x 31 milliradians. Operation of this experiment was terminated on August 7, 1980.
Orbiter Radio Science
There are four distinct sets of Viking radio science data, three using orbiter data and one primarily using lander data with calibrations from orbiter data. The orbiter tracking data, obtained from the two-way orbiter-earth S-band and X-band radio links, consist of Doppler frequencies and time-of-flight range measurements. These determined the position and motion of the orbiters, and can be used to study the Mars gravitational field, the plasma in interplanetary space, and the structure of the solar corona. The occultation data were obtained from these same radio links by analog recording of the signal when a spacecraft was passing into or out of occultation with Mars. The data can be used to produce altitude profiles of the temperature, density, and pressure of the atmosphere (including the ionosphere) and to measure the radius of the planet using a large number of surface points. The surface-properties aspect of this investigation utilized the UHF (381 MHz) signal on which the landers transmitted data to the orbiters. At the beginning or end of a data transmission session, when the orbiter was near the lander's horizon, the strength of the received signal was recorded as a function of time. These signal "fading patterns," resulting from interaction of the radio waves with the Martian surface, contain information about the physical properties of the surface near the landers. The lander tracking data from the two-way direct lander-earth S-band links permit determination of the location of the landers and studies of the motion of the planet. Operation of this experiment was terminated on August 7, 1980.