Darbant ve Genişbant Sanal İyonosonda için Sinyal İşleme Teknikleri

Abstract

One of the mostly used band for long range communication is High Frequency (HF) band. Radio waves in the HF band range (3 − 30 MHz) are reflected from ionosphere which is one of the layers of atmosphere. So, a communication link can be set between two nodes which are thousands of kilometers away from each other. Hence, it is important to characterize the ionosphere state of which depends on date, time, location, sun activities etc. Ionosondes are used for this purpose. Ionosondes can be considered as HF band radar, so the ionospheric layer heights can be calculated by measuring the time delay between transmitted and received signals through the relation between critical frequencies and reflection heights.

The construction of ionosonde stations are costly and requires special expertise. That’s why, there are limited number of stations around the world and they are sparsely distributed. So, there are still some blind zones. To compansate the missing data for blind zones, tomographical methods using satellites and interpolation methods through earth measurement points are utilized. In this thesis, this problem is handled by considering ionosonde operation and the steps of ionosonde operations are realized in simulation environment. Hence, 3D ray tracing, ITS channel model and IRI-Plas estimation tools are integrated together to construct an ionosonde structure. Total Electron Content (TEC), hmF2 and foF2 values are fed into IRI-Plas to improve the electron density estimation results. The ionosonde structure results are compared with the real ionosonde measurement results for ionospheric stormy and calm days. The results show that, as long as the ionosonde structure is fed by true hmF2, foF2 values, it is possible to take similar results with real ionosonde measurements in terms of ionogram shape, layer heights and maximum usable frequency (MUF) results.

Another problem about ionosondes is their high output power (i.e. around 100 − 600 W) and frequency sweep due to their narrow instantaneous bandwidth. Sweeping a large band in HF with relatively high output power is a problem for other HF band users, since it produces interference for other users. So in this thesis, wideband waveforms (1 MHz bandwidth), ITS channel model and time frequency analysis methods are utilised to costruct ionogram trace with relatively low output power (20 W). For this purpose, Barker, PN and Golay codes are investigated among phase coded waveforms. Linear Frequency Modulation (LFM), Costas coded and Non Linear Frequency Modulation (NLFM) waveforms are investigated among frequency modulated waveforms. All waveforms are evaluated in terms of their ambiguity functions and their effects on ionogram traces. LFM waveforms are selected because of their flat spectral view along their bandwidth, robustness to frequency variations and availability for long duration pulses. Wigner-Viller Distribution (WVD), Short Time Fourier Transform (STFT) and Reassignment methods are investigated for time frequency analysis. As a result, Reassignment method is selected due to its high resolution, distinction capability and cross term features. In the simulations, the current narrowband ionosonde waveforms and operations are compared with those of the proposed wideband method. The results show that, for low SNR values the proposed method performs better than the current narrowband method. One more comparison is performed for ionogram construction durations and it is concluded that, wideband method is able to construct the ionogram trace in 20 − 70 ms. As a result, it is shown that, in order to construct ionogram traces with low output power, without interfering other HF band users and in very short time duration is possible by using the proposed wideband method. This result emphasizes that it can be easily adapted to track sudden changes in the ionosphere, especially for stormy ionospheric days when rapid changes occur.