Background info about Radio Occultation
(for use with the Satellites and Weather Teaching Box)
In the Satellites and Weather Teaching Box, students read about and compare two types of electromagnetic radiation, visible light and radio waves. They also read about the layers of Earth's atmosphere. Finally, they view a video about the COSMIC satellites and radio occultation, and explain how the bending of radio waves beamed between satellites allows scientists to measure traits of the atmosphere.
Students may need help with some of the steps in the logic of how the radio occultation technique provides scientists with data about temperature, pressure, and humidity at different heights in the atmosphere. Specifically:
- We can observe how light rays bend when light passes from a substance with one index of refraction into another substance with a different index of refraction.
- The density of a substance and its index of refraction are often (but not always!) related. In general, higher density and higher index of refraction go together.
- Exceptions to this trend include olive oil and alcohol as compared to water. Water has a higher density than either alcohol or olive oil, however both alcohol and olive oil have a higher index of refraction than water.
- Radio waves, like light, bend when they pass between substances of different densities.
- If radio waves beamed from one satellite to another pass through the atmosphere, they bend. The amount of bending depends on the density of the portions of the atmosphere the radio waves pass through.
- The density of the atmosphere at a specific place depends on a combination of air temperature, air pressure, and humidity... therefore each of these factors influences how much radio waves bend.
- The temperature, pressure, and humidity of the atmosphere vary with altitude. In fact, they vary so much that scientists use different names for layers of the atmosphere based on the characteristics of the atmosphere at each height. Therefore, the radio occultation technique allows us to observe different layers of the atmosphere by measuring the amount of bending of radio waves as they pass through air with varying density.
The diagram below illustrates how radio occultation is used to measure properties of the atmosphere at different heights. Have students explain the features of this diagram and the science behind radio occultation based on their knowledge of refraction of electromagnetic waves as they pass through media of different densities, the vertical structure and layers of Earth's atmosphere, and the similarities and differences between visible light and radio waves.
This diagram shows how radio waves beamed from one satellite to another bend when the radio waves pass through Earth's atmosphere. The radio waves are beamed from the GPS Satellite in the higher orbit to the COSMIC satellite in the lower orbit at positions A, B and C.
- The signal from the GPS satellite to position A does NOT pass through Earth's atmosphere at all. Therefore, the radio waves DO NOT bend and simply move in a straight line between the two satellites.
- The signal from the GPS satellite to position B passes through the upper portions of Earth's atmosphere. The radio signal is bent a little bit as is passes through the upper atmosphere. The values of air pressure and density at high altitudes in the atmosphere are very small. Therefore, the index of refraction of the air at high altitudes is small, so the radio waves bend only slightly.
- The signal from the GPS satellite to position C passes through both the upper AND lower portions of Earth's atmosphere. The radio signal bends the most as it passes through the lower atmosphere. The values of air pressure and density at low altitudes in the atmosphere are much greater than they are at high altitudes. Therefore, the index of refraction of the air at low altitudes is larger, so the radio waves bend the most when they pass through the relatively dense air that is nearer to ground level.