Figure 2. Typical coverage over the island of Newfoundland for a single satellite for one day.
Satellite Data
The SAR systems that are used for iceberg detection here are located at an altitude of ~800 kilometers and orbit the earth 14 times each day. In a single day, each satellite can look at one or two thick strips of Newfoundland and Labrador as seen in Figure 2. Over the course of a few days it is possible to image the whole province. The advantage with using satellite SAR is that we can get information on where icebergs are at a given instant in time. Since icebergs move so slowly and satellite images are taken so often, we can usually find most of the icebergs that are around the coast. The two satellites used are RADARSAT-1 and ENVISAT.
Radar Versus Optical
The SAR systems used for iceberg detection operate at a frequency that lets the radar signal penetrate through the atmosphere on cloudy or rainy days and at night. Since the satellites are so high in altitude, iceberg searching can continue even during storms. The satellite can even see icebergs through the fog - that comes in handy here in Newfoundland and Labrador!
Optical satellites can provide a much higher resolution with images that often look like full colour photos. Then why are we using radar to look for icebergs? Optical imaging is passive and it relies on the sun to illuminate the region of interest in order to work. That means that we would not be able to see anything at night or on days that are cloudy or foggy. Radar is an active sensor - it provides its own source of energy to illuminate an object. So we can see things regardless of the weather or environmental conditions.
RADARSAT-1 Satellite
The RADARSAT-1 satellite was developed by Canada and launched in 1995. All satellite SAR technology allows for a trade off between resolution and area. To get accurate iceberg information, the RADARSAT-1 beam mode used is the one with the highest resolution of 8 meters and the smallest image area of 50 km by 50 km. A picture of RADARSAT-1 can be found above in Figure 1.
ENVISAT Satellite
The ENVISAT satellite was launched in 2002 after being built by the European Space Agency. ENVISAT has a number of sensors onboard, but the one being used for iceberg detection is ASAR (advanced synthetic aperture radar). The radar is called advanced because it has extra imaging modes with different polarizations. Light can be polarized (that’s why you can buy polarized sunglasses) and it is a form of electromagnetic radiation. Radar uses electromagnetic waves and can be polarized too. There are two different polarizations used for iceberg detection which give separate data sets over the same area, but provide different information. It’s kind of like putting on sunglasses on a really sunny day. With the sunglasses on you can still see the same things, but it’s easier to look at the light reflecting off the water and harder to see in the shadows.
Figure 3. ENVISAT image of Exploits River in Central Newfoundland using two data channels.
The ENVISAT data used has a resolution of 25 meters and coverage area of 100 km by 100 km. Figure 3 shows the Exploits River in Newfoundland viewed with two different polarizations. There is a bridge across the river that is circled in both images. In the first image, the bridge across the river is clearly seen as a bright line but in the other polarization the bridge looks like it is not there any more. What other differences can you see between the two images?
How do we detect icebergs?
Figure 4. RADARSAT-1 image showing land, sea ice and water - the satellite can see more than just icebergs.
Everyone has seen radar blips on a screen from a ship or aircraft radar on TV or in a movie. The satellite radar works in much the same way. Radar is based on bouncing a signal off an object and looking at the echo. Instead of marking the targets on a screen, all the data is written to a file that can be viewed as an image.
A target will appear as a bright spot or region on the radar image. This means the echo is strong. Close targets and large targets will have the strongest return and will appear the brightest in the image. The exceptions to this rule are that some objects tend to absorb the radar energy or are shaped or structured in a way that reduces the amount of energy reflected back towards the radar and will not be as bright even if they are close by or large.
With satellite SAR, the distance to the targets is always about the same – ~800km, but reflections can come from anywhere – land, islands, ships, sea ice and icebergs. How do we tell what’s what? Due to the way icebergs are formed, the radar wave tends to penetrate them and so reflections occur from somewhere inside the ice as well as at the surface. The result is that the amount of energy returned to the radar is a lot lower making icebergs harder to find than other targets, such as ships.
Sea ice doesn’t come from glaciers, but is formed in cold temperatures when the surface of the ocean freezes. It isn’t as thick as icebergs usually are, or as strong since it hasn’t been compressed for thousands of years like a glacier. For more information on sea ice, visit http://ice-glaces.ec.gc.ca/App/WsvPageDsp.cfm?ID=10160&LnId=3&Lang=eng.
Figure 5. Satellite imagery of ships and an oil platform.
When looking for icebergs in satellite images, we need to filter out the land, small islands and sea ice. Figure 4 is a RADARSAT-1 image that shows some of the things that can be mistaken for icebergs. Targets due to land are easiest to remove through the use of mapping programs. As well, the bulk of sea ice is much larger than, and has a characteristic shape which is different from, icebergs. Ships, the edges of sea ice and icebergs will appear as ‘blobs’, that is, small and clearly defined bright dots, in the image. By the time icebergs arrive at Newfoundland, there is usually no sea ice left to get confused with icebergs. The only part that remains is to distinguish ships and icebergs.
How we do tell the difference between ships and icebergs?
When the radar sees a target, it has no idea if it’s looking at a ship or an iceberg. The image has to be processed to extract that kind of information. Before classification can be done, first, a lot of icebergs and ships need to be imaged and examined. By doing this, we can categorize what ships and icebergs should look like in a SAR image. In order to classify a target as a ship or an iceberg, several characteristics of the target are used such as shape, size and brightness. The values for all of these features are combined to see if the target has properties more like a ship or an iceberg. For large ships and oil platforms, the radar return is so strong that parts of the area around the ship look bright in the image making the target look like a star (see Figure 5). Iceberg targets tend to look duller, but can still be seen with the radar (see Figure 6).
Figure 6. Precision RADARSAT-1 image with close-ups and visual comparisons.
Who else is interested in finding icebergs?
Figure 7. Area monitored by satellite for shipping and offshore oil development.
Thousands of people come to Newfoundland and Labrador from all over the world just to see our icebergs. But the sight is not a vacation for everyone. Since icebergs are so large, they have the potential to cause severe damage to offshore oil platforms and ships. To make matters worse, it is time consuming, difficult and expensive to move the oil platforms and significant shipping traffic travels back and forth between Europe and North America right through the iceberg danger zone. The oil and gas companies operating on the Canadian East Coast have gone through great expense to construct platforms that can withstand the impact of an iceberg or, in extreme circumstances, can disconnect from production wells and move off to a safe distance.
In addition to their own onboard radars, oil companies use iceberg data from flight missions and satellites to find the icebergs and determine which ones are the most hazardous. The International Ice Patrol (IIP) is a branch of the United States Coast Guard responsible for informing mariners about the locations of icebergs. The IIP also use flight data along with satellite technology to help them get their job done. The area monitored by satellite for safety reasons is shown in Figure 7.
Satellite data provided by...
