What Is Remote Sensing?
In its broadest definition, remote sensing means collecting information about
an object without being in direct physical contact with it: learning
without touching. The most familiar kind of remote sensing is the use of
our eyes to detect light. We also use remote sensing when we hear, and when we
feel heat that radiates from a warm object.
Bats sense their environments by emitting sound waves (shown above in black).
The sound waves hit objects and are reflected back (shown above in white) to
the bats. The time it takes for the reflected sound waves to get back to the
bats indicates to them how far they are from insect prey, trees, and objects
around them. We use remote sensing when we hear in everyday situations.
When a car honks its horn, we immediately focus all our attention on it to
learn whether or not we are in danger. We also use sound waves to make medical
images (ultrasound) and to look for submarines (sonar) from a ship. Animals use sound waves in sophisticated
ways. Bats use them to find insects and to find their way through their
surroundings. When we sit near a fire,
we sense its radiant energy (heat). This is a form of remote sensing! Other
animals can sense heat even better than we do. Rattlesnakes use special organs
on their heads to detect heat radiation from small prey animals such as mice.
Understanding the Advanced Very High Resolution
Radiometer (AVHRR) Images
The satellites used for this
image are the NOAA Polar Orbiters (also known as TIROS satellites). NOAA, the
National Oceanic and Atmospheric Administration, has a very nice description
of the Polar Orbiters. The Canada Centre for Remote Sensing has a very nice
web page with details on all the NOAA polar orbiter satellites, from
NOAA-1 on to future ones. Briefly, the Advanced Very High Resolution Radiometer (AVHRR)
sensor on the NOAA Polar Orbiters scans and transmits back
to earth a narrow strip across its ground track 6 times each second. Each strip
is about 1 km wide and 1462 km long (909 miles). These scans are received as
long as the satellite is in view of the ground station and all the strips
together form an image of the earth below. The along track size of the image
depends on how long the satellite is visible from the ground station. This may
be a very short time if the satellite appears to just skim the horizon, or up
to about 15 or 16 minutes if it passes directly overhead. The AVHRR image data
has 5 channels or wavelength bands, ranging from the visible to the far
infrared. Each band shows somewhat different features as discussed in the next
section. The Satellite Imagery FAQ gives a lot of details about this
topic and pointers to other websites.
Color Composite Images: Channels 2, 1, and 3 Displayed
as Red, Green, and Blue
This color combination was selected to try to match the commonly used Landsat infrared
color images (which typically uses Landsat bands 4,
3, and 1 as red, green, and blue (wavelengths not the same as AVHRR)). The
match is not perfect but the images are similar. Vegetation areas show as red
or some variations such as orange or purple. Muddy water or shallow water shows
as green. Clear water shows as black if cold or blue if warm. Clouds appear
fairly white if warm and yellow if cold. This usually gives a good indication
of the relative cloud heights, low clouds are warm and high clouds cold (and
yellow). An unfortunate side effect of this color combination is that cold snow
appears yellow. Perhaps the best way to learn what the colors in the image mean
is to check what time of year the image was taken and look at a detailed road
map or other atlas. Urban areas often look pale blue or blue-gray. However
barren regions also sometimes show a similar color. Agricultural areas show as
a bright red or somewhat orange. Barren areas with some vegetation may appear
somewhat purple from the mixture of red and blue. Mountainous areas such as in