The project INSIGHT-II is funded by the German Research Foundation (DFG) within the framework of the 2^nd phase of the Special Proirity Programme (SPP) 1788 Dynamic Earth. The work is done in cooperation with TUM's Chair of Astronomical and Physical Geodesy (IAPG), the DLR Institute for Communication and Navigation, and the Insitut für Erdmessung (IfE) of the Leibniz University Hannover. INSIGHT-II is a successor of the project INSIGHT that was running during the 1^st phase of the SPP 1788 Dynamic Earth. For the 2^nd phase, INSIGHT was split into the two projects INSIGHT-II and TIPOD.

The general objective of INSIGHT-II is the development of an interdisciplinary approach to enhance the scientific outcome of satellite missions such as Swarm and GOCE by improving the data quality and quantifying the measurement errors. The accelerometers onboard of low-Earth orbiting (LEO) satellites provide valuable information on the non-gravitational forces acting on the satellite and thus, on the thermospheric drag mainly depending on the thermospheric density. Due to coupling processes between the thermosphere and the ionosphere the thermospheric drag is also depending on the electron density of the ionosphere.

During the last years DGFI-TUM developed a model of the electron density based on GNSS, satellite altimetry, DORIS and ionospheric radio occultations (IRO). In the first part of INSIGHT-II this suite of input data will be complemented, e.g. by Langmuir probe data and GRACE K-band measurements; see Fig. 1. At the same time the project partner DLR will assimilate the thermospheric density information from LEO accelerometers into the physical thermosphere-ionosphere coupling model CTIPe, especially for selected storm times. The results from CTIPe for ionospheric key parameters such the electron density will then be validated against DGFI-TUM’s empirical ionosphere model. Finally, the model results can then be used, e.g. to improve the GOCE gravity fields by reducing the (thermospheric) noise in the measured gravity gradients.

Theoretically, a three-dimensional (3-D) empirical electron density model can be derived from a dense network of radio occultation measurements. Since these networks are up to now not available, the so-called separation approach (SA) shall be tested for approximating the electron density. The SA is based on the assumption that the 3-D electron density model can be split into the product of a 2-D map of the vertical total electron content (VTEC) – depending on latitude and longitude – and a height-dependent profile function such as the Chapman function.

The Figure summarizes the main process steps of DGFI-TUM within INSIGHT-II:

1. Development of the separation approach for electron density (N_e) modelling using VTEC maps computed from GNSS, satellite altimetry, DORIS and IRO data.

2. Assimilation of Swarm and Champ Langmuir (LP) probe, GRACE and COSMIC 2 IRO and GRACE K-Band data into the model of the electron density N_e based on the SA

3. Internal comparison of the results from CTIPe with the empirical model of DGFI-TUM; the left bottom panel shows electron density maps at different heights, the right panel visualizes a vertical electron density profile for a selected location derived from COSMIC radio occultation data.

4. External comparison of DGFI-TUM’s electron density solution with models developed in other projects, e.g., TIK and TIPOD.