Synthetic Aperture Radar (SAR)

Description

The purpose of this experiment was to obtain radar images, combined with altimetry and radiometry measurements, over the majority of Venus' surface. The radar sensor unit was contained in a single unit of overall dimensions 1.27 x 0.76 x 0.30 m. The radar system used the 3.7 m diameter high-gain antenna for synthetic aperture radar (SAR) observations and radiometry. A smaller, fan-beam antenna was used for the altimetry measurements. The antennae operated at a wavelength of 12.6 cm and shared common electronics. Between SAR bursts, pulses from the altimeter were transmitted toward the planet's surface. Radiometer data were also interleaved between the SAR bursts by having the high-gain antenna in a passive mode during which it measured the amount of radiated thermal microwave energy from the surface. Calibration of the radiometer was performed using an internal source. Thermal noise sources (e.g., the radar sensor, antenna, and cabling) were removed by making physical temperature measurements. The SAR operated at a frequency of 2.385 GHz with a pulse length of 0.0265 ms and a peak output of 325 W. In radar mode, the resolution of the instrument was about 150 m. The resolution of the altimeter was 30 m and the accuracy of the radiometer was 2 K. Mapping took place during a 37.2 minute period centering around periapsis with data transmitted during the time around apoapsis. This resulted in imaging swaths of approximately 25 km by 17,000 km which could then be combined to create a map of Venus' surface. When the radar data was combined with altimeter data, a three-dimensional surface map could be created. The radiometer data were used to determine the electrical conductivity and composition of the surface. Each mapping cycle was designed to allow the surface to rotate by one Venus day (243 Earth days or about eight months) under the spacecraft, thereby imaging a full 360 degrees of longitude in each cycle. Because both experiments required the dedicated use of the high-gain antenna, the first three cycles of operation were dedicated to the SAR with the final cycle dedicated to the gravimetry experiment. The original intent was to provide a map which covered in excess of 70% of the surface. By the end of the first mapping cycle, over 80% of the surface had been mapped. By the end of the second cycle, over 90% of the surface had been mapped at least once. By the completion of the third cycle, approximately 99% of the surface had been mapped by the SAR.