CN103281268A - Precompensation method for in-band group delay fluctuation of satellite navigation signal generating system - Google Patents

Abstract

A pre-compensation method for in-band group delay fluctuations in a satellite navigation signal generation system, (1) measure the combined S parameters of the modulator, filter and amplifier in the satellite navigation signal generation system, and determine the amplitude-frequency, Group delay characteristics; (2) Calculate the group delay characteristics of the precompensation filter according to the combined group delay characteristics; (3) Obtain the phase frequency characteristics of the precompensation filter; (4) Set the amplitude frequency of the precompensation filter The characteristic is an inverse Sinc function; (5) Using the amplitude-frequency characteristics and phase-frequency characteristics of the pre-compensation filter, the system function of the pre-compensation filter is obtained; (6) The unit impulse response function of the pre-compensation filter is obtained; (7) Setting Threshold value, all the quantities smaller than the threshold value in the unit impulse response function are set to zero, as the coefficient of the pre-compensation filter; (8) Using the coefficient of the pre-compensation filter, in the digital stand-alone of the navigation signal generation system, A precompensation filter is implemented to compensate for group delay fluctuations of the navigation signal generation system.

Description

Precompensation method for in-band group delay fluctuation of satellite navigation signal generation system

Technical Field

The invention relates to a satellite navigation technology, in particular to a precompensation method of in-band group delay fluctuation of a satellite navigation signal generation system.

Background

The satellite navigation signal generating system consists of a digital single machine, a modulator, an amplifier, a filter and an antenna. The digital single machine generates a digital baseband signal according to the agreed navigation signal format, and sends the digital baseband signal to the modulator after digital-to-analog conversion; the modulator up-converts the analog baseband signal to generate a radio frequency navigation signal, and sends the radio frequency navigation signal to the amplifier; the amplifier amplifies the radio frequency navigation signal to the required power and broadcasts the radio frequency navigation signal to the ground through the antenna.

In the generation process of the satellite navigation signal, the group delay fluctuation of a modulator, a filter and an amplifier seriously degrades the quality of the generated navigation signal, and therefore, a group delay pre-compensation method is required for a satellite navigation signal generation system.

Currently, group delay compensation can be performed by the following methods:

the method comprises the following steps: a group delay segmented compensation method. Changing the broadband signal into a group of narrow-band signals by using a comb filter, then, assuming that the group delay of each narrow-band signal is a fixed value, and compensating the delay of the narrow-band signals by using an adjustable delay unit;

the method 2 comprises the following steps: provided is a digital end filter compensation method. The method utilizes a feedback branch of a transmitter to collect the characteristics of a transmitting signal so as to fit a group delay compensation filter.

However, the group delay compensation by the above method has the following problems:

(1) the method 1 assumes that the group delay of each narrow-band signal output by the comb filter is a fixed value, and then compensates the fixed value, if the bandwidth of the comb filter is wider, the method has larger compensation error; if the bandwidth of the comb filter is narrow, the complexity and the cost are increased;

(2) the feedback loop and calibration system of this method 2 are complex to implement and are not suitable for satellite navigation signal generation payloads.

Disclosure of Invention

The technical problem to be solved by the invention is as follows: the method overcomes the defects of the prior art, provides a group delay fluctuation precompensation method suitable for the effective load of a navigation satellite, and improves the quality of the generated navigation signal.

The technical solution of the invention is as follows: a pre-compensation method for in-band group delay fluctuation of a satellite navigation signal generation system comprises the following steps:

(1) measuring the combined S parameter of a modulator, a filter and an amplifier in a satellite navigation signal generating system, and determining the group delay characteristic of the combination of the three devices according to the combined S parameter;

(2) calculating the group delay characteristic of the precompensation filter according to the combined group delay characteristic, and adding the group delay characteristic and the group delay characteristic to be equal to a constant;

(3) integrating the group delay characteristic of the pre-compensation filter to obtain the phase-frequency characteristic of the pre-compensation filter;

(4) setting the amplitude-frequency characteristic of a pre-compensation filter as an inverse Sinc function;

(5) obtaining a system function of the pre-compensation filter by using the amplitude-frequency characteristic and the phase-frequency characteristic of the pre-compensation filter;

(6) solving Fourier inverse transformation of a system function of the precompensation filter to obtain a unit impulse response function of the precompensation filter;

(7) setting a threshold value, and setting all the quantities smaller than the threshold value in the unit impulse response function as the coefficients of the pre-compensation filter to be zero;

(8) and the pre-compensation filter is realized in the digital single machine of the navigation signal generation system by utilizing the coefficient of the pre-compensation filter, so that the group delay fluctuation of the navigation signal generation system is compensated.

The constant in the step (2) is optimally the maximum combined group delay characteristic tau (omega) at different angular frequencies omega.

Compared with the prior art, the invention has the beneficial effects that:

(1) the existing compensation method is mostly realized by adopting analog devices (such as an analog comb filter and a hardware time delay device); the invention compensates the group delay fluctuation of the navigation signal generating system at the digital end, has high correction precision and flexible and convenient parameter modification;

(2) in the prior art, a broadband signal is divided into a plurality of narrow-band signals, and different fixed delay amounts are compensated for the narrow-band signals to compensate group delay fluctuation, the compensation precision is limited by the bandwidth of the narrow-band signals, the precision is poor, and signal distortion is caused by shunting and combining; the method disclosed by the invention accurately compensates the group delay through the digital FIR filter, and the in-band group delay of the FIR filter can be accurately adjusted, so that the compensation precision is improved, and the signal is not required to be shunted, thereby avoiding the signal distortion caused by shunting and combining;

(3) in the prior art, a feedback branch and a compensation effect evaluation function module are required, the implementation is complex, the weight and the power consumption of the whole system are greatly increased, and the method is not suitable for a satellite navigation signal generation system; the invention adopts the pre-calibrated group delay fluctuation characteristic and then carries out the pre-compensation method, has no feedback branch, does not need to add an additional single machine, and does not increase the weight and the power consumption of the effective load of the navigation signal generation system.

Drawings

FIG. 1 is a flow chart of a method for pre-compensating for group delay of a filter of a satellite navigation signal generating system according to the present invention;

FIG. 2 is a combined amplitude-frequency characteristic of a modulator, filter and amplifier of a navigation signal generation system;

FIG. 3 is a combined group delay characteristic of a navigation signal generation system modulator, filter and amplifier;

fig. 4 is a pre-compensation filter group delay characteristic designed for the combined group delay characteristic described in fig. 3;

fig. 5 is a pre-compensation filter parameter designed for the combined group delay characteristic described in fig. 3.

Detailed Description

Referring to fig. 1, the method for pre-compensating the group delay of the filter of the satellite navigation signal generation system in the invention comprises the following steps:

(1) the combined S-parameters of the modulator, filter and amplifier (i.e. the input-output relationship of the modulator, filter and amplifier as a whole) are measured by a vector network analyzer, and are a set of complex vectors, which can be expressed as S (ω) = S_real(ω)+is_imag(ω)，s_real(omega) and s_imag(ω) is the real and imaginary part of the S parameter, respectively, using

Calculating the combined amplitude-frequency characteristic A (omega) and phase-frequency characteristic of the three devices

Where | is | represents the absolute value calculation, arctan (-) represents the arctan calculation,

by using

Calculating the combined group delay characteristics of the modulator, filter and amplifier, wherein

For the phase-frequency characteristic, τ (ω) is the group delay characteristic, ω represents the angular frequency, an actual combined amplitude-frequency characteristic is shown in fig. 2, and a combined group delay characteristic is shown in fig. 3;

(2) from the group delay characteristic τ (ω), using

τ_comp(ω)=max[τ(ω)]-τ(ω)

Calculating group delay characteristic tau of digital compensation filter_comp(ω)，max[τ(ω)]Represents the maximum value of τ (ω), as shown in fig. 4;

(3) for group delay characteristic tau_comp(omega) integration to obtain the phase-frequency characteristic of the filter

Wherein, $\left\{\begin{array}{c}{\mathrm{\ω}}_1=f_c-2\×B\\ {\mathrm{\ω}}_2=f_c+2\×B\end{array}\right.$

wherein f is_cThe center frequency point of the filter passband is B, and the design bandwidth of the filter is B;

(4) setting the amplitude-frequency characteristic as an inverse Sinc function to compensate for zero-order hold distortion produced by the back-end digital-to-analog converter, i.e.

$A_{\mathrm{comp}}\left(\mathrm{\ω}\right)={\left\{\mathrm{Si}\left(\frac{\mathrm{\ωT}}{2}\right)e^{-\mathrm{j\ω}\frac{T}{2}}\right\}}^{-1}$

Wherein,

is a Sinc function, T is the sampling period;

(5) using amplitude-frequency characteristics A_comp(omega) and phase frequency characteristics

Obtaining a system function

(6) For system function H_comp(omega) solving N points to obtain Fourier inverse transformation to obtain unit impulse response function

h_comp(n)=IFFT(H_comp(ω))

As shown in fig. 5. Wherein, the selection of N is determined according to the order of the filter;

(7) setting a threshold value h_thr

h_thr=α·max(h_comp(n))

Wherein, alpha is generally 0.01.

Extracting the coefficient coef (n) of the digital end compensation filter according to the following formula

$\mathrm{coef}\left(n\right)=\left\{\begin{array}{cc}{h}_{\mathrm{comp}}\left(n\right)& h_{\mathrm{comp}}\left(n\right)\&GreaterEqual;h_{\mathrm{thr}}\\ 0& h_{\mathrm{comp}}\left(n\right)<h_{\mathrm{thr}}\end{array};\right.$

n is the filter coefficient number and is a natural number.

(8) Storing the filter coefficient in a memory of the spaceborne computer, and calling the filter coefficient in the memory of the spaceborne computer when generating the filter; the filter is implemented using an FPGA or DSP.

(9) The ground can modify the filter coefficient in the memory of the satellite-borne computer by utilizing the satellite upper injection instruction, thereby realizing the adjustment of the filter coefficient.

A digital single machine in the satellite navigation signal generating system generates a digital baseband signal according to an agreed navigation signal format, the digital baseband signal is filtered by a filter, and then the signal is sent to a modulator after being subjected to digital-to-analog conversion; the modulator up-converts the analog baseband signal to generate a radio frequency navigation signal, and sends the radio frequency navigation signal to the amplifier; the amplifier amplifies the radio frequency navigation signal to the required power and broadcasts the radio frequency navigation signal to the ground through the antenna.

The invention has not been described in detail in part of the common general knowledge of those skilled in the art.

Claims (2)

1. A precompensation method for in-band group delay fluctuation of a satellite navigation signal generation system is characterized by comprising the following steps:

(1) measuring the combined S parameter of a modulator, a filter and an amplifier in a satellite navigation signal generating system, and determining the group delay characteristic of the combination of the three devices according to the combined S parameter;

(2) calculating the group delay characteristic of the precompensation filter according to the combined group delay characteristic, and adding the group delay characteristic and the group delay characteristic to be equal to a constant;

(3) integrating the group delay characteristic of the pre-compensation filter to obtain the phase-frequency characteristic of the pre-compensation filter;

(4) setting the amplitude-frequency characteristic of a pre-compensation filter as an inverse Sinc function;

(5) obtaining a system function of the pre-compensation filter by using the amplitude-frequency characteristic and the phase-frequency characteristic of the pre-compensation filter;

(6) solving Fourier inverse transformation of a system function of the precompensation filter to obtain a unit impulse response function of the precompensation filter;

(7) setting a threshold value, and setting all the quantities smaller than the threshold value in the unit impulse response function as the coefficients of the pre-compensation filter to be zero;

(8) and the pre-compensation filter is realized in the digital single machine of the navigation signal generation system by utilizing the coefficient of the pre-compensation filter, so that the group delay fluctuation of the navigation signal generation system is compensated.

2. The method of claim 1, wherein the method for pre-compensating for in-band group delay fluctuation in the satellite navigation signal generating system comprises: the constant in the step (2) is optimally the maximum combined group delay characteristic tau (omega) at different angular frequencies omega.

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