Radio occultation involves the careful observation of radio signals emitted by global navigation satellite systems (GNSS) as they pass through the atmosphere to remotely measure the atmospheric properties of the stratosphere and troposphere.
Radio signals are constantly emitted by GNSS's orbiting the earth at an altitude of around 20,000 km, examples of GNSS's include: global positioning systems (GPS), Globalnaya Navigazionnaya Sputnikovaya Sistema (GLONAS), and the European Global Navigation System (GALILEO). Radio signals emitted by GNSS satellites on their way down from space to earth are influenced by the ionospheric electron density, temperature, pressure, and water vapour present in the earths atmosphere.
As radio signals pass through the earths atmosphere from satellites higher in orbits to satellites in lower in orbits, their radio waves are refracted to different degrees based on the properties of the atmosphere they are passing through. The magnitude of the refraction can be measured and used to determine atmospheric properties such as atmospheric pressure, atmospheric temperatures, and atmospheric water vapor. Radio occultation makes a vertical scan of different layers of the atmosphere is possible due the constant change of altitude and position of satellites in relation to each other. Radio occultation can also be used to determine atmospheric properties by satellites sending radio signals to receivers on planes or high mountaintops.
Climatologists, meteorologists, and other scientists use radio occultation to gain further insight into how properties of the earths atmosphere relate to weather and climate, helping them create more accurate weather prediction and climate change models.
The radio occultation technique was first developed at Stanford University in their Jet Propulsion Laboratory (JPL) for studying lunar and planetary atmospheres.
Scientists began exploring how to use GNNS to measure atmospheric properties in 1995 with the Global Positioning System/Meteorology (GPS/MET) mission. A research satellite with a radio receiver, named MicroLab1, was launched into lower earth orbit (about 750km in altitude) on April 3, 1995 with the goal of obtaining temperature soundings of the atmosphere using radio occultation.
As the MicroLab1 satellite rose and set in relation to 24 operational GPS satellites it received radio signals from each of the 24 GPS satellites. During transmission of radio signals from the GPS satellites to MicroLab1, the radio signals passed through the atmosphere and got refracted by the contents of the atmosphere on their way to MicroLab1's radio receiver. When the refraction levels of the radio signals are carefully measured this process is known as radio occultation.
From their 1995 study, scientists proved obtaining accurate vertical temperature profiles using radio occultation was accurate from about 40km to 5-7 km in altitude between their radio receiver and the GPS satellites due to negligible moisture interference. They discovered it was difficult to accurately measure temperature in the upper atmosphere (above 40km in altitude) due to inaccuracy issues determining the initial temperature, pressure, and other ionospheric refraction assumptions. They also had trouble getting accurate atmospheric temperature readings in the lower atmosphere (below 5-7km in altitude) from multipath effects caused by unpredictable refractivity gradients due to variable water vapor distribution.
Based on the results of the first radio occultation study, scientists discovered they could accurately predict temperature within 0.5 degrees celsius accuracy within the tropopause (interface between the stratosphere and troposphere), and to 1 degree celsius accuracy above 40km in altitude between the GPS satellite and radio receiver.
