RU2552530C2 - Method of obtaining ionogram - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: invention relates to radio engineering, telecommunication, radar and can be used in systems for diagnosis of plasma phenomena in the Earth's ionosphere. The method of obtaining an ionogram includes, at each probing cycle, emitting a radio pulse in the form of a discrete-frequency signal (DFS) packet, which is a sequence of N pulses with a different frequency and the same duration from an array of fixed probing frequencies; receiving and measuring parameters of the reflected radio signal during propagation from the emitter to the receiver simultaneously and independently at each of the N frequencies of the DFS packet; readjusting the DFS frequencies to a new packet of DFS frequencies from the array of fixed probing frequencies, and emitting and receiving the reflected signal on the new DFS packet; successively selecting resetting the frequencies of the array of fixed probing frequencies with new DFS packets at each probing cycle until complete resetting of all frequencies in the array of probing frequencies over the time interval ΔT_DFS=ΔT/N, where ΔT is the standard time for obtaining an ionogram.

EFFECT: obtaining an ionogram over a time interval considerably shorter than 1 s.

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Description

The invention relates to radio engineering, telecommunications, radar; can be used in systems for diagnosing plasma phenomena in the Earth's ionosphere.

Ionograms are obtained with vertical (VZ), oblique or reciprocal oblique sounding of the ionosphere. The most common OT. When the ionosphere is overwhelmed by ion probes, the ionogram is recorded in about a minute. Ionograms (distance-frequency characteristics of the radio signals reflected from the ionosphere when the ionosphere is radiated at frequencies in the range of 0.3-20 MHz) provide information on the altitude profile of the electron concentration in the ionosphere, which is considered as a quasi-stationary medium with time variations of the order of a minute or more. Methods for producing ionograms are well known [1, 2, 3]. At an EW, the ionospheric E _s layer closest to the Earth's surface, located at an altitude of ~ 110 km, gives a reflected signal with a delay of ~ 700 μs. This value limits the duration of the probe signal τ _imp , for example, in conventional analog ionosondes, the value τ _imp ~ 50-100. In modern digital ionosondes, difficultly manipulated signals are used to increase the energy potential of radar, for example, the DPS-4 ionosonde [3] uses a PCM signal with 16 element Barker code, consisting of elements (pulses) of duration Δτ ~ 33.33 μs each with manipulation of the initial phase from impulse to impulse; the entire package has a duration of τ _imp = NΔτ = 533.3, the duration of the minimized signal at the level of 0.5 33.33 μs, the energy gain in the convolution is 32 dB.

The prototype for the proposed method for producing ionograms can be selected as the most fully described in the literature ionosphere type "Basis" [1, 2]. In the prototype to obtain an ionogram produce the following basic operations:

1. Radiation of a sounding radio pulse. The pulse duration is τ _imp (usually about 100 μs). The carrier radio frequency - f ₀ is in the range of 0.1-40 MHz. The initial sounding frequency is f _min (can be selected from 0.1 to 1 MHz), the final frequency is f _max (10-40 MHz). Sounding frequency range Δf = f _max -f _min . Pulse repetition frequency f _rep (usually - 50 Hz).

2. Reception and registration of a radio signal reflected from the ionosphere at a sounding frequency f ₀ and measurement of the reflected signal parameters (intensity, group delay, arrival angles, polarization, Doppler frequency shift) during the propagation time from the emitter to the receiver.

3. Tuning the frequency of the emitted radio pulse of the transmitter and receiver to a new sounding frequency (f ₀ + δf) and performing operations 1 ÷ 2 at a new frequency (f ₀ + δf), Here δf is the frequency tuning step, which is usually chosen within 10 ÷ 100 kHz Frequencies are selected from a given grid of fixed sounding frequencies.

4. Operation 3 is repeated, starting from the initial frequency f _min to the final sounding frequency f _max with a complete exhaustion of the sounding frequencies for the time interval ΔT. The probe repetition rate f _rep = 1 / ΔT.

In VZ, the working range of sounding frequencies is usually set in the range from 1 to 15 MHz. With a uniform step δf of frequency change, in order to overlap a given sounding frequency range, it is necessary to emit and receive signals at M frequencies (M is equal to Δf / δf).

The maximum group delay of the once-reflected signal Δt is in the range 5-7 ms, therefore, the sending frequency of probe pulses f _rep can be selected, for example, equal to 50 Hz (maximum delay Δt = 20 ms), if we set f _rep = 100 Hz, then Δt = 10 ms, at f _rep = 200 Hz we get Δt = 5 ms.

The total time of recording an ionogram with a single sounding at each frequency is determined by the formula

$$\Delta T=M/f_{P\mathrm{about}\mathrm{at}t.}(\mathrm{one})$$

In the practice of probing the ionosphere, the number of frequencies M used is usually 400. Then, for example, at a pulse repetition rate f _rep = 100 Hz, the minimum time for obtaining an ionogram according to (1) will be ΔT = 4 sec, for f _rep = 200 Hz the minimum time for obtaining an ionogram will be ΔT = 2 sec, and for the standard repetition frequency of most ionosonde f _rep = 50 Hz and ΔT = 8 sec.

In studies of ionospheric disturbances associated with fast phenomena in the ionospheric plasma, such as, for example, the effect of a solar flare on the Earth's ionosphere, it becomes necessary to register processes with a characteristic change time of ~ 1 sec or less. Similar development times have the ionospheric effects of magnetic storms, as well as the effects of artificial disturbance of the ionosphere during non-linear exposure to powerful radio waves that develop in fractions of a second. A special situation arises when the ionosphere is overhead from the satellite, when the ionosonde moves in orbit at a speed of ~ 8 km / s and the ionogram cannot be assigned to geographical coordinates with sufficient accuracy.

These reasons make it important for practice to record ionograms over a period of time ΔT significantly less than 1 second.

The technical problem to be solved is obtaining an ionogram for a time interval of significantly less than 1 second.

In the proposed method for obtaining an ionogram listed in the prototype operations 1-4 are modified:

1. The first operation (radiation of a radio pulse) in the proposed method is modified as follows: on each sounding cycle, it is proposed to radiate a sounding radio pulse in the form of a packet of a discrete-frequency signal (DFS), which is a sequence of the following continuously one after another N radio pulses (elements) of different frequencies , but of the same duration Δτ. The frequencies that make up the DFS package are selected from a given grid of fixed sounding frequencies.

2. The second operation (receiving the reflected signal at the sounding frequency f ₀ ) in the proposed method is modified as follows: in each sounding step, the parameters of the reflected radio signal are simultaneously received and measured independently at each of the N frequencies making up the DPS package. With a uniform distribution of the frequencies of the emergency package over the sounding frequency range, the frequency of the emergency package can be written: f ₀ , f ₀ + Δf _n , f ₀ + 2Δf _n , f ₀ + 3Δf _n , ... f ₀ + (N-1) Δf _n , where Δf _n = Δf / N. In this case, the parameters of the reflected signal (intensity, arrival angles, polarization, Doppler frequency shift), as in the prototype, should be measured in the delay range from 700 μs to 1 / f _rep.

3. The third operation. Reconfiguration of the transmitter and receiver channels to a new set of N frequencies of the new DFS package. With a uniform distribution of the frequencies of the DFS package over the sounding frequency range and the same frequency tuning of δf for each component of the new DFS package, the frequencies of the new DFS package can be written: (f ₀ + δf), (f ₀ + δf) + Δf _n , (f ₀ + δf) + 2Δf _n , (f ₀ + δf) + 3Δf _n , ... (f ₀ + δf) + NΔf _n . Then, operations 1 and 2 are performed for a new package of emergency situations.

4. The fourth operation. Operation 3 is repeated until all sounding frequencies from a given grid of fixed frequencies are completely sorted out within the sounding frequency range Δf.

When using DFS, the time of obtaining an ionogram is determined by the formula

$$\Delta T_{dh\mathrm{from}}=M/\left(f_{P\mathrm{about}\mathrm{at}t\text{A.}}N\right)=\Delta T/N(2)$$

From a comparison of formulas (1) and (2), we can conclude that for the same regime parameters of recording the ionogram (Δf, δf, f _rep , M), the value of ΔT _dhs is N times less than the time ΔT, for which a similar ionogram is obtained in the standard way.

The decrease in the time of obtaining the ionogram is due to the fact that virtually the entire frequency range of the ionogram Δf was proposed to be divided into N separate frequency subbands and at the same time to probe in each of the N subbands. At the same time, the reception and measurement of parameters are carried out simultaneously and independently for each of the N frequencies of the DFS package.

Usually, in practice, air pulses using radio pulses with a duration of at least 50 μs are used. Since the maximum duration of the pulse of sounding of the ionosphere cannot be more than 700 μs, the duration of the DFS packet should be less than 700 μs, and N the number of frequency elements in the DFS packet is limited to no more than 14. The number 14 is the maximum reduction ratio possible in practice time to obtain an ionogram when using the proposed method. For example, to obtain an OT ionogram at 400 frequencies with a pulse sending frequency f _rep = 100 Hz, the minimum possible recording time will be (4/14) sec., I.e. less than 0.3 sec.

The proposed method is further illustrated by the following graphic images:

Figure 1. The view of the package of the DES for the case of N = 5.

Figure 2. Frequency-time diagram of the arrangement of signals during sounding by a package of emergency.

Figure 3. Functional block diagram of a transmitter synthesizer for DFS with N = 4.

Figure 4. Functional block diagram of a multi-channel RPU for receiving and registering signals of the emergency.

It can be seen from the diagram of the emitted DFS package presented in FIG. 1 that during one probe cycle during the time τ _imp = ΔτN, a broadband sequence of N pulses at discrete frequencies will be emitted. For example, with a uniform distribution of individual elements of the DFS across the entire range of sounding frequencies Δf, the frequencies of the DFS can be in the form of a series of values f ₀ , f ₀ + Δf _n , f ₀ + 2Δf _n , f ₀ + 3Δf _n , ... f ₀ + (N -1) Δf _n , where Δf _n = Δf / N.

Figure 2 shows a graph of the ionogram for a typical daytime ionosphere, against which the principle of obtaining the ionogram of the proposed method is schematically illustrated. The “O” and “X” curves are the “ordinary” and “extraordinary” components of the signals reflected from the ionosphere. The scale on the left is the effective height, on the right is the group delay scale. In the delay interval from 0 to 0.5 ms, the position of the probe pulses at the working frequencies f ₁ , f ₂ , f ₃ , f ₄ , f ₅ (filled rectangles) is shown. The rectangles on the curved lines indicate the position of the reflected pulses.

We assume that the pulse repetition rate is f _rep = 100 Hz and that at each sounding cycle various DFS packets with non-repeating frequency elements are emitted.

Then the total number of operating frequencies can be calculated by the formula

$$M=N{f}_{P\mathrm{about}\mathrm{at}t}(3)$$

For example, at N = 5 and f _rep = 100 Hz, an ionogram can be obtained in a time ΔT _dfs = 1 second, and sounding at 500 frequencies can be performed.

If, for example, the duration of the radiated EMF packet is increased to τ _imp = 600 μs, and Δτ = 60 μs is taken, then the number of elements of the EMD will take the value N = 10, then the total number of operating frequencies according to (3) will be M = 1000. If the number of frequencies in the DFD packets is limited by the number M = 500, then the ionogram can be recorded according to (2) for the time ΔT _dfs = 0.5 sec.

Figure 2 shows that the signals forming the ionogram do not go beyond the group delay of 4 ms, so you can increase the clock frequency to f _repeat = 200 Hz (in this case, the range will be 750 km). Then, with the total number of operating frequencies M = 500 and a single scan at these frequencies, an ionogram will be obtained in a time ΔT _dhs = 0.25 sec.

The implementation of the proposed method, consider an example based on the reconstruction of the ionosphere complex "Basis-2". With the broadband transmitter of the complex unchanged, it is necessary to modernize the coherent signal synthesizer for the complex transmitter so that it can radiate DFS packets. It is necessary to replace the control system and create a multi-channel radio receiving system.

For the basis of the DFS signal synthesizer, we take the principle of operation of a multi-frequency synthesizer, the functional block diagram of which is shown in Fig. 3. The PC hosts the software — the ionosphere sounding algorithm software, which issues the necessary command codes to the synchronization and control unit (BSU). In addition to BSU control codes from the main PC is supplied as a reference frequency f _0, which is generated by a highly stable secondary frequency standard (e.g., rubidium type oscillator CAS-74). Codes of the fixed operating sounding frequencies are received in the 4-channel signal synthesis block, and from the block output, the working frequency signals are sent to the DFS signal generation block, which also receives DFS codes. Formed DFS signals from the output of the synthesizer enter the radio transmitter (RPD).

The main difficulty in creating a multichannel radio receiving device (MRPU) for registering DFS signals reflected from the ionosphere is related to the need to simultaneously receive and register signals at different frequencies while maintaining the coherent properties of the received signals. A radio receiver with the necessary properties can be, for example, an N-channel receiver, each receiving channel of which, at each sounding cycle, is independently tuned to one of the N fixed frequencies of the emitted discrete sequence of pulses of the DFS package. At the next sounding step, each of the N channels of the RPU is tuned to a new DPS package: f, f + Δf _n , f + 2Δf _n , f + 3Δf _n , ... f + NΔf _n , where f = (f ₀ + Lδf). Here L is the sounding tact number. The traditional solution to the problem of creating multi-channel RPUs is to use the required number of industrially produced RPUs that make up the channels of multi-frequency RPU (MPU). Figure 4 shows a functional block diagram of a 4-channel MPPU, designed to receive 4-element packets of emergency. The PC hosts the software (software) of the ionosphere sounding algorithm, which provides the necessary command codes to the synchronization and control unit (BSU), as well as the emergency codes for the corresponding setting of the MPPU channels.

In the case of pulsed sounding considered here, the difficulty lies in the need to solve the problem of the influence of the direct (“earth”) wave on the inputs of the MRPU, since the duration of the DFS packet can approach the maximum allowable value of 700 μs. To solve this problem, a complex of hardware and software solutions should be applied, including well-known methods of suppressing a direct wave, for example, as is done in the Bazis-2 ionosonde.

At each sounding cycle, the received radio signal from the antenna-feeder system (AFS) enters the broadband antenna amplifier, which has the ability to limit signals that exceed a predetermined threshold (for example, ~ 5 V). Next, the received signal with the RF key is blocked for the duration of the blocking (gate) signal, while the strobe duration is determined by the duration (Nτ _imp ) of the radiated DPS package. From the output of the RF key, the signal goes to the input of the MPPU, from each of the 4 outputs of the MPPU the signals go to the input of the 4-channel ADC. The ADC is launched by SI clock pulses, which are generated in the synchronization and control unit (BSU). Then, the ADC codes enter the PC, where the radio sounding data is accumulated and processed. Since during the VZ diagnostics the transmitting and receiving parts of the probing equipment form a single system, the exhaust gas and personal computer blocks can be combined, which will provide the necessary synchronism of the emitted and received radio signals.

The processing of sensing data to obtain an ionogram graph does not differ from signal processing in known methods.

The above method for obtaining ionograms using broadband DFS signals leads to the need for processing the probing signals in almost real time and storing significant amounts of data. This makes quite high demands on the parameters of the ADC and PC. Currently existing multi-core processors and high-speed multi-channel ADC modules with a dynamic range of up to 14 bits allow the hardware and software to implement the proposed method.

As radio receivers in MRPU channels, industrially produced digital RPUs can be used, for example, the AR-ONE receiver from AOR (Japan) [4].

The proposed method was tested on the layout and confirmed the expected results to reduce the time to obtain an ionogram to fractions of a second. This allows you to apply the proposed method for the study of fast processes in the ionospheric plasma.

LIST OF USED SOURCES

1. Weather E.V. Diagnostic ionosphere complex "Basis" and its modifications // Experimental methods for studying the ionosphere. - M .: IZMIRAN, 1981, pp. 145-152.

2. Complex aggregate ionospheric Basis-2. Technical conditions Bt 2.009.007 TU. SKB FP AN SSSR. Moscow. 1983, p. 5-11.

3. Description of DPS-4: http://www.digisonde.com/dps-4dmanual.html.

4.http: //www.aor.ru

Claims (1)

A method for producing ionograms consisting in emitting radio pulses with a predetermined sounding frequency range, a frequency change step to cover the sounding frequency range, a pulse repetition frequency, the number of frequencies used, receiving a radio signal reflected from the ionosphere at a sounding frequency, and measuring the reflected radio signal parameters representing the radio signal intensity, angles of arrival, polarization, Doppler frequency shift, followed by frequency tuning of the emitted radioimp pulse from a given grid of fixed frequencies within the sounding frequency range with a complete exhaustion of the sounding frequencies for the time interval ΔТ, characterized in that at each sounding step, the radio pulse is emitted in the form of a packet of a discrete-frequency signal (DFS), which is a sequence of N following one after another other pulses of different frequencies of the same duration from a given grid of fixed sounding frequencies, then receive and measure the parameters of the reflected radio signal over time and propagation from the emitter to the receiver simultaneously and independently on each of the N frequencies of the DFD package, then the DFD frequencies are tuned to a new DFD package from the grid of fixed sounding frequencies, and the radiation is received and reflected on a new DFD package, then the frequencies are sequentially sorted mesh of fixed sounding frequencies with new DFS packets at each sounding cycle until all probing frequencies (in the sounding frequency grid) are completely sorted out for the time interval ΔТ _ДДС = ΔT / N.

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