CN103823206A - Satellite-ground bistatic SAR (synthetic aperture radar) time-frequency synchronization method based on navigation satellite - Google Patents

Abstract

The invention relates to a satellite-ground bistatic SAR (synthetic aperture radar) time-frequency synchronization method based on a navigation satellite, and belongs to the technical field of radar signal processing. The time-frequency synchronization of a satellite-ground bistatic SAR system can be realized without a post-event ephemeris. A parameter estimation method provided by the invention can be used for extracting navigation message information by virtue of direct waves, decoding acquisition time and satellite orbit information to calculate a theoretical time delay history and a theoretical Doppler phase history corresponding to received data, and subtracting the theoretical time delay history and the theoretical Doppler phase history from an apparent time delay history and an apparent Doppler phase history to obtain synchronous error estimation values of time and phase. According to the method, the real-time property of imaging can be improved, the time information can be accurately acquired, and the error influence caused by time deviation is lowered. The method is suitable for all navigation satellites and is wide in application range.

Description

A kind of star ground double-base SAR time-frequency synchronization method based on Navsat

Technical field

The present invention relates to a kind of star ground double-base synthetic aperture radar (SAR) time-frequency synchronization method based on Navsat, belong to Radar Signal Processing Technology field.

Background technology

Star ground double-base SAR technology based on Navsat is to realize a kind of approach of Continuous Observation over the ground.The signal that its system has certain bandwidth by transmitting is realized distance to resolution, and fly via satellite and target is formed to effective corner realize orientation to resolution, navigation number of satellite is many again, the feature that revisit time is short, thus realize the effect to the continuous two-dimensional imaging of scene.

The ground of the star based on Navsat double-base SAR system, due to bistatic, receives life period and frequency synchronization error in signal, under long synthetic aperture time conditions, time the time and the frequency synchronization error that become can cause image seriously to defocus.And traditional synchronous method by utilize direct-path signal and afterwards satellite ephemeris synchronous error is estimated and is compensated, cannot meet the requirement of real-time synchronization, because ephemeris can not obtain immediately afterwards, there is not ephemeris afterwards for Big Dipper Navsat, and traditional method can not be obtained the accurate UTC time that echo data is corresponding, this by the application of the traditional synchronous method of restriction, worsen its synchronous performance of processing.

Summary of the invention

In order to meet the synchronous requirement of real-time time-frequency, the present invention proposes a kind of star ground double-base SAR Time and Frequency Synchronization disposal route based on Navsat, can utilize navigation message but not ephemeris afterwards, conveniently realize in real time the synchronous of time-frequency, the method comprises the steps:

S1 is historical and apparent doppler phase history from described direct wave extracting data apparent time delay, comprising:

Step 101, according to the cycle of pseudo-ranging code in direct wave data to direct wave data carry out two dimension divide;

Step 102, to two dimension divide after direct wave data carry out apart from pulse pressure processing;

Step 103, extract delay history and the phase history at peak place according to the pulse pressure result of direct wave data, obtain the historical τ of apparent time delay _v(η _n), be written as time synchronization error τ _err(η _n), the historical τ of theoretical time delay _d(η _n) and time stochastic error τ _w(η _n) be added form, that is:

τ _v(η _n)＝τ _err(η _n)+τ _d(η _n)+τ _w(η _n)

And apparent doppler phase history

it is written as phase-locking error theoretical doppler phase history

and phase place stochastic error

the form being added, that is:

S2 is from direct wave extracting data navigation message information;

S3 resolves Navsat orbit information in conjunction with the spacing wave interface control document of the Navsat of described navigation message information and correspondence;

S4 is by obtaining text time corresponding to a certain frame navigation message in direct wave data, and according to this position of frame navigation message in direct wave data, thereby calculates the signal transmission time corresponding to initial time of direct wave data;

S5 obtains the position of direct wave antenna, in conjunction with signal transmission time corresponding to initial time that resolves the direct wave data that obtain in the satellite-orbit information that obtains and S4 in the position of direct wave antenna, described S3, thereby calculate the theoretical oblique distance history R between Navsat and direct wave antenna in time period corresponding to whole direct wave data _d(η _n);

S6 makes the historical R of described theoretical oblique distance _d(η _n) obtain theoretical time delay history divided by light velocity c,

make theoretical oblique distance history be multiplied by 2 π divided by wavelength X and obtain theoretical doppler phase history,

wherein, for specific satellite and wave band, wavelength is known parameters;

S7 contrast acquisition time synchronous error is estimated and phase-locking estimation of error;

Make apparent time delay history deduct theoretical time delay history, obtain time synchronization error estimated value, ${\widehat{\mathrm{\τ}}}_{\mathrm{err}}\left({\mathrm{\η}}_n\right)={\mathrm{\τ}}_{\mathrm{err}}\left({\mathrm{\η}}_n\right)+{\mathrm{\τ}}_w\left({\mathrm{\η}}_n\right)={\widehat{\mathrm{\τ}}}_v\left({\mathrm{\η}}_n\right)-{\mathrm{\τ}}_d\left({\mathrm{\η}}_n\right);$ Make apparent doppler phase history deduct theoretical doppler phase history, obtain phase-locking error estimate,

S8 compensation deals:

According to the 2D signal zoned format of direct wave data, each pseudo-ranging code cycle of echo data is carried out to shifting processing according to the estimated value of the time synchronization error obtaining in described S7, i.e. deadline synchronous error processing; The cycle of each the pseudo-ranging code to echo data is deducted corresponding phase-locking error according to the estimated value of the phase-locking error obtaining in described S7, complete phase-locking Error processing, remove simultaneously navigation message phase place and distance to Doppler shift modulation, be compensated echoed signal after treatment, realize the star ground double-base SAR Time and Frequency Synchronization based on Navsat.

In the step 101 of described S1, be PRT=1ms in the cycle of pseudo-ranging code described in Big Dipper navigational satellite system, and under different satellite systems the cycle of pseudo-ranging code choose as the case may be.

In described S2, by phaselocked loop, direct wave data are carried out to carrier wave and Phase Tracking, thereby navigation message information is extracted.

Beneficial effect: a kind of star ground double-base SAR time-frequency synchronization method based on Navsat of the present invention, directly utilize direct-path signal and navigation message information to carry out Time and Frequency Synchronization estimation of error, do not rely on almanac data afterwards, can realize the real-time synchronization between Navsat and direct wave aerial receiver and echo aerial receiver, improve the real-time of imaging processing, and obtaining of temporal information is more accurate, reduce the error effect that time deviation is introduced, be applicable to the double-base SAR system that all types of Navsats irradiate.

Accompanying drawing explanation

Fig. 1 is that star of the present invention ground double-base SAR system forms schematic diagram.

Fig. 2 is method flow diagram of the present invention.

Fig. 3 is Time and Frequency Synchronization compensation schematic diagram of the present invention, and wherein, Fig. 3 (a) is time synchronized compensation schematic diagram, and Fig. 3 (b) is phase-locking compensation schematic diagram.

Embodiment

Below in conjunction with drawings and Examples, the inventive method is elaborated.

The invention provides a kind of star ground double-base SAR time-frequency synchronization method based on Navsat, the method is utilized the coherent of direct wave passage and echo channel in multichannel receiver, by the estimation to Time and Frequency Synchronization error in direct-path signal, compensate the synchronous error in echoed signal.It is main by the carrier phase of direct wave passage is followed the tracks of, extract navigation message information, thereby decode time information and satellite orbit parameter, in conjunction with the position of echo receiving antenna, calculate the initial time of data, thereby calculate the theoretical oblique distance history of satellite corresponding to whole data and receiving antenna.Meanwhile, by direct-path signal data are carried out to pulse pressure processing, obtain apparent time delay history and apparent doppler phase history, more just can estimate Time and Frequency Synchronization error in conjunction with above-mentioned theory oblique distance is historical.Finally, according to direct-path signal form, the Time and Frequency Synchronization error of echo data is compensated, thereby complete synchronous processing.

As shown in Figure 1, double-base SAR system comprises Navsat, direct wave antenna and clawback antenna, wherein, and direct wave antenna direction satellite, the direct-path signal of reception satellite, echo antenna direction scene, the echoed signal of reception scene.Direct wave passage and echo channel share local oscillator, and two antenna putting positions are approaching, to guarantee the consistance of Time and Frequency Synchronization error in two passages.

Satellite-signal is set up to model, and Navsat transmits and is made up of on carrier wave " ranging code+navigation message " orthogonal modulation of I, two branch roads of Q, and its signal model can be expressed as:

Wherein, I represents I branch road, and Q represents Q branch road; A represents signal amplitude; C represents ranging code; D represents the numeric data code of modulating on ranging code, value be-1 or+1, f ₀represent carrier frequency,

represent data first phase.

Because two branch roads use orthogonal different pseudo-codes, therefore can only consider a road, have:

Wherein, $D\left(t\right)=\left\{\begin{array}{cc}0,& D_I\left(t\right)=1\\ \mathrm{\π},& D_I\left(t\right)=-1\end{array}\right.$

Ignore signal amplitude, introduce the theoretical time delay τ being caused by oblique distance _d(t), receiving signal can be written as:

Local oscillator frequencies can be written as:

f _L＝f ₀+Δf ₀(t)?(4)

Wherein, f ₀for nominal frequency, Δ f ₀(t) frequently float for random.

Ignore first phase, receive signal S _r(t) after demodulation, obtain:

$S_b\left(t\right)=C(t-{\mathrm{\τ}}_d\left(t\right))\exp (j2\mathrm{\π}{f}_0{\mathrm{\τ}}_d\left(t\right)+j2\mathrm{\π}{\∫}_0^t\mathrm{\Δ}{f}_0\left(t\right)\mathrm{dt}+\mathrm{jD}(t-{\mathrm{\τ}}_d\left(t\right)))---\left(5\right)$

Full-time t is written as to slow time η _nthe form of=nPRT and fast time τ sum:

t＝η _n+τ?(6)

Wherein, the cycle that PRT is pseudo-ranging code.

By theoretical time delay τ _d(t=η _n+ τ) carry out Taylor expansion at τ=0 place, obtain:

${\mathrm{\τ}}_d({\mathrm{\η}}_n,\mathrm{\τ})={\mathrm{\τ}}_d\left({\mathrm{\η}}_n\right)+{\mathrm{\τ}}_d^{\′}\left({\mathrm{\η}}_n\right)\mathrm{\τ}+\frac{{\mathrm{\τ}}_d^{\′\′}\left({\mathrm{\η}}_n\right)}{2}{\mathrm{\τ}}^2+...\left(7\right)$

Consider the feature of pseudo-ranging code signal to frequency displacement sensitivity, above formula got to 1 rank (also can get more multistage in the more high-precision situation of needs), have:

${\mathrm{\τ}}_d({\mathrm{\η}}_n,\mathrm{\τ})\≈{\mathrm{\τ}}_d\left({\mathrm{\η}}_n\right)+{\mathrm{\τ}}_d^{\′}\left({\mathrm{\η}}_n\right)\mathrm{\τ}={\mathrm{\τ}}_d\left({\mathrm{\η}}_n\right)+\frac{f_d\left({\mathrm{\η}}_n\right)}{f_0}\mathrm{\τ}---\left(8\right)$

By formula (8) substitution formula (5):

$\begin{array}{c}{S}_b({\mathrm{\η}}_n,\mathrm{\τ})=C(\mathrm{\τ}-{\mathrm{\τ}}_d\left({\mathrm{\η}}_n\right))\exp (j2\mathrm{\π}{f}_0{\mathrm{\τ}}_d\left({\mathrm{\η}}_n\right)+j2\mathrm{\π}{f}_d\left({\mathrm{\η}}_n\right)\mathrm{\τ})\\ \·\exp \left(j2\mathrm{\π}{\∫}_0^{{\mathrm{\η}}_n+\mathrm{\τ}}\mathrm{\Δ}{f}_0\left(t\right)\mathrm{dt}\right)\exp \left(\mathrm{jD}(\mathrm{\τ}-{\mathrm{\τ}}_d\left({\mathrm{\η}}_n\right))\right)\end{array}---\left(9\right)$

Visible, due to not coherent of transmitting-receiving local oscillator, the sampled signal of data acquisition equipment is also incoherent for satellite carrier frequency, and this has just caused sampling error, thereby in the time the original signal collecting being carried out to two dimension division, can introduce an extra migration error.On the other hand, because sending and receiving end lacks identical triggering clock, therefore, receive initial sampling time the unknown of signal.If the impact of the two is presented as time synchronization error τ _err(η _n), so, formula (9) can be written as:

$\begin{array}{c}{S}_b({\mathrm{\η}}_n,\mathrm{\τ})=C(\mathrm{\τ}-{\mathrm{\τ}}_d\left({\mathrm{\η}}_n\right)-{\mathrm{\τ}}_{\mathrm{err}}\left({\mathrm{\η}}_n\right))\exp (j2\mathrm{\π}{f}_0{\mathrm{\τ}}_d\left({\mathrm{\η}}_n\right)+j2\mathrm{\π}{f}_d\left({\mathrm{\η}}_n\right)\mathrm{\τ})\\ \·\exp \left(j2\mathrm{\π}{\∫}_0^{{\mathrm{\η}}_n+\mathrm{\τ}}\mathrm{\Δ}{f}_0\left(t\right)\mathrm{dt}\right)\exp \left(\mathrm{jD}(\mathrm{\τ}-{\mathrm{\τ}}_d\left({\mathrm{\η}}_n\right))\right)\end{array}---\left(10\right)$

Analysis above formula is known, and compared with traditional SAR echoed signal, the star ground double-base SAR signal based on Navsat has had more time synchronization error τ _err(η _n), frequency synchronization error Δ f ₀(η _n), frequency synchronization error is converted into phase-locking error for ease of processing, and the distance that prior art easily solves is modulated this four fractional error to frequency modulation (PFM) and orientation to text.

Synchronous processing will first estimate time synchronization error τ exactly _err(η _n) and phase-locking error

in echo data, compensate again.

The whole star ground double-base SAR time-frequency synchronization method flow process based on Navsat as shown in Figure 2, specifically comprises:

S1 is historical and apparent doppler phase history from direct wave extracting data apparent time delay.

Step 101, according to the cycle of pseudo-ranging code to direct wave data carry out two dimension divide, wherein, in Big Dipper navigational satellite system, the cycle of pseudo-ranging code is PRT=1ms, under different satellite systems, this value can be chosen as the case may be;

Step 102, to two dimension divide after direct wave data carry out apart from pulse pressure processing;

Step 103, extract delay history and the phase history at peak place according to the pulse pressure result of direct wave data, can obtain apparent time delay history it can be written as following form:

${\widehat{\mathrm{\τ}}}_v\left({\mathrm{\η}}_n\right)={\mathrm{\τ}}_{\mathrm{err}}\left({\mathrm{\η}}_n\right)+{\mathrm{\τ}}_d\left({\mathrm{\η}}_n\right)+{\mathrm{\τ}}_w\left({\mathrm{\η}}_n\right)---\left(11\right)$

Wherein, τ _err(η _n) be time synchronization error, τ _d(η _n) be theoretical time delay history, τ _w(η _n) be time stochastic error;

And apparent doppler phase history, it can be written as following form:

Wherein,

for phase-locking error,

for theoretical doppler phase history,

for phase place stochastic error.

S2 extracts navigation message information.

Navigation message information, with in the present direct wave data of phase place form body, therefore, is used phaselocked loop to carry out carrier wave and Phase Tracking to direct wave data, navigation message information can be extracted.

S3 resolves satellite-orbit information.

Spacing wave interface control (ICD) file of navigation textual information and corresponding Navsat, can decode to satellite-orbit information, and ICD file is announced by navigational satellite system supvr.

S4 calculates the signal transmission time corresponding to initial time of direct wave data.

According to text time corresponding to a certain frame navigation message in direct wave data, according to this position of frame navigation message in direct wave data, calculate the signal transmission time corresponding to initial time of direct wave data.

S5 calculates the historical R of theoretical oblique distance between interior satellite of time period corresponding to whole direct wave data and direct wave receiving antenna _d(η _n).

Obtain the position of direct wave receiving antenna by GPS or additive method, the signal transmission time corresponding to initial time of position, satellite-orbit information and the direct wave data of combined with receiving antenna, can calculate the historical R of theoretical oblique distance of direct wave _d(η _n).

S6 calculates theoretical time delay history and theoretical doppler phase history.

The historical τ of theoretical time delay _d(η _n) can be by the historical R of theoretical oblique distance _d(η _n), obtain according to formula (13):

${\mathrm{\τ}}_d\left({\mathrm{\η}}_n\right)=\frac{R_d\left({\mathrm{\η}}_n\right)}{c}---\left(13\right)$

Theoretical doppler phase history can be by the historical R of theoretical oblique distance _d(η _n), obtain according to following formula (14):

The wavelength that wherein λ is signal, for specific satellite and wave band, wavelength is known parameters.

S7 contrast acquisition time synchronous error is estimated and phase-locking estimation of error.

Make apparent time delay history

deduct the historical τ of theoretical time delay _d(η _n), obtain time synchronization error estimated value

obtain formula (15) by formula (11):

${\widehat{\mathrm{\τ}}}_{\mathrm{err}}\left({\mathrm{\η}}_n\right)={\mathrm{\τ}}_{\mathrm{err}}\left({\mathrm{\η}}_n\right)+{\mathrm{\τ}}_w\left({\mathrm{\η}}_n\right)={\widehat{\mathrm{\τ}}}_v\left({\mathrm{\η}}_n\right){-\mathrm{\τ}}_d\left({\mathrm{\η}}_n\right)---\left(15\right)$

Make apparent doppler phase history

deduct theoretical doppler phase history

obtain phase-locking error estimate

obtain formula (16) by formula (12):

S8 compensation deals.

Finally, according to direct-path signal form, according to the Time and Frequency Synchronization error estimating, echoed signal is carried out to synchronous error compensation: can be compensated the echoed signal after processing by formula (10):

S _b(η _n,τ)＝C(τ-τ _d(η _n))exp(j2πf ₀τ _d(η _n))?（17）

As shown in Figure 3, the cycle of each the pseudo-ranging code according to the 2D signal zoned format of direct wave data to echo data is carried out shifting processing according to the estimated value of the time synchronization error obtaining in described S7, i.e. deadline synchronous error processing; The cycle of each the pseudo-ranging code to echo data is deducted corresponding phase-locking error according to the estimated value of the phase-locking error obtaining in described S7, complete phase-locking Error processing, remove simultaneously navigation message phase place and distance to Doppler shift modulation, be compensated echoed signal after treatment, realize the star ground double-base SAR Time and Frequency Synchronization based on Navsat.

Time-frequency synchronization method of the present invention makes full use of the feature of navigation satellite signal, solve conventional synchronization method to the dependence of ephemeris afterwards, after completing, data acquisition can process immediately, real-time, and the estimation of the start time to data is more accurate, thereby avoid the impact of start time error.

Above-described embodiment; object of the present invention, technical scheme and beneficial effect have been carried out to further detailed description; institute is understood that; the foregoing is only specific embodiments of the invention; the protection domain being not intended to limit the present invention; all within spirit of the present invention and principle, any modification of making, be equal to replacement, improvement etc., within all should being included in protection scope of the present invention.

Claims (2)

1. the ground of the star based on a Navsat double-base SAR time-frequency synchronization method, wherein, direct wave antenna direction satellite, the direct wave data of reception Navsat, echo antenna direction scene, the echoed signal of reception scene, is characterized in that, concrete steps comprise:

S1 is historical and apparent doppler phase history from described direct wave extracting data apparent time delay, comprising:

Step 101, according to the cycle of pseudo-ranging code in direct wave data to direct wave data carry out two dimension divide;

Step 102, to two dimension divide after direct wave data carry out apart from pulse pressure processing;

Step 103, extract delay history and the phase history at peak place according to the pulse pressure result of direct wave data, obtain apparent time delay history

and apparent doppler phase history

S2 is from direct wave extracting data navigation message information;

S3 resolves Navsat orbit information in conjunction with the spacing wave interface control document of the Navsat of described navigation message information and correspondence;

S4 is by obtaining text time corresponding to a certain frame navigation message in direct wave data, and according to this position of frame navigation message in direct wave data, thereby calculates the signal transmission time corresponding to initial time of direct wave data;

S5 obtains the position of direct wave antenna, in conjunction with signal transmission time corresponding to initial time that resolves the direct wave data that obtain in the satellite-orbit information that obtains and S4 in the position of direct wave antenna, described S3, thereby calculate the theoretical oblique distance history R between Navsat and direct wave antenna in time period corresponding to whole direct wave data _d(η _n);

S6 makes the historical R of described theoretical oblique distance _d(η _n) obtain theoretical time delay history divided by light velocity c,

make theoretical oblique distance history be multiplied by 2 π divided by wavelength X and obtain theoretical doppler phase history,

S7 contrast acquisition time synchronous error is estimated and phase-locking estimation of error;

Make apparent time delay history deduct theoretical time delay history, obtain time synchronization error estimated value,

make apparent doppler phase history deduct theoretical doppler phase history, obtain phase-locking error estimate,

S8 compensation deals:

According to the 2D signal zoned format of direct wave data to each pseudo-ranging code cycle of echo data according to the time synchronization error estimated value obtaining in described S7

carry out shifting processing, i.e. deadline synchronous error processing; The cycle of each the pseudo-ranging code to echo data is according to the phase-locking error estimate obtaining in described S7

compensate corresponding phase-locking error, complete phase-locking Error processing, remove simultaneously navigation message phase place and distance to Doppler shift modulation, be compensated echoed signal after treatment, realize the star ground double-base SAR Time and Frequency Synchronization based on Navsat.

2. a kind of star based on Navsat ground double-base SAR time-frequency synchronization method as claimed in claim 1, is characterized in that, in described S2, by phaselocked loop, direct wave data is carried out to carrier wave and Phase Tracking, thereby navigation message information is extracted.

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