Introduction

[1] The Magellan mission to Venus was launched from Cape Canaveral, Florida, aboard Space Shuttle Atlantis on May 4, 1989. After an interplanetary cruise of 15 months, the spacecraft arrived at Venus on August 10, 1990, and was inserted into a near-polar elliptical orbit. Following 4 weeks of in-orbit checkout, mapping operations commenced on September 15, 1990. The primary requirements of this mission were to map at least 70% of the surface at a resolution better than 300 m [Saunders et al., 1992] and to determine the global relief at a horizontal resolution of about 10 km and a vertical accuracy of 80 m or better [Ford and Pettengill, 1992]. Data at these resolutions facilitate the detailed analysis of tectonic, volcanic, eolian, and impact features. A timeline of 243 days was necessary to achieve these "prime-mission" requirements. This timeline, termed Cycle 1, is the interval in which the planet made one full rotation under the surveillance of the Magellan orbiting radar sensor.

The scientific objectives of the Magellan mission were

(1) To provide a global characterization of landforms and tectonic features.

 

(2) To distinguish and understand impact processes.

 

(3) To define and explain erosion, deposition, and chemical processes.

 

(4) To model the interior density distribution.

At the start of Magellan mapping, the spacecraft orbit was adjusted to a period of 3.259 hours. Continued repetition of the basic mapping-orbit profile during Cycle I produced image data of about 83.7% of the planet. The extended mission added two cycles of mapping (see Chapter 2), a cycle devoted to gravity measurement, and a period of aerobraking maneuvers designed to circularize the orbit.

Pre-Magellan Observations

Radar observations of the Venusian surface were first obtained in the 1960s by Earth-based radio telescopes at Goldstone, California; Haystack, Massachusetts; and Arecibo, Puerto Rico. The data provided low-resolution images ( I to 20 km) and information about the scattering properties of the surface. From 1978 to 1981, the U.S. Pioneer-Venus orbiter obtained images from about 40°N to 10°S with a resolution of about 30 km and altimeter data from about 78°N to 63°S with a footprint size of about 100 km and an altitude accuracy of about 100 m [Pettengill et al., 1980]. These data revealed that Venus consists of about 65% rolling plains, 27% lowlands, and 8% highlands [Masursky et al., 1980]. The understanding of topography, surface properties, and the tectonic evolution of Venus that was gained from these studies, and was supplemented by surface sample data from the Soviet Veneras 8 to 10 and 13 to 14, is summarized by McGill et al. [1983].

In 1983 and 1984, the Soviet Veneras 15 and 16 acquired the first orbital synthetic-aperture radar (SAR) images of the northern hemisphere. About 25% of the surface was covered at a resolution of 1 to 2 km. Corresponding altimeter coverage was obtained with a footprint of 40 to 50 km and an altitude accuracy of 50 m. These data showed the presence of tectonic features such as comical uplifts, low ridge belts, heavily deformed terrain (tesserae), pronounced circular features (coronae), and a variety of plains and impact craters [Barsukov et al., 1986].

The results of these Earth-based-imaging, orbital-mapping and radar-altimeter experiments revealed the need for global radar data coverage of the Venusian surface at orders-of magnitude higher resolution. They provided incentives for determining the objectives of the Magellan mission, designing [2] the Magellan sensor system, and planning the mission operations.

The Magellan Radar Sensor

The Magellan sensor is a single instrument capable of acquiring radar data in three different modes. In the SAR imaging and the radiometer modes, the system is connected to a 3.7-m-diameter parabolic high-gain antenna (HGA) fixed at 25 deg off nadir in a direction normal to the spacecraft trajectory. The SAR operates at a wavelength of 12.6 cm (S-band, 2.385 GHz) with horizontal parallel transmit/receive polarization (HH). This enables the pulsed microwave energy to penetrate the thick cloud cover (mostly carbon dioxide) that envelops the planet and to discriminate small-scale surface roughness. In the altimeter mode, the system is connected to a smaller, nadir-directed altimeter horn antenna (ALTA). The geometry of data acquisition is shown in Figure 1-1.

Because of the elliptical orbit, it was necessary to vary the SAR imaging geometry (look angle/incidence angle) and viewing direction so as to obtain maximum surface coverage. The antennas, however, were fixed, and this operation was accomplished by means of spacecraft attitude adjustments. Three reaction wheels mounted in the forward module of the spacecraft (one for each possible axis of rotation) provided the momentum required to achieve such adjustments. The beamwidth of the altimeter antenna was wide enough to maintain nadir-directed transmit/receive capability within the limits of these adjustments. Details of the system design are given in Saunders et al. [1990] and Johnson [1991]. Characteristics of the sensor and the orbit are listed in Table 1-1.

Burst Mode of Data Collection

Magellan radar data were obtained by a process known as the burst mode of operation, in which the sensor acquired SAR, altimeter, and radiometer data sequentially in batches several times per second. Each batch or burst period included time-domain multiplexing of transmitted SAR and altimeter pulses, received SAR and altimeter echoes, and received

.....passive microwave energy. Using the HGA, the SAR first transmitted a burst of 150 to 800 pulses, each with a length of 26.5 µs, for a period from 25 to 250 ms. Because of the long duration of this mode, the received SAR echoes were interleaved with the transmitted pulses (Figure 1-2). After the last SAR echo had been received, the sensor switched to the altimeter mode and, using the horn antenna, transmitted a burst of 17 nadir-directed pulses. All the altimeter pulses were transmitted before the first altimeter echo was received. Switching to the radiometer mode, the sensor used the HGA to receive naturally emitted energy from the planet surface for a period of 50 ms.

Mapping-Orbit Profile

Mapping started when Magellan was located above the north pole and the HGA was pointed toward the Venusian surface. Because this occurred in the descending node of the orbit, the mapping-orbit profile proceeded clockwise in a west-to-east direction (Figure 1-3). In the left-viewing geometry of Cycle 1, the HGA was pointed east of the spacecraft nadir track. Mapping continued for 37.2 min with onboard tape recording of the radar data. During this interval, a systematic sequence of preprogrammed commands controlled the spacecraft attitude, the radar pulse repetition frequency (PRF), and timing of the echo sampling. In Cycle 1, the nominal incidence angle was varied by rotating the spacecraft attitude to obtain the best resolution at a fixed signal-to-noise ratio (see Chapter 2). In all mapping cycles, the PRF and timing of the echo sampling were adjusted continuously to compensate for the changing altitude of the spacecraft, from over 2000 km in polar latitudes to about 290 km at periapsis.

Radar mapping was followed in each orbit profile by rotation of the spacecraft and alignment of the HGA to play back the recorded radar data to a Deep Space Network (DSN) station. Deep Space Communications Complexes are situated about 120 deg apart in longitude, in California (at Goldstone, 190 km northeast of Los Angeles), Australia (40 km southwest of Canberra), and Spain (60 km west of Madrid). The playback period is divided into 56.6 min before and 57.2 min after apoapsis (8458 km), with an intervening interval of 14 min for star scan and for reaction-wheel desaturation. Desaturation reduces the velocity of the reaction wheels to zero in each orbit, and thus maintains their readiness to control the rotation of the spacecraft in each succeeding orbit.

After completion of playback, the spacecraft was rotated to orient the HGA toward Venus for a renewed mapping orbit. Thus the mapping mission consisted of programmed intervals of data collection, data playback to Earth, and spacecraft housekeeping.

Because the area of new terrain observed by the sensor in equatorial latitudes is much greater than at the poles, it is possible to map high latitudes on alternating orbits with an acceptable margin of overlap. In Cycle 1, this technique was used to reduce redundancy and maximize areal coverage. Mapping started at the north pole in each alternate orbit and continued to about 57°S latitude. These swaths are termed "immediate." In the intervening orbits, mapping started at about 70°N and extended to 74°S latitude. These swaths are "delayed." An idle time of about 7 min occurred at the end of each immediate swath and the beginning of each delayed swath (Figure 1-3). The spacecraft attitude controls and the radar command sequences described above were preprogrammed and relayed at intervals to the Magellan spacecraft to satisfy mission requirements throughout the three mapping cycles.

References

- Barsukov, V. L., et al., 1986, "The geology and geomorphology of the Venus surface as revealed by the radar images obtained by Veneras 15 and 16," Proc. Lunar Planet. Sci. Conf: 16th, Part 2, J. Geophys. Res., v. 91, suppl., p. D378-D398.

- Ford, P. G., and G. H. Pettengill, 1992, "Venus topography and kilometer-scale slopes," J. Geophys. Res., v. 97, no. E8, p. 13,103-13,114.

- Johnson, W. T. K., 1991, "Magellan imaging radar mission to Venus," IEEE Proc., v. 79, p. 777-790.

- Masursky, H., W. M. Kaula, G. E. McGill, G. H. Pettengill, G. G. Schaber, and G. Schubert, 1980, "Pioneer Venus radar results: Geology from images and altimetry," J. Geophys. Res., v. 85, p. 8232-8260.

- McGill, G. E., et al., 1983, "Topography, surface properties, and tectonic evolution," in Venus, edited by D. M. Hunten, L. Colin, T. M. Donahue, and V. I. Moroz, The University of Arizona Press, Tucson, Arizona, p. 69-130.

- Pettengill, G. H., D. B. Campbell, and H. Masursky, 1980, "The surface of Venus," Sci. Am., v. 243, no. 2, p. 54-65.

- Pettengill, G. H., P. G. Ford, W. T. K. Johnson, R. K. Raney, and L. A. Soderblom, 1991, "Magellan: Radar performance and data products," Science, v. 252, p. 260 265.

- Saunders, R. S., G. H. Pettengill, R. E. Arvidson, W. L. Sjogren, W. T. K. Johnson, and L. Pieri, 1990, "The Magellan Venus radar mapping mission," J. Geophys. Res., v. 95, p. 8339-8355.

- Saunders, R. S., et al., 1992, "Magellan Mission Summary," J. Geophys. Res., v. 97, no. E8, p. 13,067-13,090.

- Young, C., editor, 1990, The Magellan Venus Explorer's Guide, Publication 90-24, Jet Propulsion Laboratory, Pasadena, California, 197 p.

 

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