GNSS in Earth Science Research: GNSS-RO, GNSS-R, and GNSS-GR

Blog

Scientists have found ways to study Earth’s surface and atmosphere using Global Navigation Satellite Systems (GNSS), but not by using these systems’ positioning capabilities. Instead, they use satellites to measure changes in GNSS signals passing through the atmosphere and reflecting off the surface. In effect, these opportunistic techniques convert sources of positioning error into scientific data sources for a better understanding of our planet.

Early satellite remote sensing

Using space-based systems to study planetary atmospheres with radio waves has a long history dating back to NASA’s Mariner V mission to Venus. As the spacecraft flew behind Venus from Earth’s perspective, its S-band carrier signal passed through the planet’s atmosphere. Changes to the signal let planetary scientists better understand how hot and thick Venus’ atmosphere really is.

Remote sensing satellite missions have used radio signals for Earth observation. For example, synthetic aperture radar (SAR) satellites put radar systems in orbit to get high-resolution surface measurements. First used by intelligence agencies in the 1960s, NASA oceanographic missions applied these techniques in the 1970s. The drawback to this active approach is the added cost and complexity of the satellite’s powerful radio transmitter.

The deployment of GPS and other GNSS constellations made this kind of Earth observation cheaper and more straightforward. Projects get standardized, space-based radio transmissions with global availability “for free” by putting a relatively low-powered GNSS receiver into orbit.

GNSS radio occultation (GNSS-RO)

Radio signals broadcast for GNSS satellites in medium-Earth orbits (MEO) do not travel in a straight line as they penetrate deeper into the atmosphere. The denser, warmer air and increasing water vapor content refract the radio waves and increase the path length. As a result, signals arrive at receivers slightly later than they theoretically should. Basic receivers simply accept these slight inaccuracies, while more advanced receivers use atmospheric models to correct the errors.

Atmospheric scientists, on the other hand, treat signal delays as data using GNSS radio occultation. Satellites in low-Earth orbit (LEO) receive signals from GNSS satellites coming into or passing out of view over the horizon. These signals pass from MEO into the atmosphere and skim above Earth’s surface before being received by the GNSS-RO satellite in LEO. Atmospheric models turn this data into temperature, pressure, and humidity measurements along the signal’s path.

Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC-2) is a joint project of Taiwan’s National Space Organization, America’s National Oceanic and Atmospheric Administration, and other American government and university partners. COSMIC-2’s network of six LEO satellites uses GNSS-RO to create 800-kilometer-high profiles of atmospheric conditions. Compared to traditional sounding techniques, this orbital approach provides global, all-weather coverage and yields highly accurate data with better vertical resolution.

The most immediate contribution of GNSS-RO is to weather forecasting. Detailed data about atmospheric conditions helped NOAA reduce errors by up to eight percent. Climate research is another beneficiary of this technique, allowing scientists to monitor global and regional impacts of rising temperatures.

GNSS reflectometry (GNSS-R)

Multi-path is another source of positioning error with scientific value. Receivers pick up multiple versions of a GNSS satellite’s signal — the direct signal and secondary signals reflected off local terrain and buildings. By collecting these reflections, scientists turn the world’s GNSS networks into global radar systems.

For example, the ground’s surface roughness and moisture content will determine its reflectivity to GNSS radio waves. LEO satellites pick up these reflections to measure aspects of Earth’s surface. Altimetry can measure ground altitude and ocean wave height. Scientists can also study soil moisture, ocean wind speeds, ice shelves, and glaciers.

NASA’s Cyclone Global Navigation Satellite System (CYGNSS) mission is a network of eight micro-satellites (seven functional) in low-inclination orbits. Its goal is to reach a better understanding of how tropical cyclones develop by measuring wind speed and atmospheric conditions. Commercial satellite services like those from Spire and GeoOptics have built on NASA’s pioneering work and collect GNSS-R data for researchers and meteorologists.

GNSS grazing-angle reflectometry (GNSS-GR)

Grazing-angle reflectometry, like GNSS-RO, looks at GNSS satellites low on the horizon. However, this technique measures the signals that have grazed the surface. Texture has little effect on reflectivity at these shallow angles, letting scientists treat the surface as a mirror. Evaluating phase changes in the signal allows for extremely precise calculations of surface height.

The European Space Agency’s passive reflectometry and dosimetry (PRETTY) mission is a LEO CubeSat dedicated to collecting GNSS-GR data. The satellite’s two antennas will detect L5 signals broadcast by the Galileo GNSS constellation. One will detect the reflected signal, allowing the satellite’s systems to use GNSS-GR to measure surface height. The other antenna will pick up the direct signal that passes through the atmosphere. The satellite will use interferometric techniques to compare the two signals’ phases and derive surface heights to at least fifty centimeters.

The future of GNSS-based Earth observation

Next-generation GNSS constellations will increase the number of satellites and radio signals available for civilian use. These developments will let scientists create richer datasets, improve weather forecasting, and build better models of our planet’s atmospheric and oceanographic processes.

April 19, 2024