2: Why study the oceans from space?


Before we jump into the main subject of this presentation, it might be helpful to understand how we have learned what we know about the oceans in the past and how we are doing this today. Being an oceanographer and working for NASA has more often than not seriously confused people. My standard reply when asked what an oceanographer is doing at NASA is to say that I study the oceans from space. At that point, people generally shake their heads and look at me in a very puzzled way and ask "why do you have to launch a satellite to study the oceans? Wouldn't it just be easier to go out in a boat and look at it?". Well, for many things that we want to learn about the oceans that is indeed true. Getting 'up close and personal' with the thing that you want to study is generally the best way to go but sometimes, it is necessary to take a step back and get a better view. In this case, stepping back means to step way back, more than 700 kilometers back, and straight up into space.

Ship sampling with bongo nets neuston net picture Most of what we have learned about the oceans over the years has come from people going out to sea in boats and tossing things over the side and collecting whatever happens to get caught in their nets. Now to be honest, there are a few serious drawbacks to this. First, the ocean is a very big place and ships don't move all that quickly. Second, conditions in the ocean can change pretty quickly and that fact combined with their enormous size means that the chances of really characterizing the conditions in any given place are pretty difficult. And third, being out at sea for any length of time can be pretty uncomfortable even under the best of circumstances and most oceanographic research vessels can certainly NOT be classified as the best of circumstances. Although the conditions aboard ships and the ways that measurements are made have improved over the past 100 or more years since the Challenger expedition opened the era of descriptive oceanography, ship-based oceanographers are still generally limited to sampling things in a pretty small area with often a great deal of difficulty.

map of pacificCZCS image of Tasmania Satellites on the other hand are wonderful for looking at very large areas of the world in a very short time. This is called synoptic; being able to look at a large region at the same time. To give you one example that clearly demonstrates the differences between ship sampling and satellite observations, let's take a look at a pretty unique part of the world - Tasmania. Located off the southeast coast of the continent of Australia, the ocean around the island of Tasmania is in a constant state of turmoil. Ocean currents collide with one another around the island producing regions of intense mixing which ultimately result in very complex patterns of phytoplankton distributions which can be seen in this image of the plankton chlorophyll concentrations taken by the Coastal Zone Color Scanner (CZCS) way back in 1981. The success of the CZCS mission is what led NASA to develop the SeaWiFS Program...but more on that later.

This satellite image is composed of many individual measurements that the satellite acquired as it scanned the Earth from space. These individual samples (called pixels) are spaced approximately 1 kilometer apart. This particular image represents a portion of the Earth that is about 1000 kilometers wide by 1000 kilometers high which means that there are 1 MILLION individual satellite measurements in this single image. Now imagine that you are on that research ship steaming back and forth across the waters around Tasmania at a constant 10 kilometers per hour (a reasonable speed for a typical research ship as it takes samples) steaming east along a 1000 kilometer cruise track. When you reach the end of one row, you steam north for 1 kilometer and turn and the head west for another 1000 kilometers. You repeat this process of heading back and forth until you have matched the coverage that you see in the satellite image. However, while it took the satellite just a little more than 1 MINUTE to make all those 1 million measurements, it would take you MORE THAN 10 YEARS to make the same number of measurements from the ship.

Not only would you NOT want to spend the next 10 years of your life steaming back and forth on the waters around Tasmania, but you can easily see that it would be impossible to capture the kind of variability that you can see in that satellite image over such a large area from the traditional shipboard survey. However, the real strength in satellite measurements come when they are combined with the very detailed measurements that can only be made by sampling the ocean directly.

More Information:

  1. Remote Sensing - A Guide for Teachers - a concise introduction to remote sensing principles, from the University of Texas, El Paso, Pan-American Center for Earth & Environmental Studies.
  2. Exploring Satellite Oceanography - a set of lesson plans for high school science students from the University of Rhode Island.
  3. The Remote Sensing Core Curriculum - from the University of Maryland.

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gene carl feldman (gene@seawifs.gsfc.nasa.gov) (301) 286-9428