Glacier in arctic ocean, arctic circle, north pole

Radio Occultation

Beyond Gravity is a leading provider of radio occultation (RO) instruments and has maintained the RO golden reference since 2006 with its first-generation GRAS instrument on the MetOp spacecrafts. The company's second-generation GRAS instrument is scheduled to launch on the six MetOp SG spacecrafts soon, ensuring the continued delivery of high-quality RO data for weather forecasting and climate monitoring.

Annalena Johansson
Annalena Johansson Senior Sales Manager Digital

What will the weather be like?

The importance of Radio Occultation (RO) sounding has significantly increased over the years, becoming a crucial data source for operational weather forecasting. RO provides bias-free and calibration-free soundings with global coverage and high vertical resolution. These measurements offer valuable information about atmospheric temperature, humidity in the troposphere and stratosphere, as well as electron content and the state of the ionosphere.

Today, RO soundings have become indispensable for global weather forecasting, climate monitoring, and space weather applications. In particular, RO data has been used operationally since 2006 and is now one of the most important data sources feeding Numerical Weather Prediction models (NWP). These models rely heavily on the high-quality data provided by RO to generate accurate weather forecasts.

Europe’s first meteorological satellite in polar orbit

GRAS: Setting New Standards in Radio Occultation Performance

Beyond Gravity is a pioneer in Radio Occultation (RO) technology, with our first-generation GRAS instrument on the MetOp satellites demonstrating outstanding performance and reliability since 2006. 

The two MetOp-GRAS instruments in orbit today generate a near-constant flow of 700 occultations per day each, providing high-quality data for a wide range of scientific and operational applications.

The accuracy of bending angle measurements is fundamental to RO soundings. Our MetOp-GRAS instruments have consistently outperformed other RO data sources in terms of bending angle accuracy, with much lower and tighter distributed measurement errors. This is a testament to our commitment to excellence in space workmanship and state-of-the-art application knowledge.

MetOp-SG A is at Airbus' facilities in Friedrichshafen, Germany undergoing final tests to solar panels. Copyright: Airbus

Introducing GRAS-2: The Future of RO Instrumentation

Beyond Gravity's GRAS-2 radio occultation (RO) instrument, launching on the upcoming six MetOp-SG spacecraft, will revolutionize RO data with state-of-the-art performance, an extended number of soundings, and continuous global coverage. 

Receiving signals from GPS, Galileo, and BeiDou, GRAS-2 will triple the number of occultations to 2100 per day per instrument, tripling its value. Chosen by Europe for RO sounding from MetOp-SG, GRAS-2 will be employed on all MetOp-SG satellites to provide RO data until 2050.

GRAS-2 is designed to continue the success of the first generation MetOp-GRAS instruments, which have been providing essential data for weather forecasting, climate monitoring, and space weather research since 2006. The instrument's modular architecture allows for easy adaptation to future mission requirements.

Metop - Second Generation (Metop-SG) is EUMETSAT's next generation of polar-orbiting satellites. Copyright: Eumetsat.

GRAS-2 RO Performance on MetOp-SG

The GRAS-2 instrument on the MetOp-SG satellite provides measurement coverage from the surface up to 500 km altitude, with improved Level 1b performance relative to the current MetOp-GRAS instrument.

For weather observations, the predicted typical performance of stratospheric bending angle accuracy is better than 0.4 micro-radians. GRAS-2's proprietary open-loop tracking concept ensures measurement coverage all the way down to the surface under all weather conditions.

Level 1b performance for GRAS-2 was analyzed using an end-to-end simulator that included realistic atmospheric conditions, error characteristics of the transmitting GNSS satellites, instrument errors corresponding to the GRAS-2 design, and state-of-the-art measurement retrieval algorithms.

Improvements over MetOp-GRAS

GRAS-2 offers a number of improvements over the first generation MetOp-GRAS instruments, including:

  • More occultations: Up to 2100 per day, more than double the current number.
  • Continuous open loop tracking: Measures GNSS signals in all weather conditions.
  • Multi-correlator tracking to lower altitudes: More accurate measurements of the troposphere.
  • Ionosphere coverage to at least 500 km: New data on the ionosphere for space weather forecasting.
  • Reduced bending angle noise: More accurate measurements of atmospheric temperature and pressure.
  • Improved reliability: Continuous data for weather forecasting and climate monitoring.
  • Highly selective filtering: Reduced interference from other radio signals.
  • Active interference mitigation: Accurate measurements even in the presence of strong interference.


GRAS-2 Occultation Antennas for MetOp-SG

The GRAS-2 instrument on the MetOp-SG satellite includes two identical six-element occultation antennas, mounted to point in the satellite's velocity and anti-velocity directions. These antennas are designated the GRAS-2 Velocity Antenna (GVA) and the GRAS-2 Anti-Velocity Antenna (GAVA).

The GRAS-2 Zenith Antenna (GZA) is mounted on the satellite's zenith-facing panel. Each antenna is connected to a Low Noise Amplifier (LNA) unit, which is mounted close to the antenna to minimize signal loss. The LNA filters and amplifies the received GNSS signals.

The LNA units are connected to the GRAS-2 Electronics Unit (GEU), which implements the following functions:

  • Frequency Generator Module (FGM): Provides stable frequency references.
  • Ultra-Stable Oscillator (USO): Provides a 10 MHz reference to the FGM.
  • GNSS Receiver Modules (GRM): Down converts the RF signals to baseband and converts the signals to the digital domain. The GRM also includes an advanced GNSS channel matrix with signal correlators as well as a general purpose processor for Software execution and communication. The Software includes modules for Instrument Control, Radio Occultation, Navigation, Services and Boot.
  • Power and Interface Module (PIM): Contains secondary power conditioning and SpaceWire links to the Spacecraft platform.