Radio occultation

The radio occultation instrument has a direct heritage from GRAS flying on EPS.

The main objective of the Global Navigation Satellite (GNSS) radio occultation mission is to support numerical weather prediction (NWP) and climate monitoring, by providing measurements of bending angle profiles in the troposphere and the stratosphere with a high vertical resolution and accuracy.

The RO will provide high-resolution temperature and water vapour profiles by gaining measurements of bending angle profiles in the troposphere and the stratosphere with high vertical resolution and accuracy. The measurement of bending angles can be used to obtain information on refractivity profiles, which can be used to retrieve atmospheric temperature and humidity profiles, as well as surface pressure. A secondary objective is to provide space-weather information through measurement of electron density and its profile in the middle and high atmosphere.

The unique combination of global coverage; high precision, high vertical resolution; long-term stability, and all-weather viewing enabled by the long Global Navigation Satellite system (GNSS) wavelengths, demonstrated by RO measurements, complement and enhance datasets obtained by existing in situ and other remote sounding techniques. This is established to have a direct impact in the improvement of operational NWP forecasts and climate monitoring.

Innovative features of this new RO instrument receiver are the possibility to observe, acquire and track signals from the modernised GPS and from the Galileo navigation systems, by handling the combinations of the dual-frequency signals from the same GNSS satellite (see table below), up to 500km height (enabling the ionospheric monitoring).

Tracking of CDMA-based GLONASS and the COMPASS/Beidou signals might also be possible, depending on the actual GNSS signal structures implemented. The RO Instrument will acquire, as quickly as possible, all the signals exploiting a new full open-loop tracking strategy, aiding both the code phase and the carrier phase tracking through the use of a range/Doppler model. The standard closed-loop tracking is also implemented.

GNSS signals and frequencies that will be tracked by RO instrument.

Measurement principle

RO observations are based on measuring GNSS signal characteristics after they have propagated through the atmosphere in a limb sounding geometry. The overall geometry is depicted in the figure below, showing a single radio occultation ray travelling from the transmitter onboard a GNSS satellite (on the left) through the Earth’s atmosphere. Signals are observed by a GNSS receiver onboard a LEO satellite (on the right) while, from the point of view of the observing LEO satellite, the GNSS satellite is either setting or rising behind the Earth’s horizon.

With signals gradually entering the atmosphere, refraction increases, bending the ray towards the Earth’s surface, before the rays leaves the atmosphere again on its way to the receiver. The point along the ray path which is closest to the Earth’s surface is known as tangential point; the angle˛ between the ray asymptotes is the bending angle which is the primary output of the Level 1 processing of radio occultation soundings. The bending angle depends on the refractive index and of the atmosphere, which is determined by the temperature, pressure and amount of water vapour. In the ionosphere, the refractive index is driven by the electron density.