Global Positioning System (GPS) radio occultation is an increasingly critical technique for meteorology and atmospheric science. By analyzing changes in GPS signals passing through the atmosphere, radio occultation sensors onboard satellites provide valuable data on weather conditions and climate trends.
This article explores the basics of GPS radio occultation, its history and scientific value, occultation measurement processes, new technical developments, and some of the companies utilizing the technique today. This article also examines the growing applications of radio occultation data across weather forecasting, climate monitoring, and space weather.
Background on Radio Occultation
Radio occultation refers to the phenomenon where a radio signal from one celestial body passes through the atmosphere of another body, causing bending, delay, and distortion of the signal. By precisely analyzing these changes, properties of the atmosphere that the signal passed through can be determined.
For GPS radio occultation, receivers onboard low Earth orbiting (LEO) satellites detect and characterize changes in GPS signals from navigation satellite transmitters as they pass through Earth’s atmosphere. Based on measured shifts in the signal properties, vertical profiles of atmospheric temperature, pressure, moisture, and ionospheric electron density can be derived.
The concept of leveraging radio occultation for atmospheric profiling was first proposed in the 1960s. But practical applications had to wait for the deployment of GPS satellites starting in 1978 to provide a permanent, high quality signal source from space for radio occultation study.
Importance for Meteorology
Today, GPS radio occultation stands as one of the most significant advancements in meteorology over the past few decades. The accurate, high vertical resolution data captured by occultation sensors provides key advantages:
- Enhances weather forecast skill by improving initialization of models
- Provides calibration/validation data for other weather satellites
- Delivers all-weather temperature and moisture soundings of high accuracy
- Enables monitoring of long term climate trends in the atmosphere
- Profiles Parts of atmosphere not easily measured (e.g. planetary boundary layer)
Compared to traditional weather balloons, radio occultation offers much higher global coverage and vertical resolution into the troposphere and ionosphere. It provides a vital data source for both operational weather prediction and climatology.
How Radio Occultation Measures Atmosphere
A radio occultation event occurs when a LEO satellite passes behind or near the atmosphere of Earth relative to a GPS satellite. This causes the GPS signals to be refracted and slowed as they travel through the atmosphere before reaching the LEO satellite.
The LEO satellite records precise Doppler shifts and signal delay. advanced inversion algorithms then extract profiles of atmospheric properties like temperature, pressure, and moisture from the raw radio occultation measurements.
By combining occultation data captured from many GPS-LEO viewing geometries as satellites circle Earth, a global representation of atmospheric conditions is assembled. This can detect fine vertical structure and variability.
Radio occultation directly captures vertical thermodynamic profiles, complementing horizontal data from other satellite sensors. The self-calibrating nature of measuring calibrated GPS signals also gives radio occultation high accuracy and stability.
New Technical Developments
As radio occultation matures, a number of innovations are improving performance and applications:
- New antennas, receivers, and processing systems allow capturing higher quality signals at faster rates for more uniform global coverage and higher vertical resolution.
- Alternative positioning signals like Galileo and BeiDou can supplement GPS for expanded radio occultation capacity.
- Assimilating radio occultation in weather models is progressing, especially for initializing hurricane forecast models.
- Monitoring changes in polar ionospheric conditions is helping track space weather phenomena.
- Cubesats and CubeSat constellations enable low-cost radio occultation deployments.
- Data is becoming rapidly available, even directly from satellites in orbit, benefiting operational weather agencies.
With these improvements, the amount and utility of radio occultation data for both research and real-time weather prediction is increasing.
Organizations Using Radio Occultation Technology
Given its valuable applications, radio occultation sensors have been deployed on various operational weather satellites over the past two decades. Key examples include:
- COSMIC – Pioneering constellation that demonstrated the value of multi-satellite radio occultation.
- MetOp – European weather satellites with radio occultation packages.
- COSMIC-2 – New fleet of U.S. radio occultation satellites launched in 2019.
In addition, emerging private companies like Spire Global, GeoOptics and PlanetiQ are developing and deploying radio occultation sensor systems aboard small satellite constellations to provide datasets commercially.
As radio occultation data becomes increasingly available, adoption by weather agencies, researchers, and companies is expected to grow.
Conclusion
Starting from its early conceptual roots, GPS radio occultation has become an invaluable atmospheric profiling technique today. Its high accuracy and spatial coverage provides critical weather and climate data unmatched by other observation methods.
As sensors and data analysis continue advancing, radio occultation promises even greater applications in operational meteorology, climatology, space weather monitoring, and research. New satellite constellations are also enabling widespread commercial use of radio occultation data. Harnessing radio waves that streak across the solar system, radio occultation unlocks an innovative view of Earth’s atmosphere and environment.
