Accurate continuous mapping of Arctic ice floes by airborne imaging radar has been proven feasible by team of Jet Propulsion Laboratory scientists, working jointing on National Aeronautics and Space Administration program with colleagues from the United States and Canada.
In series of flights over the Beaufort Sea north of Alaska, radar imagery from 30,000-foot altitude determined that ice floes drifted up to 40 kilometers (25 miles) in five days of mid-summer.
But more importantly, according to Dr. Charles Elachi, JPL team leader, the radar experiment showed that this type of geographic measuring could be done from an Earth-orbiting satellite such as SEASAT, which JPL will launch for NASA in the spring of 1978. The SEASAT will have onboard an imaging radar which is being developed at JPL.
The accuracy of measurement is expected to be the same-over 95 percent--even though SEASAT will be in an 800-kilometer (500-mile) high polar orbit. Working with well-defined ground points may make the radar accuracy virtually 100 percent, project scientists believe.
Dr. William Campbell of the U.S. Geological Survey and Dr. Rene Ramseier of the Canadian Department of the Environment were prominent co-investigators on Project Aidjex (Arctic Ice Dynamics Joint Experiment).
For the August, 1975 flights the JPL L-band radar was used on the NASA CV-990 aircraft from Ames Research Center, operating from Fairbanks.
Average daily ice floe drift recorded was 6.5 kilometers, roughly four miles, eastwardly and 2.9 km (1.1 miles) in southerly direction. It was even possible for the radar to determine the amount of rotation of individual ice floes induced by winds and currents. The analytical technique used in the determination of the ice motion was developed by Dr. Franz Leberl, an Austrian scientist in residence at JPL. This technique was first developed for lunar cartography using Apollo 17 radar imagery.
With the opening of the north Alaska coast to oil exploration, maintenance of ports and shipping lanes will require better knowledge of ice problems in the Artic Ocean, of which the Beaufort Sea is part. The AIDJEX missions are establishing radar's capability to differentiate between new (first-year) sea ice, older sea ice, and open water, as well as rate of drift on global basis.
Elachi's JPL colleagues, besides Dr. Leberl, include Dr. M. L. Bryan, Tom G. Farr and Elmer McMillan. Dave Billiue and Gene Samuel were contractor technicians on the project.
The team predicted satellite radar would be more accurate and convenient for mapping the top of the world. Among the reasons given:
~<~ The swath width of SEASAT radar image will be 100 km (62 miles) compared to the 12 km (7.5 miles) swath width of the aircraft radar.
~<~ Satellite radar's angle of incidence will vary only 6 degrees compared to 55-degree variation in some aircraft radar scans.
~<~ The orbit of SEASAT will be smoother than the flight path of the plane and its radar will have an internal geometry system, which will help reduce mapping error possibilities.
Earlier in April, 1975, the JPL radar flew over the Bering Sea area and studied the ice-covered tundra lakes of southwest Alaska. Elachi and Bryan reported that the equipment was able to determine whether or not most of these lakes were frozen completely to the bottom. Their colleague on this study was Dr. W. F. Weeks of the Cold Regions Research and Engineering Laboratories, Hanover, N.H.
Similar measurements of lakes in northern Alaska this spring verified the technique. The experimenters say that radar will help to determine which of the shallow lakes are suitable as year-round sources of fresh water or should be considered for the possible stocking of fish.
Another potential application is to pinpoint the lakes frozen all the way to the bottom so that they may be used as landing strips for heavy transport aircraft. This activity is expected to grow drastically in the next few years as more oilfields in north Alaska are opened for exploration.
SEASAT, the scientists predicted, will be able to provide complete radar map of Alaska in few weeks time, permitting such lakes to be mapped on continuous basis.
The JPL radar system can discern objects as small as 25 meters across, irrespective of cloud cover, sun illumination, or platform altitude. The L-band instrument operates at 1200 megahertz frequency (25 centimeter wavelength). The final outputs of the system are recorded on 70-millimeter negative film transparencies which are arranged to form mosaics for mapping purposes. Computer processing produces other data required--such as relative and absolute ice motion during the period between different flights.
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