AIR-MoPSy

Satellite-based navigation systems (GNSS from Global Navigation Satellite System) provide essential services for positioning and time synchronisation. They offer a wide range of technical applications, but also create dependencies. Natural or man-made interference with GNSS signals can have catastrophic consequences for air and sea traffic. In the current political situation in Europe, GNSS signals in the Baltic Sea are repeatedly subject to artificial interference. A backup system that provides sufficiently accurate position and time data in the event of GNSS interference is therefore of crucial importance. The DLR Institute of Communications and Navigation has been continuously developing such a system since 2017 with the terrestrial navigation system R-Mode (Ranging Mode). R-Mode is a cost-efficient approach in which existing maritime radio stations transmit R-Mode signals in the medium-wave range (300 kHz).

Each R-mode transmitter emits a ground wave and a space wave. The ground wave is used to determine the distance between the transmitter and receiver. The space wave is reflected in the E-layer (90-130 km), whereby it is significantly attenuated during the day when it passes through the D-layer (60-90 km). At night, but also during fluctuations in the D layer, this attenuation is cancelled or reduced. At the location of the R-mode receiver, the superposition of ground and space waves leads to a deterioration in R-mode accuracy depending on the state of the atmosphere. Due to insufficient scientific understanding and atmospheric variability, position determination using R-mode at night and during the day in certain conditions of the D-layer does not yet fulfil the required accuracy of 10 m for harbour approaches and 100 m for coastal navigation.

The main objectives of this project are therefore a) to better understand the physical processes behind the propagation of the space wave depending on the atmospheric state, b) to investigate and quantify the variability of all relevant physical parameters, and c) to provide realistic error estimates for position determination with R-Mode. In addition, a warning system concept will be developed to inform R-Mode users about the limited accuracy of the R-Mode service and suggestions will be made to improve the R-Mode system based on the knowledge gained.

The main task of the DLR Institute of Solar-Terrestrial Physics is to describe the state of the lower ionosphere as a function of atmospheric variability, focussing on the propagation of radio waves. To this end, the relationship between the R-mode signal and the VLF measurements (e.g. GIFDS) is to be investigated in order to be able to draw conclusions for the R-mode signal in the event of changes in the lower ionosphere. In addition, the state of the lower ionosphere under different atmospheric conditions is to be derived from the VLF measurements. With the help of the knowledge provided by the partners, an empirical model of the lower ionosphere will be developed, taking atmospheric variability into account. This work will make it possible to derive the accuracy of the R-mode method depending on the state of the lower ionosphere and make it available to IMPC users.

In addition to DLR-SO, the following institutes are involved in the project:

• University of Greifswald
• DLR-KN
• IAP Kühlungsborn
• IOW Warnemünde