A doppler positioning method based on frequency hopping spread spectrum

Abstract

The application provides a Doppler positioning method based on frequency hopping spread spectrum, and belongs to the technical field of radio positioning, to solve the problem that the fixed carrier frequency mode adopted by the traditional Doppler positioning technology makes the signal extremely easy to be intercepted, oriented and interfered. The method comprises the following steps: step 1: describing an application scenario; step 2: in the application scenario, a frequency hopping measurement signal is sent by a transmitting end; step 3: in the application scenario, a receiving end performs frequency hopping synchronization and measures a Doppler shift; step 4: a Doppler shift is preprocessed; and step 5: the preprocessed Doppler shift is used to calculate the position of a terminal to be positioned in combination with the relative motion of the transmitting end and the receiving end. The method can obtain the low interception rate and anti-interference ability given by the rapid change of the carrier frequency, and can also use the frequency diversity characteristics of frequency hopping to reduce the positioning performance deterioration caused by the multipath effect, and is suitable for application scenarios with high requirements for robustness, positioning accuracy and other performances.

Description

A Doppler localization method based on frequency hopping spread spectrum Technical Field

[0001] This invention belongs to the field of radio positioning technology, specifically relating to a Doppler positioning method based on frequency hopping spread spectrum. Background Technology

[0002] Location information, as a crucial foundational information, plays an increasingly important role in both civilian and military fields. Radio positioning technologies, represented by satellite navigation and radar, have become an important technical means of acquiring location information due to their advantages of high precision, all-weather operation, and wide coverage.

[0003] During radio signal propagation, the relative radial motion between the transmitter and receiver causes a Doppler frequency shift in the received signal. Integrating this Doppler frequency shift in the time domain and dividing it by the carrier wavelength of the transmitted signal yields the change in distance between the transmitter and receiver during this time (distance difference). If the target device (either the transmitter or the receiver) is stationary, while the motion state information (position and velocity) of the moving device is known, the position of the target device can be calculated based on the distance difference positioning principle. This positioning method is called Doppler positioning and has two important advantages: first, the target device has a simple structure and low cost; second, positioning can be achieved with only a single station and a single antenna. Therefore, it is widely used in various application scenarios such as navigation, surveillance, and search and rescue.

[0004] However, the fixed carrier frequency operating mode adopted by traditional Doppler positioning technology makes the signal easily intercepted, directed, and interfered with; at the same time, the multipath effect in the wireless channel will cause a large error in the Doppler positioning results. Summary of the Invention

[0005] To address the aforementioned problems, this invention proposes a Doppler positioning method based on frequency hopping spread spectrum. This method not only achieves low intercept rate and anti-interference capability due to rapid carrier frequency hopping, but also utilizes the inherent frequency diversity characteristics of frequency hopping to mitigate the positioning performance degradation caused by multipath effects. It is suitable for application scenarios with high requirements for robustness and positioning accuracy.

[0006] The technical solution adopted in this invention is as follows:

[0007] In a first aspect, the present invention protects a Doppler localization method based on frequency hopping spread spectrum, the method comprising the following steps:

[0008] Step 1: Describe the application scenario

[0009] Application scenarios can be broadly categorized into two types: navigation (passive positioning) and surveillance / search and rescue (active positioning). In navigation applications, the base station is in motion, continuously transmitting measurement signals via a transmitter. These signals are modulated with navigation messages (helping the terminal to be located calculate the base station's position, velocity, and other motion state information, which are then used for its own position calculation). The terminal to be located is stationary; after receiving the measurement signals, it processes them to calculate its own position. In surveillance / search and rescue applications, the terminal to be located is stationary, transmitting measurement signals via a transmitter. The base station is in motion, detecting and receiving the measurement signals, and then combining this information with its own motion state information to calculate the terminal's position.

[0010] Step 2: In the application scenario, the transmitting end sends a frequency hopping measurement signal;

[0011] Step 3: In the application scenario, the receiver performs frequency hopping synchronization and measures the Doppler frequency shift;

[0012] Step 4: Preprocess the Doppler translation;

[0013] Step 5: Calculate the position of the terminal to be located by using the pre-processed Doppler translation and the relative motion of the transmitter and receiver.

[0014] Furthermore, in step 2, the transmitting end sends the frequency hopping measurement signal in broadcast form, specifically as follows:

[0015] During the K frequency-hopping time slots over the terminal to be located, the low-frequency satellite broadcasts frequency-hopping measurement signals, with carrier frequencies f0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f0 k (k=1,…,K), the complex envelope function of the signal is s k (t), the duration of each frequency hopping time slot is T, and the signal is modulated with navigation messages.

[0016] Furthermore, in step 3, after the receiver detects the measurement signal, it first completes frequency hopping synchronization, and then performs frequency estimation on the intermediate frequency signal after frequency hopping synchronization to obtain the Doppler frequency shift caused by the relative motion between the receiver and the transmitter.

[0017] Furthermore, the measurement signal received by the receiver is represented as the product of a relatively slowly changing envelope and a sinusoidal carrier exhibiting Doppler frequency shift, i.e.:

[0018] r ck (t)=b k s k (t)e j2πf′(t)t +n k (t) (1)

[0019] In the above formula, b k n represents the signal propagation coefficient from the satellite to the terminal within the k-th frequency hopping time slot; k(t) represents the k-th frequency hopping time slot with a mean of 0 and a variance of σ. 2 Gaussian noise; f′(t) represents the carrier frequency of the signal received by the terminal at time t.

[0020] Furthermore, the Doppler frequency shift f d The expression for (t) is as follows:

[0021]

[0022] In the above formula, c is the speed of light, p k (t) and v k (t) represent the satellite's position and velocity at time t, respectively.

[0023] Furthermore, the preprocessing operation for the Doppler frequency shift in step 4 is as follows: First, divide by the frequency-hopping carrier frequency to normalize the Doppler frequency shift. Then, smooth the normalized Doppler frequency shift to eliminate noise and outliers. The preprocessed Doppler frequency shift becomes a quantity that changes continuously with time, expressed as μ. k (p0) means:

[0024]

[0025] The satellite's motion is continuous and stable, with a normalized Doppler frequency shift of μ. k (p0) is a continuously varying doping amount.

[0026] Furthermore, in step 5, the integral of the Doppler frequency shift of the preprocessed axis over a period of time, multiplied by the radio propagation speed, is the distance difference between the receiver and transmitter during that period. By performing Doppler integration over multiple time periods, multiple distance difference data can be obtained. Combining the distance difference information and motion state information, the location of the terminal to be located is obtained using the distance difference positioning algorithm.

[0027] Furthermore, the Doppler frequency shift reflects the radial motion of both the radio transmitter and receiver, and the normalized Doppler frequency shift μ k (p0) Integrating over the time interval [t1,t2], we have

[0028]

[0029] In the formula, D1 and D2 are the distances from the satellite to the terminal to be located at times t1 and t2, respectively, and the terminal position p0 is located at times p1 and t2. k (t1) and p k On a hyperboloid of revolution with focus (t2), any point on the hyperboloid is connected to p. k (t1) and p kThe distance difference (t2) is D2-D1. By integrating the normalized Doppler frequency shift measurement values ​​piecewise, multiple hyperboloids are obtained. The terminal position can be obtained by solving the intersection points of these hyperboloids.

[0030] Secondly, the present invention also protects an electronic device, including a processor and a memory, wherein computer instructions are stored in the memory, and the processor is used to execute the computer instructions stored in the memory to implement the steps of a frequency hopping spread spectrum-based Doppler localization method.

[0031] In a sixth aspect, the present invention also protects a computer-readable storage medium storing computer instructions for performing steps of a frequency-hopping spread spectrum-based Doppler localization method.

[0032] Compared with existing fixed-frequency Doppler positioning methods, the advantages of this invention mainly include two aspects:

[0033] 1) Low interception rate and strong anti-interference capability

[0034] Frequency hopping technology, with its high-speed carrier frequency and pseudo-random hopping characteristics, makes it difficult for the enemy to detect, intercept, and interfere with.

[0035] 2) Multipath error is reduced

[0036] Multipath propagation is a major source of error in radio positioning. According to radio wave propagation theory, signals transmitted at different frequencies on the same channel exhibit different multipath fading characteristics; this phenomenon is known as the frequency selectivity of multipath channels. To combat this, wireless communication systems often employ frequency diversity techniques, transmitting signals on different carriers. For frequency-hopping Doppler positioning, when measuring the Doppler shift on randomly hopping carrier frequencies, the frequency estimation error caused by multipath propagation is rapidly "whitened," exhibiting strong randomness, thus being partially offset during Doppler integration. Therefore, overall, frequency-hopping spread spectrum also enhances the multipath resistance of Doppler positioning through frequency diversity, and the "whitening" effect becomes more significant with increasing frequency hopping bandwidth and the number of frequency points, effectively mitigating multipath errors. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 is a flowchart of the Doppler localization method based on frequency hopping spread spectrum according to the present invention.

[0039] Figure 2 is a schematic diagram of the working process of the Doppler positioning method based on frequency hopping spread spectrum of the present invention in a low-orbit satellite navigation scenario.

[0040] Figure 3 is a comparison of the positioning results of the traditional method and the method of the present invention.

[0041] Figure 4 is a comparison of the cumulative distribution function of the positioning error between the traditional method and the method of the present invention. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0043] Low Earth orbit (LEO) satellite systems have developed rapidly in recent years. Due to their closer proximity to Earth compared to conventional navigation satellites (MTO), their stronger signal and faster overhead speed have made them widely used for Doppler positioning, serving as an important backup and supplement to the Global Navigation Satellite System (GNSS). The following describes the specific implementation of this invention in detail, using a LEO satellite navigation scenario as an example and in conjunction with Figures 1-4.

[0044] Step 1: Description of Low Earth Orbit Satellite Navigation Application Scenarios

[0045] As shown in Figure 2, in the application scenario of low-orbit satellite navigation, when the low-orbit satellite, which serves as a navigation base station, flies over the positioning terminal, it broadcasts a frequency-hopping measurement signal; the actual position coordinates of the positioning terminal device are given, and after receiving the measurement signal, it can calculate its own position.

[0046] Step 2: The transmitter sends a frequency hopping measurement signal.

[0047] The radio transmitter broadcasts frequency-hopping measurement signals according to the frequency-hopping method and parameters (hopping rate, frequency-hopping pattern, etc.). For navigation applications, the measurement signal needs to be modulated with navigation messages.

[0048] During the K frequency-hopping time slots in which the low-Earth orbit satellite passes over the terminal to be located, it broadcasts frequency-hopping measurement signals with carrier frequencies f0, f1, f2, f3, f4, f5, f6, f7, f8, f9 k (k=1,…,K), the complex envelope function of the signal is s k (t), the duration of each frequency hopping time slot (the reciprocal of the hopping rate) is T, and the signal is modulated with navigation messages; the satellite's position and velocity at time t are represented by p. k (t) and v k (t) indicates that after the positioning terminal demodulates the data in the navigation message, p can be calculated. k (t) and v k (t), therefore it is considered a known quantity here.

[0049] Step 3: Frequency hopping synchronization at the receiver and measurement of Doppler frequency shift.

[0050] After the radio terminal detects the measurement signal, it first completes the establishment and maintenance (de-hopping) of frequency hopping synchronization; then, it uses the frequency estimation method to estimate the frequency of the de-hopped intermediate frequency signal to obtain the Doppler frequency shift caused by the relative motion between the radio transmitter and receiver.

[0051] When a satellite flies over the terminal to be positioned, the terminal first establishes and maintains frequency hopping synchronization, and then begins receiving measurement signals. This invention mainly relates to a Doppler positioning method under a narrowband signal model. Since the bandwidth of a narrowband signal is much smaller than the carrier frequency, the envelope function s can be ignored. k The Doppler effect in (t) is considered, and it is assumed that Doppler information mainly exists in the carrier component. Therefore, the measurement signal received by the terminal can be expressed as the product of a relatively slowly changing envelope and a sinusoidal carrier with Doppler frequency shift, that is:

[0052] The measurement signal received by the receiver is represented as the product of a relatively slowly changing envelope and a sinusoidal carrier exhibiting Doppler frequency shift, i.e.:

[0053] r ck (t)=b k s k (t)e j2πf′(t)t +n k (t) (1)

[0054] In the above formula, b k n represents the signal propagation coefficient from the satellite to the terminal within the k-th frequency hopping time slot; k (t) represents the k-th frequency hopping time slot with a mean of 0 and a variance of σ. 2 Gaussian noise; f′(t) represents the carrier frequency of the signal received by the terminal at time t.

[0055] According to the Doppler principle, the Doppler frequency shift f d The expression for (t) is as follows:

[0056]

[0057] In the above formula, c is the speed of light, p k (t) and v k (t) represent the satellite's position and velocity at time t, respectively.

[0058] Estimating the Doppler frequency shift f from the received signal d The methods (t) include those based on Fast Fourier Transform (FFT), Phase-Locked Loop (PLL), and a combination of both. FFT-based methods are accurate and efficient, but their performance is limited by spectral resolution, which is Δf = fs / N is determined by the sampling rate f s The frequency shift is determined by both the sampling rate and the sample length N. While meeting basic sampling rate requirements, increasing the sample length is necessary to improve resolution, inevitably leading to processing delays. Phase-locked loop (PLL) methods compare the phase of the received signal with the phase of a local reference signal, adjusting the frequency of the local reference signal via a feedback control loop to estimate and correct the frequency offset. This method offers good real-time performance and high accuracy, and is commonly used for real-time frequency shift estimation. Because the carrier frequency of a frequency-hopping system is constantly changing, traditional carrier tracking loops are forced to continuously switch to a step response state, leading to increased tracking errors or even loss of lock. Frequency-hopping systems typically employ PLLs based on frequency-hopping patterns to continuously estimate the Doppler frequency. This method uses a known frequency-hopping pattern and the current normalized Doppler frequency shift to estimate the Doppler frequency shift transient at the start of the next frequency-hopping time slot, compensating for it in the numerically controlled oscillator (NCO).

[0059] Step 4: Normalization and smoothing of Doppler frequency shift

[0060] For fixed-frequency Doppler positioning, since the carrier frequency remains constant and the relative motion between the radio transmitter and receiver is continuous, the measured Doppler frequency shift is also a quantity that changes continuously with time. For frequency-hopping Doppler positioning, however, the carrier frequency is constantly changing, causing the Doppler frequency shift to be a discontinuous, step-changing quantity that cannot be directly integrated. Therefore, the Doppler frequency shift measured in the previous step needs to be preprocessed. First, it is divided by the frequency-hopping carrier frequency to normalize the frequency shift; then, the normalized Doppler frequency shift is smoothed to eliminate noise and outliers, and the preprocessed Doppler frequency shift becomes a quantity that changes continuously with time again.

[0061] The Doppler frequency shift measured in the previous step needs to be preprocessed. First, it is divided by the frequency-hopping carrier frequency to normalize the frequency shift. Then, the normalized Doppler frequency shift is smoothed to eliminate noise and outliers. The preprocessed Doppler frequency shift becomes a continuously varying quantity over time again, expressed as μ. k (p0) means:

[0062]

[0063] The satellite's motion is continuous and stable, with a normalized Doppler frequency shift of μ. k (p0) is a continuously varying doping amount.

[0064] Step 5: Calculate the location of the terminal to be located

[0065] The integral of the preprocessed Doppler frequency shift over a period of time, multiplied by the radio propagation speed (approximately the speed of light), yields the change in distance between the transmitter and receiver over that period (distance difference). Since the motion state information of the moving party is acquireable (navigation) or known (surveillance and search and rescue) in Doppler positioning, multiple distance difference data points can be obtained by performing Doppler integration over multiple time periods. Combining the distance difference information with the motion state information, the location of the terminal to be located can be determined using a distance difference positioning algorithm.

[0066] Doppler shift reflects the radial motion of both the radio transmitter and receiver, and is related to the normalized Doppler shift μ. k (p0) Integrating over the time interval [t1,t2], we have

[0067]

[0068] In the formula, D1 and D2 are the distances from the satellite to the terminal to be located at times t1 and t2, respectively, and the terminal position p0 is located at times p1 and t2. k (t1) and p k On a hyperboloid of revolution with focus (t2), any point on the hyperboloid is connected to p. k (t1) and p k The distance difference (t2) is D2-D1. By integrating the normalized Doppler frequency shift measurement values ​​piecewise, multiple hyperboloids are obtained. Solving for the intersection of these hyperboloids will yield the terminal position (distance difference positioning).

[0069] Since the hyperboloid foci are all on the same plane of the arc when the satellite passes overhead once, the geometric diversity is relatively lacking. Therefore, the coordinates of the intersection point may have a large error. In practical applications, the measurement accuracy can be further improved by means of elevation information assistance and multiple overhead measurements.

[0070] The method of this invention was simulated using MATLAB to evaluate its effect on mitigating multipath errors.

[0071] The actual location of the terminal to be positioned was taken as 34°N, 108°E. The Tiangong space station, which passed overhead at 9:47:23 AM UTC on March 26, 2023 (highest elevation angle 43°), was selected as the hypothetical low-Earth orbit navigation satellite. The transmitter's initial carrier frequency was set to 400MHz, with a frequency hopping interval of 1 second. The frequency hopping sequence was randomly generated from the frequency range [400MHz, 400MHz + BW], where BW is the frequency hopping bandwidth, defaulted to 10MHz. Frequency shift estimation was performed using a Costas loop based on a frequency hopping pattern. The obtained Doppler frequency shift was divided into four equal segments in time and integrated to obtain four hyperboloid equations. The second step was to solve this nonlinear equation system using Newton's method (with the initial position estimate set to the actual value) to obtain the final position estimate.

[0072] Figure 3 shows a schematic diagram of the positioning results obtained from a single simulation; Figure 4 shows the cumulative distribution function of the positioning error under two different conditions, BW=5MHz and BW=10MHz (25 simulations). It can be seen that frequency hopping has a significant effect on reducing multipath error, and as the frequency hopping frequency range expands, the "whitening" effect of frequency measurement error becomes more significant, and the positioning error is further reduced.

[0073] The above-described embodiments are merely preferred embodiments of the present invention, and the scope of protection of the present invention is not limited thereto. Any simple changes or equivalent substitutions of the technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention shall fall within the scope of protection of the present invention.

Claims

1. A Doppler localization method based on frequency hopping spread spectrum, characterized in that, The method includes the following steps: Step 1: Describe the application scenario; Step 2: In the application scenario, the transmitting end sends a frequency hopping measurement signal; Step 3: In the application scenario, the receiving end performs frequency hopping synchronization and measures the Doppler frequency shift; Step 4: Preprocess the Doppler shift; Step 5: Calculate the position of the terminal to be located using the preprocessed Doppler shift and the relative motion of the transmitting and receiving ends; Step 4, the preprocessing operation of the Doppler frequency shift, is as follows: First, divide by the frequency hopping carrier frequency to normalize the Doppler frequency shift; then, smooth the normalized Doppler frequency shift to eliminate noise and outliers. The preprocessed Doppler frequency shift becomes a quantity that changes continuously with time. express: (3) The satellite's motion is continuous and stable, and the normalized Doppler frequency shift is normalized. This is a continuously changing doping level; in step 5, the integral of the preprocessed axis's Doppler frequency shift over a period of time, multiplied by the radio propagation speed, represents the distance difference between the receiver and transmitter during that period. Multiple distance difference data can be obtained by performing Doppler integration over multiple time periods. Combining the distance difference information with motion state information, the location of the terminal to be located is obtained using a distance difference positioning algorithm. The Doppler frequency shift reflects the radial motion of both the radio transmitter and receiver. The normalized Doppler frequency shift... During the time period [ , Earn points, there are (4) In the formula, and They are respectively and Distance from the satellite to the terminal to be located at any given time; terminal location Located in and On a hyperboloid of revolution with focus , any point on the hyperboloid... and The distance difference is By integrating the normalized Doppler frequency shift measurement values ​​piecewise, multiple hyperboloids of revolution are obtained. The terminal position can be obtained by solving the intersection points of these hyperboloids.

2. The Doppler localization method based on frequency hopping spread spectrum according to claim 1, characterized in that, In step 2, the transmitting end sends frequency hopping measurement signals in broadcast form. Specifically, this involves the low-frequency satellite passing over the terminal to be located... In each frequency hopping time slot, the broadcast transmits frequency hopping measurement signals, with carrier frequencies of respectively. The complex envelope function of the signal is The duration of each frequency hopping time slot is The signal is modulated with navigation messages.

3. The Doppler localization method based on frequency hopping spread spectrum according to claim 2, characterized in that, In step 3, after the receiver detects the measurement signal, it first completes frequency hopping synchronization, and then performs frequency estimation on the intermediate frequency signal after frequency hopping synchronization to obtain the Doppler frequency shift caused by the relative motion between the receiver and the transmitter.

4. The Doppler localization method based on frequency hopping spread spectrum according to claim 3, characterized in that, The measurement signal received by the receiver is represented as the product of a relatively slowly changing envelope and a sinusoidal carrier exhibiting Doppler frequency shift, i.e.: (1) In the above formula, Indicates the first The signal propagation coefficient from satellite to terminal within a frequency hopping time slot; Indicates the first The mean is 0 and the variance is within each frequency hopping time slot. Gaussian noise; Indicates that the terminal is in The carrier frequency of the signal received at any given time.

5. The Doppler localization method based on frequency hopping spread spectrum according to claim 4, characterized in that, Doppler shift The expression is as follows: (2) In the above formula, At the speed of light, and These respectively represent the satellite in Position and velocity at any given moment.

6. An electronic device comprising a processor and a memory, characterized in that, The memory stores computer instructions, and the processor executes the computer instructions stored in the memory to implement the steps of the method as described in any one of claims 1-5.

7. A computer-readable storage medium storing computer instructions, characterized in that, Computer instructions are used to perform the steps of the method as described in any one of claims 1-5.

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