CN102073041A - Top ionosphere detection space-borne MIMO radar system - Google Patents

Abstract

The invention discloses a space-borne MIMO radar system for top-level ionosphere detection, which belongs to the field of radar technology and includes a complete complementary sequence generator, a pulse period delayer, a modulator, a transmitting antenna, a receiving antenna, a demodulator, a pulse compression system and Electron density distribution generation system; the complete complementary sequence generator generates N pairs of complementary sequences at the same time, and the pulse cycle delayer makes the two complementary sequences alternately input to the modulator, and the N pairs of signals are respectively modulated by N carrier frequencies, and are transmitted by N The antenna is transmitted, the echo signal is received by N receiving antennas, the demodulator demodulates the signals on N carrier frequencies, and the signal is sent to the electron density distribution generation system after pulse compression to generate a two-dimensional ionization map. The spaceborne MIMO radar system of the present invention is used for top-level ionosphere detection, and compared with existing ionosphere detectors, it has ultra-high azimuth resolution and can generate a two-dimensional ionogram; compared with existing ionosphere detectors than, there is a high work efficiency.

Description

A spaceborne MIMO radar system for top-level ionosphere detection

technical field

The invention relates to a space-borne MIMO (Multiple-Input Multiple-Output) radar system for detecting the top ionosphere, belonging to the technical field of radar.

Background technique

The ionosphere is one of the important components of the earth's space environment, and its altitude extends from about 60 kilometers to 1000 kilometers above the ground. The ionosphere will cause serious interference to the propagation of electromagnetic waves, and have a great impact on space information systems such as satellite communications, navigation and positioning, and microwave remote sensing. Therefore, detecting the structure of the ionosphere is conducive to improving the information acquisition and application of the above-mentioned space information systems. quality. In addition, by detecting changes in ionospheric physical parameters (electron density, etc.), early warnings of natural disasters such as earthquakes and tsunamis can be provided. Taiwanese scholars have used relevant satellite data to analyze earthquakes with magnitudes above 5 in Taiwan in the past 10 years, and found that there will be a decrease in electron density in the high-altitude ionosphere before earthquakes. Therefore, ionospheric detection is of great significance to scientific research and disaster warning.

The vertical structure of the ionosphere is divided into D, E, F1, and F2 layers according to the electron density, among which the F2 layer has the highest electron density. Generally, the ionosphere is divided into the bottom ionosphere and the top ionosphere according to the distribution of electron density. The bottom ionosphere refers to the area below the F2 layer to the bottom of the ionosphere; the top ionosphere refers to the area above the F2 layer to the top of the ionosphere, about 200 kilometers to 1,000 kilometers above sea level. Similarly, ionospheric detection is also divided into two categories: bottom layer detection and top layer detection. Bottom detection is to use ground detection equipment to observe the bottom ionosphere. Top-level detection refers to the observation of the top-level ionosphere by using satellites and other spacecraft platforms to carry ionospheric detection payloads. Spaceborne ionospheric detectors are mainly used to detect the top ionosphere. Early ionospheric soundings were bottom-level soundings. With the emergence and rapid development of launch vehicles and artificial satellites, people can use spaceborne detection equipment to detect the top ionosphere from space. In 1962, the first ionospheric sounding satellite, Alouette-I, was launched and obtained the first ionograms of the top ionosphere. Its working mode is the same as that of ground detection. It uses frequency scanning to measure the one-dimensional distribution of ionosphere vertical electron density. The frequency signal will be reflected at different heights of the ionosphere, and the delay time of each frequency echo signal is measured in a frequency sweep cycle to obtain the one-dimensional distribution of the electron density in the height direction (variation with the virtual height); after that, The ISIS (International Satellites for Ionospheric Studies)-I&II of the United States and the ISS (Ionosphere Sounding Satellite)-I of Japan appeared again. They have the same working mode as Alouette-I and have high transmission power. After the 1990s, with the development of large-scale integrated circuits, high-performance computers and signal processing technology, spaceborne ionospheric detectors have developed in the direction of miniaturization and intelligence. The ionospheric detection satellites that appeared during this period have been greatly improved, mainly reflected in: the use of small satellite technology reduces the launch cost; the launch pulse adopts linear frequency modulation signal or pulse code signal, which improves the signal-to-noise ratio and reduces the transmit power. . Typical representatives include the magnetospheric detection IMAGE satellite launched by the National Aeronautics and Space Administration (NASA) in 2000 and the top ionosphere detection WARNING satellite launched by Ukraine in 2001. ) two advanced ionospheric sounding radars. RPI mainly obtains the electron density map of the magnetosphere, and its height direction resolution (distance resolution) is 480 kilometers, while TOPADS mainly obtains the electron density map of the top ionosphere, and its height direction resolution is 5 kilometers. However, their working mode is still the traditional sweeping working mode. The scanning period of this scanning mode is relatively long, resulting in almost no ability to resolve the electron density in the azimuth direction (along the flight direction of the satellite). For example, the scanning period of TOPADS is 10 seconds, and its azimuth resolution is as low as 75 kilometers.

In recent years, with the research of multiple-input multiple-output (MIMO) communication systems, people have extended the idea of MIMO to the field of radar system design and radar signal processing, and proposed the concept of MIMO radar. Its working mode is through multiple transmitting antennas simultaneously. Transmit multiple waveform signals and receive echo signals through multiple receive antennas. It can use the orthogonality between signals to restore each waveform signal, and the commonly used signals include phase encoding signals and so on. Complete complementary sequence (CC-S, complete complementary sequence), as a phase encoding signal, can ensure complete orthogonality between signals and high processing gain, and has been used in MIMO radar research in recent years. Taking advantage of the multiple transmission and multiple reception of MIMO radar signals, MIMO radar can be used for ionospheric detection.

Contents of the invention

The purpose of the present invention is to break through the limitations of the frequency sweep mode in traditional ionospheric detection, improve the azimuth resolution, and propose a MIMO radar system for top-level ionospheric detection. The system transmits multiple different frequencies simultaneously by N transmitting antennas. Different phase encoding pulse signals of different phases, and the echo signals are received by N receiving antennas. This system can greatly improve the azimuth resolution and obtain a two-dimensional ionization map (the distribution of electron density in the height and azimuth directions).

A spaceborne MIMO radar system for top-level ionosphere detection, comprising a complete complementary sequence generator, a pulse period delayer, a modulator, a transmitting antenna, a receiving antenna, a demodulator, a pulse compression system and an electron density distribution generation system;

The complete complementary sequence generator generates N pairs of complete complementary sequences {A _i , B _i }, i=1, ..., N, A _i sequence and B _i sequence at the same time every other pulse repetition period T _prf of a sequence pair is:

$\left\{\begin{array}{c}{\mathrm{<span\; class="notranslate">\; A</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; ,</span>{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; ,</span>{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>\\ {\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; b</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; ,</span>{\mathrm{<span\; class="notranslate">\; b</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; ,</span>{\mathrm{<span\; class="notranslate">\; b</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>\end{array}\right.$

和

代表子脉冲码，i＝1，…，N，k＝0，…，L-1，L表示序列的长度；In the formula,

and

Represent sub-pulse code, i=1,..., N, k=0,..., L-1, L represents the length of the sequence;

其中H_s表示卫星轨道高度，c表示真空中光速；将每个通道的解调频率f_i换算成电子密度值Ne_i，Ne_i＝(f_i/9)²；则在一个序列对的脉冲重复周期T_prf里，得到N个电子密度值对应的虚高，生成电子密度随虚高的一维变化；综合多个脉冲重复周期的一维结果，得到电子密度随虚高和方位距离的二维变化，即二维电离图。The pulse cycle delayer delays the output sequence B _i of the complete complementary sequence generator by one pulse cycle, that is, half the pulse repetition period T _prf /2 of the delayed sequence pair, so that A _i and B _i are alternately input to N modulators; the modulator Perform carrier frequency f _i modulation on the input sequences A _i and B _i , where f ₁ =f _min , f ₂ =f ₁ +Δ, f ₃ =f ₂ +Δ,..., f _N =f _max , f _max and f _min represent the maximum and minimum values of the transmitting frequency, respectively, and Δ represents the frequency interval The modulated N signals with frequencies f ₁ , f ₂ , ...f _N are transmitted through N transmitting antennas respectively; each receiving antenna receives signals of all frequencies reflected by the ionosphere, and N receiving antennas form N receiving antenna channels; the demodulator performs frequency mixing and low-pass filtering on the signal of the receiving antenna channel, and demodulates two complementary signals on the carrier frequency f _i ; the pulse compression system demodulates the complementary two signals The signals are matched and filtered, and then superimposed to achieve pulse compression; the electron density distribution generation system measures the delay time t _i of the signal after pulse compression in each channel, and calculates the corresponding virtual height

Where H _s represents the height of the satellite orbit, c represents the speed of light in vacuum; the demodulation frequency f _i of each channel is converted into the electron density value Ne _i , Ne _i = (f _i /9) ² ; then in a sequence of pulses In the repetition period T _prf , the virtual height corresponding to N electron density values is obtained, and the one-dimensional change of the electron density with the virtual height is generated; the one-dimensional results of multiple pulse repetition periods are combined to obtain the two-dimensional variation of the electron density with the virtual height and the azimuth distance. Dimensional changes, that is, two-dimensional ionograms.

The advantages of the present invention are:

(1) The spaceborne MIMO radar system of the present invention is used for top-layer ionospheric detection, compared with existing ionospheric detectors, has ultra-high azimuth resolution, and can generate a two-dimensional ionogram;

(2) Compared with the existing ionospheric detectors, the spaceborne MIMO radar system of the present invention has very high working efficiency.

Description of drawings

Fig. 1 is the structural representation of a kind of top layer ionosphere detection space-borne MIMO radar system of the present invention;

Fig. 2 is a schematic diagram of waveforms at nodes H ₁ , H ₂ , ..., H _N in Fig. 1;

Figure 3a is a two-dimensional view display of the electron density distribution in an embodiment of the present invention;

Figure 3b is a three-dimensional view display of the electron density distribution in the embodiment of the present invention;

Fig. 4a is a two-dimensional view display of a two-dimensional ionogram of a system detection result in an embodiment of the present invention;

Fig. 4b is a three-dimensional display of the two-dimensional ionogram of the system detection result in the embodiment of the present invention.

In the picture:

1-Complementary Complementary Sequence Generator 2-Pulse Period Delayer 3-Modulator

4-Transmitting Antenna 5-Receiving Antenna 6-Demodulator

7-pulse compression system 8-electron density distribution generation system

The symbols of the device in the figure are as follows:

混频器；

求和器T: pulse cycle delayer; Tx: transmit antenna; Rx: receive antenna; LPF: low-pass filter; MF: matched filter;

Mixer;

summer

Detailed ways

The present invention will be further described in detail with reference to the accompanying drawings and embodiments.

A kind of top ionosphere detection spaceborne MIMO radar system of the present invention, as shown in Figure 1, comprises complete complementary sequence generator 1, pulse period delayer 2, modulator 3, transmitting antenna 4, receiving antenna 5, demodulator 6. Pulse compression system 7 and electron density distribution generation system 8 .

The complete complementary sequence generator 1 (CC-S generator) _generates N pairs of complete complementary sequences {A _i , B _i }, i=1, . . .

In order to ensure that the frequency spectrum between transmitted signals does not alias, the value of N is limited by formula (1):

$\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; <</span>\frac{{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}}{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; min</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

In the formula, ρ _r represents the distance resolution, f _max and f _min represent the maximum and minimum values of the emission frequency, respectively, and c represents the speed of light in vacuum.

A _i sequence and B _i sequence are specifically:

$\left\{\begin{array}{c}{\mathrm{<span\; class="notranslate">\; A</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="0"\; label="i"\; filenames="HDA0000031588310000032.png"\; state="\{\{state\}\}">i</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; ,</span>{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; ,</span>{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>\\ {\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="0"\; label="b\; i"\; filenames="HDA0000031588310000032.png"\; state="\{\{state\}\}">b</figure-callout></span><figure-callout\; id="0"\; label="b\; i"\; filenames="HDA0000031588310000032.png"\; state="\{\{state\}\}">\; </figure-callout>}}_{\mathrm{<span\; class="notranslate">\; </span>}}^{\mathrm{<span\; class="notranslate">i</span>}}<span\; class="notranslate">\; ,</span>{\mathrm{<span\; class="notranslate">\; b</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; ,</span>{\mathrm{<span\; class="notranslate">\; b</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>\end{array}\right.<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

和

代表子脉冲码，i＝1，…，N，k＝0，…，L-1。L表示序列的长度。A_i序列、B_i序列具有高处理增益并且满足完全正交性，如式(3)和式(4)所示。In the formula,

and

Represents sub-pulse codes, i=1,...,N, k=0,...,L-1. L represents the length of the sequence. A _i sequence and B _i sequence have high processing gain and satisfy complete orthogonality, as shown in formula (3) and formula (4).

$R_{A_m{A}_m}\left(\mathrm{\&tau;}\right)+R_{B_m{B}_m}\left(\mathrm{\&tau;}\right)=\left\{\begin{array}{cc}2L& \mathrm{\&tau;}=0\\ 0& \mathrm{\&tau;}=\&PlusMinus;1,\&PlusMinus;2,...,\&PlusMinus;(L-1)\end{array}\right.,$ m=1, 2, ..., N (3)

$R_{A_m{A}_{\mathrm{no}}}\left(\mathrm{\&tau;}\right)+R_{B_m{B}_{\mathrm{no}}}\left(\mathrm{\&tau;}\right)=0$ τ=0, ±1, ±2,..., ±(L-1), m=1, 2,..., N, n=1, 2,..., N, m≠n (4)

分别表示序列A_m和A_n、序列B_m和B_n的非周期相关函数。In formulas (3) and (4),

Denote the aperiodic correlation functions of sequences A _m and A _n , sequences B _m and B _n respectively.

The pulse period delayer 2 delays the output sequence B _i (i=1, ..., N) of the complete complementary sequence generator 1 by one pulse period, that is, half the pulse repetition period T _prf /2 of the delay sequence pair, and realizes A _i , Bi _is alternately input to the modulator 3 . Assume that the input end of modulator 3 with modulation frequency f _i is node H _i , as shown in FIG. 1 , and FIG. 2 is a schematic diagram of waveforms at nodes H ₁ , H ₂ , . . . , H _N in FIG. 1 . At the node H _i, there are alternating pulse sequences of A _i and B _i , and the pulse repetition period of the sequence pair is T _prf .

Then, N modulators 3 perform carrier frequency f _i modulation on the input sequences A _i and B _i through the mixer, i=1,...,N, where f ₁ =f _min , f ₂ =f ₁ +Δ , f ₃ ＝f ₂ +Δ,..., f _N ＝f _max , f _max and f _min represent the maximum value and minimum value of the transmitting frequency respectively, Δ represents the frequency interval

N channels of modulated signals with frequencies f ₁ , f ₂ , . . . f _N are transmitted through N transmitting antennas 4 respectively.

The signal with frequency f _i passes through the ionosphere and will be reflected at the height where the electron density is Ne _i =(f _i /9) ² .

Each receiving antenna 5 receives signals of all frequencies reflected by the ionosphere, and N receiving antennas 5 form N receiving antenna channels.

Demodulator 6 comprises mixer and low-pass filter, enters mixer and low-pass filter successively by the signal of receiving antenna channel, carries out frequency mixing and low-pass filtering process, demodulates carrier frequency f _i (i= 1, . . . , N) complementary two signals.

The pulse compression system 7 performs matched filtering on the two demodulated complementary signals, and then superimposes to realize pulse compression. The pulse compression system 7 specifically includes a matched filter and a summer, and the matched filter pairs the demodulated complementary two The signals are matched filtered, and then superimposed by the summer.

H_s表示卫星轨道高度，c表示真空中光速。再将每个通道的解调频率f_i换算成电子密度值Ne_i，Ne_i＝(f_i/9)²。这样，在一个序列对的脉冲重复周期T_prf里，得到N个电子密度值对应的虚高，生成电子密度随虚高的一维变化。综合多个脉冲重复周期的一维结果，得到电子密度随虚高和方位距离的二维变化，也就是二维电离图。方位分辨率为V_s·T_prf，也就是一个序列对的脉冲重复周期卫星飞行的距离，V_s表示卫星速度大小。The electron density distribution generation system 8 measures the delay time t _i of the pulse-compressed signal of each channel, i=1,...,N, and calculates the corresponding virtual height

H _s represents the altitude of the satellite orbit, and c represents the speed of light in vacuum. Then the demodulation frequency f _i of each channel is converted into an electron density value Ne _i , Ne _i =(f _i /9) ² . In this way, in the pulse repetition period T _prf of a sequence pair, imaginary heights corresponding to N electron density values are obtained, and a one-dimensional change of electron density with the imaginary height is generated. Combining the one-dimensional results of multiple pulse repetition periods, the two-dimensional variation of electron density with false height and azimuth distance is obtained, that is, the two-dimensional ionization map. The azimuth resolution is V _s ·T _prf , that is, the distance that the satellite flies in the pulse repetition period of a sequence pair, and V _s represents the satellite speed.

Example:

According to the parameters shown in Table 1, a computer simulation was carried out on the system. In the simulation, the sub-pulse codes of the complete complementary sequence take values from {1, -1, j, -j}, where j ² =-1.

Table 1 Simulation parameters

In the simulation, the ionospheric electron density distribution N _e (h, x) is approximately regarded as the product of the vertical distribution part and the horizontal disturbance part, assuming that the vertical distribution N _ver (h) satisfies the Chapman model, and the horizontal disturbance part F _hor (x) is a Gaussian function:

N _e (h, x) = N _ver (h) · F _hor (x) (5)

${\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; ver</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; h</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="0"\; label="N"\; filenames="HDA0000031588310000032.png"\; state="\{\{state\}\}">N</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; \{</span>\mathrm{<span\; class="notranslate">\; 0.5</span>}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; h</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; h</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\mathrm{<span\; class="notranslate">\; h</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span><span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; h</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; h</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\mathrm{<span\; class="notranslate">\; h</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \}</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; hor</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; -</span>\frac{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&mu;</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 7</span>}<span\; class="notranslate">\; )</span>$

In the formula, h and x represent the height and horizontal distance, N ₀ is the peak electron density, h ₀ is the height of the electron density peak, H is the elevation, μ and σ represent the center and scale of the Gaussian function, respectively. Take N ₀ =1×10 ¹² /m ³ , h ₀ =350km, H=50km, μ=0, σ=10km. Fig. 3 is an electron density distribution diagram directly generated by formula (5), Fig. 3a is a two-dimensional view, and Fig. 3b is a three-dimensional view.

The signal delay time in the simulation is calculated by formula (8), which expresses the round-trip delay time t _i of the electromagnetic wave with frequency f _i reflected by the ionosphere:

${\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}{\mathrm{<span\; class="notranslate">\; c</span>}}{<span\; class="notranslate">\; \&Integral;</span>}_{{\mathrm{<span\; class="notranslate">\; h</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}^{{\mathrm{<span\; class="notranslate">\; h</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}\frac{\mathrm{<span\; class="notranslate">\; d\; h</span>}}{\sqrt{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 8</span>}<span\; class="notranslate">\; )</span>$

In the formula, H _s represents the height of the satellite orbit, h _i represents the height where the electromagnetic wave is reflected, that is, the height where the electron density is (f _i /9) ² , f _p represents the cut-off frequency

According to the parameters in Table 1, the azimuth resolution in the simulation is as high as 0.28km, that is, every time the satellite flies 0.28km, that is, every time the horizontal distance x increases by 0.28km, the system generates a one-dimensional change result of electron density with false height, and the comprehensive horizontal distance From the result of x from -10km to 10km, the two-dimensional distribution of electron density - two-dimensional ionization map is obtained, as shown in Figure 4, Figure 4a is a two-dimensional view, and Figure 4b is a three-dimensional view.

Claims (6)

1. spaceborne MIMO radar system of top layer ionospheric probing, it is characterized in that, comprise fully-complementary sequence maker, recurrence interval delayer, modulator, emitting antenna, receiving antenna, detuner, pulse compression system and electron density distribution generation system;

The fully-complementary sequence maker is every a pulse repetition time T that sequence is right _Prf, generate N simultaneously to fully-complementary sequence { A _i, B _i, i=1 ..., N, A _iSequence, B _iSequence is specially:

$\left\{\begin{array}{c}{A}_i=(a_i^0,a_i^1,...,a_i^{L-1})\\ B_i=(b_i^0,b_i^1,...,b_i^{L-1})\end{array}\right.---\left(1\right)$

In the formula,

With

Represent the subpulse sign indicating number, i=1 ..., N, k=0 ..., L-1, L represent the length of sequence;

The recurrence interval delayer is to fully-complementary sequence maker output sequence B _iPostpone a recurrence interval, i.e. half right pulse repetition time T of delayed sequence _Prf/ 2, realize A _i, B _iAlternately input to N modulator; Modulator is to the sequence A of input _i, B _iCarry out carrier frequency f _iModulation, wherein, f ₁=f _Min, f ₂=f ₁+ Δ, f ₃=f ₂+ Δ ..., f _N=f _Max, f _MaxAnd f _MinRepresent the maximal value and the minimum value of transmission frequency respectively, Δ is represented frequency interval

N road frequency after the modulation is respectively f ₁, f ₂... f _NSignal go out via N transmission antennas transmit respectively; Each receiving antenna receives the signal that layer reflects that is ionized of all frequencies, and N receiving antenna forms N receiving antenna passage; Detuner carries out mixing and low-pass filtering treatment to the signal that receives antenna channels, demodulates carrier frequency f _iOn two signals of complementation; Pulse compression system is carried out matched filtering to two signals of the complementation that demodulates, and stack again realizes pulse compression; The electron density distribution generation system is measured t time delay of each channel pulse compression back signal _i, calculate corresponding virtual height

H wherein _sThe expression satellite orbital altitude, c represents the light velocity in the vacuum; Frequency, demodulation frequency f with each passage _iBe converted into electron density value Ne _i, Ne _i=(f _i/ 9) ²Then at a pulse repetition time T that sequence is right _PrfIn, obtain the virtual height of N electron density value correspondence, generate electron density and change with the one dimension of virtual height; The one dimension result of comprehensive a plurality of pulse repetition times obtains the two dimension variation of electron density with virtual height and azran, promptly two-dimentional ionogram.

2. the spaceborne MIMO radar system of a kind of top layer ionospheric probing according to claim 1 is characterized in that, the fully-complementary sequence maker generates N to fully-complementary sequence, and wherein the value of N is limited by formula (2):

$N<\frac{{\mathrm{\&rho;}}_r}{c}(f_{\max }-f_{\min })+1---\left(2\right)$

In the formula, ρ _rThe expression range resolution, f _MaxAnd f _MinRepresent the maximal value and the minimum value of transmission frequency respectively, c represents the light velocity in the vacuum.

3. the spaceborne MIMO radar system of a kind of top layer ionospheric probing according to claim 1 is characterized in that described A _iSequence, B _iSequence has high processing gain and satisfies complete orthogonality, shown in (3) and formula (4);

$R_{A_m{A}_m}\left(\mathrm{\&tau;}\right)+R_{B_m{B}_m}\left(\mathrm{\&tau;}\right)=\left\{\begin{array}{cc}2L& \mathrm{\&tau;}=0\\ 0& \mathrm{\&tau;}=\&PlusMinus;1,\&PlusMinus;2,...,\&PlusMinus;(L-1)\end{array}\right.,$ m＝1，2，…N， (3)

$R_{A_m{A}_n}\left(\mathrm{\&tau;}\right)+R_{B_m{B}_n}\left(\mathrm{\&tau;}\right)=0$ τ＝0，±1，±2，…，±(L-1)，m＝1，2，…，N，n＝1，2，…，N，m≠n (4)

In formula (3) and (4),

Represent sequence A respectively _mAnd A _n, sequence B _mAnd B _nRelated function non-periodic.

4. the spaceborne MIMO radar system of a kind of top layer ionospheric probing according to claim 1, it is characterized in that, described detuner comprises frequency mixer and low-pass filter, and frequency mixer carries out mixing to the signal that receives antenna channels, and the signal of low-pass filter after to mixing carries out low-pass filtering treatment.

5. the spaceborne MIMO radar system of a kind of top layer ionospheric probing according to claim 1, it is characterized in that, described pulse compression system comprises matched filter and summer, and matched filter carries out matched filtering to two signals of the complementation that detuner demodulates, and summer superposes again.

6. the spaceborne MIMO radar system of a kind of top layer ionospheric probing according to claim 1 is characterized in that the azimuthal resolution of described two-dimentional ionogram is V _sT _Prf, V _sExpression satellite velocities size.

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