Sandia LabNews

Sandia SAR technology seeks water on the moon


converted PNM file

In late June, a little bit of Sandia flew to the Moon — a working model of the operational heart of the Labs’ miniature synthetic aperture radar (miniSAR).

The miniSAR is an established frequent-flier on reconnaissance drones. But as a special passenger aboard NASA’s Lunar Reconnaissance Orbiter (LRO), a fundamental element of Sandia’s miniSAR technologies has become a component of the spacecraft’s miniature radio frequency imaging radar, called Mini-RF for short.

Its prime mission is to search for subsurface water-ice deposits in the moon’s craters, and collect high-resolution imagery of regions that lie permanently in shadow (see “Water on the moon — what’s the fuss?” below).

Since the LRO entered lunar orbit June 23, mission scientists have been bringing its suite of instruments online one by one.

The Sandia LRO team, which has been working on the lunar project since 2002, has found the opportunity to work with NASA to be highly motivational.

“It is a truly special accomplishment of the Sandia SAR development organizations to apply our technology and capabilities to a high-profile science mission such as NASA’s LRO,” says Dale

Dubbert (5345), the technical lead for Sandia’s piece of the LRO mission.

Steve Becker, manager of Embedded Radar Processing Dept. 5348 and program manager of the Sandia LRO effort, says Dale’s leadership was “essential and indispensable.”

“He was the glue for the entire program,” says Steve.

In addition to the Mini-RF package, the LRO is carrying six other instruments to provide scientists with detailed maps of the lunar surface and enhance understanding of the moon’s topography, lighting conditions, mineralogical composition, and natural resources. This information will be used to select safe landing sites, determine locations for future lunar outposts, and help mitigate radiation dangers to astronauts.

Aside from the Mini-RF technology demonstration and its search for subsurface water, six full-scale LRO experiments will probe the Moon’s radiation environment and map its thermal, topographical, and hydrogen distribution characteristics. For example the Lyman Alpha Mapping Project and the Lunar Reconnaissance Orbiter Camera will produce images from far-ultraviolet and visible-light/ultraviolet scans, respectively. The polar regions of the moon are the main focus of the mission because continuous access to sunlight may be possible and water ice may exist in permanently shadowed areas of the poles.

Trapped ice particles?

Of particular interest is Shackleton Crater at the Lunar south pole. At roughly 12 miles wide and more than a mile deep, Shackleton Crater is permanently shadowed. Here the temperature may be as cold as 50 to 70 kelvin (minus 223 to minus 203 Celsius) in areas that some believe may contain trapped ice particles.

During observation opportunities, LRO’s Mini-RF instrument will image both poles in two bands (S-band and X-band) with a spatial resolution down to 15 meters, covering the entire polar regions in a single 28-day window. The Mini-RF also has a supplemental list of goals that may be executed after successful completion of its primary mission. One of these supplemental goals is to conduct a spacecraft-to-spacecraft “bistatic” imaging experiment in which a Mini-RF signal would be transmitted from India’s Chandrayaan-1 (launched in 2008) and received by a sister unit on LRO. Sandia mission planners believe that analysis of the returned backscatter signal potentially could provide the most definitive remote technique for discriminating between ice and rock.

Backup communications capability

If necessary, Mini-RF could provide a backup communication capability for the entire spacecraft. This multimode capability is enabled by a Sandia-designed reconfigurable digital synthesizer and digital receiver module tailored from equivalent technology developed for the miniSAR generation of radars.

All raw data as well as processed data (including higher-order products such as mosaics) will be made available to the scientific community. This will be achieved by archiving the data to the geosciences node of the Planetary Data System (PDS).

Sandia’s participation in LRO resulted from its long-term commitment to miniaturizing synthetic aperture radar technology. The LRO SAR includes a digital waveform synthesizer and digital receiver module, which uses modern field programmable gate array (FPGA) technology from Electronic Systems Center 5300. (Sandia-designed antenna-gimbal assembly and miniaturized RF/microwave electronics, not provided for Mini-RF, are also parts of the miniSARs used in fixed wing aircraft, along with commercial off-the-shelf components.)

The Labs’ partner on the Mini-RF waveform generator/digital receiver is Raytheon, which also provided the systems antenna and analog receiver and exciter. Center 5300 also undertook system performance analysis and aided in the development of operational scenarios; Engineering Sciences Center 1500 supported the project with shock and vibration testing and model verification; Monitoring and Systems Technology Center 5700 with radar system and component level specifications and synergy with other satellite sensor technologies; and, Systems Assessment and Research Center 5900 with signal-processing algorithms.

Water on the moon — what’s the fuss?

Why the fuss over water on the moon? Because, if astronauts are ever to return to Earth’s natural satellite, they will need water for an extended stay. LRO is expected to provide fundamental data to enable a human return to the moon, and may in fact be part of a new “race to the moon” involving the US, Japan, China, and India. The Japanese and Chinese launched separate water-seeking lunar spacecraft in the fall of 2007; India’s Chandrayaan took off in October 2008.

In addition to its critical function as a source of water for human visitors, lunar water could be broken down into hydrogen and oxygen for use as rocket fuel and breathable air. Even sufficient concentrations of hydrogen by itself would be valuable because it could be used as fuel or combined with oxygen from the soil to make water. Taking water to the moon for human use might prove prohibitively expensive, so the prospect of finding “cheap” water on-site is more than seductive.

Some critics think the idea of finding frozen water on the moon is no more than wishful thinking. However, previous probes have provided tantalizing hints that there may indeed be water on the moon. Clementine, a small probe launched by NASA and DoD in 1994, picked up radio echoes that may have indicated the presence of ice. Then in 1998 NASA’s Lunar Prospector mission detected hydrogen, a component of ice, in a neutron backscattering experiment. Ironically, since it is so close to the Sun, the planet Mercury has so far produced the strongest radar indications of fully formed extraterrestrial water ice. (In 2002, hydrogen that could indicate water-ice was identified just below Mars’ surface by both a neutron spectrometer and a gamma-ray spectrometer aboard NASA’s Mars Odyssey orbiter. Last year, tests aboard NASA’s Phoenix Mars Lander identified water in a soil sample.)