GNSS Radio Occultation
Science
Earth Observation
Positioning and Navigation
Measurement Types
GNSS (Global Navigation Satellite System) Radio Occultation (GNSS-RO) is a remote sensing technique that uses GNSS measurements from low-Earth orbiting satellites. The word occultation is derived from the Latin word occultare, meaning to cover something. GNSS-RO allows for a long-term, global coverage profile of Earth’s atmosphere and ionosphere with high accuracy (averaged profiles to < 0.1 K) and resolution (~100 m in the lower troposphere) in all-weather conditions. As measurements can be made in any weather this technique is ideally suited to weather forecasting, understanding atmospheric processes, climate monitoring, space weather, and ionospheric research. This measurement technique is relatively novel, with its first application in 1995 by the US GPS/MET (GPS/Meteorology) instrument onboard GeoEye’s OrbView-1 satellite to provide vertical profiling of Earth’s lower and upper atmosphere. GNSS-RO is unique in its data products, as it provides independent height, pressure, and temperature data with global coverage of the atmosphere ranging from the Earth’s surface to an altitude of 40 km. Electron density profiles are measured from 60 km up to Low Earth Orbit (LEO) altitude. 1) 2)
GNSS-RO works by taking advantage of radio signals broadcast from GNSS satellites already in orbit, using limb sounding techniques. GNSS-RO satellites intercept these radio signals and measure the refraction and signal delay caused by molecules and electrons in the atmosphere. If the position of both the transmitting and receiving satellites are known, the atmospheric density can be inferred. Refractive occultations occur when density gradients within the atmosphere cause refraction of the signals, creating curved paths through the atmosphere. The strength of the signal bending changes depending on the vertical variation of the atmospheric refractive index. In the neutral atmosphere, the refractive index depends on temperature and humidity, while in the ionosphere it depends on the electron density. Hence, this data allows the retrieval of profiles of refractivity, temperature, pressure, water vapour, and electron density in the ionosphere. 1) 2) 3)
GNSS-RO can be demonstrated on rising or setting satellites observed from a satellite-borne receiver, as shown in Figure 2. Setting occultations occur when the scanning of the atmosphere is done from top to bottom and are tracked by processing the signal in reverse, and rising events occur when the scanning of the atmosphere is done bottom to top. 5)
Example Products
GNSS-RO techniques result in the retrieval of the bending angle of radio signals as a function of impact parameter and refractivity as a function of altitude. From this, refractivity data can be retrieved via statistical methods to understand atmospheric pressure and temperature for altitudes up to 40 km, and for water vapour concentration in the lower troposphere, as well as electron density profiles from 60 km up to the LEO orbital altitude. 2) 6)
Refractivity gradient
The atmosphere, as a dielectric medium, can be characterised by the distribution of the refractive index of the air within it, which is dependent on pressure, temperature, and humidity. As signals are transmitted through the atmosphere by GNSS satellites they are refracted due to this refractivity gradient of Earth’s atmosphere. The refraction angle of this signal can be measured to derive a value for the refractivity gradient of the section of the atmosphere it passes through. This refractivity gradient varies depending on the time of year due to temperature, pressure, humidity, and water vapour changes. It is important to measure the spatial distribution of atmospheric refractivity as it affects the propagation of electromagnetic waves. 1) 5) 7) 8)
Atmospheric properties: pressure, temperature, and water vapour
By using the Doppler shift of transmitted radio waves through the atmosphere, the bending angle of the signals can be calculated. This angle is related to the refractive index, which can be used to estimate atmospheric properties such as pressure, temperature, and water vapour concentration. Various factors impact atmosphere pressure profiles such as altitude and the extent of aerosol scattering. Accurate atmospheric pressure measurements are essential for applications in weather prediction and measuring air pollutant spread, as well as improving the accuracy of GPS systems and monitoring aircraft and satellite altitudes. Long term temperature profiles of Earth’s atmosphere through GNSS-RO can support a greater understanding of climate change over time, as well as aiding weather forecasting models. 10)
Ionospheric properties: electron density profiles
The GNSS-RO signals are distorted when passing through the planet’s ionosphere due to the electrons and ions in the upper atmosphere. This distortion can be studied to retrieve electron density profiles. A method based on a Chapman layer function and an exponential decay function can be used to model the topside ionosphere and plasmasphere. 12)
Related Missions
MetOp (Meteorological Operational Satellite Program of Europe)
The Meteorological Operational satellite program (MetOp), launched in October 2006 and retired in 2021, was a collaboration between the European Space Agency (ESA) and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) with the aim to provide satellite observation and data services for weather prediction and climate monitoring. The onboard GRAS (GNSS Receiver for Atmospheric Sounding) instrument, designed by Saab Ericsson Space (SES) of Sweden and Austrian Aerospace (AAE), used Radio Occultation techniques to acquire data on the pressure, temperature and humidity of the atmosphere, using both the GPS and GLONASS constellations.
Spire Global Nanosatellite Constellation
In 2014, commercial company Spire Global introduced the Lemur nanosatellite constellation project with the launch of the Low Earth Multi-Use Receiver-1 (Lemur-1) prototype satellite. The first Lemur-2 satellites were launched in 2015, which were followed by over 180 flight units over the next ten years. STRATOS, an instrument onboard Lemur-2, is the Spire Global Navigation Satellite System (GNSS) receiver for remote sensing and precision orbit determination, and is used for weather model assimilation of radio occultation, space weather monitoring, ionosphere corrections for navigation, and thermospheric density measurements.
GRACE (Gravity Recovery And Climate Experiment)
GRACE, a collaborative twin satellite mission of NASA and the German Space Agency (DLR), was launched in March 2002. Ending in October 2017, GRACE provided routine collection of GPS atmospheric radio occultation (50 Hz) data using the BlackJack GPS Flight Receiver provided by JPL (the same instrument flown on CHAMP). This data was used to demonstrate that through measuring subtle temporal variations in gravity, satellites can detect groundwater variations which has advanced our understanding of Earth’s water dynamics.
GRACE-FO
GRACE-FO, launched in May 2018, is NASA and DLR’s successor to the GRACE mission. GRACE-FO consists of two satellites that focus on tracking water movement and global surface mass changes to allow for continually changing high-resolution monthly global models of Earth’s gravitational field. The secondary objective of the GRACE-FO mission is to continue the measurements of GRACE’s radio occultations for the operational provision of vertical temperature and humidity profiles to weather services. GRACE-FO carries a (TriG-RO) RO receiver onboard.
TSX (TerraSAR-X) and TDX (TanDEM-X)
TerraSAR-X (TSX) and TanDEM-X (TDX) are interferometric SAR satellite missions of the German Aerospace Centre (DLR), launched in June 2007 and June 2010 respectively. Both missions include a Tracking, Occultation and Ranging payload (TOR), that consists of a dual-frequency Integrated GPS Occultation Receiver (IGOR) that collects atmospheric radio occultation (RO) data to be used in the improvement of weather forecasting, climate change studies and space weather monitoring. Through RO techniques, TSX yielded a daily total of about 250 neutral atmospheric profiles for temperature and humidity as well as additional ionospheric data of the vertical electron density distribution.
FormoSat-7 (Formosa Satellite 7)/COSMIC-2 (Constellation Observing System for Meteorology, Ionosphere and Climate)
The FormoSat-7/COSMIC-2 constellation is an international collaboration between Taiwan (NSPO/TASA) and the United States (NOAA) that consists of 12 remote sensing microsatellites, six of which were launched June 2019. This mission is a follow-on to the FormoSat-3/COSMIC-1 mission, which aimed to collect further GPS RO data for weather forecasting. The RO capabilities of this mission are provided by the TGRS (Tri-band GNSS Receiver System) instrument which receives all L-band GNSS signals.
FormoSat-3 (Formosa Satellite 3)/COSMIC-1 (Constellation Observing System for Meteorology, Ionosphere and Climate)
The FormoSat-3/COSMIC-1, previously named ROCSat-3 (Republic of China Satellite-3), was an international collaborative project between NSPO (National Space Program Office, now Taiwan Space Agency (TASA)) of Taiwan and UCAR (University Corporation for Atmospheric Research) of the United States of America. Launched into LEO (Low Earth Orbit) in April 2006, the constellation of six microsatellites successfully collected atmospheric remote sensing data for operational weather prediction, climate, ionospheric (space weather monitoring), and geodesy research. This mission utilised the Radio Occultation technique to receive the L-band signals from the GPS satellites which were then processed to to derive important weather and climate research parameters, including atmospheric temperature, moisture, and pressure, to form the near-real time global weather observation network.
CHAMP
DLR’s (German Aerospace Centre) Challenging Minisatellite Payload (CHAMP) was operational from July 2000 to May 2020. The mission aimed to provide atmosphere and ionosphere sounding by GPS radio occultation with applications in weather forecasting, navigation, space weather, and global climate change. BlackJack, the onboard GPS Flight Receiver, observed in parallel ionospheric electron content and provided atmospheric soundings permitting the derivation of atmospheric vertical profiles of density, pressure, temperature of water vapour, and ionospheric electron density profiles (refractive occultation monitoring). The receiver system had an occultation mode in which the receiver software scheduled every 50HZ tracking of setting occultations of up to four GPS satellites.
OrbView-1 (formerly Microlab-1)
GeoEye’s OrbView-1, an imaging microsatellite mission, was launched in April 1995. It had the GPS/MET (GPS/Meteorology) instrument of UCAR/JLP ((University Consortium for Atmospheric Research, with collaboration by JPL) onboard, an atmospheric measurement system that utilised radio occultation techniques. The objective of this instrument was to provide limb sounding of the atmosphere using radio occultation measurements provided by the signals of the GPS constellation.
References
1) “Constellation Observing System for Meteorology Ionosphere and Climate,” GNSS Radio Occultation. URL: https://www.cosmic.ucar.edu/what-we-do/gnss-radio-occultation
2) “GNSS Radio Occultation,” GGOS. URL: https://ggos.org/item/gnss-radio-occultation/
3) “OrbView-1 (formerly Microlab-1),” eoPortal, 11 June 2012. URL: https://www.eoportal.org/satellite-missions/orbview-1
4) “FormoSat-7 / COSMIC-2 (Constellation Observing System for Meteorology, Ionosphere and Climate),” eoPortal, 29 April 2013. URL: https://www.eoportal.org/satellite-missions/stp2-formosat-7
5) Zhran M, “An evaluation of GNSS radio occultation atmospheric profiles from Sentinel-6,” The Egyptian Journal of Remote Sensing and Space Sciences, December 2023. URL: https://doi.org/10.1016/j.ejrs.2023.07.004
6) Leroy SS, McVey AE, Leidner SM, Zhang H, Gleisner H, “GNSS Radio Occultation Data in the AWS Cloud,” Earth and Space Science, February 2024, URL: https://doi.org/10.1029/2023EA003021 .
7) Bettouche Y, Agba B. L, Kouki A. B, “Geoclimatic factor and point refractivity evaluation in Quebec-Canada,” 2014 XXXIth URSI General Assembly and Scientific Symposium (URSI GASS), 16-23 August 2014. URL: https://doi.org/10.1109/URSIGASS.2014.6929621
8) Grabner M, Pechac P, Valtr P, “On horizontal distribution of vertical gradient of atmospheric refractivity,” Atmospheric Science Letters, 13 June 2017. URL: https://doi.org/10.1002/asl.755
9) Kirincich A, Emery B, “Revisiting HF Ground Wave Propagation Losses Over the Ocean: A Comparison of Long‐Term Observations and Models,” Radio Science, 31 March 2023. URL: https://doi.org/10.1029/2022RS007550
10) “Air Pressure,” National Oceanic and Atmospheric Administration. 18 December 2023. URL: https://www.noaa.gov/jetstream/atmosphere/air-pressure
11) He Y, Zhang S, Guo S, Wu Y. “Quality Assessment of the Atmospheric Radio Occultation Profiles from FY-3E/GNOS-II BDS and GPS Measurements,” Remote Sensing, 10 November 2023. URL: https://doi.org/10.3390/rs15225313
12) Lyu H, Herandez-Pajares M, Monte-Moreno E, Cardellach E. “Electron Density Retrieval From Truncated Radio Occultation GNSS Data,” Journal of Geophysical Research: Space Physics, 31 May 2019. URL: https://doi.org/10.1029/2019JA026744
13) Jakowski N, Kutiev I, Heise S, Wehrenpfennig A. “A Topside Ionosphere/Plasmasphere Model for Operational Applications,” January 2002. URL: https://www.ursi.org/proceedings/procGA02/papers/p2174.pdf