VERTICAL IONOSPHERE SOUNDING USING CONTINUOUS SIGNALS WITH LINEAR FREQUENCY MODULATION

A.V. Podlesny, V.I. Kurkin, A.V. Medvedev, K.G. Ratovsky

Institute of Solar-Terrestrial Physics SB RAS, 664033, Irkutsk, Lermontov st., 126a, P.O. Box 291, Russia pav1986@rambler.ru, kurkin@iszf.irk.ru, medvedev@iszf.irk.ru, ratovsky@iszf.irk.ru

Abstract

Consideration of the digital chirp sounder design for the modern systems of a geophysical monitoring, communication channels state forecasting and the research problems solution is given.

Given the comparative results of pulsed and of continuous signal vertical sounding ionosondes.

1. Introduction

The high interference immunity of a chirp ionosonde and its good electromagnetic compatibility compared with a pulsed ionosonde predetermined wide use of chirp ionosondes for fundamental and applied research in the field of physics of the ionosphere and radio-wave propagation.

Ionosondes for vertical and slightly oblique sounding are used for ionosphere monitoring in the vicinity of a

diagnostic facility as well as for a study of physical processes in the ionosphere subject to natural and artificial disturbances. Oblique-sounding ionosondes are intended for a study of the ionosphere along the propagation path and the behavior of decameter radio waves propagated under different geophysical conditions and are employed in systems of frequency provision of adaptive communication systems for a prompt choice of optimal operating frequencies (optimal radio channel). Chirp ionosondes of oblique backscatter sounding are used in systems of frequency provision of HF communication and over-the-horizon HF radars [1]. Theory of the chirp sounding and the main characteristics can be found in [2].

An experimental network of chirp ionosondes for oblique sounding of the ionosphere, equipped with software

and hardware developed at the Mari State Technical University (MariSTU), Radiophysical Research Institute of Nizhny Novgorod (NIFRI), and Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences (ISTP RAS SB) has been operated since 1988 in Russia. Using this network of a chirp ionosondes was made extensive data on the state of a natural and artificially disturbed ionosphere and to examine the features of formation of an HF signal field on midlatitude, subauroral, and transequatorial paths of different length.

In mono-static chirp observations, coupling between the transmitting and receiving antennas is inevitable in

HF band used by ionosonde, such that a considerable part of emitted power from the transmitted antenna sneaks into the receiving antenna and suppresses the receiver. Because of it for vertical sounding applied a pulse transmission ionosondes and a pulsed chirp ionosondes [3]. But high transmitted power of this ionosondes cause of interference with neighboring radio systems, making the system unsuitable for continuous observations and denies the possibility of simultaneous operation in the mode of oblique sounding. Sodankyla Alpha-Wolf ionosonde with continuous chirp signal have a minimal known distance between receiving and transmitting antennas of 930 meters [4]. However, applying of digital signal processing allows for simultaneous operation of the ionosonde for transmission and reception of a signal without overloading the receiver when placing the antennas at a distance of less than 100 meters. This was made possible thanks to a new generation of multichannel digital receiver that has superior performance results from its innovative, direct-sampling, digital down-converter architecture along with the use of leading-edge components and design concepts. These all result in a very high IP3, wide dynamic range, excellent sensitivity, selectivity and tuning accuracy. These key features create a receiver in a class of its own, with wide application potential, at a very affordable price. We have developed a mono-static ionosonde, allowing for a small (about 4 watts) of power to obtain information about the state of the ionosphere. This ionosonde can be an excellent basis for creating the All-Russian network for geophysical monitoring and forecasting propagation of radio waves.

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2. New ionosonde of vertical ionosphere sounding using continuous signals with linear frequency modulation

The summer of 2010 near the village of Tori (51.8 N, 103.1 E), the Republic of Buryatia, was launched a

prototype of a new ionosonde vertical sounding (short title Monostat), developed in the ISTP SB RAS. During the eight months trial of continuous operation of the new ionosonde amassed hundreds of thousands of ionograms of oblique and vertical sounding data, confirming availability of a new instrument in all major modes of his work.

2.1. Construction

Structurally Monostat made in the form of two independent blocks of the receiver and transmitter. Monostat’s

transmitter, performed on GPS controlled DDS, 12 bit amplitude accuracy, 100 MHz clock rate, 4 watts peak power semiconductor amlifier runs on a vertical rhombic antenna with a vertical depth of 21 m in height, oriented in the east-west direction. Receiver performed on the basis of internal PCI cards direct sampling, software defined radio, works on delta-loop with the overall depth of 10 m, oriented in a north-south direction and located at a distance of 60 m from the transmitter antenna. This modular construction allows not only vertical sounding, but also near-vertical and oblique sounding simultaneously, that can not be implemented in other types of ionosonde.

All data received by a new instrument, are stored in the original raw data allows computation of ionograms

using different, non-standard resolutions and better noise rejection if new better technique and filters are available. In normal mode vertical sounding measurements are carried out every minute at rate of 200 kHz / s from 1.3

to 10 MHz and a peak power of 4 watts. Also simultaneously with the vertical sounding receives a signal every minute near-vertical sounding of chirp of the transmitter power of 100 watts, which is located at a distance of 160 km near the town of Usolie-Siberian and working at rate of 200 kHz / s from 1.5 to 10 MHz. During coordinated observations Monostat also receives signals from the oblique-sonding chirp stations located in Norilsk, Khabarovsk and Magadan.

2.2. Filtration

After measuring the received raw data is processed and filtered by different algorithms. Figure 1a shows a

typical ionogram received the Monostat without filtration. The first stage of processing make is FIR filtering to suppress signals from the transmitted antenna sneaking into the receiving antenna. At the second stage make filtration radio station interference by amplitude limitation. Figure 1b presents a typical ionogram after the last stage of processing estimate background noise and subtraction it.

a) b) Fig. 1. Filtering system of Monostat a) without filtration, b) after FIR, station suppress and noise subtraction.

Spectral purity digital synthesis of the transmitted signal, a wide linear dynamic range of digital receiver’s provides modern chirp-ionosonde ionograms extremely high quality. At the same time a good temporal and spatial resolution make digital ionosonde one of the most informative radio-physical tools for investigating the nature of small and medium-scale ionospheric formations.

2.3. Comparison of DPS and Monostat

The DPS-4 [5] utilize coherent transmission and reception of HF radio pulses at appropriate repetition frequencies to be unambiguous in range and/or Doppler for the ionospheric regions under observation. This instrument can rely on the signal processing gain from eight-bit complementary phase-coded pulse compression (12 dB) and coherent Doppler integration (providing 9–21 dB additional processing gain), allowing the transmission of low power RF without compromising echo detection. DPS-4 is capable of measuring group range, amplitude and phase, Doppler shift and spread, angle-of-arrival (AOA) and polarization of echoes reflected by the ionosphere.

Monostat does not transmit in pulses of different frequencies. Instead, a frequency-modulated countinuous-

wave chirp is performed at the rate of 200 kHz/s from 1.3 to 10 MHz. The instrument is thus capable of performing one sounding per minute versus 15 minutes for DPS. Table 1 summarizes the main characteristics of both instruments.

Table 1. Instrumentation characteristics

Features DPS-4 Monostat Tx power/antenna 2 x 150 W/cross-rhombic 1 x 4 W/rhombic Rx antenna Cross-loop Single delta-loop Tx polarization Elliptical left or right Linear Rx polarization Circular left or right Linear Interferometer Equilateral triangle 60 m No Max. PRF 200 Hz Continuous signal Min. PRF 50 Hz/Nmultiplexed Continuous signal Height resolution 2.5, 5, 10 km Selected by user, typical 1.8 km Frequency range 1-40 MHz 1-40 MHz

The possibility of obtaining ionograms of vertical sounding was used for comparison of results obtained by DPS-4, located in Irkutsk, and Monostat’s located in Tory at a distance of 100 km. Figure 2 shows the typical diurnal variations of critical frequencies (foF2, foF1, foE) and heights of maxima (hmF2, hmF1, hmE) ionospheric layer F2, F1, and E, respectively, obtained by Monostat (gray dots) and DPS-4 (black points) 15.08.2010. In Table 2 shows the statistics of deviations between the data obtained by Monostat ionosonde and DPS-4. Figure 2 and Table 2 shows that the characteristics of the different instruments are close to each other, the differences between the data is much smaller than the values of characteristics. Differences appear due to space diversity of tools and their analysis requires the individual studies. Thus, trial experiments showed that the pulsed ionosonde (Digizond DPS-4) and Monostat have approximately the same potential, though the power of the transmitter chirp ionosonde (4W) by very smaller magnitude than the power of the transmitter DPS-4 (300 W).

Figure 2. Results of Monostat and DPS-4 comparison

Table 2. Dispersion of data obtained Monostat and DPS-4

Features ΔfoF2 (MHz) ΔfoF1 (MHz) ΔfoE (MHz) ΔhmF2 (km) ΔhmF1 (km) ΔhmE (km) Average -0.02 0.06 0.09 -5.7 5.7 1.5 Dispersion 0.15 0.14 0.05 14.5 17.2 2.0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24UT1

2

3

4

5

6

7foF2, foF1, foE, fmin (MHz) DPS-4 Monostat 15.08.2010

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24UT100

150

200

250

300

350hmF2, hmF1, hmE(km) DPS-4 Monostat 15.08.2010 ã.

3. Results Below in Figure 3 shows examples ionograms of vertical, near-vertical and oblique sounding, obtained during

the coordinated observation on a network chirp sounding of the ISTP SB RAS in the first quarter of 2011. All ionograms, obtained from the Monostat’s receiver are automatic cleanup of the station interference and for ionograms of vertical sounding also removed the traces of the sneaks of the ground wave.

a) b) d) e) Figure 3. Ionograms of oblique (a, b), vertical (d) and near-vertical sounding (e) obtained by Monostat

Modern digital chirp ionosonde has a distinct advantage over traditional pulse ionosonde. It features high noise immunity, high time resolution of group delay, has small dimensions, weight and power consumption.

References

1. V.A. Ivanov, V.I. Kurkin, V.E. Nosov, V.P. Uryadov, and V.V. Shumaev, “Chirp Ionosonde and Its Application in the Ionospheric Research,” Radiophysics and Quantum Electronics, Vol. 46, No. 11, 2003, pp. 821-851. 2. N.V. Ilyin, V.V. Khakhinov, V.I. Kurkin, I.I. Orlov, and S.N. Ponomarchuk, “The theory of chirp-signal sounding,” Proceeding of ISAP’96, Chiba, Japan, 1996, pp.689-692 3. A.W.V. Pool, “On the use of pseudorandom codes for “chirp” radar,” IEEE transaction on antennas and propagation 1979. v.AP27, n. 4, pp. 480-485. 4. T. Ulich, T. Turunen, E. Turunen, “The new Sodankula ionosonde,” 38th COSPAR Scientific Assembly, Bremen, Germany, 2010. 5. D.M. Haines, and B.W. Reinisch, “Digisonde Portable Sounder System Manual,” University of Massachusetts Lowell Center for Atmospheric Research, 1995.