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 minutes.
Another option for ground truth is the S'COOL project. Students and citizen scientists around the world can be involved by making a "ground truth" observation within +/- 15 minutes of the time that a satellite views their area.
View the results from an initial data comparison between ground truth observations and satellite data.