A High-Orbit SAR Satellite Ionospheric Measurement Method Based on P / L Dual-Frequency Joint Measurement

Abstract

This invention provides a method for measuring the ionosphere of a high-orbit SAR satellite based on P / L dual-frequency combined measurement. Utilizing the characteristic that the influence of the ionosphere on SAR signals increases with decreasing radio frequency, the method involves a high-orbit SAR satellite simultaneously transmitting pulse signals in both P and L bands, while a ground station simultaneously receives both signals. Based on the differences in the ionospheric influence on the P and L signals, the absolute and relative changes in the ionospheric TEC (Technical Temperature Coefficient) can be measured more accurately. Furthermore, the absolute and relative changes in the ionospheric TEC measured by this invention can be used for imaging compensation, thereby reducing the effects of phase changes during SAR signal propagation, such as image shift, resolution degradation, pulse broadening, and decreased peak-to-sidelobe ratio.

Description

Technical Field

[0001] This invention belongs to the fields of microwave imaging technology and radar detection technology, and particularly relates to a method for measuring the ionosphere of high-orbit SAR satellites based on P / L dual-frequency joint measurement. Background Technology

[0002] Currently, spaceborne SAR ionospheric calibration techniques are mainly divided into two categories: 1) calibrating the ionosphere using ionospheric measurement networks such as GPS and BeiDou systems. This method is characterized by high accuracy and relatively mature technology, but it has a large spatial sampling interval and a different observation path than spaceborne SAR, resulting in certain deficiencies in accurately compensating for SAR echo signals; 2) using a mathematical model of the influence of ionospheric TEC on SAR echo signals, and estimating ionospheric TEC (Total Electron Content) by analyzing spaceborne SAR echo data or calibration data. This method is simple to implement in engineering, and the ionospheric TEC measurement path is the same as the propagation path of the SAR signal, but the measurement accuracy is often not as high as that of the dual-frequency GPS method. Summary of the Invention

[0003] To address the aforementioned issues, this invention provides a high-orbit SAR satellite ionospheric measurement method based on P / L dual-frequency combined measurement. This method can more accurately measure the absolute and relative changes in ionospheric TEC based on the differences in the influence of the ionosphere on the P and L dual-frequency signals.

[0004] A method for measuring the ionosphere using high-orbit SAR satellites based on P / L dual-frequency joint measurement includes the following steps:

[0005] The high-orbit SAR satellite simultaneously transmits P-band and L-band pulse signals, and the ground station simultaneously receives the P-band and L-band pulse signals; the P-band and L-band pulse signals use the same signal source.

[0006] The ground station performs pulse compression processing on the received P-band and L-band pulse signals, and records the P-band and L-band pulse signals received at the same time and after pulse compression processing as s, respectively. P (t1), s L (t1), the pulse signals in the P-band and L-band received at the next time t2 adjacent to time t1, after pulse compression processing, are denoted as s respectively. P (t2), s L (t2);

[0007] s P (t1) and s L (t1) Perform correlation, and then obtain the absolute value of TEC based on the maximum point of the obtained correlation function; or, extract s respectively. P (t1) and sL The absolute value of TEC is obtained by taking the peak position of the pulse pressure (t1) and then using the difference between the two peak positions of the pulse pressure.

[0008] Get s P (t1) and s P The phase difference ΔΦ between (t2) P s L (t1) and s L The phase difference ΔΦ between (t2) L Then according to ΔΦ P With ΔΦ L The difference between them yields the relative change in TEC.

[0009] Furthermore, the absolute value of TEC is obtained as follows:

[0010]

[0011] Where c is the speed of light, t P For s P The peak pulse pressure position of (t1), t L For s L The peak pulse pressure position of (t1), f p f is the frequency of the pulse signal in the P-band. L The frequency of the pulse signal in the L-band is given.

[0012] Furthermore, the method for obtaining the relative change in TEC is as follows:

[0013]

[0014] Where c is the speed of light, f p f is the frequency of the pulse signal in the P-band. L The frequency of the pulse signal in the L-band;

[0015] Furthermore, the L-band pulse signal and P-band pulse signal transmitted by the high-orbit SAR satellite based on P / L dual-frequency joint use the same signal source. The signal source first generates the L-band pulse signal, and the L-band pulse signal is down-converted to obtain the P-band L-band pulse signal.

[0016] Furthermore, the pulse signal in the P-band is circularly polarized and the pulse signal in the L-band is HH polarized before being transmitted.

[0017] Furthermore, the absolute value of TEC and the relative change of TEC are used to compensate for the imaging of P-band SAR and L-band SAR.

[0018] Furthermore, when there are two or more ground stations, and the distance between the ground stations is less than the spatial correlation scale of the ionosphere, the absolute value and relative change of TEC measured by each ground station are averaged.

[0019] Beneficial effects:

[0020] 1. This invention provides a method for measuring the ionosphere of a high-orbit SAR satellite based on P / L dual-frequency joint measurement. Utilizing the characteristic that the influence of the ionosphere on SAR signals increases with decreasing radio frequency, the high-orbit SAR satellite simultaneously transmits pulse signals in both P and L bands, while the ground station simultaneously receives both signals. Based on the differences in the ionospheric influence on the P and L signals, the absolute and relative changes in the ionospheric TEC can be measured more accurately. Furthermore, the absolute and relative changes in the ionospheric TEC measured by this invention can be used for imaging compensation, thereby reducing the effects of phase changes during SAR signal propagation, such as image shift, resolution degradation, pulse broadening, and decreased peak-to-sidelobe ratio.

[0021] 2. This invention provides a method for measuring the ionosphere of high-orbit SAR satellites based on P / L dual-frequency joint measurement. The L-band SAR uses HH polarization, and the P-band uses circular polarization, which can reduce the impact of the ionosphere on signal power attenuation.

[0022] 3. This invention provides a method for measuring the ionosphere of high-orbit SAR satellites based on P / L dual-frequency joint measurement. It can measure the ionospheric TEC during the imaging process of P-band SAR and L-band SAR respectively, and then use the measured value to compensate for the P-band SAR and L-band SAR with high accuracy. That is, the real-time ionospheric measurement value of the same path provided by this invention can be used to compensate for the influence of the ionosphere on imaging, reduce the influence of phase changes during SAR signal propagation, and improve the SAR image quality.

[0023] 4. This invention provides a high-orbit SAR satellite ionosphere measurement method based on P / L dual-frequency joint measurement. Multiple receiving stations on the ground can receive signals, which is beneficial for simultaneously measuring the ionosphere at different locations. In addition, if the distance between the ground stations is less than the spatial correlation scale of the ionosphere, the ionosphere measurements from multiple ground stations can be processed together to improve the measurement accuracy of the ionosphere. Attached Figure Description

[0024] Figure 1 A flowchart of a high-orbit SAR satellite ionospheric measurement method based on P / L dual-frequency joint measurement is provided for this invention;

[0025] Figure 2 This is a schematic diagram of the ionospheric measurement mode of the present invention;

[0026] Figure 3This is a schematic diagram of the high-orbit SAR satellite ionospheric measurement system based on P / L dual-frequency joint measurement according to the present invention.

[0027] Figure 4 This is a flowchart illustrating the measurement process of the absolute value of TEC according to the present invention.

[0028] Figure 5 This is a flowchart illustrating the measurement process of the relative change in TEC according to the present invention. Detailed Implementation

[0029] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings.

[0030] Ionospheric measurements based on ionospheric measurement networks such as GPS and BeiDou can achieve high-precision ionospheric measurements, but they suffer from large spatial sampling intervals and differ from the observation paths of spaceborne SAR, resulting in insufficient compensation for SAR imaging. Traditional mathematical models utilizing the influence of ionospheric TEC on SAR echo signals can provide preliminary estimates of ionospheric TEC, but the measurement accuracy is low and cannot meet the image processing requirements of high-orbit SAR. To better compensate for the influence of the ionosphere on high-orbit SAR imaging, this invention proposes a high-orbit SAR satellite ionospheric measurement system based on P / L dual-frequency joint measurement. The high-orbit SAR satellite simultaneously transmits pulse signals in both the P and L bands, and the ground station simultaneously receives both signals. Real-time ionospheric measurements along the same path can be used to compensate for the influence of the ionosphere on P and L band imaging.

[0031] Specifically, such as Figure 1 As shown, a method for measuring the ionosphere of a high-orbit SAR satellite based on P / L dual-frequency joint measurement includes the following steps:

[0032] A high-orbit SAR satellite simultaneously transmits P-band and L-band pulse signals, and a ground station simultaneously receives the P-band and L-band pulse signals. The P-band and L-band pulse signals use the same signal source. Figure 2 As shown.

[0033] The ground station performs pulse compression processing on the received P-band and L-band pulse signals, and records the P-band and L-band pulse signals received at the same time and after pulse compression processing as s, respectively. P (t1), s L (t1), the pulse signals in the P-band and L-band received at the next time point adjacent to time t1, after pulse compression processing, are denoted as s respectively. P (t2), s L (t2).

[0034] s P (t1) and sL (t1) Perform correlation, and then obtain the absolute value of TEC based on the maximum point of the obtained correlation function; or, extract s respectively. P (t1) and s L The absolute value of TEC is obtained by determining the peak position of the pulse pressure (t1) and then using the difference between the two peak positions of the pulse pressure.

[0035] Get s P (t1) and s P The phase difference ΔΦ between (t2) P s L (t1) and s L The phase difference ΔΦ between (t2) L Then according to ΔΦ P With ΔΦ L The difference between them yields the relative change in TEC.

[0036] It should be noted that a high-orbit SAR satellite ionospheric measurement method based on P / L dual-frequency joint measurement can be based on, for example... Figure 3 The system shown is a high-orbit SAR satellite ionospheric measurement system based on P / L dual-frequency joint measurement. This system mainly consists of a rubidium clock, SAR frequency source, radar central control processor, P-band transceiver link, L-band transceiver link, P-band feed, L-band feed array, and a large reflector antenna. The P-band and L-band share links with the radar central control processor, receiver, and large reflector antenna, enabling the system to perform both P-band and L-band SAR. It can also simultaneously measure the absolute and relative changes in ionospheric TEC along the radar signal propagation path while performing P-band or L-band SAR imaging. The system uses HH polarization for L-band SAR and circular polarization for P-band to reduce the impact of ionospheric attenuation on signal power.

[0037] The method for measuring the absolute value of TEC is described in detail below.

[0038] The measurement procedure for the absolute value of TEC is as follows: Figure 4 As shown. Assuming that at time t1, the P-band and L-band signals received by the ground station's dual-frequency receiver, after pulse compression processing, are expressed as follows:

[0039]

[0040]

[0041] Among them, s P (t1) represents the P-band pulse signal received at time t1 after pulse compression processing, s L (t1) represents the L-band pulse signal received at time t1 after pulse compression processing. This represents the phase error of the radar's transmission channel corresponding to the P-band. φ represents the phase error of the radar's transmission channel corresponding to the L-band. p φ represents the error of the P-band signal source. L This represents the error of the L-band signal source. Since the P-band and L-band use the same signal source, φ p =φ L , R represents the signal amplitude, B represents the Doppler bandwidth, R1 represents the slant range at time t1, c represents the speed of light, and f represents the speed of light. p f represents the frequency of the pulse signal in the P-band. L The frequency of the pulse signal in the L-band is represented by TEC, the absolute value of TEC is represented by j, and the imaginary part is represented by j.

[0042] From the above two equations, it can be seen that without the influence of the ionosphere, the peak positions of the dual-frequency signals after pulse compression should be the same. However, due to the influence of the ionosphere, the peak positions of the pulse compressions are different. By correlating the signals from the above two equations and finding the maximum point of the correlation function, or by extracting the peak positions of the dual-frequency pulse compressions separately, the absolute value of TEC can be estimated from the difference in peak positions. The specific estimation formula for estimating the absolute value of TEC from the difference in peak positions is as follows:

[0043]

[0044]

[0045]

[0046]

[0047] Among them, t P The pulse signal s received at time t1, after pulse compression processing, belongs to the P-band. P The peak pulse pressure position of (t1), t L The pulse signal s received at time t1, after pulse compression processing, is in the L-band. P The peak position of the pulse pressure (t1) is given by equation (6), which is the absolute value of the ionospheric TEC estimated by the time delay method.

[0048] The following section details the method for measuring the relative change in TEC.

[0049] For synthetic aperture radar (SAR) imaging, the change in TEC (transformation potential) between adjacent pulses, ΔTEC, affects image focusing. TEC values ​​are typically estimated using time delay, but this method has relatively low accuracy. Phase-based measurement methods can estimate the change in TEC values ​​with high precision.

[0050] The measurement procedure for the relative change of TEC is as follows: Figure 5 As shown. The radar's repetition rate is as high as 100Hz or more. Between adjacent pulses, the radar's transmit and receive channel characteristics can be considered unchanged; the changes in echo characteristics between adjacent pulses are mainly caused by ionospheric variations. The echoes of adjacent pulses from a dual-frequency radar are shown in the following equations:

[0051]

[0052]

[0053]

[0054]

[0055] Among them, s P (t2) represents the pulse signal in the P-band received at the next time t2 adjacent to time t1, after pulse compression processing. L (t2) represents the L-band pulse signal received at the next time t2 adjacent to time t1 after pulse compression processing, R2 represents the slant distance at time t2, φ1 represents the error of the signal source at time t1, and φ2 represents the error of the signal source at time t2.

[0056] Extracting the phases from equations (7) and (8) respectively, and subtracting them, we get:

[0057]

[0058] Where, ΔΦ P For s P (t1) and s P The phase difference between (t2) and (t2).

[0059] Extracting the phases from equations (9) and (10) respectively and subtracting them yields:

[0060]

[0061] Where, ΔΦ L For s L (t1) and s L The phase difference between (t2) and (t2).

[0062] Subtracting equation (11) from equation (12), we get:

[0063]

[0064] From equation (13), we can see that ΔΦ LPIt no longer contains errors from the signal source or the radar transceiver channel. The slant ranges R2 and R1 can be calculated from the satellite orbit determination data and the position coordinates of the ground receiver. The 3-axis orbit determination accuracy of a high-orbit SAR satellite is better than 2 meters. The standard deviation of the phase error caused by the slant range error is 10.6°, and the introduced ΔTEC error is 0.04 Tecu, which has a minor impact and can be ignored. Therefore, the formula for calculating ΔTEC is:

[0065]

[0066] In SAR systems, the initial phase of each transmitted signal is stable. The method for measuring the relative change in TEC assumes that the phase change caused by the TEC change between adjacent pulses will not exceed 2π. Based on the actual conditions of the ionosphere, this assumption is acceptable (except for ionospheric storms). This measurement method utilizes the phase subtraction of adjacent pulse signals from a co-frequency radar to eliminate phase errors in the transmit and receive channels, and utilizes the phase subtraction of a dual-frequency radar to eliminate initial errors in the signal source, thus achieving high-precision measurement of the relative change in ionospheric TEC.

[0067] Therefore, the ionospheric measurement method for high-orbit SAR satellites based on P / L dual-frequency joint measurement provided by this invention uses the same signal source for both the L-band and P-band signals. Specifically, the signal source generates the L-band transmission signal, which is then down-converted to generate the P-band transmission signal. The L-band and P-band signals are transmitted simultaneously, ensuring that the atmospheric environment traversed by both frequency bands is identical, and that their initial phase and initial error are the same. The ground station simultaneously receives both frequency signals and measures the absolute and relative changes in ionospheric TEC based on the differences in ionospheric influence on the P and L frequency signals. This invention allows for simultaneous measurement of ionospheric TEC during the imaging processes of P-band SAR and L-band SAR, enabling high-precision compensation for P-band and L-band SAR signals, reducing the impact of phase changes during SAR signal propagation, and improving SAR image quality.

[0068] In summary, the advantages of this invention compared to the prior art are as follows:

[0069] (1) By sharing related equipment links between the P-band and L-band, the capability of P-band SAR, L-band SAR imaging and ionospheric measurement was realized.

[0070] (2) It can take into account imaging, that is, the ionosphere is measured at the same time as imaging. The real-time ionosphere measurement value along the same path can be used to compensate for the influence of the ionosphere on imaging.

[0071] (3) Launching from the satellite allows for multiple receiving stations on the ground to receive the signal, which is beneficial for simultaneously measuring the ionosphere at different locations and is more in line with the original intention of "measuring the ionosphere along the same path as imaging". At the same time, if the distance between the ground stations is less than the spatial correlation scale of the ionosphere, the ionosphere measurements from multiple ground stations can be processed together to improve the measurement accuracy of the ionosphere.

[0072] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the appended claims.

Claims

1. A method for measuring the ionosphere of a high-orbit SAR satellite based on P / L dual-frequency joint measurement, characterized in that, Includes the following steps: The high-orbit SAR satellite simultaneously transmits P-band and L-band pulse signals, and the ground station simultaneously receives the P-band and L-band pulse signals; the P-band and L-band pulse signals use the same signal source. The ground station performs pulse compression processing on the received P-band and L-band pulse signals, and records the P-band and L-band pulse signals received at the same time and after pulse compression processing as s, respectively. P (t1), s L (t1), the pulse signals in the P-band and L-band received at the next time t2 adjacent to time t1, after pulse compression processing, are denoted as s respectively. P (t2), s L (t2); s P (t1) and s L (t1) Perform correlation, and then obtain the absolute value of TEC based on the maximum point of the obtained correlation function; or, extract s respectively. P (t1) and s L The absolute value of TEC is obtained by taking the peak position of the pulse pressure (t1) and then using the difference between the two peak positions of the pulse pressure. Get s P (t1) and s P The phase difference ΔΦ between (t2) P s L (t1) and s L The phase difference ΔΦ between (t2) L Then according to ΔΦ P With ΔΦ L The difference between them yields the relative change in TEC.

2. The method for measuring the ionosphere of a high-orbit SAR satellite based on P / L dual-frequency joint measurement as described in claim 1, characterized in that, The absolute value of TEC is obtained as follows: Where c is the speed of light, t P For s P The peak pulse pressure position of (t1), t L For s L The peak pulse pressure position of (t1), f p f is the frequency of the pulse signal in the P-band. L The frequency of the pulse signal in the L-band is given.

3. The method for measuring the ionosphere of a high-orbit SAR satellite based on P / L dual-frequency joint measurement as described in claim 1, characterized in that, The method for obtaining the relative change in TEC is as follows: Where c is the speed of light, f p f is the frequency of the pulse signal in the P-band. L The frequency of the pulse signal in the L-band is given.

4. The method for measuring the ionosphere of a high-orbit SAR satellite based on P / L dual-frequency joint measurement as described in claim 1, characterized in that, The L-band pulse signal and P-band pulse signal transmitted by the high-orbit SAR satellite based on P / L dual-frequency use the same signal source. The signal source first generates the L-band pulse signal, and the L-band pulse signal is down-converted to obtain the P-band L-band pulse signal.

5. A method for measuring the ionosphere of a high-orbit SAR satellite based on P / L dual-frequency joint measurement as described in any one of claims 1 to 4, characterized in that, P-band pulse signals are circularly polarized, and L-band pulse signals are HH-polarized before being transmitted.

6. A method for measuring the ionosphere of a high-orbit SAR satellite based on P / L dual-frequency joint measurement as described in any one of claims 1 to 4, characterized in that, The absolute value and relative change of TEC are used to compensate for the imaging of P-band SAR and L-band SAR.

7. A method for measuring the ionosphere of a high-orbit SAR satellite based on P / L dual-frequency joint measurement as described in any one of claims 1 to 4, characterized in that, When there are two or more ground stations, and the distance between the ground stations is less than the spatial correlation scale of the ionosphere, the absolute value and relative change of TEC measured by each ground station are averaged.

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