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The S (2.3 GHz) and Xband (8.4 GHz) tracking links with the Viking orbiters have been used to study the atmosphere and topography of Mars at latitudes ranging from 74øS to 73øN. Data acquired in the troposphere show large meteorological changes with near-surface temperatures ranging from 150 ø to 250øK. Inversion layers were observed above the polar caps and in areas engulfed by dust storms. At other locations the temperature was found to decrease with increasing height at a rate equal to the dry adiabatic lapse rate. Seasonal pressure variations, presumably caused by changes in the polar frost deposits, were observed. At the 5-km altitude level the atmospheric pressure ranged from about 3.5 to 4.8 mbar during the Martian year. The measurements in the upper atmosphere yielded double-and single-layered electron density profiles on the sunlit and dark sides of the planet, respectively. A comparison of the Viking occultation data with earlier Mariner measurements has revealed that the temperature and plasma scale height of the ionosphere appear to be functions of solar activity. The topographic occultation data agree well with the elevation contours that are shown on U.S. Geological Survey map M 25M 3 RMC except in a few areas such as the south polar region and the Alba Patera region of Mars. In the south polar region the occultation measurements yielded surface elevations ranging from 3 to 6 km relative to the reference gravity equipotential surface or reference areoid. The terrain around Alba Patera was found to be 6 to 7 km high.• G. F. L. has officially changed his family name from Fjeldbo (as it appears in the reference list) to Lindal.When the spacecraft receiver lost phase lock during occultation by Mars, the downlink carrier frequencies were derived from a free-running crystal oscillator on board the vehicle. This operating mode was used during egress measurements and is called one-way tracking. Two-way ingress measurements provided the best occultation data, since all carrier frequencies in that tracking mode were derived from a stable earth-based reference oscillator. Both spacecraft revolved in their orbits around Mars in the same direction that the planet turned about its spin axis. At equatorial and intermediate latitudes the ingress and egressdata were therefore always acquired on the evening and morning side of the planet, respectively.The prime tracking stations involved in recording the signals from the Viking orbiters were the NASA deep space stations 14, 43, and 63, located in California, Australia, and Spain, respectively. A more detailed description of the radio instrumentation has been given in an earlier report [Michael et al., 1972]. DATA ANALYSISDuring the occultation of the Viking orbiters by the Martian atmosphere the carrier frequencies of the radio tracking links were perturbed due to refraction. As is described in our primary mission report, we utilize these frequency perturbations to determine the vertical distribution of the tropospheric gas refractivity and the ionospheric electron d...

The 1976 Mars solar conjunction resulted in complete occultations of the Viking spacecraft by the sun at solar minimum. During the conjunction period, coherent 3.5‐ and 13‐cm wavelength radio waves from the orbiters passed through the solar corona and were received with the 64‐m antennas of the NASA Deep Space Network. Data were obtained within at least 0.3 and 0.8 Rs of the photosphere at the 3.5‐ and 13‐ cm wavelengths, respectively. The data can be used to determine the plasma density integrated along the radio path, the velocity of density irregularities in the coronal plasma, and the spectrum of the density fluctuations in the plasma. Observations of integrated plasma density near the south pole of the sun generally agree with a model of the corona which has an 8:1 decrease in plasma density from the equator to the pole. Power spectra of the 3.5‐ and 13‐cm signals at a heliocentric radial distance of about 2 Rs have a ½‐power width of several hundred hertz and vary sharply with proximate geometric miss distance. Spectral broadening indicates a marked progressive increase in plasma irregularities with decreasing ray altitude at scales between about 1 and 100 km.

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Measurements of the round‐trip time of flight of radio signals transmitted from the earth to the Viking spacecraft are being analyzed to test the predictions of Einstein's theory of general relativity. According to this theory the signals will be delayed by up to ∼250 μs owing to the direct effect of solar gravity on the propagation. A very preliminary qualitative analysis of the Viking data obtained near the 1976 superior conjunction of Mars indicates agreement with the predictions to within the estimated uncertainty of 0.5%.