Oblique Vertical

1. Summary

Travelling ionospheric disturbances (TIDs) constitute a threat for operational systems using ground-based HF and trans-ionospheric VHF-UHF radiowave propagation. TIDs can impose disturbances with amplitudes of up to ~20% of the ambient electron density, and a Doppler frequency shifts of the order of 0.5 Hz on HF signals. Therefore, their identification and tracking is important for the reliable operation of critical systems using the ionosphere as an essential part or for systems for which the ionosphere is fundamentally a nuisance.

The Net-TIDE project has developed a warning system for real-time identification of TIDs using skywave Doppler frequency and angle-of-arrival measurements archived in a Digisonde-to-Digisonde (D2D) format. Data are collected from network-coordinated HF sounding between pairs of European DPS4D and processed in real-time for the calculation of the angles-of-arrival and Doppler frequencies of ionospherically reflected high-frequency (HF) radio signals. In the Net-TIDE methodology, the ionosphere is represented by a moving undulated mirror, to relate HF signal parameters to TID characteristics, using the Doppler-Frequency-Angular-Sounding (FAS) technique.

The raw data (D2D skymaps and ionograms) are deposited in near real time via Internet in corresponding databases at the Lowell GIRO Data Center (LGDC). During this initial phase of the project, all data are processed in real-time for the calculation of the angles-of-arrival and Doppler frequencies of ionospherically reflected high-frequency (HF) radio signals. A new database, TIDBase has been set up at LGDC for the ingestion of the new type of skymap data files, “DOP”, containing the D2D skymap data. TIDBase also stores the results of the FAS analysis providing information on the reference time, the location of the TID, the observation metadata (location of instruments, operating frequency, expected maximum group path of 1E signal), extracted time series of the RF signal characteristics, the derived TID characteristics, and the uncertainty/quality flags.

Visualization of the TID amplitude, velocity, wave period, and propagation direction at all reflection points in the network provides a near real-time comprehensive picture of TID activity over Europe. The NET-TIDE identification and warning service, provides the user with an overview of the evolution of perturbation within the last 3 hours displaying the time plots of group path, Doppler frequency, and arrival angles. The TID wave amplitudes measured at different links and frequencies will of course be different and therefore appropriate reference levels are needed to facilitate the assessment of the importance of the perturbation from the user side.

In this deliverable, we describe the methodology implemented to develop the warning system, its functionality and several cases when TID are detected. The system is under development, comments are collected from the users and stakeholders and the final release will be available in November 2017.

2. Net-TIDE warning system methodology

All measured D2D ionogram and skymap data are ingested in real time via Internet in databases at the Lowell GIRO Data Center (LGDC), http://giro.uml.edu/. The D2D skymap data and the FAS analysis results are archived in a new database, TIDBase. During this initial phase of the project, all signal processing of the measured skymap data and the FAS analysis have been performed at LGDC. To enable retrospective analysis of past data TIDBase also stores complimentary information on the reference time, the location of the TIDs (mid-point of the D2D links), the observation metadata like the coordinates of the link endpoints, operating frequency, etc.), and the extracted time series of the RF signal characteristics {ρ, δ, ε, β}. The derived TID characteristics AN, TΩ, Vp, and Θ for the dominant (maximum amplitude) TID wave component are also deposited in TIDBase. The TID characteristics and all required metadata and data from the D2D soundings are harvested by the NOA client service every 5 minutes. For TID alert applications real time Net-TIDE FAS results are displayed in graphical form at http://tid.space.noa.gr; the display is refreshed every 5 min. The chain of operations is displayed in Figure 1.

fig1
Figure 1: The Chain of Net-TIDE operations from the D2D and vertical soundings to the retrieval of raw data and finally to the graphical representation of the results.