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Published on October 30, 2007

Author: Davidson

Source: authorstream.com

AIS-INGV IONOSONDE:  AIS-INGV IONOSONDE C. Bianchi 1, U. Sciacca1, G. Tutone 1, E. Zuccheretti 1 and B. J. Arokiasamy2 (1) Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy (2) TRIL fellow, International Center for Theoretical Physics Trieste, Italy Slide2:  The first prototype was tested for more than 1 year at the Ionospheric Observatory of Rome and the ionograms produced have been compared with other analog and digital ionosondes. The characteristics of this ionosonde also fulfils the requirement of the remote ionospheric Observatory of Gibilmanna (Sicily) of INGV to produce good quality data especially for routine service. A digital low power pulse compressed ionosonde was developed at the Laboratory of the Istituto Nazionale di Geofisica e Vulcanologia (INGV), Rome, (Italy). The objective of this ionosonde was to reduce the transmitted power (less than 100-200 W) and consequently weight and size, power consumption and hardware complexity. Slide3:  AIS-INGV Slide4:  AIS-INGV Slide5:  This ionosonde employs a 16 bit complementary phase code and exploits the most advanced HF-radar techniques such as the pulse compression and the phase coherent integration. Technical Characteristics The ionosonde is completely programmable and data acquisition, control, storage and on-line processing of the acquired data are supported by a PC to which it is directly interfaced. Slide6:  AIS-INGV IONOSONDE Block Diagrams of AIS-INGV Ionosonde:  Block Diagrams of AIS-INGV Ionosonde The physical functional units are realised in printed circuit (board) inserted in a common bus. Two board are inside the PC (Control & DSP boards). The remaining boards are assembled in 19” rack (switching Filter, receiver, ADC, frequency synthesis Code&Timing, Power Supply and Synthetic Echo Boards) that constitute the MAIN UNIT. Slide8:      Main PC program flow chart Slide9:      Slide10:  90° shifter Tx Oscillator 125MHz Reference Oscillator CRF (to PWA) Trigger Generator  12 Code 1 Generator Programmable Counters Impedance Adapter Code 2 Generator AMP trig (to PWA) CODE 30s clock 400kHz_CK (400k) C1/C2 toggle CTM 40MHz Oscillator Rx Oscillator  10  10 1P6T switch SWF FSY RCV  2  2 RAM Addresses Generator ADC (I) ADC (Q) RAM (Q) RAM (I) ADC PC Bus (to PC / BCT board) DSP Bus (to PC / DSP card) 36.955.9 MHz 40 MHz 4 MHz 120 MHz TF#1 TF#2 TF#3 RF echo Rx on/off FRF RF#1 @IF#1 RF#3 LO#1 LO#2 LO#3 RF#2 @IF#2 IF#3 200kHz 100kHz Q_CK I_CK Data out (8 bit) LBI RFF NBS RFF Local bus AMP trig C1/C2 400k 400k                                                                       What’s new in AIS-INGV:  What’s new in AIS-INGV As other Adavanced Ionospheric Sounders AIS-INGV exploits the computational power of recent and most advanced microprocessor especially RISC processor. AIS-INGV employs TMS 320 TI to perform the on-line analysis of the signal Slide12:    The DSP board block diagram   Signal Analysis :  Signal Analysis After the sampling an A/D conversion of the received signal the following operation are performed FFT of the acquired signal Filtering Correlation in frequency domain Phase Coherent Integration IFFT Power signal reconstruction Slide14:                                                                                                          Fig.3.5 – DSP program flow chart   No Correlation for Code 2 Sum of the Correlation Integration Yes Time Domain Amplitude Interrupt PC Complex Inverse Fourier Transform Is code info.=Code1 ? Read Code info - Read 512 Words From ADC Complex Fourier Transform Filter Correlation for Code 1 Yes No Increment Integration Counter End Integration? No DSP program flow chart Yes Is BIO Low?   Processing gain:  Processing gain More or less 30 dB About 15 dB due to correlation process About 15 dB due to phase coherent integration Biphase modulation:  Biphase modulation Code reconstruction:  The reconstruction of the code, baseband, starting from the echo signal is the following: the analog signal at the output of the third IF (100 kHz) is sampled at the same frequency both in phase and quadrature the quadrature sampling allows to obtain the amplitude and, more important, the phase of the signal to reconstruct the baseband after the A/D conversion the signal is fed through the digital matched filer implemented on the DSP board the process is repeated for the even and odd code. Code reconstruction Complementary code:  Complementary code A 16 bit complementary phase code has been used. The code length is 480 microseconds with sub-pulses of 30 microsecond. This particular code eliminate the side lobes of the correlation process and in a certain measure the noise superimposed to the signal Pulse compression technique The energy (E) of the echo signal from the ionosphere, under certain propagative condition, is proportional to the transmitted power (P) and the pulse length (T) E=PT In the old ionosonde the power P was of the order of 10000 W while the pulse length, to have the desired high resolution, was 30 s, so the energy was of the order of 0.3 J. The AIS-INGV as the most advanced ionosonde uses a pulse length of about 500 s and a power of 200 W and the energy of the pulse is 0.1 J (same order of magnitude). The resolution is maintained because an adequate number of sub-pulse  constitute the pulse length T. The particular sequence of the sub-pulses is what we say a code. The pulse compression consists to input the pulse T in a matched filter (which is sensitive only to the chosen code) whose output concentrate its energy in a time . So the matched filter magnifies only the segment of the signal that contains the code sequence.   :  Pulse compression technique The energy (E) of the echo signal from the ionosphere, under certain propagative condition, is proportional to the transmitted power (P) and the pulse length (T) E=PT In the old ionosonde the power P was of the order of 10000 W while the pulse length, to have the desired high resolution, was 30 s, so the energy was of the order of 0.3 J. The AIS-INGV as the most advanced ionosonde uses a pulse length of about 500 s and a power of 200 W and the energy of the pulse is 0.1 J (same order of magnitude). The resolution is maintained because an adequate number of sub-pulse  constitute the pulse length T. The particular sequence of the sub-pulses is what we say a code. The pulse compression consists to input the pulse T in a matched filter (which is sensitive only to the chosen code) whose output concentrate its energy in a time . So the matched filter magnifies only the segment of the signal that contains the code sequence.   Filtering and integration :  Filtering and integration After the FFT, a digital filter, to reduce the amplitude of the strongest frequency component, has been implemented. This filter cut out the frequency of the interfering radio broadcasting etc.. This filter follows empirical criteria and can be modified according to the particular sounding site. The phase coherent integration is a sum in the frequency domain lasting till the phase difference between the first and the last echoes of the incoming signal is less than 90 degrees. After that the integration process is not useful. This process takes into account the time coherence of the reflection process in the ionosphere. Noise rejection:  Noise rejection Hardware consideration:  Hardware consideration     The hardware has been designed on the bases of the following criteria: to employ the most recent and common IC to avoid circuits complexity to use printed circuits and reduce the wiring including data and address busses to facilitate exchange and insert new boards with ease to have good isolation and shielding Board examples:  Board examples Baord layout:  Baord layout Ionogram examples:  Ionogram examples

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