CN113029161B - Spatial VLBI signal enhancement method and system based on cross-correlation spectrum synthesis - Google Patents
Spatial VLBI signal enhancement method and system based on cross-correlation spectrum synthesis Download PDFInfo
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Abstract
The invention provides a spatial VLBI signal enhancement method based on cross-correlation spectrum synthesis, which comprises the following steps: the space telescope and the ground telescope form a VLBI interference system, and original observation data from the space telescope and the ground telescope are acquired; based on the correlation processing of the original observation data between the space telescope and the ground telescope and the correlation processing of the original observation data between the ground telescope, the enhanced VLBI signal is obtained through coherent synthesis. The spatial VLBI signal enhancement method is based on a spatial VLBI signal enhancement technology of VLBI cross-correlation spectrum synthesis, and simultaneously completes the synthesis of original signals observed by a foundation VLBI telescope and the correlation processing of a spatial-ground VLBI baseline, so that the effect of performing lunar VLBI on a single foundation caliber telescope and a space small caliber telescope is realized.
Description
Technical Field
The invention relates to a spatial VLBI signal enhancement method, in particular to a spatial VLBI signal enhancement technical method and system based on VLBI cross-correlation spectrum synthesis.
Background
Astronomical observations ultimately require the pursuit of sensitivity or spatial resolution. The very long baseline interferometry (Very Long Baseline Interferometry, VLBI) technique has very high angular resolution and sensitivity, and is widely used in astronomical physics, astronomical measurements, astronomical earth dynamics, and other fields. According to the basic principles of electromagnetism, the angular resolution of a telescope is approximately equal to the observation wavelength divided by the effective caliber.
Description of the principles of the related processor of the existing VLBI (FX type):
The input to the FX VLBI correlation processor is the time domain signal obtained by the stations at both ends of the baseline and the output is the inter-station cross-correlation spectrum (called the visibility function). As shown in fig. 1, after two paths of signals enter an FX-type correlation processor, the signals are firstly decoded according to a frequency channel; then, carrying out integer bit delay compensation according to a pre-calculated delay model; after stripe rotation and fast Fourier transformation, the signal is transformed from a time domain to a frequency domain, and fractional bit time delay compensation is completed; and finally, performing conjugate multiplication and accumulation integration on the two paths of signals to obtain visible data.
Because of the limited speed of light, the signal wave fronts emitted by the power supply respectively reach the two telescopes at different moments, and the time difference between the two is called VLBI time delay and can be expressed as:
Where τ is the total delay, τ 0 and Is the theoretical time delay and the time delay rate of the moment calculated according to the time delay model. The correlation processor corrects the time delay and the time delay rate according to the theoretical model, and particularly aligns the two paths of signals in the time domain and the frequency domain through integer bit compensation, stripe rotation and fractional bit compensation. Because the accuracy of the time delay model is limited, the output visible data still has residual time delay and residual time delay rate, namely the slope of the interference fringe frequency-phase function is not zero, and the interference fringe frequency-phase function changes along with the time. The VLBI post-processing software obtains the residual time delay and the residual time delay rate through a stripe fitting method; together with the model delay and delay rate and the clock speed, they form the total delay and delay rate, so as to obtain the observed quantity of the total delay and the total delay rate of the VLBI.
For VLBI, its highest angular resolution is approximately equal to the wavelength of the observation divided by the baseline length (i.e., the distance between the two radio telescopes). Currently, the VLBI facility with the highest ground angle resolution is a black hole telescope with the largest equivalent caliber approaching the earth diameter. Astronomists have long recognized that the maximum baseline length of foundation VLBI is unlikely to exceed the earth diameter. If the radio telescope is transmitted to space, and the space or space VLBI array is formed by the radio telescope and the foundation or space, the base line length can exceed the earth diameter, so that higher angular resolution is obtained. This is space VLBI (Space VLBI) or space-earth VLBI (space-earth VLBI).
However, technical and cost factors limit the caliber size of space telescopes, resulting in sensitivity of space-ground baselines that is often lower than that of ground baselines. In addition, because the space VLBI baseline is longer than the ground VLBI, the angular resolution is high, resulting in a large portion of the structure of the target source being decomposed, and thus the detected source flow is low, the space VLBI is more difficult to obtain clear fringes than conventional ground VLBI for the same observation source. How to increase the sensitivity of a spatial VLBI telescope is therefore a technical problem to be solved.
It is common practice for the space VLBI to increase the diameter of the space VLBI telescope as much as possible while at the same time increasing the effective area of the ground VLBI telescope as much as possible. In order to increase the effective area of the foundation telescope, the existing method adopts the foundation telescope with ultra-large caliber or the ground line interference telescope array mode as much as possible to participate in the space VLBI networking observation. Currently, it is not known from the disclosure to synthesize the received signals of the foundation space VLBI telescope and to perform VLBI interference processing with the space telescope. In the existing space VLBI observation, an ultra-large caliber foundation telescope facility is often required.
For example, russian space VLBI project RadioAstron is the space VLBI project that has been more successful to date, and the caliber of space telescope Spektr-R is 10 meters. The project utilizes the large-caliber telescope on the ground of each country and Spektr-R to form a space VLBI system through international cooperation. The RadioAstron project utilizes large foundation telescope equipment such as an Arrayleigh telescope with the ground diameter of 300 meters, a green shore telescope with the ground diameter of 110 meters, a Boen telescope with the ground diameter of 100 meters, a Westerbork synthetic aperture telescope array and the like, and improves the space VLBI observation sensitivity by improving the receiving area of the ground telescope. The Westerbork-class synthetic aperture telescope array adopts a method of synthesizing the signals of the connecting line short baseline telescope to form the 66-meter equivalent aperture telescope, wherein the telescope signal synthesis means that signals received by two radio telescopes are coherently synthesized by a technical means, and the effect similar to that of a signal received by a single-aperture radio telescope is achieved.
In the lunar exploration of goddess Chang E No. seven, a relay star is used for carrying a telescope with the caliber of 4.2 meters and is provided with VLBI special equipment to form a VLBI telescope running on a lunar orbit, an X-band ground-moon space VLBI test system with the baseline exceeding 30 ten thousand km is formed by the relay star and a ground telescope of the Chinese VLBI net (the length of the formed ground-moon baseline is about 38 ten thousand km), and the observation study of the ground-moon VLBI baseline is carried out. However, in the space VLBI at present, the caliber of the space telescope is smaller, the baseline signal to noise ratio is often improved by combining the observation of the foundation large-caliber VLBI telescope, and as the X-band foundation radiotelescope with the caliber of hundred meters is not available in China at present, the Chinese VLBI network which can be used in the future only has the medium caliber telescope with the caliber of 25-65 meters, so that the caliber of the space telescope and the caliber of the foundation VLBI telescope are smaller compared with that of the telescope abroad, and the observation sensitivity is weak.
Furthermore, at present, there are 5 existing signal synthesis schemes that can be used to synthesize signals from deep space probes, respectively referred to as: (1) full spectrum synthesis (FSC); (2) complex symbol synthesis (CSC); (3) symbol Stream Synthesis (SSC); (4) baseband synthesis (BC); (5) carrier grouping (CA). The following table 1 shows the advantages and disadvantages of the signal synthesis scheme of the 5 antenna array systems. (see [ Rogowski et al, antenna array technology for deep space networks, [ M ]. University of Qinghua Press, 2005) ]
TABLE 1 advantages and disadvantages of the Signal Synthesis scheme presented
Wherein CSC, SSC, BC, and CA must propose an array scheme according to the spectral characteristics of the received signal of the deep space probe and the existing equipment. In these schemes, the received signal is converted into complex symbols, complex symbol streams, complex baseband, respectively, by existing demodulation synchronization equipment, and then signal synthesis is performed. These schemes require varying degrees of carrier tracking equipment, subcarrier tracking equipment and symbol synchronization equipment and are therefore only used for modulating signals with well-defined characteristics, such as detector signals, but are not applicable to various types of signals of unknown or similar noise (cosmic power is such a signal).
The existing spacecraft has more tasks adopting an FSC scheme. FSC directly synthesizes intermediate frequency signals or baseband signals from each telescope, in order to ensure coherence, signals are subjected to time delay and phase compensation before synthesis, and signal correlation is used for determining the compensation quantity of the time delay and the phase of the signals among the array telescopes.
However, the above-described full spectrum synthesis (FSC) signal synthesis method is often used for in-line interferometry or short baseline, for detector signal synthesis, and is not applicable to long baseline radio source signal synthesis where VLBI telescopes are far away.
Currently, there is a scheme to extend FSC into VLBI telescope signal synthesis at far distances, known as very long baseline full spectrum signal synthesis. The flow chart of the long baseline full spectrum signal synthesis method is shown in fig. 1, a correlation processing result is obtained through a prediction time delay model, a near field model is usually adopted as a time delay model of a detector, and the time delay model is obtained through calculation by taking the earth center as a reference. And then, obtaining residual time delay, time delay rate and phase difference according to the stripe search, and performing a series of time delay compensation and phase alignment operations on each path of signals from different telescopes. The method comprises the steps of calculating residual time delay and time delay rate to obtain time delay which needs to be compensated in addition to a time delay model, and dividing the time delay into two steps of integer bit shift and hour bit correction in the compensation process. Because of the digital signal sampling points, only integer bits can be shifted. In fig. 1, τ b is the delay compensated for by the integer bit shift step. τ f is the delay that remains after the delay value to be compensated is subtracted by τ b, which is the delay compensated for in the fractional bit correction step.Is phase compensation generated by earth rotation when the signals received by the two telescopes are synchronous with the same wave front. This τ b、τf,/>The method is obtained by calculating the residual time delay, the time delay rate and the phase difference. Prior to synthesis, signal-to-noise ratio estimation is often required to maximize the signal-to-noise ratio of the synthesized signal. The method mainly comprises the following steps: cross-correlation and stripe search, delay compensation and stripe rotation, signal to noise ratio estimation and calculation of weighting coefficients.
However, this scheme operates directly on the original signal, mainly for telemetry and data signal synthesis. In addition, the existing schemes are all direct synthesis of the original signal, such as FSC, CSC, SSC, BC.
In summary, the full spectrum signal synthesis method is based on original signal synthesis before correlation processing, so that the calculated amount and the implementation difficulty are high.
Therefore, a scheme for performing array and signal synthesis by using the VLBI telescope to equivalent the large caliber telescope and simultaneously completing the interference processing is needed to be applied to the space VLBI project.
Disclosure of Invention
The invention provides a spatial VLBI signal enhancement technical method and a spatial VLBI signal enhancement technical system based on VLBI cross-correlation spectrum synthesis, so as to enhance the signal sensitivity of a space-to-ground baseline of VLBI.
In order to achieve the above object, the present invention provides a spatial VLBI signal enhancement method based on cross-correlation spectrum synthesis, comprising:
S1: providing a space telescope and a plurality of ground telescopes to form a VLBI interference system, and acquiring original observation data from the space telescope and from the plurality of ground telescopes;
S2: based on the correlation processing of the original observation data of the space telescope and the ground telescope and the correlation processing between the original observation data of the ground telescope, the enhanced VLBI signal is obtained through coherent synthesis.
In the step S2, the correlation processing of the original observation data between the space telescope and the ground telescope is to perform FX-type VLBI correlation processing on the original observation data of the space telescope and the original observation data of the ground telescope to obtain cross-correlation spectrums of a plurality of space base lines, and then synthesize the cross-correlation spectrums of the plurality of space base lines.
The step S2 is based on the synthesis of cross-correlation spectra of a plurality of space-earth baselines, and includes:
S21: carrying out correlation processing on the original observation data received by the ground telescope to obtain a residual time delay compensation time delay model, and carrying out correlation processing again to flatten stripes of base lines among the ground telescopes;
S22: FX type VLBI correlation processing is carried out on the space telescope and the original observation data of the ground telescope respectively to obtain cross-correlation spectrums of the space base lines, the space base lines are aligned in phase and the stripes are leveled through phase compensation and time delay compensation, and then the cross-correlation spectrums of different space base lines are synthesized.
The step S21 includes:
S211: performing FX-type VLBI correlation processing on the original observation data of the two ground telescopes by adopting a correlation processor to obtain a cross-correlation spectrum between the two ground telescopes;
s212: a correlation post-processing method is adopted for the cross-correlation spectrum between the two ground telescopes to obtain residual time delay and residual time delay rate;
S213: and compensating the residual time delay and the residual time delay rate to a time delay model, and performing correlation processing again to obtain a cross-correlation spectrum.
After said step S213, further comprising a step S213': and repeating the step S212, judging whether the residual time delay and the residual time delay rate reach enough precision, if so, ending the flow, otherwise, continuing to execute the step S213 and the step S213'.
The step S22 includes:
S221: performing FX-type VLBI (very-large-scale binary-space) correlation processing on the original observation data of the space telescope and the original observation data of a plurality of ground telescopes respectively by adopting a correlation processor to obtain cross-correlation spectrums of a plurality of space-to-ground baselines;
S222: constructing Q-functions using cross-correlation spectra of space-to-ground baselines And at Q function/>Searching to obtain the coordinate/>, corresponding to the point with the maximum function value And τ is respectively used as a compensation phase and a compensation time delay;
The Q function The method comprises the following steps:
wherein k represents a frequency point; v AC、VBC represents data of cross-correlation spectra of AC baseline and BC baseline, respectively; is the phase; Δf is the frequency resolution; τ is the time delay;
S223: by compensating for phase Aligning the phases of the two space base lines, and leveling the stripes of the two space base lines by compensating the time delay tau;
s224: and coherently summing the cross-correlation spectrums of different space-earth baselines to obtain a synthesized signal, wherein the obtained synthesized signal is an enhanced VLBI signal.
When the number of the ground telescopes is greater than 2, the step S21 further includes a step S214: steps S211-S213 are repeated until the residual delay of the baseline between any two ground telescopes reaches sufficient accuracy, and the fringes flatten out.
The spatial VLBI signal enhancement method based on cross-correlation spectrum synthesis further comprises the following step S3: the signal to noise ratio of the enhanced VLBI signal was determined and used as a criterion for baseline synthesis efficiency.
In another aspect, the present invention provides a spatial VLBI signal enhancement system based on cross-correlation spectrum synthesis, comprising a spatial telescope, a plurality of terrestrial telescopes, and a signal enhancement processor arranged to perform the spatial VLBI signal enhancement method based on cross-correlation spectrum synthesis as described above.
The spatial VLBI signal enhancement method based on cross-correlation spectrum synthesis is based on cross-correlation spectrum after the spatial foundation VLBI correlation processing, so that a plurality of foundation telescopes are synthesized into a telescope with larger equivalent caliber, interference fringes similar to a base line formed by a large-caliber telescope and a spatial telescope can be obtained, the effect of enhancing the sensitivity of the VLBI space and the ground base line is achieved, the method can be applied to the field of earth-moon space VLBI or other VLBIs, and the requirement on the large-caliber foundation telescope is reduced.
The spatial VLBI signal enhancement method based on cross-correlation spectrum synthesis of the invention relates to a signal synthesis method which is different from the existing signal synthesis method. The method comprises the steps of synthesizing a cross-correlation spectrum of an air-ground baseline obtained after VLBI correlation processing, so as to simultaneously complete original observation signal synthesis of a foundation VLBI telescope and space-ground VLBI baseline correlation processing, achieve a VLBI signal with enhanced space VLBI signal synthesis effect, greatly reduce the processed data volume, achieve the effect of completing interference processing of an equivalent large-caliber telescope, and be used for synthesizing a radio source signal and a detector signal; and from the original observation data to the synthesized result, the whole process only needs to carry out correlation processing twice, so that the data processing steps are reduced. Meanwhile, distortion phenomena caused by VLBI decimal delay compensation operation and the like in the long-baseline full-spectrum signal synthesis technology are avoided. In conclusion, the calculated amount and the implementation difficulty in the scheme are far smaller than those of the existing scheme.
Drawings
FIG. 1 is a flow chart of data processing of a conventional FX type VLBI correlation processor.
Fig. 2 is a flow diagram of a prior art method of long baseline full spectrum signal synthesis.
FIG. 3 is a flow chart of a method for spatial VLBI signal enhancement based on VLBI cross-correlation spectrum synthesis, according to an embodiment of the invention.
FIG. 4 is a schematic structural diagram of a spatial VLBI signal enhancement system based on VLBI cross-correlation spectrum synthesis, showing that a plurality of ground telescopes are equivalent to one equivalent large-caliber ground telescope, according to one embodiment of the invention.
Detailed Description
Embodiments of the present invention are described in detail below with reference to the accompanying drawings.
As shown in fig. 3, the spatial VLBI signal enhancement method based on VLBI cross-correlation spectrum synthesis of the present invention specifically includes the following steps:
Step S1: providing a space telescope and a plurality of ground telescopes to form a VLBI interference system, and acquiring original observation data from the space telescope and from the plurality of ground telescopes;
the ground telescopes may or may not be uniform in size. The caliber difference of the ground telescope is small, so that the synthesis efficiency can be improved; the aperture difference of the ground telescope is large, and the signal-to-noise ratio enhancement effect is not obvious.
The original observation data are all time domain signal data. In this embodiment, the number of the ground telescopes is 2, namely, the ground telescope a and the ground telescope B, and the space telescope is the space telescope C. However, in other embodiments, the number of ground telescopes may be any number of not less than 2.
Step S2: the enhanced VLBI signal is obtained through coherent synthesis based on the correlation processing of the original observation data of the space telescope and the ground telescope and the correlation processing of the original observation data between the ground telescope.
In this embodiment, in the step S2, the correlation processing of the original observation data between the space telescope and the ground telescope is to perform FX-type VLBI correlation processing on the space telescope and the original observation data of the ground telescope to obtain cross-correlation spectrums of the plurality of space base lines, and then synthesize the cross-correlation spectrums of the plurality of space base lines.
Thus, the step S2 is based on the synthesis of cross-correlation spectra of a plurality of space-earth baselines, and specifically includes:
step S21: leveling the stripes of the base lines among the ground telescopes through time delay compensation;
In this embodiment, the ground telescope is the ground telescope a and the ground telescope B, that is, the residual delay Δτ AB =0 and the residual delay rate of the baseline between the ground telescope a and the ground telescope B (i.e., AB baseline) are also 0 through delay compensation.
In the step S21, the time delay compensation is performed on the base lines between the ground telescopes, mainly to facilitate the correlation addition of the cross-correlation spectrum of the space-to-ground base line (the cross-correlation spectrum of the AC base line and the cross-correlation spectrum of the BC base line) at a later time, which is based on the following principle:
Baseline synthesis takes advantage of the property of three baseline delay closures. For the ground telescope a, the ground telescope B, and the space telescope C, assuming that the time when the same wavefront from the same signal source arrives at each station is denoted as t A、tB、tC, the time delay of each baseline is defined as:
The time delay of the AB baseline is as follows:
τAB＝tB-tA＝τm,AB+ΔτAB (11)
Where τ m,AB is the theoretical delay value of the AB baseline and Δτ AB is the residual delay of the AB baseline. The theoretical time delay values are obtained by calculation according to a theoretical model of time delay.
Similarly, the delay of the BC baseline is:
τBC＝tB-tC＝τm,BC+ΔτBC (12)
Where τ m,BC is the theoretical delay value of the BC baseline and Δτ BC is the residual delay of the BC baseline.
Similarly, the delay of the CA baseline is:
τCA＝tC-tA＝τm,CA+ΔτCA (13)
where τ m,CA is the theoretical delay value of the CA baseline and Δτ CA is the residual delay of the CA baseline.
From the baseline total and model delay closures, it is known that:
τAB+τBC+τCA＝0 (14)
τm,ΑΒ+τm,BC+τm,CA＝0 (15)
Finally, the method can obtain:
ΔτAB+ΔτBC+ΔτCA＝0 (16)
If let the residual delay of the AB baseline Δτ AB =0, i.e. the residual delay of the AB baseline is leveled, Δτ BC+ΔτCA =0, i.e. Δτ BC＝ΔτAC,ΔτAC is the residual delay of the AC baseline. Therefore, the step S21 realizes the equality of the residual time delay of the AC baseline and the residual time delay of the BC baseline, and is helpful for phase alignment and time delay compensation in the subsequent steps so as to flatten stripes, thereby achieving the purpose of improving the signal-to-noise ratio of the baseline.
The step S21 is implemented based on a correlation processor.
The step S21 specifically includes:
Step S211: performing FX type VLBI related processing on the original observation data of the two ground telescopes by adopting an FX type VLBI related processor to obtain a cross-correlation spectrum (namely visibility data) between the two ground telescopes;
In this example, the FX type VLBI related processor is a software related processor of the China VLBI Network (CVN). At this time, in the output cross-correlation spectrum, there are residual delay and residual delay rate because of errors in the theoretical delay model.
Step S212: a correlation post-processing method is adopted for the cross-correlation spectrum between the two ground telescopes to obtain residual time delay and residual time delay rate;
the VLBI post-processing software obtains the residual time delay and the residual time delay rate by adopting a method of stripe fitting.
Step S213: and compensating the residual time delay and the residual time delay rate to a time delay model of a correlation processor, and performing correlation processing again to obtain a cross-correlation spectrum. The fringes of the baseline between the two ground antenna stations are flattened at this time.
In this embodiment, after the step S213, step S213' may further include: and repeating the step S212, judging whether the residual time delay and the residual time delay rate reach enough precision, if so, ending the flow, otherwise, continuing to execute the step S213 and the step S213'. The step S213' is used to determine whether the compensated residual delay and the residual delay rate reach sufficient accuracy, which plays a role in improving accuracy. However, in other embodiments where particularly high precision is not required, step S213' may also be omitted.
In other embodiments, when the number of ground telescopes is greater than 2, the step S21 further includes: step S214: and repeating the steps S211-S213 until the residual time delay of the base line between any two ground telescopes reaches enough precision, and leveling the stripes.
The residual time delay and the residual time delay rate form VLBI total time delay and total time delay rate observables by adjusting a clock difference and clock speed compensation time delay model and the residual time delay, the residual time delay rate, the clock difference, the clock speed and the model time delay.
Step S22: FX type VLBI correlation processing is carried out on the space telescope and the original observation data of the ground telescope respectively to obtain cross-correlation spectrums of the space base lines, the space base lines are aligned in phase and the stripes are leveled through phase compensation and time delay compensation, and then the cross-correlation spectrums of different space base lines are synthesized. The space-ground base line is a base line between the space telescope and different ground telescopes.
In order to fully utilize the data of a plurality of space-based lines to improve the baseline signal-to-noise ratio, the cross-correlation spectrum of the space-based lines needs to be synthesized. The problem of different baseline phase alignment and stripe leveling needs to be addressed during this synthesis. That is, by adjusting the phase of the BC baseline, the phases of the two baselines are aligned. The fringes are leveled by compensating for the time delay of the baseline.
The step S22 includes:
Step S221: performing FX-type VLBI (very-large-scale binary-space) correlation processing on the original observation data of the space telescope and the original observation data of a plurality of ground telescopes respectively by adopting a correlation processor to obtain cross-correlation spectrums of a plurality of space-to-ground baselines;
In this embodiment, the number of space-to-ground baselines is 2, AC baseline and BC baseline, respectively.
Step S222: constructing Q-function using cross-correlation spectra of two of the space-earth baselinesAnd at Q functionSearching to obtain the coordinate/>, corresponding to the point with the maximum function value And τ is respectively used as a compensation phase and a compensation time delay;
Wherein the Q function The formula of (2) is as follows:
wherein k represents a frequency point; v AC、VBC represents data of cross-correlation spectra of AC baseline and BC baseline, respectively; Is the phase; Δf is the frequency resolution; τ is the delay.
According to formula (17), the method only needs to search one compensation phase for the space-to-ground baselineAnd a compensating delay τ. This is premised on that the fringes of the ground base line have all been leveled, i.e. the delta tau AB = 0 described above is satisfied.
Step S223: by compensating for phaseAligning phases of two empty ground base lines (namely an AC base line and a BC base line), and leveling stripes of the two empty ground base lines by compensating the time delay tau;
The invention uses the time delay tau and the phase between two baselines formed by two foundation telescopes and the space telescope respectively The residual time delay of the cross-correlation spectrum on the two base lines is approximately 0, and the initial phases are consistent, so that after the combination, the signal to noise ratio and the related fringe quality are improved, and the interference measurement effect of the large caliber telescope and the space telescope is achieved.
In this embodiment, after the step S223, step S223' may further include: step S222 is repeated to determine whether the compensation phase and the compensation delay reach sufficient accuracy (i.e. whether the phases are aligned and the stripes are leveled), if so, the flow is ended, otherwise, step S223 and step S223' are continuously performed. The step S223' is used for determining whether the compensated residual delay and the residual delay rate reach sufficient accuracy, which plays a role in improving accuracy. However, in other embodiments, where particularly high precision is not required, step S223' may also be omitted.
Step S224: and coherently summing the cross-correlation spectrums of different space-earth baselines to obtain a synthesized signal, wherein the obtained synthesized signal is an enhanced VLBI signal.
In addition, when the number of the ground telescopes is greater than 2, before the step S224, the method further includes: steps S221-S223 are repeated until the cross-correlation spectra of any two space-earth baselines are phase aligned and the fringes are leveled.
Step S3: the signal to noise ratio of the enhanced VLBI signal was determined and used as a criterion for baseline synthesis efficiency.
The signal-to-noise ratio (SNR) employed herein is calculated in accordance with the HOPS by the following formula:
Wherein A represents the amplitude obtained by adding all frequency points in all integration periods, and is applied to the amplitude of the synthesized signal obtained above; b is the channel bandwidth; t ap is the integration period; nlags is the frequency point number of the channel; n ap is the sum of the weights of all integration periods.
The more the signal-to-noise ratio increases before and after synthesis, the higher the synthesis efficiency.
In summary, the invention adopts the correlation processor to process the original data of the ground telescope to obtain the correlation spectrum, the result output by the software correlation processor for the first time often cannot enable Deltaτ ΑΒ =0, and the residual time delay rate are obtained through the calculation of the correlation post-processing; and then modifying the clock speed of the clock in the parameter file according to the residual time delay and the residual time delay rate, and carrying out correlation processing on the original data again to obtain a correlation spectrum, wherein in the obtained result of the correlation processor, delta tau ΑΒ =0.
Then, the invention also synthesizes the two space base lines after the phase compensation and the time delay compensation treatment, and obtains the signal to noise ratio for the synthesized base lines, and the calculated base line signal to noise ratio can be used as a judging standard of the base line synthesis efficiency.
Experimental effect:
the equivalent telescope caliber theory estimation after signal synthesis and the existing signal synthesis method are described below. From theoretical estimation, the equivalent large caliber telescope VLBI observation effect can be obtained through signal synthesis. Compared with the existing signal synthesis method applied to the spatial VLBI signal enhancement, the method has obvious advantages in terms of calculation amount and implementation difficulty by utilizing the existing FX type VLBI related processing software.
After the signal synthesis, the calculation of the equivalent telescope caliber is deduced as follows:
Baseline signal-to-noise ratio:
Wherein B is the signal recording bandwidth, T is the integration time, and ρ 0 is the original correlation strength of a single baseline;
The original correlation intensity ρ 0 for a single baseline is:
Where T ax,Tnx,Tay,Tny refers to the effective signal temperature and noise temperature of the stations x, y, respectively.
Slightly processing ρ 0 to obtain:
When the telescope is synthesized, the K telescopes are arranged to synthesize an equivalent telescope (K is more than or equal to 2), and for convenience in processing, the effective signal temperature T ax of the equivalent telescope is as follows, assuming that the signal to noise ratios of the K telescopes are identical:
Tax′＝K2Tax (4)
the noise temperature T nx′ is:
Tnx′＝K2Tnx (5)
The correlation intensity ρ' 0 of the new baseline constituted by the equivalent telescope is:
T ax<<Tnx in the usual case, so:
On the basis of the above, the equivalent area of the synthesized telescope signals of 1 telescope of 65 meters, 4 telescope signals of 40 meters and 2 telescope signals of 25 meters is analyzed. Here, it is assumed that the telescope units are different in size and have identical other parameters, so that the change of the telescope gain directly determines the change of the signal-to-noise ratio. The idea of calculating the equivalent area is to calculate the equivalent area of the signals of all the 25 meters telescope by taking the 25 meters telescope as a reference and calculating the equivalent of the 40 meters telescope and the 60 meters telescope corresponding to the 25 meters telescope respectively.
The calculation formula of the gain of the parabolic telescope is as follows:
Wherein M is the effective area of the telescope, lambda is the wavelength, and eta is the telescope efficiency.
According to the convention, only area change brings about gain improvement, so if K telescopes with the area of M are synthesized into a telescope with the equivalent area of N, according to (1) (2), the signal to noise ratio is improved to 10log K (db), and the gain improvement of a single telescope is synthesized to 10log K (db), and the following relation is obtained by the formula (3):
10logN-10logM＝10logK (9)
The above formula is simplified, and the square relationship between the area and the caliber is considered, so there are:
Wherein d M、dN refers to the caliber of the original telescope and the equivalent telescope.
Based on the above derivation, 140 meters telescope is equivalent to 2.56 25 meters telescope (40 2/252 =2.56), 1 65 meters telescope is equivalent to 6.76 25 meters telescope (65 2/252 =6.76), so 1 65 meters, 4 40 meters, 2 25 meters telescope is equivalent to 19 (6.76+4×2.56+2=19) 25 meters telescope, and again using formula T nx′＝K2Tnx, 19 25 meters telescope is equivalent to 1 109 meters telescope.
From the theoretical deduction, the signals received by the telescope with small and medium caliber can be equivalently received by the telescope with large caliber.
The spatial VLBI signal enhancement system based on cross-correlation spectrum synthesis implemented based on the spatial VLBI signal enhancement method based on cross-correlation spectrum synthesis described above comprises a spatial telescope 10, a plurality of terrestrial telescopes 20 and a signal enhancement processor arranged to perform the spatial VLBI signal enhancement method based on cross-correlation spectrum synthesis described above, whereby the plurality of terrestrial telescopes 20 are equivalent to one large caliber foundation telescope 30 and the spatial telescope 10 and the plurality of terrestrial telescopes 20 constitute one component VLBI interference system 40.
The invention provides a space VLBI interference system which is formed by networking and signal synthesis by using a domestic multi-surface 40-65 m medium caliber foundation VLBI telescope and a space 4.2m telescope. Through a spatial VLBI signal enhancement technology based on VLBI cross-correlation spectrum synthesis, cross-correlation power spectrum synthesis is performed on a plurality of space base lines, and finally interference fringes (the interference fringes are based on a VLBI cross-correlation function, particularly a 'phase-frequency' function) of which the base lines are formed by the class 100-meter large-caliber telescope and the space telescope can be obtained. The objects of observation may be a radio source and a deep space probe. The invention solves the two key technical problems of combining the original signal observed by the foundation VLBI telescope and processing the space-ground VLBI baseline correlation by skillfully utilizing the structural characteristics of the VLBI correlation processor under the condition that the X-band large-caliber telescope is not provided in China, and achieves the effect of utilizing a plurality of medium-caliber foundation telescopes to achieve a single 100-meter-level large-scale VLBI foundation telescope.
The foregoing description is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention, and various modifications can be made to the above-described embodiments of the present invention. All simple, equivalent changes and modifications made in accordance with the claims and the specification of this application fall within the scope of the patent claims. The present invention is not described in detail in the conventional art.
Claims (7)
1. A method for spatial VLBI signal enhancement based on cross-correlation spectrum synthesis, comprising:
Step S1: providing a space telescope and a plurality of ground telescopes to form a VLBI interference system, and acquiring original observation data from the space telescope and from the plurality of ground telescopes;
step S2: based on the correlation processing of the original observation data between the space telescope and the ground telescope and the correlation processing of the original observation data between the ground telescope, obtaining an enhanced VLBI signal through coherent synthesis;
in the step S2, the correlation processing of the original observation data between the space telescope and the ground telescope is to perform FX-type VLBI correlation processing on the original observation data of the space telescope and the original observation data of the ground telescope respectively to obtain cross-correlation spectrums of a plurality of space base lines, and then synthesize the cross-correlation spectrums of the plurality of space base lines;
The step S2 is based on the synthesis of cross-correlation spectra of a plurality of space-earth baselines, and includes:
Step S21: carrying out correlation processing on the original observation data received by the ground telescope to obtain a residual time delay compensation time delay model, and carrying out correlation processing again to flatten stripes of base lines among the ground telescopes;
Step S22: and performing FX-type VLBI correlation processing on the original observation data of the space telescope and the original observation data of the ground telescope respectively to obtain cross-correlation spectrums of a plurality of space base lines, enabling the plurality of space base lines to be in phase alignment and stripe leveling through phase compensation and time delay compensation, and then synthesizing the cross-correlation spectrums of different space base lines.
2. The method for spatial VLBI signal enhancement based on cross-correlation spectrum synthesis according to claim 1, wherein said step S21 comprises:
Step S211: performing FX-type VLBI correlation processing on the original observation data of the two ground telescopes by adopting a correlation processor to obtain a cross-correlation spectrum between the two ground telescopes;
Step S212: a correlation post-processing method is adopted for the cross-correlation spectrum between the two ground telescopes to obtain residual time delay and residual time delay rate;
Step S213: and compensating the residual time delay and the residual time delay rate to a time delay model, and performing correlation processing again to obtain a cross-correlation spectrum.
3. The method for spatial VLBI signal enhancement based on cross-correlation spectrum synthesis according to claim 2, further comprising, after said step S213, a step S213': and repeating the step S212, judging whether the residual time delay and the residual time delay rate reach enough precision, if so, ending the flow, otherwise, continuing to execute the step S213 and the step S213'.
4. The method for spatial VLBI signal enhancement based on cross-correlation spectrum synthesis according to claim 2, wherein said step S22 comprises:
Step S221: performing FX-type VLBI (very-large-scale binary-space) correlation processing on the original observation data of the space telescope and the original observation data of a plurality of ground telescopes respectively by adopting a correlation processor to obtain cross-correlation spectrums of a plurality of space-to-ground baselines;
Step S222: constructing Q-functions using cross-correlation spectra of space-to-ground baselines And at Q function/>Searching to obtain the coordinate/>, corresponding to the point with the maximum function value And τ is respectively used as a compensation phase and a compensation time delay;
The Q function The method comprises the following steps:
wherein k represents a frequency point; v AC、VBC represents data of cross-correlation spectra of AC baseline and BC baseline, respectively; is the phase; Δf is the frequency resolution; τ is the time delay;
Step S223: by compensating for phase Aligning the phases of the two space base lines, and leveling the stripes of the two space base lines by compensating the time delay tau;
Step S224: and coherently summing the cross-correlation spectrums of different space-earth baselines to obtain a synthesized signal, wherein the obtained synthesized signal is an enhanced VLBI signal.
5. The method of claim 4, wherein when the number of the terrestrial telescopes is greater than 2,
The step S21 further includes a step S214: steps S211-S213 are repeated until the residual delay of the baseline between any two ground telescopes reaches sufficient accuracy, and the fringes flatten out.
6. The method for spatial VLBI signal enhancement based on cross-correlation spectrum synthesis according to claim 1, further comprising step S3: the signal to noise ratio of the enhanced VLBI signal was determined and used as a criterion for baseline synthesis efficiency.
7. A spatial VLBI signal enhancement system based on cross-correlation spectrum synthesis, characterized in that it comprises a spatial telescope, a plurality of terrestrial telescopes and a signal enhancement processor arranged to perform the spatial VLBI signal enhancement method based on cross-correlation spectrum synthesis according to one of claims 1-6.
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