warm Gulf Stream in the region adversely 

 affects ship and aircraft operations. For ships, 

 the combination of bad visibility and the prox- 

 imity of icebergs creates an obvious hazard to 

 safe navigation. Less obvious is the severe 

 limitation poor visibility places on aircraft re- 

 connaissance. Indeed, conditions suitable for 

 visual reconnaissance are rare. During the 

 iceberg season, the cloud ceiling on the north- 

 ern Grand Banks is less than 1000 ft. or 

 visibility less than 2.5 nm roughly half the time 

 (Mortsch et al., 1 985). During the period from 

 1948 through 1982, when Ice Patrol relied on 

 aerial visual reconnaissance, an aircraft was 

 stationed in Newfoundland, Canada for the 

 entire iceberg season. Patrols were flown 

 when and where visibility conditions permit- 

 ted. To maximize reconnaissance opportuni- 

 ties in such poor visibility conditions. Ice Patrol 

 sought new airborne remote sensing tech- 

 nologies able to detect and identify icebergs. 



SLAR RECONNAISSANCE 



In 1983, Ice Patrol beganto use the AN/ 

 APS-135, Side-Looking Airborne Radar 

 (SLAR), an X-band (9250 MHz), real aperture 

 surveillance radar manufactured by f^otorola. 

 It was acquired by the Coast Guard primarily 

 for the task of locating and tracking oil spills, 

 however, its usefulness to the Ice Patrol mis- 

 sion was clear. The SLAR imagery is pro- 

 duced on 9 in. dry process photographic film. 

 The image is not visible to the operator in real 

 time; the processing time is about five min- 

 utes. Gridded film is the only georegistration. 

 Field studies (Robe et al, 1985; Alfultis et al, 

 1988; and Rossiter, et al., 1985) have shown 



the SLAR to be an effective iceberg (>15 m 

 long) detector at typical Ice Patrol search 

 altitudes (6000-8000 ft)^ The ability to detect 

 smaller pieces of ice such as growlers (<1 5 m) 

 seems to be strongly dependent on sea state; 

 the larger the seas, the less likely that a 

 growler will be seen by the SLAR. How 

 effective the SLAR is at discriminating be- 

 tween icebergs and vessels is not as well 

 known. 



In the absence of visual confirmation 

 there are several ways to infer whether a 

 SLAR radar target is an iceberg or a vessel. 

 The best cue is gross target movement. If the 

 target is moving at significant speed (>1 kts), 

 it is clearly a ship. The presence of ship's 

 wake can sometimes be detected by radar, 

 also indicating the target is a ship. Radar 

 shadows (an area of no radar return on the 

 side away from the radar) can sometimes be 

 used to suggest that the target is a relatively 

 tall target, and therefore more likely to be an 

 iceberg. Finally, the intensity of the radar 

 return is used to add to the evidence that a 

 target is a ship. "Hard" targets are more likely 

 to be ships. 



Other than gross target movement, 

 none of the cues are very compelling and are 

 dependent on the experience of the film inter- 

 preter. Target identification with SLAR is 

 somewhat of an art, and the ice observers are 

 left with many ambiguous targets. The many 

 stationary fishing vessels in the Grand Banks 

 present severe identification problems. Their 

 small size and lack of substantial motion make 

 them difficult to differentiate from icebergs. 



' Because of the chronically low-visibility conditions over the Grand Banks, the Coast 

 Guard desires to fly in controlled airspace. In international oceanic airspace, 5500 ft is the 

 lowest controlled flight level. This altitude range (6000-8000 ft) has proven to optimize 

 visual and radar detection. 



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