Annals 48, 3, 2005+app1 445 ANNALS OF GEOPHYSICS, VOL. 48, N. 3, June 2005 Key words ionosonde – automatic ionogram scal- ing – ionospheric data – vertical soundings 1. Introduction Ionospheric observations contributed to the knowledge of physical phenomena such as the radio propagation in ionised media, physical and chemical processes in upper atmosphere, ionosphere and magnetosphere coupling, solar- terrestrial relations, etc. Nowadays the scientif- ic interest remains but the ionospheric observa- tions are much more focused on radio propaga- tion forecasting. To achieve this objective the new ionospheric sounders should have some distinctive characteristics especially oriented towards routine service with network link and automatic scaling of the ionograms. Like other recent sounders, AIS-INGV ionosonde is practically built around a PC that constitutes the most important part (fig. 1). This ionosonde has been designed to fulfil certain physical characteristics such as the power re- duction (around 200 W against several kilo- watts of traditional systems) and consequently weight, size, power consumption and hardware complexity. It exploits the computer resources to manage the sounding, real time signal pro- cessing, data storing and sharing; it also has the capability to be remotely programmed. The basic work of this prototype is to gen- erate an ionogram from which virtual heights and critical frequencies can be scaled. This ba- sic system allows future expansions including polarization measurement and doppler analysis. This first prototype has been installed at IN- GV Gibilmanna Ionospheric Observatory locat- ed in the centre of the Mediterranean area where no systematic ionospheric observations have been performed. In recent years the growing in- terest in real time mapping and short term fore- casts produced efforts to achieve real time scal- New low power pulse compressed ionosonde at Gibilmanna Ionospheric Observatory Baskaradas James Arokiasamy (2), Cesidio Bianchi (1), Michael Pezzopane (1), Vincenzo Romano (1), Umberto Sciacca (1), Carlo Scotto (1), Giuseppe Tutone (1) and Enrico Zuccheretti (1) (1) Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy (2) TRIL fellow, The Abdus Salam International Centre for Theoretical Physics (ICTP), Trieste, Italy Abstract A digital low power pulse compressed ionosonde was developed at the Istituto Nazionale di Geofisica e Vul- canologia (INGV), Rome, Italy. The aim of this Advanced Ionospheric Sounder, AIS-INGV, is to reduce the trans- mitted power and, consequently, weight, size, power consumption and hardware complexity. To compensate the power reduction the most advanced HF radar techniques such as the pulse compression and a phase coherent inte- gration are used. The ionosonde is completely programmable and a PC supports the data acquisition, control, stor- age and on-line processing. The first prototype was installed at Gibilmanna Ionospheric Observatory (Sicily), an interesting location in the center of Mediterranean area. The new ionosonde will contribute to ionospheric database and real time knowledge of South European ionospheric conditions for space weather applications. In this work the first results (ionograms and autoscaled characteristics) are presented and briefly discussed. Mailing address: Dr. Enrico Zuccheretti, Istituto Na- zionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, Italy; e-mail: zuccheretti@ingv.it 446 Baskaradas James Arokiasamy et al. ing of ionograms which led to many softwares (e.g., Reinisch and Huang, 1983; Fox and Blun- dell, 1989; Igi et al., 1993; Tsai and Berkey, 2000). Among these, widely used and tested (Gilbert and Smith, 1988) is the ARTIST system, developed at the University of Lowell, Center for Atmospheric Research. The INGV ionospheric laboratory designed and developed software to scale ionospheric pa- rameters foF2 and MUF(3000)F2 automatically within a few minutes after every sounding. To date, autoscaled characteristics as well as iono- grams are available real time at the site . Fig. 1. Block diagram of the AIS-INGV ionosonde: red lines are digital signals while blue lines are analog. 447 New low power pulse compressed ionosonde at Gibilmanna Ionospheric Observatory 2. Description of the basic principle and system characteristics The specifications of the new system are re- ported in Zuccheretti et al. (2003) and Aroki- asamy et al. (2002). The new ionosonde was designed on the base of radar systems theory (Skolnik, 1980, 1997) applied to the study of the ionosphere (Hunsucker, 1991). We exploited the information of the trans- mitted code, as in other phase-coded HF-VHF radars, to perform the pulse compression and the coherent integration (a more detailed de- scription of the mathematical processes is giv- en in Bianchi et al., 2003). According to a general radar equation (Le Chevalier, 2002) the received power Pr is P r P G L G 4 1 4r t t p r 2 2 $ $= r r m (2.1) where Pt is transmitted power, r is the range, Gt and Gr are the transmitting and receiving anten- na gain, λ is the wavelength, and Lp represents all the losses. The bandwidth of the receiver, being less than the thermal noise, can be neglected with respect to the environmental noise N. The min- imum S/N useful for detection is /S N N P G Gr a process$ $ = (2.2) where Ga includes all the analog gain in the re- ceiver chain and Gprocess is the processing gain. To achieve the desired S/N, the processing gain Gprocess must be greater than 20-30 dB, be- cause of various factors like poor antenna gain (less than 2 dBi), the reduced transmitted pow- er and the maximum required range. This con- straint imposes limits on the system parameters like the Pulse Repetition Frequency (PRF), pulse width T, subpulse number M, phase-code and modulation characteristics (Bianchi et al., 2003). A 16 chips bi-phase complementary code has been employed (Golay, 1961); this particular code theoretically eliminates the side lobes as they are opposite in phase (fig. 2). The processing gain after the pulse compression, expressed as S/N, is given by 10log(M) that for M = 16 is approximately 12 dB. A further con- tribution to the gain comes from the integration process based on the ionospheric coherence. The process of the phase coherent integration, depending on the ionospheric variability, can be performed till the phase of echoes sequence dif- fers less than π / 2, which limits the number N of the integrations. At Gibilmanna Ionospheric Ob- Fig. 2. Correlation results for code 1, code 2 and their addition. On x axis distance of an echo in km is indicated, while on y axes an arbitrary scale is used. Table I. Processing and phase code features. Code Specifications Type Bi-phase complementary Code length T 480 µs Subpulse number M 16 Subpulse length τ 30 µs Pulse Repetition 30 Hz Frequency (PRF) Processing gain due 12 dB to the correlation M Processing gain due 8-18 dB to the integration N 448 Baskaradas James Arokiasamy et al. servatory different tests suggested a maximum number of integrations of 30. Table I summarizes the main parameters of the phase code and pro- cessing gain; other parameters have been chosen on the basis of the radar techniques. Assuming light speed as the radio wave ve- locity and 750 km as the range, the maximum PRF should be around 200 Hz, but processing time limits this parameter to 30 Hz; this value of PRF still makes the integration process ad- vantageous. Other design parameters are related to spa- tial resolution and minimum detectable dis- tance from the ground (Arokiasamy et al., 2002). A pulse width of 480 µs and 16 chips code with a chip length of 30 µs lead to a min- imum range of 72 km and a radar resolution of around 5 km (table II). 3. The INGV software for automatic scaling of ionograms The INGV software is based on a technique of image recognition and is able to work without polarization information. Hence it can be used Table II. AIS-INGV programming parameters. Parameter Requirement Height range (90–750) km Distance resolution 5 km Peak transmitted power 200 W (5 ∼ 10 W) (medium power) Receiver sensitivity ∼ –85 dBm for 0 dB S/N Input dynamic range ∼ 80 dB Frequency range (1–20) MHz Frequency resolution 25 kHz, 50 kHz, 100 kHz Scan duration 3 min (1.5 MHz-20 MHz, 50 kHz step) Acquisition sampling rate 100 kHz Acquisition quantization 8 bit Storage data rate (max) 60 kbytes (with 50 kHz step) Fig. 3a-c. a) A typical AIS-INGV ionogram. b) Se- lected element of the family of functions superposed on the ionogram. The automatically detected traces are reported (in red the ordinary trace and in yellow the extraordinary trace). c) Vertical asymptote and tangent transmission curve of the automatically ordi- nary trace detected in correspondence of which foF2 and MUF(3000)F2 are calculated. a b c 449 New low power pulse compressed ionosonde at Gibilmanna Ionospheric Observatory with both single antenna system and crossed an- tenna system. A family of empirical functions having the typical shape of the F2 trace is con- sidered. A particular element of this family is se- lected by a maximum contrast technique and it is assumed as representative of the F2-layer trace. The vertical asymptote of the selected function corresponds to the critical frequency foF2; the MUF(3000)F2 is calculated numerically find- ing the transmission curve tangent to the select- ed function (fig. 3a-c). Using different families of empirical functions this method can in princi- ple be applied for the identification of the other ionospheric layers. For radiopropagation pur- poses the real time scaling of E sporadic and F1- layers would also be important. With respect to the previous version of INGV software for automatic scaling of ionograms (Scotto and Pezzopane, 2002), two main im- provements have been introduced: 1) The parameterization of the girofrequen- cy that makes this version able to scale iono- grams recorded in any location. 2) The capability to identify ionograms with sufficient information, to make them properly scaled. If the ionogram is identified by the soft- ware to have insufficient information it is dis- carded by the program and neither the foF2 nor the MUF(3000)F2 are given as an output. To test the software a comparison between the values scaled by an operator and by the pro- gram was performed considering 1124 iono- grams recorded by the AIS-INGV installed at Gibilmanna Ionospheric Observatory in 2002 from December 1st to December 15th. 3.1. Quantitative estimation of the software’s capability to identify ionograms with sufficient information To test the capability of the software to properly identify the ionograms with sufficient information, the processed ionograms were di- vided into two subsets: subset S, that contains the ionograms considered by the program with sufficient information, hence scaled, and subset N containing ionograms considered by the pro- gram with insufficient information, hence dis- carded. For each subset we considered: a) the num- ber of ionograms for which the operator was able to scale neither the foF2 nor the MUF(3000)F2; b) the number of ionograms for which the oper- ator was able to scale foF2 only; c) the number of ionograms for which the operator was able to scale MUF(3000)F2 only; d) the number of iono- grams for which the operator was able to scale both foF2 and MUF(3000)F2. The results of this analysis are reported in table III. It can be observed that 9 ionograms are considered with sufficient information with- out having it, while 7 ionograms are improper- ly considered with insufficient information. This shows that the error percentage in recognising the trace of the ionogram by the software is 1.4% (16 out of 1124). With refer- ence to this it is important to emphasize that for the software the lack of trace near the F2 region asymptote makes the choice of a particular ele- ment of the family of functions more difficult. This lack of trace sometimes leads the software to discard the ionogram even if an operator could scale MUF(3000)F2. On the contrary if the trace near the F2 region asymptote is clear- ly visible the program scales the ionogram for both while an operator could scale only foF2. We can therefore conclude that the software capability to identify ionograms with sufficient information is quite good. 3.2. Test of accuracy and acceptability of the automatically scaled parameters In this work an accurate value is considered to lie within ± 0.1 MHz of the value obtained by the operator for foF2 and ± 0.5 MHz for MUF(3000)F2. An acceptable value is consid- ered to lie within ± 0.5 MHz for foF2 and ± 2.5 MHz for MUF(3000)F2. Limits of acceptability have been adopted in accordance with the URSI limits of ± 5 ∆ (∆ is the reading accuracy). With reference to the ionograms scaled by the INGV software, the following three subsets have been considered (see table III): subset A, containing the ionograms in which the operator was able to scale both the critical frequency foF2 and the MUF(3000)F2; subset B, contain- ing the ionograms in which the operator was 450 Baskaradas James Arokiasamy et al. able to scale the MUF(3000)F2 only; subset C, containing the ionograms in which the operator was able to scale the foF2 only. A quantitative comparison between the val- ues scaled automatically and manually has been performed and the results of the comparison are reported in table IV. 4. Observatory and data management The software performs all the system oper- ations and data management. Sounding param- eters, number of integrations and sounding time scheduling, can be remotely changed at any time writing a new configuration file down- loadable by the ionosonde, so remote control is also possible. The system is completed with a device able to turn the amplifier on and to connect the an- tenna system just during the sounding time, in order to use less energy and to protect the system against ESD due to storms. At present the ionosonde performs soundings every fifteen minutes using a frequency range from 1.5 MHz to 13.0 MHz with a step of 50 kHz. At the end of every sounding the acquired data file is imme- diately available on , while processed ionograms and autoscaled data are available at the web site within few minutes as GIF file and TXT file re- spectively. The research team makes also a man- ual validation of the hourly data that can be shared on demand. 5. Conclusions Nowadays ionosondes are intended to be ionospheric conditions monitoring systems, rather than being just scientific instruments for studying the physics of the ionosphere. In fact short-term ionospheric forecasts are based on real-time data from vertical sounders and best results could be obtained connecting several ionosondes in a net covering an area and obtaining data from it. This means that a modern efficient ionosonde has to be low cost, easy to install and remotely programmable; in addition it should have the possibility to auto- Table IV. Subset A, subset B and ubset C: acceptability and accuracy percentages of autoscaled parameters. Subset A Subset B Subset C foF2 MUF(3000)F2 MUF(3000)F2 foF2 N° [%] N° [%] N° [%] N° [%] Total 847 100.0 847 100.0 182 100.0 12 100.0 Acceptable 833 98.3 847 100.0 179 98.5 9 75.0 Accurate 638 75.3 827 97.5 170 93.5 6 50.0 Table III. Test results of the software capability in identifying ionograms with sufficient information. No. of cases scaled No. of cases discarded by the INGV software by the INGV software (subset S) (subset N) The operator scaled neither foF2 nor MUF(3000)F2 9 46 The operator scaled both foF2 and MUF(3000)F2 847 (subset A) 7 The operator scaled MUF(3000)F2 only 182 (subset B) 21 The operator scaled foF2 only 12 (subset C) 0 Total 1050 74 451 New low power pulse compressed ionosonde at Gibilmanna Ionospheric Observatory matically scale the main characteristics giving a contribution to have reliable forecasts within few minutes after sounding. 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