Ionospheric impact on space-borne GNSS reflectometry: studying satellite and sounding rocket scenarios

Abstract

Earth-reflected signals of Global Navigation Satellite Systems (GNSS) provide remote sensing opportunities to study the Earth system. Some of these reflectometry retrievals exploit the reflection itself to estimate ocean- and land surface parameter, e.g. sea surface height, sea-ice concentration or soil moisture. Other retrievals exploit the atmospheric refraction experienced by the signal while travelling from the transmitting GNSS satellite to the Earth surface and the GNSS receiver platform. Atmospheric parameter of the neutral gas (total zenith delay) and of the ionosphere (total electron content) can be retrieved. Monitoring of threats in terrestrial weather (precipitation and flooding)and space weather (ionospheric irregularities) can be improved by such retrievals. A main challenge of reflectometry applications arises from the separation of Earth surface, neutral gas or ionosphere effects, as well as, instrumental biases. A dedicated modeling of the different effects and biases is required to ensure precise retrievals. In this paper we report on two GNSS reflectometry studies that involve spaceborne observations from a satellite and from a sounding rocket. The focus of both studies lies on the retrieval of ionosphere parameters. The primary goal is to retrieve ionospheric delay and Doppler shift in the GNSS links based on observation data and to compare the retrieval results with computations using ionospheric electron density models.The first study on ionospheric effects involves observation data of ESA's Passive REflecTometry and dosimeTrY (PRETTY) nano satellite mission in a scientific cooperation of DLR-SO, GFZ and NTNU. The satellite mission was successfully launched on 9th October 2023 and provides reflectometry observations from a polar orbit of roughly 550 km altitude. Signal acquisition is confined to grazing geometry, it means geometries with high incidence angle at the surface (between 75° and 85°) and long propagation paths (up to several thousands of km) in the atmosphere. The second study started recently with simulations to prepare a reflectometry experiment during a sounding rocket flight thatis planned for launch in November 2024. This experiment is part of space weather studies conducted by DLR-SO to strengthen RESIlient TEchnologies for disaster control (RESITEK project) in cooperation with DLR-MP and DLR-FT. The reflectometry payload shall provide data in an altitude range of the receiver between 100 and 240 km. The major atmospheric contributions in reflectometry belong to altitudes below the receiver level. Observations from satellites and sounding rockets are essentially different in altitude. For the low earth orbit satellite at about 550 km altitude, the ionospheric F-layer contribution exceeds other ionospheric contributions. The rocket flight trajectory will stay lower (up to 240 km). In major parts it will be below the F-layer peak height but still above E-layer. A crucial question is whether a characterization of E-layer ionosphere is possible from such rocket flight observations or not. The reflectometry instruments used on the satellite and on the rocket are slightly different. The satellite mission runs a single-frequency (L5) reflectometry receiver developed by Beyond Gravity Austria. The receiver is capable to generate coherent waveforms in orbit. The mission is expected for at least one year of operation. Further estimation of delay and Doppler from the satellite data is done on ground. The rocket flight will carry a dual frequency (L1, L5) front-end recorder provided by Syntony GNSS, Toulouse. This receiver collects raw data during flight duration (about 15 minutes). Coherent waveforms, as well as, delay and Doppler estimates shall be generated from the raw data in post-processing after the flight. A software-defined GNSS reflectometry receiver, that was developed for flight scenarios in a cooperation of ULCO, GFZ and DLR-SO, will be adapted to process the data.