How does the spacecraft send us data?

The next interesting question that we have to ask about remote sensing images from space is "How do we get the pictures?". In the case of the Space Shuttle photos, the answer is simple -- the astronauts use regular film, which they bring back to Earth for processing. Images from spacecraft in orbit are different, in that the spacecraft is still up there even after we get the picture.

Obviously, as you are using a computer to read this text, you are aware that many images we use are digital pictures. Most of the satellite images on Virtually Hawaii are digital when they are obtained. This includes Landsat, SPOT and SIR-C radar images. These digital images can be sent back by radio as a digital signal that we pick up on via a radio antenna on Earth.

Since all parts of Virtually Hawaii show digital images, it is now possible to discuss how we can tell the computer which parts of the image should be dark, and which ones are bright. To do this, we use binary numbers, which are either 0's or 1's (think of them as a switch that is either "off" or "on", with nothing in between). Converting from our decimal system to binary numbers, 00 = 0, 01 = 1, 10 = 2, 11 = 3. Note that we cannot use decimal numbers since all computers are fussy -- they only like "on" and "off".

Let us start with a simple example of this, using an image that is made up of 8 columns by 5 rows of points (each called a "pixel"). This is shown in the figure above. In this figure, we have four shades: black, dark gray, light gray and white. We can assign the darkest point the binary number 00, dark gray as 01, light gray as 10, and the brightest part the binary number 11. In the Figure, we therefore have four pixels (B5, C4, D7 and E2) that the spacecraft says are 00. There are three dark gray pixels (B3, C2, C6 and E6) assigned the binary number 01, three light gray pixels (D3, D6 and E5) that are binary number 10, and 29 white pixels are assigned the binary number 11.

Four shades between white and black would produce images with a lot of contrast, so instead of using binary numbers between 00 and 11, spacecraft use a string of 8 binary numbers (called "8-bit data"), which can range from 00000000 to 11111111. These numbers correspond to 0 to 255 in the decimal system. With 8-bit data, we can assign the darkest point in an image to the number 00000000, and the brightest point in the image to 11111111. This produces 256 shades of gray between black and white. It is these binary numbers between 0 and 255 that the spacecraft sends back to each for each pixel in every row and column -- and it takes a computer to keep track of every number for every pixel!

We now end up with a black and white image similar to the ones we see in Page 3 of this tutorial. So, it is now time to think about what these different shades of gray mean.