^ UAVSAR Mounted Below NASA’s Research Aircraft (Source: http://www.jpl.nasa.gov/missions/uninhabited-aerial-vehicle-synthetic-aperture-radar-uavsar/)
Ancient Wisdom, Modern Technology
En route a stormy voyage to Britain in 1757, Benjamin Franklin observed that only two ships of the convoy were sailing smoothly. Apparently, cooks had offloaded oils from the sides of these ships. Back then, pouring olive oil to calm down rough seas was a routine practice, a practice not completely disassociated from superstition.
Some 2,500 years ago, ancient Greek philosopher and scientist Aristotle had noted the same smoothing effect of oil on water surfaces. This is precisely why radars detect oil spills in water. Backscatter i.e. reflected radar signals from oil-covered water are darker than those from normal water.
The problem is, the same mishap can create light spills 1/100th-inch thick as well as deep, fat spills. How then do you tell between a minor spill and a fat one? From up there, both look alike. Mind you, this is a million dollar question for damage control authorities have to prioritize the cleaning of the thickest, most destructive spills.
Now we have just the kind of radar, NASA’s Unmanned Aerial Vehicle Synthetic Aperture Radar (UAVSAR). And, it has just proved its mettle at the Oil on Water 2015 exercise at the abandoned Frigg Oilfield, 140miles northwest of Stavanger, Norway.
Gulf Oil Spill 2010: Blessing in Disguise for the UAVSAR
During the 2010 Deepwater Horizon Oil Spill in the Gulf of Mexico, NASA deployed the UAVSAR for damage assessment. The results were fantastic for the UAVSAR indicated its ability to distinguish between thick and thin oil films.
Now they had to confirm its accuracy. The Gulf Oil Spill did not permit such checks as scientists could only make rough estimates on the quantity of spilled oil and its flow rate. The opportunity came this year.
Norway has been hosting Oil and Water annually since the 1980s and is among the few countries that legally permit controlled oil discharge in open seas for testing new oil-spill-cleanup technologies and practicing the use of existing ones. Oil and Water 2015 released emulsions of differing thicknesses for calibrating UAVSAR data.
Technical & Operational Details
Researchers have to quickly measure crustal deformations during earthquakes, volcanoes, and other dynamic geophysical events as deformations change within seconds. Plus, they have to do this on diverse time scales ranging from seconds to decades.
This is exactly what the UAVSAR does. It specializes in:
- precision crustal deformation measurements
- vertical structure mapping of vegetated areas
- forest structure and topography measurements
UAVSAR is airborne, reconfigurable, polarimetric L-band synthetic aperture radar (SAR) with emergency response potential. It studies earth science viz. hydrology, earthquakes, volcanoes, ice, vegetation etc. by collecting repeat-track SAR data for differential interferometric measurements.
Repeat tracking involves repeated flying of an aircraft over the same area. Differences in observations are the magnitude of change. Repeat-track interferometry via air-borne platform is tougher because:
- turbulence and gusty winds hinder repeat passes
- changing crosswinds hamper the maintaining of antenna position during repeat passes
For Oil and Water 2015, participants flew the UAVSAR on C-20A, NASA’s piloted research aircraft. It is designed to be flown aboard NASA’s Gulfstream III aircraft and eventually on uninhabited aerial vehicles.
SAR combines the long-range propagation ability of radar signals with complex information processing features of modern digital electronics to provide broad-area imaging even during night and bad weather.
Deployed from aircrafts, SARs produce two-dimensional images. One dimension is range and the other is azimuth. Range is the ‘line-of-sight’ distance from the radar to the target and is perpendicular to the direction of aircraft velocity. The duration between a transmitted pulse and its received echo determines the range.
Minimum interfaces with the aircraft improve the UAVSAR’s portability. NASA added a Precision Autopilot to the Gulfstream-III to control its trajectory from a precision real-time, differential Ground Positioning System (GPS). This improves the UAVSAR’s accuracy by enabling:
- the aircraft to fly within a 10m diameter cylindrical zone
- radar antenna steering within 10 accuracy over a +200 range in azimuth
Ground Data System creates flight trajectories and loads them into the Precision Autopilot and the Automatic Radar Controller (ARC). The ARC is the primary control computer that operates automatically as well as manually via the Radar Operator Workstation (ROW).
- Control and Timing Unit (CTU) supervises the timing of all transmitted-received signals
- Digital Electronics Subsystem (DES) supplies overall timing and control signals
- RF Subsystem provides a 10MHz reference signal
- Navigation Subsystem upgrades position accuracy to 1m vis-à-vis 5-10m through real-time software and correction information
Developments in low-cost electronics will further slash SAR prices and open its applicability for budget applications.
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