CN113029161A - 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 plurality of ground telescopes form a VLBI interference system, and original observation data from the space telescope and the plurality of ground telescopes 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, simultaneously completes the correlation processing of original signal synthesis observed by a foundation VLBI telescope and a spatial-ground VLBI baseline, and realizes the effect of performing lunar VLBI on a single foundation caliber telescope and a single spatial small caliber telescope.
Description
Technical Field
The invention relates to a spatial VLBI signal enhancement method, in particular to a spatial VLBI signal enhancement technical method and a spatial VLBI signal enhancement technical system based on VLBI cross-correlation spectrum synthesis.
Background
Astronomical observations are ultimately required to pursue sensitivity or spatial resolution. Very Long Baseline Interferometry (VLBI) technology has Very high angular resolution and sensitivity, and is widely applied in the fields of celestial physics, celestial measurement, astronomical geodynamics, and the like. According to the basic principles of electromagnetism, the angular resolution of a telescope is approximately equal to the observation wavelength divided by the effective aperture.
Description of the principle of the existing VLBI correlation handler (FX type):
the input to the FX-type VLBI correlation processor is the time domain signal obtained at the stations at both ends of the baseline, and the output is the cross-correlation spectrum between stations (called the visibility function). As shown in fig. 1, two signals enter an FX type correlation processor and are decoded according to frequency channels; then, carrying out integer bit delay compensation according to a pre-calculated delay model; after stripe rotation and fast Fourier transform, the signal is transformed from a time domain to a frequency domain, and fractional bit delay compensation is completed; and finally, carrying out conjugate multiplication and accumulation integration on the two paths of signals to obtain visibility data.
Due to the limited speed of light, the wavefront of the signal emitted by the radio source reaches the two telescopes at different times, and the time difference between the two telescopes is called VLBI time delay and can be expressed as:
where τ is the total time delay, τ0And
the theoretical time delay and the time delay rate of the moment are calculated according to the time delay model. And the correlation processor corrects time delay and time delay rate according to a theoretical model, and specifically aligns the two paths of signals on a time domain and a frequency domain through integer bit compensation, fringe rotation and decimal bit compensation. Because the accuracy of the time delay model is limited, the output visibility 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 changes along with the time change. VLBI post-processing software obtains residual time delay and residue by a fringe fitting methodA delay rate; the model delay, the delay rate and the clock speed form the total delay and the delay rate together, so that VLBI total delay and total delay rate observed quantities are obtained.
For VLBI, the highest angular resolution is approximately equal to the observed wavelength divided by the baseline length (i.e., the distance between the two radio telescopes). At present, the VLBI facility with the highest ground angle resolution is a black hole field telescope, and the maximum equivalent caliber of the VLBI facility is close to the diameter of the earth. Astronomers have long recognized that the maximum base length of a foundation VLBI is unlikely to exceed the diameter of the earth. If the radio telescope is launched into space, forming an open space or space VLBI array with a ground or space radio telescope, the base length may exceed the earth's diameter, thereby achieving higher angular resolution. This is the Space VLBI (Space VLBI) or Space VLBI (Space-Earth VLBI).
However, technical and cost factors limit the aperture size of the space telescope, resulting in a space-ground baseline sensitivity that is often lower than that of the ground baseline. Furthermore, since the open space VLBI baseline is longer than the ground based VLBI, and the angular resolution is high, resulting in the target source being mostly structurally decomposed, the detected source flow is low, and the spatial VLBI is more difficult to obtain sharp streaks for the same observed source than the conventional ground-based VLBI. Therefore, how to improve the sensitivity of the spatial VLBI telescope is a technical problem to be solved.
Spatial VLBI is a common practice to maximize the diameter of the spatial VLBI telescope while maximizing the effective area of the ground VLBI telescope. In order to increase the effective area of the ground-based telescope, the existing method adopts the ground-based telescope with an ultra-large caliber as much as possible or utilizes a ground line interference telescope array mode to participate in spatial VLBI networking observation. At present, no existing ground-based space VLBI telescope received signal is synthesized and subjected to VLBI interference processing with a space telescope from the public. In the existing space VLBI observation, an ultra-large-caliber foundation telescope facility is often needed.
For example, the Russian space VLBI project RadioAstron is a more successful space VLBI project so far, and the caliber of a space telescope Spektr-R is 10 meters. The project utilizes the land large-caliber telescopes and Spektr-R of each country to form a space VLBI system through international cooperation. The radio astron project utilizes large-scale ground-based telescope equipment such as an arresibo telescope with the ground diameter of 300 meters, a green shore 110-meter telescope, a Bonn 100-meter telescope, a Westerbork synthetic aperture telescope array and the like to improve the space VLBI observation sensitivity by increasing the receiving area of the ground telescope. The Westerbork and other synthetic aperture telescope arrays adopt a method of line short baseline telescope signal synthesis to form a 66-meter equivalent aperture telescope, wherein telescope signal synthesis refers to coherent synthesis of signals received by two radio telescopes through technical means, and the effect similar to the effect of the signals received by one single aperture radio telescope is achieved.
In the moon detection of ChangE seven, China plans to carry a telescope with the caliber of 4.2 meters by using a relay star and is provided with VLBI special equipment to form a VLBI telescope running on a moon track, and the VLBI telescope and a ground telescope of a China VLBI net form an X-band moon space VLBI test system with the baseline exceeding 30 km (the length of the formed moon baseline is about 38 km), so as to carry out observation research on the moon VLBI baseline. However, in the current space VLBI, the aperture of the space telescope is small, the baseline signal-to-noise ratio is often improved by combining the observation of the foundation large-aperture VLBI telescope, and because China does not have an X-frequency-band foundation radio telescope with the aperture of one hundred meters at present, China VLbi network which can be used in the future only has a medium-aperture telescope with the aperture of 25-65 meters, the aperture of the space and the aperture of the foundation VLbi telescope are smaller compared with that of the foreign countries, and the weak point exists in the aspect of observation sensitivity.
Furthermore, there are currently 5 existing signal synthesis schemes that can be used to synthesize the signals from the deep space probe, respectively called: (1) full spectrum synthesis (FSC); (2) complex symbol synthesis (CSC); (3) symbol Stream Synthesis (SSC); (4) baseband synthesis (BC); (5) carrier group array (CA). Table 1 below gives the advantages and disadvantages of the signal synthesis scheme for 5 antenna array systems. (see [ Rogerstota et al, antenna array technology for deep space networks, [ M ]. Qinghua university Press, 2005 ])
TABLE 1 advantages and disadvantages of the existing signal synthesis schemes
The CSC, SSC, BC and CA must propose a constellation scheme according to the spectrum 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, a complex symbol stream, and a complex baseband, respectively, by an existing demodulation synchronization apparatus, and then signal synthesis is performed. These solutions require different degrees of carrier tracking equipment, subcarrier tracking equipment and symbol synchronization equipment and are therefore only used for signals with well-defined modulation characteristics, such as detector signals, and are not suitable for various types of signals that are unknown or noise-like (cosmic radiation sources are such signals).
The existing spacecraft tasks mostly adopt FSC scheme. The FSC directly synthesizes the intermediate frequency signals or baseband signals from each telescope, and in order to ensure coherence, the signals are subjected to time delay and phase compensation before synthesis, and the compensation quantity of the time delay and phase of the signals among the array telescopes is determined by utilizing signal correlation.
However, the full spectrum synthesis (FSC) signal synthesis method described above is often used for line-bound interferometers or short baselines for detector signal synthesis, and is not applicable to long-baseline radio source signal synthesis where the VLBI telescope is at a long distance.
Currently, there is a solution to extend FSC to far distance VLBI telescope signal synthesis, referred to as very long baseline full spectrum signal synthesis. The flow diagram of the very long baseline full-spectrum signal synthesis method is shown in fig. 1, and a related processing result is obtained through a forecast delay model, and a delay model of a detector is generally obtained by adopting a near-field model and calculating with the geocentric as a reference. And then, obtaining residual time delay, time delay rate and phase difference according to the fringe search, and carrying out a series of time delay compensation and phase alignment operations on each path of signal from different telescopes. Wherein, the model except the time delay is obtained by calculating the residual time delay and the time delay rateIn order to further need the time delay of compensation, the compensation process is divided into two steps of integer bit shift and hour bit correction. Only an integer number of bits can be shifted because of the sampling point of the digital signal. In FIG. 1, τbIs the time delay compensated by this step of integer bit shifting. Tau isfIs to subtract tau from the value of the time delay to be compensatedbThe delay remaining thereafter is the delay compensated by the fractional bit correction step.
The phase compensation is generated by the rotation of the earth when the signals received by the two telescopes are synchronous and have the same wave front. This taub、τf、
And calculating the residual time delay, the time delay rate and the phase difference. Before synthesis, it is often necessary to make a signal-to-noise ratio estimate so that the signal-to-noise ratio of the synthesized signal is maximized. The method mainly comprises the following steps: cross-correlation and stripe search, time delay compensation and stripe rotation, signal-to-noise ratio estimation and weighting coefficient calculation.
However, the scheme directly operates on the original signal and is mainly applied to telemetry and data transmission signal synthesis. Furthermore, existing schemes are directed to the synthesis of the original signal, such as FSC, CSC, SSC, BC, etc.
In summary, the full-spectrum signal synthesis method is based on the original signal synthesis before the correlation processing, so the calculation amount and the realization difficulty are high.
Therefore, a scheme for performing array combining and signal synthesis by using a VLBI telescope to achieve equivalent large-aperture telescopes and simultaneously complete interference processing is urgently needed to be provided, and the scheme is applied to a spatial 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, which are used for enhancing the signal sensitivity of a space-to-ground baseline of VLBI.
In order to achieve the above object, the present invention provides a method for enhancing a spatial VLBI signal 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 the 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 step S2, the correlation processing of the raw observation data between the space telescope and the ground telescope is to perform FX-type VLBI correlation processing on the raw observation data of the space telescope and the raw observation data of the ground telescopes to obtain a plurality of air-ground baseline cross-correlation spectra, and then synthesize the plurality of air-ground baseline cross-correlation spectra.
The step S2 is based on the synthesis of cross-correlation spectra of a plurality of space-ground baselines, and includes:
s21: performing correlation processing on original observation data received by the ground telescopes to obtain residual time delay, compensating a time delay model, and performing correlation processing again to flatten the stripes of the base lines among the plurality of ground telescopes;
s22: FX-type VLBI correlation processing is carried out on the original observation data of the space telescope and the ground telescopes respectively to obtain cross-correlation spectrums of the space baselines, the space baselines are enabled to be aligned in phase and have flattened stripes through phase compensation and time delay compensation, and then the cross-correlation spectrums of different space baselines are synthesized.
The step S21 includes:
s211: performing FX type VLBI (very high performance building Block) related processing on the original observation data of the two ground telescopes by using a related processing machine to obtain a cross-correlation spectrum between the two ground telescopes;
s212: obtaining residual time delay and residual time delay rate by adopting a correlation post-processing method for the cross-correlation spectrum between the two ground telescopes;
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 the step S213, a step S213': and repeating the step S212 and determining whether the residual delay and the residual delay rate reach sufficient accuracy, if yes, ending the process, otherwise, continuing to execute the steps S213 and S213'.
The step S22 includes:
s221: performing FX type VLBI (very high performance building information) related processing on the original observation data of the space telescope and the original observation data of the plurality of ground telescopes by using a related processing machine to obtain cross-correlation spectrums of the plurality of space baselines;
s222: construction of Q function using cross-correlation spectra of space-ground baselines
And in the Q function
Searching to obtain the coordinate corresponding to the point with the maximum function value
And τ are used as a compensation phase and a compensation time delay respectively;
the Q function
Comprises the following steps:
in the formula, k represents a frequency point; vAC、VBCData representing cross-correlation spectra of the AC baseline and the BC baseline, respectively;
is the phase; Δ f is the frequency resolution; tau is time delay;
s223: by compensating for phase
Aligning the phases of the two space-ground baselines, and flattening the stripes of the two space-ground baselines by compensating the time delay tau;
s224: and coherently adding the cross-correlation spectra of different space-ground baselines to obtain a synthetic signal, wherein the obtained synthetic signal is an enhanced VLBI signal.
When the number of the ground telescopes is greater than 2, the step S21 further includes the step S214: steps S211-S213 are repeated until the residual time delay of the baseline between any two ground telescopes reaches sufficient accuracy, and the fringes are leveled.
The method for enhancing a spatial VLBI signal based on cross-correlation spectrum synthesis further comprises 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 cross-correlation synthesis based spatial VLBI signal enhancement system comprising a spatial telescope, a plurality of terrestrial telescopes, and a signal enhancement processor configured to perform the cross-correlation synthesis based spatial VLBI signal enhancement method as described above.
The space VLBI signal enhancement method based on cross-correlation spectrum synthesis is based on the cross-correlation spectrum after the space-ground baseline VLBI correlation processing is synthesized, so that a plurality of ground telescopes are synthesized into a telescope with a larger equivalent caliber, interference fringes similar to the baseline formed by a large-caliber telescope and a space telescope can be obtained, the effect of enhancing the sensitivity of a VLBI space and a ground baseline is achieved, the method can be applied to the field of the moon-ground space VLBI or other VLBI, and the requirement for the large-caliber ground telescope is reduced.
The signal synthesis method related to the spatial VLBI signal enhancement method based on cross-correlation spectrum synthesis is different from the existing signal synthesis method. The scheme synthesizes the cross-correlation spectrum of the open space base line obtained after VLBI correlation processing so as to simultaneously complete original observation signal synthesis of a foundation VLBI telescope and space-ground VLBI base line correlation processing, thereby achieving the effect of synthesizing space VLBI signals to obtain enhanced VLBI signals, greatly reducing the processed data volume, achieving the effect of completing interference processing by an equivalent large-caliber telescope, and being applicable to synthesis of radio source signals and detector signals; and from the original observation data to the synthesized result, the whole process only needs to carry out two times of related processing, thereby reducing the data processing steps. Meanwhile, distortion phenomena caused by VLBI fractional time delay compensation operation and the like in a very long baseline full-spectrum signal synthesis technology are avoided. In conclusion, the calculation amount and the realization difficulty in the scheme are far smaller than those of the existing scheme.
Drawings
FIG. 1 is a flow chart of data processing for a conventional FX-type VLBI correlation handler.
Fig. 2 is a flow chart of a conventional very long baseline full-spectrum signal synthesis method.
Fig. 3 is a flow diagram of a spatial VLBI signal enhancement method based on VLBI cross-correlation spectrum synthesis according to an embodiment of the present invention.
Fig. 4 is a schematic structural diagram of a spatial VLBI signal enhancement system based on VLBI cross-correlation spectrum synthesis according to an embodiment of the present invention, showing that a plurality of ground telescopes are equivalent to one equivalent large caliber ground-based telescope.
Detailed Description
Embodiments of the present invention are provided below and described in detail with reference to the accompanying drawings.
As shown in fig. 3, the method for spatial VLBI signal enhancement 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 the ground telescopes;
wherein, the sizes of the ground telescopes can be consistent or inconsistent. The aperture 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, which are the ground telescope a and the ground telescope B, respectively, and the space telescope is the space telescope C. However, in other embodiments, the number of ground telescopes may be any number not less than 2.
Step S2: 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 telescopes, the enhanced VLBI signal is obtained through coherent synthesis.
In this embodiment, in step S2, the correlation of the raw observation data between the space telescope and the ground telescope is performed by performing FX-type VLBI correlation on the raw observation data of the space telescope and the ground telescopes to obtain a plurality of cross-correlation spectra of the space baselines, and then combining the cross-correlation spectra of the space baselines.
Therefore, the step S2 is based on the synthesis of the cross-correlation spectrum of the plurality of space baselines, and specifically includes:
step S21: through time delay compensation, the stripes of the base lines among the ground telescopes are flattened;
in the present embodiment, the ground telescopes are the ground telescope a and the ground telescope B, that is, the residual time delay Δ τ of the base line (i.e., AB base line) between the ground telescope a and the ground telescope B is compensated by the time delayAB0 and the residual delay rate is also 0.
In the step S21, the time delay compensation is performed on the base lines between the plurality of ground telescopes, mainly to facilitate the correlation addition of the cross-correlation spectra of the subsequent space-ground base lines (the cross-correlation spectrum of the AC base line and the cross-correlation spectrum of the BC base line), which is based on the following principle:
baseline synthesis utilizes the property of three baseline time delays closed. For the ground telescope A, the ground telescope B and the space telescope C, the time when the same wavefront from the same signal source signal reaches each station is assumed to be recorded as tA、tB、tCThe time delay for each baseline is defined as:
the AB baseline time delay is:
τAB＝tB-tA＝τm，AB+ΔτAB (11)
wherein，τm，ABIs the theoretical time delay value of the AB base line, Δ τABThe residual time delay at AB baseline. And the theoretical time delay values are obtained by calculation according to a theoretical model of the time delay.
Similarly, the delay of the BC baseline is:
τBC＝tB-tC＝τm，BC+ΔτBC (12)
wherein, taum，BCIs the theoretical time delay value, Δ τ, of the BC baselineBCIs the residual latency of the BC baseline.
Similarly, the time delay of the CA baseline is:
τCA＝tC-tA＝τm，CA+ΔτCA (13)
wherein, taum，CAIs the theoretical time delay value, Δ τ, of the CA baselineCAResidual time delay at the CA baseline.
From the baseline total delay closure and the model delay closure:
τAB+τBC+τCA＝0 (14)
τm,ΑΒ+τm,BC+τm,CA＝0 (15)
finally, the following can be obtained:
ΔτAB+ΔτBC+ΔτCA＝0 (16)
if let the residual time delay delta tau of AB base lineAB0, i.e. the residual time delay of the AB base line is leveled, then Δ τBC+ΔτCA0, i.e. Δ τBC＝ΔτAC，ΔτACIs the residual time delay of the AC baseline. Therefore, step S21 realizes the equality between the residual delay of the AC baseline and the residual delay of the BC baseline, and facilitates the phase alignment and delay compensation in the subsequent steps to flatten the fringes, thereby achieving the purpose of improving the baseline signal-to-noise ratio.
The step S21 is implemented based on a correlation handler.
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 processing machine to obtain a cross-correlation spectrum (namely visibility data) between the two ground telescopes;
in this embodiment, the FX-type VLBI processor is a software processor of the Chinese VLBI Network (CVN). At this time, in the output cross-correlation spectrum, because the theoretical delay model has an error, a residual delay and a residual delay rate exist.
Step S212: obtaining residual time delay and residual time delay rate by adopting a correlation post-processing method for the cross-correlation spectrum between the two ground telescopes;
and the VLBI post-processing software acquires the residual time delay and the residual time delay rate by adopting a fringe fitting method.
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 baseline fringe between the two ground antenna stations is now leveled.
In this embodiment, after the step S213, a step S213': and repeating the step S212 and determining whether the residual delay and the residual delay rate reach sufficient accuracy, if yes, ending the process, otherwise, continuing to execute the steps S213 and S213'. The step S213' is used to determine whether the compensated residual delay and 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 the stripes are leveled.
The residual time delay and the residual time delay rate form VLBI total time delay and total time delay rate observed quantity by adjusting a clock difference and clock speed compensation time delay model together with the clock difference, the clock speed and the model time delay.
Step S22: FX-type VLBI correlation processing is carried out on the original observation data of the space telescope and the ground telescopes respectively to obtain cross-correlation spectrums of the space baselines, the space baselines are enabled to be aligned in phase and have flattened stripes through phase compensation and time delay compensation, and then the cross-correlation spectrums of different space baselines are synthesized. Wherein the air-ground baseline is a baseline between the space telescope and the different ground telescopes.
In order to fully utilize the data of a plurality of space-ground baselines to improve the baseline signal-to-noise ratio, the cross-correlation spectrum of the space-ground baselines needs to be synthesized. The problems of phase alignment of different baselines and leveling of stripes need to be solved in the synthesis process. That is, the phases of the two baselines are aligned by adjusting the phase of the BC baseline. The fringes are leveled by compensating for the baseline delay.
The step S22 includes:
step S221: performing FX type VLBI (very high performance building information) related processing on the original observation data of the space telescope and the original observation data of the plurality of ground telescopes by using a related processing machine to obtain cross-correlation spectrums of the plurality of space baselines;
in this embodiment, the number of air-to-ground baselines is 2, and AC baselines and BC baselines are provided, respectively.
Step S222: construction of Q function by using cross-correlation spectrum of two space-ground baselines
And in the Q function
Searching to obtain the coordinate corresponding to the point with the maximum function value
And τ are used as a compensation phase and a compensation time delay respectively;
wherein the Q function
The formula of (1) is as follows:
in the formula, k represents a frequency point; vAC、VBCData representing cross-correlation spectra of the AC baseline and the BC baseline, respectively;
is the phase; Δ f is the frequency resolution; τ is the time delay.
According to the formula (17), the method only needs to search one compensation phase for the air-ground baseline
And a compensating time delay tau. This is done on the premise that the ground base lines have been leveled, i.e. satisfying the above-mentioned Δ τAB＝0。
Step S223: by compensating for phase
Aligning the phases of the two space baselines (namely an AC base line and a BC base line), and flattening the stripes of the two space baselines by compensating the time delay tau;
the invention relates to a time delay tau and a phase between two base lines which are respectively formed by two ground-based telescopes and a space telescope
The searching and compensating of the method enables the residual time delay of the cross-correlation spectra on the two base lines to be approximate to 0, and the initial phases to be consistent, so that after the two base lines are combined and synthesized, the signal-to-noise ratio and the quality of the related fringes are improved, and the effect of interference measurement of a large-aperture telescope and a space telescope is achieved.
In this embodiment, after the step S223, a step S223': and repeating the step S222 and determining whether the compensation phase and the compensation delay reach sufficient accuracy (i.e. whether the phase is aligned and the fringe is leveled), if so, ending the process, otherwise, continuing to execute the steps S223 and S223'. The step S223' is used to determine whether the compensated residual delay and 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 adding the cross-correlation spectra of different space-ground baselines to obtain a synthetic signal, wherein the obtained synthetic signal is an enhanced VLBI signal.
Further, 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-ground 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) calculation used herein is consistent with hoss, and the formula is as follows:
in the formula, a represents the amplitude obtained by adding all frequency points in all integration periods, and the amplitude is applied to the amplitude of the synthesized signal obtained above; b is the channel bandwidth; t isapIs an integration period; nlags is the number of frequency points of the channel; n isapIs 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 correlation processor is used for processing the original data of the ground telescope to obtain the correlation spectrum, and the result output by the software correlation processor for the first time often cannot enable the delta tau to be outputΑΒObtaining the residual time delay and the residual time delay rate by the calculation of relevant post-processing; and then, modifying the clock speed of the clock difference in the parameter file according to the residual time delay and the residual time delay rate, and performing correlation processing on the original data again to obtain a correlation spectrum, wherein the delta tau in the obtained result of the correlation processorΑΒ＝0。
And then, the invention also synthesizes the two air-ground baselines after carrying out phase compensation and time delay compensation processing, obtains the signal-to-noise ratio of the synthesized baselines, and can use the calculated signal-to-noise ratio of the baselines as the judgment standard of the synthesis efficiency of the baselines.
The experimental effect is as follows:
the theoretical estimation of the equivalent telescope aperture after signal synthesis and the existing signal synthesis method will be described below. From theoretical estimation, the observation effect of the equivalent large-aperture telescope VLBI can be obtained through signal synthesis. Compared with the application of the existing signal synthesis method to spatial VLBI signal enhancement, the scheme utilizes the existing FX type VLBI related processing software, and has obvious advantages in terms of calculation amount and realization difficulty.
After signal synthesis, the calculation of the equivalent telescope aperture is derived as follows:
baseline signal-to-noise ratio:
where B is the signal recording bandwidth, T is the integration time, p0Raw correlation strength for a single baseline;
raw correlation strength p of a single baseline0Comprises the following steps:
wherein, Tax,Tnx,Tay,TnyRespectively, the effective signal temperature and the noise temperature of the stations x, y.
For rho0Slightly processed to obtain:
when the telescopes are synthesized, K telescopes are arranged to synthesize an equivalent telescope (K is more than or equal to 2), and for convenient processing, the effective signal temperature T of the equivalent telescope is assumed to be the same as the signal-to-noise ratio of the K telescopesaxThe method comprises the following steps:
Tax′＝K2Tax (4)
noise temperature Tnx′Namely:
Tnx′＝K2Tnx (5)
correlation intensity ρ 'of new baseline composed of the equivalent telescope'0Comprises the following steps:
in the normal case Tax<<TnxTherefore:
on the basis, the equivalent area of 1 telescope signal with 65 meters, 4 telescopes with 40 meters and 2 telescopes with 25 meters after synthesis is analyzed. It is assumed here that the telescopes are identical in other parameters except for the size, and therefore 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 respectively calculate how many 25-meter telescopes 40-meter telescopes and 60-meter telescopes are equivalent to based on the 25-meter telescope, and then calculate the equivalent area synthesized by all 25-meter telescope signals.
The gain of the parabolic telescope is calculated by the following formula:
wherein M is the effective area of the telescope, lambda is the wavelength, and eta is the telescope efficiency.
According to the convention, only the area change brings the gain improvement, so if K telescopes with the area M are combined into a telescope with the equivalent area N, according to (1) and (2), the signal-to-noise ratio is improved to 10logK (db), the gain of the combined telescope is improved to 10logK (db), and the following relation is given by the formula (3):
10logN-10logM＝10logK (9)
the above equation is simplified, and the square relation between the area and the caliber is considered, so that the following equation is provided:
wherein d isM、dNRefers to the caliber of the original telescope and the equivalent telescope.
Based on the above derivation, it can be calculated that 1 40-meter telescope is equivalent to 2.56 25-meter telescopes (40)2/2522.56), 1 telescope of 65 meters is equivalent to 6.76 telescopes of 25 meters (65)2/2526.76) so 1 65 meters, 4 40 meters, 2 25 meters of telescopes is equivalent to 19(6.76+4 × 2.56+ 2-19) 25 meters of telescopes, again using the formula Tnx′＝K2Tnx19 25 meters of telescope is equivalent to 1 109 meters of telescope.
According to the theoretical derivation, the signals received by synthesizing the medium and small caliber telescopes are equivalent to the signals received by the large caliber telescope.
Based on the above-mentioned cross-correlation-spectrum-synthesis-based spatial VLBI signal enhancement method, the implemented cross-correlation-spectrum-synthesis-based spatial VLBI signal enhancement system comprises a spatial telescope 10, a plurality of ground telescopes 20 and a signal enhancement processor, wherein the signal enhancement processor is configured to execute the above-mentioned cross-correlation-spectrum-synthesis-based spatial VLBI signal enhancement method, so that the plurality of ground telescopes 20 are equivalent to one large-caliber ground-based telescope 30, and the spatial telescope 10 and the plurality of ground telescopes 20 form a combined VLBI interference system 40.
The invention provides a spatial VLBI interference system which is formed by using domestic multi-surface 40-65 m medium-caliber foundation VLBI telescopes for networking and signal synthesis and a spatial 4.2m telescope. By a space VLBI signal enhancement technology based on VLBI cross-correlation spectrum synthesis, cross-correlation power spectrum synthesis is carried out on a plurality of space baselines, and finally interference fringes (based on a VLBI cross-correlation function, particularly a phase-frequency function) of baselines formed by a 100-class large-caliber telescope and the space telescope can be obtained. The observation object can be a radio source and a deep space probe. The invention solves the two key technical problems of original signal synthesis of foundation VLBI telescope observation and space-ground VLBI baseline correlation processing simultaneously by skillfully utilizing the structural characteristics of a VLBI correlation processor under the condition that China does not have an X-band large-caliber telescope, and achieves the effect of achieving a single 100-meter large-scale VLBI foundation telescope by utilizing a plurality of medium-caliber foundation telescopes.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not intended to limit the scope of the present invention, but rather, the present invention may be implemented in various forms. All simple and equivalent changes and modifications made according to the claims and the content of the specification of the present application fall within the scope of the claims of the present patent application. The invention has not been described in detail in order to avoid obscuring the invention.
Claims (9)
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 the 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, the enhanced VLBI signal is obtained through coherent synthesis.
2. The method of claim 1, wherein in step S2, the correlation of the raw observation data between the space telescope and the ground telescope is performed by performing FX-type VLBI correlation on the raw observation data of the space telescope and the raw observation data of the ground telescopes to obtain a plurality of space-based cross-correlation spectra, and then combining the plurality of space-based cross-correlation spectra.
3. The method of claim 1, wherein the step S2 is based on the synthesis of cross-correlation spectra of a plurality of space-ground baselines, and comprises:
step S21: performing correlation processing on original observation data received by the ground telescopes to obtain residual time delay, compensating a time delay model, and performing correlation processing again to flatten the stripes of the base lines among the plurality of ground telescopes;
step S22: FX-type VLBI correlation processing is carried out on the original observation data of the space telescope and the ground telescopes respectively to obtain cross-correlation spectrums of the space baselines, the space baselines are enabled to be aligned in phase and have flattened stripes through phase compensation and time delay compensation, and then the cross-correlation spectrums of different space baselines are synthesized.
4. The method of claim 3, wherein the step S21 comprises:
step S211: performing FX type VLBI (very high performance building Block) related processing on the original observation data of the two ground telescopes by using a related processing machine to obtain a cross-correlation spectrum between the two ground telescopes;
step S212: obtaining residual time delay and residual time delay rate by adopting a correlation post-processing method for the cross-correlation spectrum between the two ground telescopes;
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.
5. The method for spatial VLBI signal enhancement based on cross-correlation spectrum synthesis according to claim 4, further comprising step S213' after said step S213: and repeating the step S212 and determining whether the residual delay and the residual delay rate reach sufficient accuracy, if yes, ending the process, otherwise, continuing to execute the steps S213 and S213'.
6. The method of claim 4, wherein the step S22 includes:
step S221: performing FX type VLBI (very high performance building information) related processing on the original observation data of the space telescope and the original observation data of the plurality of ground telescopes by using a related processing machine to obtain cross-correlation spectrums of the plurality of space baselines;
step S222: construction of Q function using cross-correlation spectra of space-ground baselines
And in the Q function
Searching to obtain the coordinate corresponding to the point with the maximum function value
And τ are used as a compensation phase and a compensation time delay respectively;
the Q function
Comprises the following steps:
in the formula, k represents a frequency point; vAC、VBCData representing cross-correlation spectra of the AC baseline and the BC baseline, respectively;
is the phase; Δ f is the frequency resolution; tau is time delay;
step S223: by compensating for phase
Phase-aligning the two air-ground baselines and passingThe compensation time delay tau flattens the stripes of the two air-ground baselines;
step S224: and coherently adding the cross-correlation spectra of different space-ground baselines to obtain a synthetic signal, wherein the obtained synthetic signal is an enhanced VLBI signal.
7. The method of claim 6, wherein when the number of ground telescopes is greater than 2,
the step S21 further includes a 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 the stripe is leveled.
8. The method for spatial VLBI signal enhancement based on cross-correlation spectrum synthesis as claimed in 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.
9. 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 to 8.
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