Doppler positioning method based on frequency hopping spectrum spreading

Abstract

The invention provides a Doppler positioning method based on frequency hopping and spectrum spreading, belongs to the technical field of radio positioning, and aims to solve the problem that a fixed carrier frequency working mode adopted by a traditional Doppler positioning technology enables a signal to be easily intercepted, orientated and interfered. The method comprises the following steps of 1, describing an application scene; 2, in an application scene, the transmitting end transmits a frequency hopping measurement signal; 3, in an application scene, a receiving end performs frequency hopping synchronization and measures Doppler frequency shift; step 4, preprocessing the Doppler translation; and 5, calculating the position of the to-be-positioned terminal by utilizing the preprocessed Doppler translation and combining the relative movement condition of the transmitting end and the receiving end. According to the method, the low interception rate and the anti-interference capability endowed by the rapid hopping of the carrier frequency are obtained, the positioning performance deterioration caused by the multipath effect can be reduced by using the natural frequency diversity characteristic of the frequency hopping, and the method is suitable for application scenes with relatively high requirements on the performance such as robustness and positioning precision.

Description

Technical Field

[0001] The invention belongs to the technical field of radio positioning, and in particular relates to a Doppler positioning method based on frequency hopping and spread spectrum. Background Art

[0002] As a key basic information, location information plays an increasingly important role in the civil and military fields. Radio positioning technology represented by satellite navigation and radar has become an important technical means to obtain location information due to its advantages of high precision, all-weather and wide coverage.

[0003] During the propagation of radio signals, the relative radial motion between the transmitter and the receiver will cause the Doppler frequency shift of the received signal. The Doppler frequency shift is integrated in the time domain and divided by the carrier wavelength of the transmitted signal to obtain the distance change (distance difference) between the radio transmitter and receiver during this period of time. If the party to be located (either the transmitter or the receiver) is stationary, and the motion state information (position, speed) of the moving party is known, then the position of the terminal to be located can be calculated based on the distance difference positioning principle. This positioning method is called Doppler positioning, which has two important advantages: first, the terminal to be located has a simple structure and low cost; second, only a single station and a single antenna are needed to achieve positioning, so it is widely used in different application scenarios such as navigation, surveillance, and search and rescue.

[0004] However, the fixed carrier frequency working mode adopted by traditional Doppler positioning technology makes the signal extremely easy to be intercepted, directed and interfered; at the same time, the multipath effect in the wireless channel will cause large errors in the results of Doppler positioning. Summary of the invention

[0005] In response to the above-mentioned problems, the present invention proposes a Doppler positioning method based on frequency hopping and spread spectrum. While obtaining the low interception rate and anti-interference capability endowed by the rapid hopping of the carrier frequency, the method can also utilize the frequency diversity characteristics naturally possessed by frequency hopping to alleviate the deterioration of positioning performance caused by the multipath effect. The method is suitable for application scenarios with high requirements on performance such as robustness and positioning accuracy.

[0006] The technical solution adopted by the present invention is:

[0007] In a first aspect, the present invention protects a Doppler positioning 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 divided into two categories: navigation (passive positioning) and surveillance and search and rescue (active positioning). In navigation applications, the base station is in motion and continuously sends measurement signals through the transmitter. The signal is modulated with navigation messages (which help the terminal to be located to calculate the location, speed and other motion status information of the base station, which is subsequently used for its own position solution); the terminal to be located is in a stationary state, and after receiving the measurement signal, it processes it and calculates its own position. In surveillance and search and rescue applications, the terminal to be located is in a stationary state and sends measurement signals through the transmitter; the base station is in motion, and after detecting and receiving the measurement signal, it calculates the position of the terminal to be located in combination with its own motion status information.

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

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

[0012] Step 4: Preprocess the Doppler shift;

[0013] Step 5: Utilize the preprocessed Doppler shift and the relative motion between the transmitter and the receiver to calculate the position of the terminal to be located.

[0014] Furthermore, in step 2, the transmitter sends the frequency hopping measurement signal in a broadcast form, specifically:

[0015] The low-frequency satellite broadcasts and transmits frequency-hopping measurement signals in the K frequency-hopping time slots passing over the terminal to be positioned. The carrier frequencies are f 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 navigation message is modulated in the signal.

[0016] Furthermore, after the receiving end detects the measurement signal in step 3, 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 receiving end and the transmitting end.

[0017] Furthermore, the measurement signal received by the receiving end is expressed as the product of a relatively slow-changing envelope and a sinusoidal carrier with Doppler frequency shift, that is:

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

[0019] In the above formula, b k represents the signal propagation coefficient from the satellite to the terminal in the kth frequency hopping time slot; n k(t) indicates that the mean value in the kth frequency hopping time slot is 0 and the variance is 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 (t) is expressed as follows:

[0021]

[0022] In the above formula, c is the speed of light, p is k (t) and v k (t) represent the position and velocity of the satellite at time t respectively.

[0023] Furthermore, the operation of preprocessing the Doppler frequency shift in step 4 is: first, divide it by the frequency hopping carrier frequency to complete the normalization of the Doppler frequency shift, and 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, and is expressed by μ k (p0) means:

[0024]

[0025] The satellite's motion is continuous and stable, with a normalized Doppler shift μ k (p0) is a continuously changing dosage.

[0026] Furthermore, the integral of the Doppler frequency shift of the preprocessing axis in step 5 over a period of time multiplied by the radio propagation speed is the distance difference between the receiving end and the transmitting end during this period of time. By performing Doppler integration over multiple time periods, multiple distance difference data can be obtained. By combining the distance difference information and the motion state information, the distance difference positioning algorithm is used to obtain the position of the terminal to be located.

[0027] Furthermore, the Doppler shift reflects the radial motion of the radio transmitter and receiver. k (p0) is integrated over the time period [t1, t2],

[0028]

[0029] Where D1 and D2 are the distances from the satellite to the terminal to be located at time t1 and t2 respectively, and the terminal position p0 is located at p k (t1) and p k (t2) is a rotation hyperboloid with focus. Any point on the hyperboloid is k (t1) and p kThe distance difference of (t2) is D2-D1. The normalized Doppler frequency shift measurement value is integrated in sections to obtain multiple rotating hyperboloids. The terminal position can be obtained by solving the intersection of these hyperboloids.

[0030] In a second aspect, the present invention also protects an electronic device, including a processor and a memory, wherein the memory stores computer instructions, and the processor is used to execute the computer instructions stored in the memory to implement the steps of a Doppler positioning method based on frequency hopping spread spectrum.

[0031] In a sixth aspect, the present invention also protects a computer-readable storage medium storing computer instructions, where the computer instructions are used to execute the steps of a Doppler positioning method based on frequency hopping spread spectrum.

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

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

[0034] The high-speed, pseudo-random hopping characteristics of the carrier frequency of frequency hopping technology make it difficult for the enemy to detect, intercept and interfere.

[0035] 2) Multipath error reduction

[0036] The multipath effect is the main source of error in radio positioning. According to the theory of radio wave propagation, the multipath fading characteristics of signals of different frequencies transmitted on the same channel will be different. This phenomenon is called the frequency selection characteristic of the multipath channel. In view of this characteristic, wireless communication systems often use frequency diversity technology to transmit signals on different carriers to combat multipath fading. For frequency hopping Doppler positioning, when measuring the Doppler frequency shift on different carrier frequencies that hop randomly, the frequency estimation error caused by multipath will also be quickly "whitened", showing a strong randomness, so that it can be offset to a certain extent when performing Doppler integration. Therefore, from a general point of view, frequency hopping spread spectrum will also enhance the anti-multipath capability of Doppler positioning through frequency diversity, and with the increase of frequency hopping bandwidth and the number of frequency points, the "whitening" effect will be more significant, which can effectively reduce multipath errors. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0038] Figure 1 The figure is a flow chart of the Doppler positioning method based on frequency hopping and spread spectrum of the present invention.

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

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

[0041] Figure 4 The figure is a comparison diagram of the cumulative distribution function of the positioning error between the traditional method and the method of the present invention. DETAILED DESCRIPTION

[0042] To make the purpose, technical solution and advantages of the embodiments of the present invention more clear, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0043] Low-orbit satellite systems have developed rapidly in recent years. As they are closer to the earth than general navigation satellites (medium-orbit), they have the advantages of strong signals and fast overhead speed, making them widely used in Doppler positioning and an important backup and supplement to the Global Navigation Satellite System (GNSS). Figure 1-4 The specific embodiments of the present invention are described in detail.

[0044] Step 1: Description of low-orbit satellite navigation application scenarios

[0045] like Figure 2 As shown in the figure, in the low-orbit satellite navigation application scenario, the low-orbit satellite serving as a navigation base station broadcasts a frequency-hopping measurement signal when flying over the terminal with positioning; the actual position coordinates of the terminal device to be positioned are, and after receiving the measurement signal, its own position can be calculated.

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

[0047] The radio transmitter sends the frequency hopping measurement signal in the form of broadcast according to the frequency hopping mode and parameters (hopping rate, frequency hopping pattern, etc.). For navigation applications, the measurement signal needs to be modulated with the navigation message.

[0048] When the low-orbit satellite passes over the terminal to be positioned, it broadcasts and transmits frequency-hopping measurement signals in K frequency-hopping time slots, with carrier frequencies of f and k (k=1,…,K), the complex envelope function of the signal is s k (t), the duration of each frequency hopping time slot (the inverse of the hopping speed) is T, and the signal is modulated with a navigation message; the position and speed of the satellite 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), so it is regarded as a known quantity here.

[0049] Step 3: Receiver frequency hopping synchronization and measurement of Doppler shift

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

[0051] When the satellite flies over the terminal to be positioned, the positioning terminal first completes the establishment and maintenance of frequency hopping synchronization, and then starts to receive the measurement signal. The present invention mainly relates to a Doppler positioning method under a narrowband signal model, and the bandwidth of the narrowband signal is much smaller than the signal carrier frequency, and the envelope function s can be ignored. k (t), and it is believed that the Doppler information mainly exists in the carrier component. Therefore, the measurement signal received by the terminal can be expressed as the product of the relatively slow-changing envelope and the sinusoidal carrier with Doppler frequency shift, that is:

[0052] The measurement signal received by the receiver is expressed as the product of a relatively slow-changing envelope and a sinusoidal carrier with Doppler frequency shift, that is:

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

[0054] In the above formula, b k represents the signal propagation coefficient from the satellite to the terminal in the kth frequency hopping time slot; n k (t) indicates that the mean value in the kth frequency hopping time slot is 0 and the variance is 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 (t) is expressed as follows:

[0056]

[0057] In the above formula, c is the speed of light, p is k (t) and v k (t) represent the position and velocity of the satellite at time t respectively.

[0058] Estimate the Doppler frequency shift f from the received signal dThe methods of (t) include those based on fast Fourier transform, those based on phase-locked loop, and the combination of the two. The method based on fast Fourier transform is accurate and efficient, but its performance is limited by the spectrum resolution, and the spectrum resolution Δf = f s / N is determined by the sampling rate f s and the sample length N. When the sampling rate meets the basic requirements, the sample length must be increased to improve the resolution, which will inevitably lead to processing delays. The method based on the phase-locked loop compares the phase of the received signal with the phase of the local reference signal, and adjusts the frequency of the local reference signal through a feedback control loop to estimate and correct the frequency deviation. This method has good real-time performance and high accuracy, and is often used for real-time estimation of frequency shift. Since the carrier frequency of the frequency hopping system is always in a state of continuous jumping, the traditional carrier tracking loop will be forced to continuously switch to the step response state, resulting in an increase in the loop tracking error or even loss of lock. The frequency hopping system usually uses a phase-locked loop assisted by a frequency hopping pattern to achieve continuous estimation of the Doppler frequency. This method uses the known frequency hopping pattern and the normalized Doppler frequency shift at the current moment to estimate the Doppler frequency shift transient at the start of the next frequency hopping time slot, and compensates it into the numerically controlled oscillator NCO.

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

[0060] For fixed-frequency Doppler positioning, since the carrier frequency remains unchanged 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, the carrier frequency is constantly jumping, which causes the Doppler frequency shift to become 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 complete the normalization of the frequency shift; then the normalized Doppler frequency shift is smoothed to eliminate noise and outliers. 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 complete the normalization of the frequency shift. Then the normalized Doppler frequency shift is smoothed to eliminate noise and outliers. The preprocessed Doppler frequency shift is converted back into a quantity that changes continuously with time, and is expressed by μ k (p0) means:

[0062]

[0063] The satellite's motion is continuous and stable, with a normalized Doppler shift μ k (p0) is a continuously changing dosage.

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

[0065] The integral of the pre-processed Doppler frequency shift over a period of time multiplied by the radio propagation speed (approximately the speed of light) is the change in the distance between the sender and the receiver during this period (distance difference). Since the motion state information of the moving party is available (navigation) or known (surveillance and search and rescue) in Doppler positioning, multiple distance difference data can be obtained by performing Doppler integration over multiple time periods. Combining the distance difference information and the motion state information, the position of the terminal to be located can be obtained by using the distance difference positioning algorithm.

[0066] The Doppler shift reflects the radial motion of the radio transmitter and receiver. k (p0) is integrated over the time period [t1, t2],

[0067]

[0068] Where D1 and D2 are the distances from the satellite to the terminal to be located at time t1 and t2 respectively, and the terminal position p0 is located at p k (t1) and p k (t2) is a rotation hyperboloid with focus. Any point on the hyperboloid is k (t1) and p k The distance difference of (t2) is D2-D1. The normalized Doppler frequency shift measurement value is integrated in sections to obtain multiple rotating hyperboloids. The terminal position (distance difference positioning) can be obtained by solving the intersection of these hyperboloids.

[0069] Because the foci of the hyperbolic surface are all on the arc of the same plane when the satellite passes overhead at one time, there is a lack of geometric diversity. Therefore, there may be large errors in the coordinates of the intersection position. In practical applications, the measurement accuracy can be further improved through the assistance of elevation information, multiple over-the-top measurements, and other means.

[0070] The method of the present invention is simulated based on MATLAB to evaluate its multipath error mitigation effect.

[0071] The ground at 34 degrees north latitude and 108 degrees east longitude is taken as the actual position of the terminal to be positioned; the Tiangong space station that will pass overhead at 9:47:23 UTC on March 26, 2023 (the highest point elevation angle is 43°) is selected as the hypothetical low-orbit navigation satellite. The transmitter's starting carrier frequency is set to 400MHz, the frequency hopping interval is 1 second, and the frequency hopping sequence is randomly selected and generated from the frequency range of [400MHz, 400MHz+BW]. BW is the frequency hopping bandwidth, which is set to 10MHz by default. The frequency shift estimation is completed by the Costas loop assisted by the frequency hopping pattern. The obtained Doppler frequency shift is divided into 4 equal segments in time and integrated separately to obtain 4 rotating hyperbolic equations; the second step is to solve the nonlinear equation group by Newton's method (the initial value of the position estimate is set to the true value) to obtain the final position estimate.

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

[0073] The embodiments described above are only preferred specific implementation modes of the present invention, and the protection scope of the present invention is not limited thereto. Any simple changes or equivalent replacements of the technical solutions that can be obviously obtained by any technician familiar with the field within the technical scope disclosed in the present invention belong to the protection scope of the present invention.

Claims

1. A Doppler positioning method based on frequency hopping spread spectrum, characterized in that: The method comprises the following steps: Step 1: Describe the application scenario; Step 2: In the application scenario, the transmitter sends a frequency hopping measurement signal; Step 3: In the application scenario, the receiving end performs frequency hopping synchronization and measures the Doppler shift; Step 4: Preprocess the Doppler shift; Step 5: Utilize the preprocessed Doppler shift and the relative motion between the transmitter and the receiver to calculate the position of the terminal to be located.

2. A Doppler positioning method based on frequency hopping spread spectrum according to claim 1, characterized in that: In step 2, the transmitter sends the frequency hopping measurement signal in the form of broadcast, specifically: The low-frequency satellite broadcasts and transmits frequency-hopping measurement signals in the K frequency-hopping time slots passing over the terminal to be positioned. The carrier frequencies are f 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 navigation message is modulated in the signal.

3. A Doppler positioning method based on frequency hopping spread spectrum according to claim 2, characterized in that: In step 3, after the receiving end 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 receiving end and the transmitting end.

4. A Doppler positioning method based on frequency hopping spread spectrum according to claim 3, characterized in that: The measurement signal received by the receiver is expressed as the product of a relatively slow-changing envelope and a sinusoidal carrier with Doppler frequency shift, that is: r ck (t)=b k s k (t)e j2πf′(t)t +n k (t) (1) In the above formula, b k represents the signal propagation coefficient from the satellite to the terminal in the kth frequency hopping time slot; n k (t) indicates that the mean value in the kth frequency hopping time slot is 0 and the variance is σ 2 Gaussian noise; f′(t) represents the carrier frequency of the signal received by the terminal at time t.

5. A Doppler positioning method based on frequency hopping spread spectrum according to claim 4, characterized in that: Doppler frequency shift f d (t) is expressed as follows: In the above formula, c is the speed of light, p is k (t) and v k (t) represent the position and velocity of the satellite at time t respectively.

6. A Doppler positioning method based on frequency hopping spread spectrum according to claim 5, characterized in that: The operation of preprocessing the Doppler frequency shift in step 4 is: first, divide it by the frequency hopping carrier frequency to complete the normalization of 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, and is expressed by μ k (p0) means: The satellite's motion is continuous and stable, with a normalized Doppler shift μ k (p0) is a continuously changing dosage.

7. A Doppler positioning method based on frequency hopping spread spectrum according to claim 6, characterized in that: The integral of the Doppler frequency shift of the preprocessing axis in step 5 over a period of time multiplied by the radio propagation speed is the distance difference between the receiving end and the transmitting end during this period of time. By performing Doppler integration over multiple time periods, multiple distance difference data can be obtained. Combining the distance difference information and the motion state information, the distance difference positioning algorithm is used to obtain the position of the terminal to be located.

8. A Doppler positioning method based on frequency hopping spread spectrum according to claim 7, characterized in that: The Doppler shift reflects the radial motion of the radio transmitter and receiver. k (p0) is integrated over the time period [t1, t2], Where D1 and D2 are the distances from the satellite to the terminal to be located at time t1 and t2 respectively, and the terminal position p0 is located at p k (t 1) and p k (t2) is a rotation hyperboloid with focus. Any point on the hyperboloid is k (t1) and p k The distance difference of (t2) is D2-D1. The normalized Doppler frequency shift measurement value is integrated in sections to obtain multiple rotating hyperboloids. The terminal position can be obtained by solving the intersection of these hyperboloids.

9. An electronic device comprising a processor and a memory, characterized in that: The memory stores computer instructions, and the processor is used to execute the computer instructions stored in the memory to implement the steps of the method as described in any one of claims 1 to 8.

10. A computer-readable storage medium storing computer instructions, characterized in that: The computer instructions are used to execute the steps of the method as claimed in any one of claims 1 to 8.

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