How does IIP detect North Atlantic icebergs?

ICEBERG DETECTION

International Ice Patrol (IIP) monitors iceberg danger in the vicinity of the Grand Banks of Newfoundland and broadcasts the southeastern, southern, and southwestern limits of all known ice (LAKI). Because of frequent fog and poor visibility over the Grand Banks, IIP relies heavily on radar onboard the USCG HC-130H aircraft for iceberg reconnaissance. Since 1983, IIP's primary detection radar has been the AN/APS-135 Side-Looking Airborne Radar (SLAR). In 1993, IIP added the AN/APS-137 Forward-Looking Airborne Radar (FLAR) as an additional sensor. Our operational experience and two tests (1991 and 1993) established that the strength of the FLAR is its ability to distinguish between icebergs and ships. The field tests, however, showed that in some cases FLAR failed to detect small and medium icebergs (50m and 100m) at ranges from which SLAR had routinely detected targets. Therefore, to avoid the smaller geographic coverage of a FLAR-only-equipped aircraft, Ice Patrol uses the two radars together to form a much-improved sensor suite for iceberg reconnaissance.

SLAR RECONNAISSANCE

In 1983, Ice Patrol began to use the AN/APS-135 Side-Looking Airborne Radar—an X-band (9250 MHz), real aperture surveillance radar manufactured by Motorola—for iceberg reconnaissance. The Coast Guard had acquired SLAR primarily to locate and track oil spills, but its usefulness to the Ice Patrol mission was clear. The SLAR imagery was originally produced on 9 in. dry process photographic film. The image was not available to the operator in real time because it required approximately five minutes to process the film. Gridded film was the only geo-reference. In 2001, the Coast Guard implemented a digital user-interface for the SLAR called the Maritime Surveillance System 5000 (MSS 5000). The MSS 5000 dramatically improved the usefulness of SLAR for iceberg reconnaissance by providing real-time return, data recording/storage, and Global-Positioning-System (GPS) input for geo-reference. As a drawback, however, much of the resolution and clarity of the film image was lost through data compression for the MSS 5000’s digital display.

Field studies (Robe et al., 1985; Alfultis et al., 1988; and Rossiter, et al., 1985) have shown SLAR to be an effective iceberg (>l5 m long) detector at typical Ice Patrol search altitudes (6000-8000 ft). The ability to detect smaller pieces of ice, such as growlers (£ l5 m), seems to strongly depend on sea state. Larger seas, for example, decrease the likelihood that SLAR will detect a growler.

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 clue is gross target movement. If the target is moving at a significant speed (>l0 knots), it is clearly a ship. Sometimes radar can detect a ship’s wake, thus indicating that the target is a ship. Radar shadows (an area of no radar return on the far range side of the target) indicate that a target is relatively tall and therefore more likely to be an iceberg. Finally, the intensity of the radar return adds to the evidence that a target is a ship. "Hard" targets (solid, smooth-edged, and uniformly intense) are likely ships.

Other than gross target movement, none of the clues provide sure identification, which depends on the operator’s experience. Target identification with SLAR is somewhat of an art, so ice observers are left with many ambiguous targets. Fishing vessels, either stationary or slow moving, are hard to identify because their small size and little motion make them difficult to differentiate from icebergs.

The above SLAR image shows an iceberg.

The above SLAR image shows vessel and its wake.

Because of prevalent low visibility over the Grand Banks, the Coast Guard usually flies in controlled airspace, which in international oceanic airspace starts at 5,500 ft. Therefore, Ice Patrol flies at 6,000 to 8,000 ft because that altitude optimizes visual and radar detection. The SLAR is a poor resource for identifying a target based on its size and determining the size of an identified target. Consequently, Ice Patrol has designed its reconnaissance strategy to take maximum advantage of SLAR's all-weather capability, while also recognizing its detection and target-discrimination uncertainties. Ice Patrol searches for icebergs using a USCG HC-130H long-range surveillance aircraft that operates out of St. John's, Newfoundland, Canada for seven days every other week. It takes approximately four flight days to investigate a 120 nm swath along the entire LAKI. Daily patrols are conducted using a parallel search pattern with 30 nm track spacing and the SLAR range set at 27 nm. Thus, SLAR gets two looks at most of the search area. This 200% coverage ensures that no growlers or small icebergs are missed and enables Ice Patrol to get target movement information (course and speed for ships), which can be determined by the target's displacement between successive search legs.

The addition of SLAR tremendously improved Ice Patrol's reconnaissance efficiency by facilitating greater coverage per patrol. Unfortunately, SLAR target identification remains problematic because some track legs allow the radar to sweep an area—less than one third of the total search area—only once. In such instances, a SLAR operator cannot deduce the drift information that would aid identification. For more detailed description on the SLAR and how the IIP uses it click here.

When the Coast Guard began evaluating the AN/APS-137 Forward-Looking Airborne Radar as a search and rescue target detector, Ice Patrol recognized its potential to detect and identify icebergs. The FLAR, which was developed by Texas Instruments to detect small targets in high sea states, is an X-band air-to-surface radar capable of Inverse Synthetic Aperture Radar (ISAR) mode and seemed ideally suited for the Ice Patrol environment. It is a high-power radar that integrates long-range detection and target-imaging capabilities into one system.

The AN/APS-137 has four operating modes, three of which are for surface search (search, navigate, and periscope) and one for imaging. In the surface search modes, the radar uses a real aperture, while the imaging mode uses a synthetic aperture. The following is a brief summary of the individual modes:

1. Search mode: designed for wide-area searches.

2. Periscope mode: designed for short-range, low-altitude (less than 3000 ft) searches for small targets. The high antenna-scan rate, the radar pulse frequency and duration, and sophisticated data processing permit reduction in sea clutter and an amplification of small target return.

3. Navigate mode: wide-area search, but low antenna scan rate, which is suitable for navigation and can be used for target detection.

4. Imaging mode: The ISAR is a synthetic aperture radar mode that takes advantage of target motion relative to the antenna. In the imaging mode, the radar's antenna stops rotating and directs its radar beam at the target. While imaging, the radar processes only range data and generates a range versus a Doppler display, which shows target details, including outline and prominent features, such as king posts, exhaust stacks, etc.

FLAR RECONNAISSANCE

The FLAR automatically tracks targets and calculates target course and speed. This automatic process is far superior to the manual method of determining the location and movement of targets using repeat SLAR detections. However, this process is affected by position errors from the aircraft's inertial navigation system (INS). The errors associated with the INS are typically small and can be minimized through periodic INS updates with GPS position information during ice reconnaissance patrols.

In 1991 and 1993, Ice Patrol conducted two tests of FLAR's ability to detect icebergs (Ezman et al., 1993; Trivers and Murphy, 1994). Both tests focused solely on the FLAR navigate mode. These tests indicated that FLAR failed to detect small and medium icebergs at the same ranges from which SLAR had been highly effective. Presumably, this is due to the head-on nature of the FLAR. O'Brien et al. (1993) demonstrated that the best life-raft detection performance for FLAR was between 350° and 010°R (relative to the antenna in the aircraft’s nose) and that the performance dropped off significantly at relative bearings of greater than ± 45 degrees. All the data reported in O'Brien et al. (1993) were collected with the radar in the periscope mode and at altitudes much lower than Ice Patrol altitudes (i.e., 500 and 1500 ft).

Trivers and Murphy (1994) indicated a slight increase in FLAR iceberg-detection range with altitude and hinted at a decrease in iceberg detection-range with sea state. In a test with HU-25 radars (AN/APS-127 FLAR and AN/APS-131 SLAR), Lewandowski et al. (1989) computed much smaller FLAR-only liferaft sweep widths than SLAR-only liferaft sweep widths. This result and those of the tests in 1991 and 1993 seem to indicate that FLAR is far less efficient at poor radar reflective target detection than SLAR. Presumably, this is due in part to the spreading of FLAR power over a much larger beam width. The AN/APS135 SLAR has twice as much peak power to azimuthal beam width as the AN/APS-137 FLAR. The multiple "looks" of the FLAR does little good if the radar cannot generate enough power to get a return signal. However, Ezman et al (1993), Trivers and Murphy (1994), and operational experience demonstrate that FLAR is very discriminating, especially between icebergs and ships. No attempt was made to test the ability of FLAR to discriminate between various iceberg sizes.

COMBINED FLAR/SLAR OPERATIONS

Rather than 30 nm track spacing, Ice Patrol could rely solely on FLAR-equipped aircraft by using 15 nm track spacing. However, the shorter track spacing would result in a dramatic reduction in search area (approximately 50%), and Ice Patrol would lose SLAR’s detection capability. Consequently, IIP uses a combined SLAR-FLAR-visual reconnaissance strategy. The figure below depicts a typical Ice Patrol search pattern. Essentially, Ice Patrol relies on SLAR (and the FLAR search mode to a lesser extent) for wide-area searching and employs the ISAR mode of FLAR to positively identify as many targets as possible. Of course, Ice Patrol prefers visual confirmation, but FLAR’s image mode alone enables Ice Patrol to identify most targets. But in the event of no visual confirmation and ambiguous radar information, a patrol may choose to divert from searching and descend to facilitate visual identification.

Currently, the two radars operate independently. The FLAR and SLAR radar repeaters are located next to each other in the cargo compartment of the HC-130H, allowing for easy correlation of the radar information. This combination has been very successful and is arguably one of the best and most effective iceberg reconnaissance systems in the world.

CONCLUSION

The AN/APS-137 FLAR has proven to be a valuable addition to the combined AN/APS-135 SLAR/ visual sensor suite. This radar does not replace SLAR for iceberg detection, but its identification capability has significantly improved Ice Patrol's ability to identify targets and has made Ice Patrol's iceberg danger warnings more accurate. The two radars are not fully integrated because no combined radar-data logging system is installed on the aircraft. Because SLAR and FLAR are becoming increasingly expensive to maintain as they age, Ice Patrol does not plan to integrate the two systems. While the digital integration of FLAR and SLAR could improve Ice Patrol's reconnaissance, it would not be a prudent use of funds since there are newer, more effective systems on the market. As Ice Patrol moves forward, it continues to research means other than Coast Guard airborne radar for ice reconnaissance. Satellite-based resources, land-based radar systems, commercial reconnaissance assets, or a combination of these are all technologies that Ice Patrol might use for iceberg detection in the future.

Written by Dr. Don Murphy and Geoff Trivers (1993)
Edited and Updated by LT Scott Stoermer and MST3 William Tootle (Dec 2004)