TEC Retrieval from Sub-orbital Rocket Flight Data for Ionospheric E-layer Detection

Sub-orbital flights of research rockets provide unique opportunities for science by access to near-Earth space. In the MAPHEUS program of DLR (German Aerospace Center) such flights are conducted for the main purpose of micro-gravity experiments (almost free of residual force). On Nov 11th, 2024 at 7h38 UTC MAPHEUS-15 was launched from Esrange space port (Sweden) to send a scientific payload to about 7 minutes of micro-gravity. During the flight, the payload passed altitudes between 80 km and 310 km at a rather constant attitude with angular rate of change below 1° per second. We use these conditions to study the ionospheric E-layer that can form at altitudes of 90-120 km. E-layer remote sensing is challenging as its contributions are often masked by stronger contributions of F-layer above (250-400 km).Passing the E-layer during the rocket flight will induce changes of Total Electron Content (TEC) for the specific GNSS satellite links. The payload on the MAPHEUS rocket included two different GNSS receiver setups that recorded GNSS data: a navigation receiver (Septentrio AsteRx-m3 Pro+) and remote sensing receiver (based on a Syntony GNSS bit-grabber). A geometry-free linear combination is applied to dual-frequency GNSS phase observations in order to retrieve uncalibrated TEC. The retrieved TEC is geo-referenced with a GNSS-based trajectory of the payload. Phase wind-up effects have to be considered and corrected using attitude data from the on-board inertial navigation system. Unfortunately, radio interference limits the number of useful GNSS links: three Galileo satellites provide TEC results (L1-L5 combination) and four GPS satellite (L2-L5 combination).In parallel to the rocket flight, the near-by EISCAT UHF incoherent scatter radar in Tromsø, Norway was used to measure the ionospheric electron density over Northern Scandinavia. These data and the Neustrelitz Electron Density Model (NEDM) allow to retrieve ancillary TEC. The comparison shows good agreement. Standard deviation of residuals between ancillary TEC and GNSS rocket results are in most cases below 1 TECU. Profiles of TEC rate and TEC gradient are determined along the up-leg of the rocket flight for the low noise Galileo observations (L1-L5 combination). These profiles resolve significant anomalies at E-layer altitudes (90-120 km) that indicate the presence of an E-layer in the local ionosphere above Northern Scandinavia. The climatological data of NEDM underestimates the E-layer presence. However, the comparison with TEC derivates from the EISCAT measurements validate the E-layer presence.