In order to "anchor" the satellite measurements, we need to compare
them to something we know. One way to do this is by what we call
"ground truth", which is one part of the calibration process. This is
where a person on the ground (or sometimes in an airplane) makes a measurement
of the same thing the satellite is trying to measure, at the same time
the satellite is measuring it. The two answers are then compared to help
evaluate how well the satellite instrument is performing. Usually we believe
the ground truth more than the satellite, because we have more experience
making measurements on the ground and sometimes we can see what
we are measuring with the naked eye.
There are a number of ways to take ground truth measurements.
The first is what we call a "field campaign". This is where
several scientists and technicians take lots of equipment and set it up
somewhere for a short but intense period of measurement. Often they go
some place rather simple, like the middle of the Great Plains in the United
States, or an island or ship in the middle of the ocean, or an ice shelf
at one of the poles. We get a lot of information from field campaigns,
but they are expensive and only run for a short time.
Another source of ground truth is the on-going work of
the National Weather Service. They have a record of weather conditions
stretching back for over 100 years. Observations are made at regular intervals
at offices around the country. These provide a nice record but are not
necessarily taken at the same time a satellite passes over the spot. As
clouds are very changeable, things can change completely in even a few
Another option for ground truth is S'COOL.
Students at schools around the world can be involved by making an
observation within a few minutes of the time that a satellite views their
View the results
from an initial data comparison between ground truth observations and