A Coast Guard C-130 on ice patrol.
A U.S. Coast Guard C-130 on ice patrol.

Sea Ice and Iceberg Detection by SLAR

In cooperation with the U.S. Coast Guard, ETL installed its RADS (Radar Acquisition and Display System) on an HC-130H aircraft as a demonstration for International Ice Patrol personnel. RADS was slaved to, and operated in parallel with, the existing data system which used dry film technology to record its images. The radar used was the Coast Guard's AN/APS-135 X-band Side-Looking Airborne Radar (SLAR).

In February 2000 the system was flown off the coast of Newfoundland and produced the real-time images presented here. These images are screen dumps done by the operators on the plane and have not been altered or enhanced in any way.

Link to full sized figure of sea ice.
Figure 1. Light blue sea ice with open water displayed in green.
Link to full sized figure of sea ice.
Figure 2. Black and blue areas indicate open water while sea ice appears as green.
Link to full sized figure of sea ice.
Figure 3. Icebergs moving through sea ice.

Reading an Image

The images are snapshots of what the operator sees on the aircraft. New data enters the image at the top, scrolls down and eventually falls off the bottom of the screen.

The color of the images represents the strength of the radar echo per the color bar at the bottom, with weaker echoes being represented by blues and greens and stronger echoes by oranges and reds. In addition very strong echoes, those that go off the top of the red scale, are painted as black. The fact that these particular images tend to show open water as blue or green is only a coincidence, and cannot be relied upon in general, since the radar return from water is highly dependant on the sea state.

The scale at the bottom is the range in kilometers, looking out one side of the aircraft. Whether it is the left or right side can be discerned by whether the zero range point is on the left or right side of the image. There is a blind spot from about 0 to 3 km, and there may be artifacts in the image here. Along one side of the image is the time in GMT, which indicates that these images took about 3-4 minutes to acquire. Other information included is the latitude and longitude of the aircraft at the current time, which is the zero range point at the top of the image.

Sea Ice Images

The (mostly) blue areas in Figure 1 represent sea ice (ice resulting from the freezing of sea water), with the green areas representing open water.

In Figure 2, the black and dark blue areas are open water, and the green area is sea ice. The wavy lines parallel to the aircraft's motion are an artifact and caused by small variations in the plane's attitude. The image ends abruptly at the 15:21:50 time mark when the aircraft banked to make a turn.

Iceberg Image

In Figure 3, the sea is mostly covered in sea ice (yellow, orange and red) but icebergs, driven more by undersea currents than the wind, have opened up leads (channels) in the ice field. Here the open water is mostly blue or green. If you look between the 15:51:27 and 15:52:02 time marks, you will see leads with black dots in the middle, or at the upper right, which represent the icebergs (some icebergs show as deep red). There is also an iceberg in the middle of the image right on the 15:52:37 time mark.

Technical Details

The AN/APS-135 radar operates at 9250 MHz and has a pulse width of 200 ns, which gives a 30-meter resolution. This is a simple non-Doppler radar. RADS is normally used on Doppler radars with polarization diversity. The images were produced from 768 range gates spaced at 200 ns.