William T. K. Johnson

The lakes of Titan

The surface of Saturn’s haze-shrouded moon Titan has long been proposed to have oceans or lakes, on the basis of the stability of liquid methane at the surface. Initial visible and radar imaging failed to find any evidence of an ocean, although abundant evidence was found that flowing liquids have existed on the surface. Here we provide definitive evidence for the presence of lakes on the surface of Titan, obtained during the Cassini Radar flyby of Titan on 22 July 2006 (T16). The radar imaging polewards of 70° north shows more than 75 circular to irregular radar-dark patches, in a region where liquid methane and ethane are expected to be abundant and stable on the surface. The radar-dark patches are interpreted as lakes on the basis of their very low radar reflectivity and morphological similarities to lakes, including associated channels and location in topographic depressions. Some of the lakes do not completely fill the depressions in which they lie, and apparently dry depressions are present. We interpret this to indicate that lakes are present in a number of states, including partly dry and liquid-filled. These northern-hemisphere lakes constitute the strongest evidence yet that a condensable-liquid hydrological cycle is active in Titan’s surface and atmosphere, in which the lakes are filled through rainfall and/or intersection with the subsurface ‘liquid methane’ table.

Radar Soundings of the Subsurface of Mars

The martian subsurface has been probed to kilometer depths by the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument aboard the Mars Express orbiter. Signals penetrate the polar layered deposits, probably imaging the base of the deposits. Data from the northern lowlands of Chryse Planitia have revealed a shallowly buried quasi-circular structure about 250 kilometers in diameter that is interpreted to be an impact basin. In addition, a planar reflector associated with the basin structure may indicate the presence of a low-loss deposit that is more than 1 kilometer thick.

Block adaptive quantization of Magellan SAR data

A report is presented on a data compression scheme that will be used to reduce the SAR (synthetic-aperture radar) data rate on the US NASA Magellan (MGN) mission to Venus. The MGN spacecraft has only one scientific instrument, namely, a radar system that is used for imaging the surface. for altimetric profiling of the planet topography, and for measuring radiation of the planet surface. A straightforward implementation of the scientific requirements of the mission results in a data rate higher that can be accommodated by the available system bandwidth. A data-rate-reduction scheme which includes operation of the radar in burst mode and block adaptive quantization of the SAR data is selected to satisfy the scientific requirements. >

Magellan: radar performance and data products.

The Magellan Venus orbiter carries only one scientific instrument: a 12.6-centimeter wavelength radar system shared among three data-taking modes. The synthetic-aperture mode images radar echoes from the Venus surface at a resolution of between 120 and 300 meters, depending on spacecraft altitude. In the altimetric mode, relative height measurement accuracies may approach 5 meters, depending on the terrains roughness, although orbital uncertainties place a floor of about 50 meters on the absolute uncertainty. In areas of extremely rough topography, accuracy is limited by the inherent line-of-sight radar resolution of about 88 meters. The maximum elevation observed to date, corresponding to a planetary radius of 6062 kilometers, lies within Maxwell Mons. When used as a thermal emission radiometer, the system can determine surface emissivities to an absolute accuracy of about 0.02. Mosaicked and archival digital data products will be released in compact disk (CDROM) format.

The "Myth" of the minimum SAR antenna area constraint

A design constraint traceable to the early days of spaceborne synthetic aperture radar (SAR) is known as the minimum antenna area constraint for SAR. In this paper, it is confirmed that this constraint strictly applies only to the case in which both the best possible resolution and the widest possible swath are the design goals. SAR antennas with area smaller than the constraint allows are shown to be possible, have been used on spaceborne SAR missions in the past, and should permit further, lower-cost SAR missions in the future.

Magellan imaging radar mission to Venus

The Magellan imaging radar mapping mission has collected and processed data from the spacecraft in an elliptical orbit around Venus. A brief description is given of the mission and the spacecraft, followed by a more detailed description of the radar system design, which used Earth-orbiting synthetic aperture radar (SAR) experience and several innovations in its design to operate from an orbit around another planet. The radar sensor, ground processing, and data products are described. This multimode radar is the only science instrument on the mission and has the objective of mapping at least 70% of the planet surface. It has three modes: SAR, altimetry, and passive radiometry. The radar system has produced maps of almost all of the Venusian surface with a resolution better than 600 m equivalent optical line pair, and the best resolution obtained is equivalent to less than 300 m. Some of the early radar images are shown. >

Cassini RADAR Sequence Planning and Instrument Performance

The Cassini RADAR is a multimode instrument used to map the surface of Titan, the atmosphere of Saturn, the Saturn ring system, and to explore the properties of the icy satellites. Four different active mode bandwidths and a passive radiometer mode provide a wide range of flexibility in taking measurements. The scatterometer mode is used for real aperture imaging of Titan, high-altitude (around 20 000 km) synthetic aperture imaging of Titan and Iapetus, and long range (up to 700 000 km) detection of disk integrated albedos for satellites in the Saturn system. Two SAR modes are used for high- and medium-resolution (300-1000 m) imaging of Titans surface during close flybys. A high-bandwidth altimeter mode is used for topographic profiling in selected areas with a range resolution of about 35 m. The passive radiometer mode is used to map emission from Titan, from Saturns atmosphere, from the rings, and from the icy satellites. Repeated scans with differing polarizations using both active and passive data provide data that can usefully constrain models of surface composition and structure. The radar and radiometer receivers show very good stability, and calibration observations have provided an absolute calibration good to about 1.3 dB. Relative uncertainties within a pass and between passes can be even smaller. Data are currently being processed and delivered to the planetary data system at quarterly intervals one year after being acquired.

Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS): subsurface performances evaluation

According to the Mars Express mission, the MARSIS primary scientific objectives are to map the distribution of water, both liquid and solid, in the upper pot-lions of the crust of Mars. Three secondary objectives are also defined subsurface geologic probing, surface characterization, and ionosphere sounding. In order to obtain the primary objectives the Radar Sounder design was based on the Ice/water interface and Dry/ice interface scenario: defining the material composition of the first layers and porosity and the pore filling materials. Concerning the surface, we have characterized the geometric structure in terms of a large-scale morphology, on which a small-scale geometric structure, due to rocks, is superimposed, taking into account also that recently the structure of the planets surface was described by means of fractals and in particular the new MARS surface models obtained by processing of the MOLA data. According to these models, this paper provides a description of the operational planning approach and expected performances of MARSIS.

Cassini Radio Detection and Ranging (RADAR): Earth and Venus observations

The Cassini Radio Detection and Ranging (RADAR) was operated in scatterometric and radiometric modes during the Venus 1 and Earth swingbys to verify its functionality. At Venus, only the thermal emission from the thick absorbing atmosphere was detected. At Earth both the radar echo and the microwave emission from the surface were detected and reveal ocean surface disturbances, the rough, high, and cold Andes mountains, and surface features including a small reservoir in Brazil. Instrument performance appears to be excellent.

Ground processing of Cassini RADAR imagery of Titan

The Cassini RADAR instrument onboard the Cassini Orbiter is currently collecting SAR imagery of the surface of Saturns largest Moon, Titan. This paper describes the ground processing of Cassini SAR data. We focus upon the unusual features of the data and how these features impact the processing. We exhibit a data dependent mechanism we have implemented for eliminating artifacts due to attitude and ephemeris knowledge error. Finally we describe how we trade-off SAR performance vs. area of coverage when we design our spacecraft pointing profiles.