ROC2, the GPS atmospheric sounder delivered to STRATEOLE-2

ROC2 is an atmospheric sounder using GPS radio occultation

This instrument is capable of performing under balloon radio-occultation measurements from GPS signals, similar to those performed by the instrument on board the COSMIC space mission.

ROC2 makes a very precise acquisition of the signals phase emitted by the satellites composing the GPS constellation. These measurements enable to deduce the signals delay on the path between the satellite and the balloon, associated with the refractivity of the atmosphere, which essentially depends on its temperature and on the water vapour concentration.

The ROC2 instrument scans the atmosphere surrounding the balloon to extract its properties and measure the temperature profiles from the balloon’s altitude (approximately 20 km) up to around 7 km of altitude. It does so by following a GPS satellite setting or rising on the horizon from the balloon’s point of view. The complexity of the signal propagation linked to the effects of water vapour variation in the atmospheric refractivity makes it impossible to retrieve sufficiently accurate information under 7 km of altitude.

ROC2 is the evolution of the instrument which had participated in the Concoardiasi balloon project over Antarctica in 2010.

Both of the STRATEOLE-2 scientific campaigns will be provided with four instruments. They will take place during winters 2020-2021 and 2023-2024. A first instrument will be used beforehand on the preparatory campaign by the end of 2018-early 2019. The 3-month flights of the pressurised balloons over the entire equatorial belt of the Earth will thus provide an unprecedented description of the thermal structure of the upper troposphere and lower stratosphere, each instrument performing dozens of radio-occultations a day.

These observations will be used to study water vapour transport at the interface between the troposphere and the stratosphere as well as to better characterise the wave phenomena produced by the intense convective movements characteristic of low latitudes.

Meanwhile, in the Paris region and in Toulouse, the ROC2 instrument will undergo a series of tests intending to ensure its long-term operation in the hostile conditions that it will meet during the flights of the pressurised balloons.