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GPS radio occultation (GPS-RO) is a satellite remote-sensing method: a receiver in low Earth orbit tracks GPS signals as a GPS satellite rises or sets behind Earth’s atmospheric limb. The atmosphere bends and delays the signal, and scientists use those changes to reconstruct vertical profiles of atmospheric and ionospheric conditions.
What “occultation” means
In this context, occultation happens when a GPS satellite moves toward or behind Earth’s edge as seen by a receiver aboard a low-Earth-orbit (LEO) satellite. The transmitter, the atmospheric limb, and the receiver move relative to one another. As the radio path grazes the atmosphere, it samples successive heights.
This is not ordinary GPS positioning from a phone or a ground receiver. The LEO instrument listens to signals sent by GPS satellites and analyzes how the atmosphere changes their path and travel time. GPS-RO is one form of the broader technique called GNSS radio occultation (GNSS-RO), which can use signals from global navigation satellite systems beyond GPS. UCAR explains GNSS radio occultation.
How GPS radio occultation works
- A GPS satellite transmits a radio signal. A receiver on a LEO satellite tracks the signal while the GPS satellite rises or sets relative to the receiver’s view.
- The signal passes through the atmospheric limb. Neutral atmospheric molecules and ionospheric electrons alter the signal as it travels through the atmosphere.
- The receiver records the changing signal. Its phase and path behavior reveal effects such as bending and delay.
- Processing derives vertical profiles. Retrieval algorithms use the observed signal effects to estimate atmospheric and ionospheric properties at different heights. The overview sources describe this process but do not provide a full mathematical account of the inversion.
What GPS-RO measures—and what it retrieves
The instrument observes changes in the radio signal; it does not directly measure temperature or water vapor. Processing turns the observed bending and delay into geophysical estimates.
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| Stage | Examples |
|---|---|
| Signal effects observed and estimated from the received radio signal | Bending and delay |
| Atmospheric properties retrieved | Refractivity, temperature, pressure, and water vapor |
| Ionospheric property retrieved | Electron density |
UCAR lists these atmospheric and ionospheric products, while NOAA’s COSMIC-2 overview also names temperature, pressure, density, and water vapor. UCAR’s overview and NOAA’s COSMIC-2 overview describe the retrieved variables.
Why weather forecasters use the profiles
Temperature, pressure, and moisture profiles help describe the atmosphere’s vertical structure, information used in weather forecasting and atmospheric research. Radio occultation observations are incorporated into or used by weather-prediction systems. They add measurements across the globe, including places where conventional in-situ observing networks are sparse; they complement other observations rather than replace them. UCAR describes the technique as all-weather and notes its global three-dimensional coverage, extending from 40 km to the surface as a program-level characteristic—not a guarantee of uniform performance at every location and altitude. UCAR’s description discusses its applications and coverage.
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Other uses: climate and space weather
- Climate monitoring and model verification: Profiles provide observations that researchers can use to monitor atmospheric conditions and assess models.
- Ionospheric and space-weather research: Retrieved electron-density profiles help characterize the ionosphere. NOAA lists space-weather monitoring among the uses of COSMIC-2 data.
GPS-RO is useful alongside other atmospheric sounding systems. UCAR President Emeritus Rick Anthes describes radio occultation soundings as “highly complementary” to infrared and microwave sounders. The available overview supports the all-weather and complementary nature of the observations, but does not establish a complete quantitative comparison of their resolution, sampling, calibration needs, or retrieved variables.
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UCAR’s COSMIC Program overview reports approximately 100 m vertical resolution in the lower troposphere and better than 0.1 K accuracy for averaged profiles. The accuracy statement applies to averages, not necessarily to every individual occultation. UCAR does not state a year for these figures on the consulted page, and they should not be read as uniform specifications across all altitudes or conditions. UCAR’s overview gives the figures and coverage description.
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How GPS-RO developed
The GPS/MET experiment demonstrated atmospheric sounding with GPS radio occultation from 1995 to 1997. COSMIC-2, also known as FORMOSAT-7, is a network of six remote-sensing small satellites collecting data for weather forecasting, space-weather monitoring, and climate research, according to NOAA. NOAA announced that COSMIC-2 achieved Full Operational Capability after a review in September 2021; that dated milestone does not, by itself, establish a 2026 mission-status update. NOAA’s COSMIC-2 mission overview and its October 12, 2021 announcement provide the mission details. UCAR’s overview documents the GPS/MET demonstration period.
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