Radio altimeter with pre-biased local oscillator

Abstract

The invention provides a radio altimeter with a pre-biased local oscillator. The radio altimeter comprises a transmitting signal source module used for generating a frequency-modulated continuous wave signal; the pre-bias local oscillation module is used for generating a pre-bias frequency-modulated continuous wave local oscillation signal which is synchronous with the transmitting signal source module but lagged in phase; the radio frequency control module is used for controlling the transmitting signal source module and the pre-bias local oscillator module; the frequency mixer module is used for mixing the pre-bias local oscillator signal with a received ground echo signal to generate an intermediate frequency signal containing a pre-bias frequency; and the calculation circuit module is used for calculating the current height value according to the intermediate frequency signal. The system is simple and easy to implement; the lowest height measurement range of the frequency-modulated continuous wave altimeter can be effectively reduced; the original radio frequency feeder line is shortened or cancelled, the link loss is reduced, and the transmitting power is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of radar navigation, and in particular to a radio altimeter with a pre-biased local oscillator. Background Art

[0002] The radio altimeter is a commonly used aviation equipment used to measure the vertical distance between an aircraft and the ground (sea) surface. Radio altimeters with a maximum range of less than 1500m usually use a frequency modulated continuous wave system.

[0003] A frequency modulated continuous wave radio altimeter generally consists of a transmitter-receiver, a transmitting antenna, a receiving antenna, a transmitting cable, and a receiving cable.

[0004] The basic principle block diagram of the FM radio altimeter is as follows: Figure 1 As shown in the figure, the FM radio altimeter transmitter is an FM oscillator, which is modulated by a modulator to generate a continuous wave FM signal. The modulation form can be triangle wave modulation, sawtooth wave modulation or sine wave modulation. The first two are linear FM, and the latter is nonlinear FM. The transmitter outputs a FM signal such as Figure 2 As shown (for the convenience of explanation, the transmitting signal is simplified into a single spectrum signal). In the figure, ΔF is the difference between the maximum frequency F2 and the minimum frequency F1 of the FM signal, which is called the FM frequency deviation; τ is the delay time caused by the radio wave from the transmitting antenna to the ground and then reflected back to the receiving antenna, τ = 2H / C (C is the propagation speed of the radio wave in the air, H is the height of the aircraft); fb is the beat signal, which is the difference between the transmitting signal frequency (direct signal frequency) and the echo signal frequency at a certain moment; Tm is the sweep time.

[0005] Most of the energy of the transmitter output signal is fed to the transmitting antenna through the transmitting cable and transmitted to the ground. In addition, a small part of the energy (also called direct signal) is coupled out through the coupler and input to the mixer as the local oscillator signal. It should be pointed out here that the properties of the direct signal are exactly the same as the transmitting signal except for the energy size. When the transmitter sends out a signal with a frequency of F, it returns to the altimeter after τ=2H / C after the near-ground reflection. The frequency-modulated continuous wave signal in the aircraft continues to sweep at a speed of △F / Tm. At this time, its frequency is F+2(H / C)*(ΔF / Tm). The mixed signal obtained by the mixer is fb=(2H / C)*(ΔF / Tm).

[0006] After sorting, we get:

[0007]

[0008] This formula is the basic height measurement formula of the FM radio altimeter. Since the speed of light C is a constant, it can be seen that the height H is a three-variable function of Tm, fh and △F. Therefore, as long as two of the three parameters are determined, the measured height H must be linearly or inversely proportional to the third parameter. The formula can also be transformed as follows:

[0009]

[0010] According to the relevant requirements of national radio frequency management, the operating frequency of the altimeter is 4200MHz~4400MHz, that is, the maximum △F is 200MHz, usually △F is 150MHz. Substituting the speed of light and △F into (2), and changing the sweep time Tm to the sweep frequency Fm, we can get f b =H*F m (3)

[0011] The altimeter needs to obtain the accurate frequency of fb to convert it into height, which requires at least two periodic signals of fb to accurately measure the frequency of fb. From (3), it can be seen from the above formula that when the height is less than 2m, the intermediate frequency fb is already lower than the sweep frequency Fm, that is, at this time, the intermediate frequency fb can no longer obtain two complete periodic signals within one sweep cycle. That is, the minimum measurement height of this method can only reach 2m. In fact, in order to ensure the accuracy of the measurement, the minimum measurement height of this method is generally limited to more than 4m.

[0012] There are several ways to solve the above problems: changing the operating frequency, usually using the millimeter wave frequency band to obtain a larger sweep bandwidth △F, but the operating frequency of this method does not meet national regulations. This limits the application of this method on most aircraft. By measuring the frequency accumulation within multiple Tm cycles to obtain the intermediate frequency fb, but this method will be affected by the interference signal of the phase mutation point during triangle wave modulation or sawtooth wave modulation, the fb measurement accuracy error is large, thereby reducing the low altitude measurement accuracy. Figure 1 The transmitting feeder and receiving feeder in the actual work will also cause delay to the electromagnetic wave, and the delay will be included in the height. This height is called the "residual height", and formula 4 becomes

[0013]

[0014] Where L is the cable length; ε is the cable dielectric constant. Through this method, the frequency of fb can be increased, thereby effectively reducing the minimum measurement altitude of the altimeter. This is also the mainstream solution for frequency modulation continuous altimeters. However, this method increases the cable length and weight, which limits its application in small or micro aircraft that are extremely sensitive to volume and weight.

[0015] The traditional radio altimeter has limited measurement accuracy at low altitudes, and the lowest measurement altitude is generally limited to more than 4 meters. In addition, the increase in cable length leads to a large volume and weight. Therefore, a new technical solution is needed to improve the above technical problems. Summary of the invention

[0016] In view of the defects in the prior art, an object of the present invention is to provide a radio altimeter with a pre-biased local oscillator.

[0017] A radio altimeter with a pre-biased local oscillator provided by the present invention comprises:

[0018] Transmitting signal source module: used to generate frequency modulated continuous wave signal;

[0019] Pre-biased local oscillator module: used to generate a pre-biased frequency modulated continuous wave local oscillator signal that is synchronized with the transmitting signal source module but has a phase lag;

[0020] RF control module: used to control the transmission signal source module and the pre-biased local oscillator module;

[0021] Mixer module: used to mix the pre-biased local oscillator signal with the received ground echo signal to generate an intermediate frequency signal containing the pre-biased frequency;

[0022] Calculation circuit module: used to calculate the current height value based on the intermediate frequency signal.

[0023] Preferably, the transmission signal source module and the pre-biased local oscillator module operate under the same clock source, and the two operate synchronously.

[0024] Preferably, the modulation period of the pre-biased local oscillator module is the same as the modulation period of the transmission signal source module, and the modulation phase of the pre-biased local oscillator module lags behind the modulation phase of the transmission signal source module.

[0025] Preferably, the lag time is determined according to the minimum height to be measured and the parameters of the frequency modulation signal.

[0026] Preferably, it also includes a power amplification module, which is used to amplify the radio frequency signal of the transmission signal source and connect it to the transmission antenna through a transmission feeder.

[0027] Preferably, the transmitting feeder can be eliminated and the transmitting part is designed as an integrated unit.

[0028] Preferably, it also includes a receiving feeder for connecting the receiving antenna and the mixer module. The receiving feeder can be eliminated and the receiving part is designed in an integrated manner.

[0029] Preferably, the calculation circuit module calculates the current altitude value by processing the intermediate frequency signal generated by the mixer module and using the basic altitude measurement formula of the FM radio altimeter.

[0030] Preferably, the transmission signal source module and the pre-biased local oscillator module both adopt a fractional frequency synthesizer based on a high-performance PLL platform, and the fractional frequency synthesizer includes HMC703LP4E.

[0031] Preferably, the fractional frequency synthesizer is configured such that the user writes the start frequency, end frequency and modulation rate parameters of the modulation signal into the modulated signal, and starts modulating the rising and falling edges of the triangle wave under the control of the TRIG signal.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. The present invention effectively solves the problem that the traditional altimeter has a low intermediate frequency and cannot accurately measure at low altitudes by introducing a pre-biased local oscillator signal; the use of the pre-biased local oscillator signal increases the intermediate frequency fb by a fixed pre-biased frequency f, thereby ensuring that the intermediate frequency is high enough even at low altitudes to complete the measurement within one modulation cycle; the minimum altitude measurement range of the altimeter is significantly reduced, and its altitude measurement accuracy and reliability in low-altitude flight or landing stages are improved;

[0034] 2. The present invention eliminates or greatly shortens the RF feed line according to actual needs through the design of pre-biased local oscillator signals, thereby simplifying the system structure, reducing the complexity and cost of the system, and significantly reducing the volume and weight, making the altimeter more suitable for small or micro aircraft with strict requirements on volume and weight;

[0035] 3. The elimination or shortening of the RF feeder of the present invention not only reduces the volume and weight, but also reduces the feeder loss, thereby increasing the transmission power; higher transmission power means stronger signal strength and longer transmission distance, which is of great significance for improving the measurement accuracy and anti-interference ability of the altimeter; reducing link loss also helps to reduce the overall power consumption of the system and extend the flight time of the aircraft;

[0036] 4. The technical solution of the present invention is improved based on the existing frequency modulated continuous wave radio altimeter, and can be realized by adding a pre-biased local oscillator signal module and corresponding control logic; it does not involve complex hardware or software design, so it is easy to implement and promote application; the altimeter maintains an operating frequency of 4200 to 4400 MHz, making it widely applicable and can be applied to various types of aircraft. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0038] Figure 1 This is the simplest principle block diagram of the radio altimeter;

[0039] Figure 2 It is a schematic diagram of the transmitted signal and the echo signal;

[0040] Figure 3 It is a schematic diagram of the transmission signal, echo signal and biased local oscillator of the present invention;

[0041] Figure 4 It is a principle block diagram of the present invention;

[0042] Figure 5 This is an engineering implementation diagram of the radio frequency source part of the present invention;

[0043] Figure 6 This is an engineering implementation diagram of the radio frequency source part of the present invention;

[0044] Figure 7 It is a schematic diagram of module functions of the present invention;

[0045] Figure 8 Schematic diagram of the triangle wave modulation signal of the present invention. DETAILED DESCRIPTION

[0046] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0047] Embodiment 1:

[0048] Reference Figure 3 and Figure 4 , a radio altimeter with a pre-biased local oscillator provided by the present invention comprises:

[0049] Transmitting signal source module: used to generate frequency modulated continuous wave signal;

[0050] Pre-biased local oscillator module: used to generate a pre-biased frequency modulated continuous wave local oscillator signal that is synchronized with the transmitting signal source module but has a phase lag;

[0051] RF control module: used to control the transmission signal source module and the pre-biased local oscillator module;

[0052] Mixer module: used to mix the pre-biased local oscillator signal with the received ground echo signal to generate an intermediate frequency signal containing the pre-biased frequency;

[0053] Calculation circuit module: used to calculate the current height value based on the intermediate frequency signal.

[0054] The transmission signal source module and the pre-biased local oscillator module work under the same clock source, and the two work synchronously. By inputting different modulation parameters into the local oscillator module and the transmission signal source module, the modulation period of the pre-biased local oscillator module is the same as the modulation period of the transmission signal source module, and the modulation phase of the pre-biased local oscillator module lags behind the modulation phase of the transmission signal source module. The lag time is determined according to the minimum height to be measured and the parameters of the frequency modulation signal.

[0055] The radio altimeter with pre-biased local oscillator also includes a power amplifier module, which is used to amplify the radio frequency signal of the transmission signal source and connect it to the transmission antenna through a transmission feeder. The transmission feeder can be eliminated, and the transmission part is designed in an integrated manner.

[0056] The radio altimeter with pre-biased local oscillator also includes a receiving feeder for connecting the receiving antenna and the mixer module, and the receiving feeder can be eliminated, and the receiving part is designed in an integrated manner. After the transmitting feeder and the receiving feeder are eliminated, the antenna can be directly installed on the RF port of the transmitter and the receiver without the need for feeder connection, thereby achieving the purpose of integrated design.

[0057] The calculation circuit module processes the intermediate frequency signal generated by the mixer module and calculates the current altitude value using the basic altitude measurement formula of the FM radio altimeter. Both the transmitting signal source module and the pre-biased local oscillator module use a fractional frequency synthesizer based on a high-performance PLL platform, and the fractional frequency synthesizer includes HMC703LP4E. The user writes the start frequency, end frequency, and modulation rate parameters of the modulation signal into the fractional frequency synthesizer, and starts the modulation of the rising and falling edges of the triangle wave under the control of the TRIG signal.

[0058] Embodiment 2:

[0059] The present invention includes the following modules:

[0060] Transmitting signal source module: used to generate the frequency modulated continuous wave signal of the transmitter.

[0061] Pre-biased LO: A pre-biased FM CW LO signal used for use with a mixer.

[0062] RF control: controls the transmitting signal module and the pre-bias module.

[0063] Mixer: Mixes the biased local oscillator signal with the received ground echo signal to generate an intermediate frequency fb signal, which contains the pre-biased frequency.

[0064] Power amplification: Amplify the RF signal from the transmitting signal source.

[0065] Transmitting feeder: used to connect the power amplifier and the transmitting antenna. The feeder can also be eliminated to achieve an integrated design.

[0066] Transmitting antenna: transmits the frequency modulated continuous signal.

[0067] Receiving antenna: Receives ground echo signals.

[0068] Receiving feeder: connects the receiving antenna and mixer. The feeder can also be eliminated to achieve an integrated receiving design.

[0069] fb signal amplifier: amplifies the fb signal of the mixer.

[0070] Calculation circuit: calculate the current height value.

[0071] The present invention changes the method of coupling a part of the transmitter signal as the mixer local oscillator signal in the traditional frequency modulation continuous wave radio altimeter, and replaces the previous method of increasing the remaining height by lengthening the cable in the altimeter and increasing the fb signal to reduce the minimum measurement height of the altimeter by adding a pre-biased local oscillator source. The cable can be greatly shortened or even eliminated to maximize the reduction of the volume and weight of the altimeter, obtain higher transmission power, and improve the performance of the altimeter.

[0072] First, the transmitting signal source and the pre-biased local oscillator work on the same clock source to ensure that the two work synchronously. Under the control of the RF control module, the transmitting signal source and the pre-biased local oscillator simultaneously send out a RF signal with the same △F, the same and synchronous Tm, and the same modulation method, but the phase of the modulation period Tm of the pre-biased local oscillator lags behind the phase of the transmitting signal source for a period of time. Figure 3 As shown in the figure, since the local oscillator signal is delayed for a period of time, when the transmitting signal source sends a signal with a frequency of F, the pre-biased local oscillator synchronously sends a signal with a frequency of Ff. The signal sent by the transmitting signal source is reflected by the ground and returns to the altimeter after a period of τ=2H / C. The pre-biased local oscillator in the aircraft continues to sweep the frequency at a speed of △F / Tm based on Ff. At this time, the frequency is F-f+2(H / C)*(ΔF / Tm). The mixed signal obtained by the mixer is fb=(2H / C)*(ΔF / Tm)+f.

[0073] After sorting, we get:

[0074]

[0075] It can be seen that compared with the original formula, the intermediate frequency at this time has increased a pre-bias frequency f, thus avoiding the problem mentioned above that the fb frequency is too low to be measured at low altitudes, lowering the minimum measurement altitude of the altimeter, achieving the purpose of shortening or eliminating the RF feeder, reducing the volume and weight, and at the same time reducing the feeder loss, and obtaining additional greater transmission power.

[0076] The system of the present invention is simple and easy to implement; it can effectively reduce the minimum height measurement range of the frequency modulated continuous wave altimeter; shorten or cancel the original radio frequency feeder, reduce link loss, and increase transmission power. The present invention maintains the operating frequency of the altimeter at 4200-4400MHz, making it widely applicable. It ensures that the measurement of the intermediate frequency is completed within one modulation cycle. The length of the radio frequency cable is canceled or reserved according to actual needs, the volume and weight are reduced, and an integrated design can be achieved.

[0077] The HMC703LP4E fractional frequency synthesizer is based on a high performance PLL platform that provides the industry's best phase noise and spurious performance, enabling higher order modulation schemes while minimizing the blocking effects of high performance radios.

[0078] In addition, the HMC703LP4E also provides sweep and modulation, external triggering, double buffering, precise frequency control, phase modulation and other features. The precise frequency mode using a 24-bit fractional modulator can produce fractional frequencies with zero frequency error and very low channel spurious.

[0079] In this system, the modulation signal of the altimeter frequency-modulated continuous wave adopts triangular wave modulation, which requires the signal modulation phase of the transmitter to be ahead of the phase of the local oscillator for a period of time. This requires that the modulation signals of the two are synchronized. When the triangular wave is in the rising stage, the transmitter frequency is higher than the local oscillator signal by f, and when the triangular wave is in the falling stage, the transmitter frequency is lower than the local oscillator signal by f.

[0080] HMC703LP4E allows the user to write the starting frequency, ending frequency, modulation rate and other parameters of the triangle wave modulation signal, and start the rising edge and falling edge under the control of the TRIG signal. According to the above characteristics, the corresponding start frequency, end frequency and modulation rate are written to the transmitter through MCU, and the start frequency written to the local oscillator is f higher than the transmitter parameters, and the end frequency is f lower than the transmitter parameters respectively, and the modulation rate is the same as the transmitter. When the transmitter and the local oscillator receive the same TRIG signal to start the rising edge modulation of the triangle wave, since the start frequency of the local oscillator signal is f higher than that of the transmitter, the frequency of the local oscillator is always f higher than that of the transmitter in the entire rising cycle of the triangle wave. It can be seen from the figure that this method realizes that the transmitter signal is ahead of the local oscillator in the phase of the triangle wave. After the rising edge modulation is completed, the end frequency at this time is the start frequency of the falling edge. Since the end frequency of the local oscillator is f lower than the transmitter parameter, the starting frequency of the falling edge of the local oscillator is f lower than the starting frequency of the transmitter. When the two HMC703LP4E are controlled by the TRIG signal at the same time again to start the triangle wave downward sweep, the frequency of the local oscillator is always f lower than that of the transmitter in the entire rising cycle of the triangle wave, realizing that the transmitter signal is ahead of the local oscillator in the phase of the falling cycle of the triangle wave. Through the above method, the phase of the transmitter is ahead of the local oscillator source for a period of time during the entire rising and falling stages of the triangle wave, thereby achieving the purpose of the present invention.

[0081] Those skilled in the art may understand this embodiment as a more specific description of Embodiment 1.

[0082] Those skilled in the art know that, in addition to realizing the system and its various devices, modules, and units provided by the present invention in a purely computer-readable program code, it is entirely possible to realize the same functions in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system and its various devices, modules, and units provided by the present invention can be considered as a hardware component, and the devices, modules, and units included therein for realizing various functions can also be regarded as structures within the hardware component; the devices, modules, and units for realizing various functions can also be regarded as both software modules for realizing the method and structures within the hardware component.

[0083] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essence of the present invention. In the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A radio altimeter with a pre-biased local oscillator, characterized in that: include: Transmitting signal source module: used to generate frequency modulated continuous wave signal; Pre-biased local oscillator module: used to generate a pre-biased frequency modulated continuous wave local oscillator signal that is synchronized with the transmitting signal source module but has a phase lag; RF control module: used to control the transmission signal source module and the pre-biased local oscillator module; Mixer module: used to mix the pre-biased local oscillator signal with the received ground echo signal to generate an intermediate frequency signal containing the pre-biased frequency; Calculation circuit module: used to calculate the current height value based on the intermediate frequency signal.

2. The radio altimeter with pre-biased local oscillator according to claim 1, characterized in that: The transmission signal source module and the pre-biased local oscillator module work under the same clock source, and the two work synchronously.

3. The radio altimeter with pre-biased local oscillator according to claim 1, characterized in that: The modulation period of the pre-biased local oscillator module is the same as the modulation period of the transmission signal source module, and the modulation phase of the pre-biased local oscillator module lags behind the modulation phase of the transmission signal source module.

4. The radio altimeter with pre-biased local oscillator according to claim 3, characterized in that: The lag time is determined according to the minimum height to be measured and the parameters of the frequency modulation signal.

5. The radio altimeter with pre-biased local oscillator according to claim 1, characterized in that: It also includes a power amplifier module, which is used to amplify the radio frequency signal of the transmitting signal source and connect it to the transmitting antenna through a transmitting feeder.

6. The radio altimeter with pre-biased local oscillator according to claim 5, characterized in that: The transmitting feeder can be eliminated, and the transmitting part is of integrated design.

7. The radio altimeter with pre-biased local oscillator according to claim 1, characterized in that: It also includes a receiving feeder line for connecting the receiving antenna and the mixer module. The receiving feeder line can be eliminated, and the receiving part is of integrated design.

8. The radio altimeter with pre-biased local oscillator according to claim 1, characterized in that: The calculation circuit module calculates the current altitude value by processing the intermediate frequency signal generated by the mixer module and using the basic altitude measurement formula of the FM radio altimeter.

9. The radio altimeter with pre-biased local oscillator according to claim 1, characterized in that: The transmission signal source module and the pre-biased local oscillator module both adopt a fractional frequency synthesizer based on a high-performance PLL platform, and the fractional frequency synthesizer includes HMC703LP4E.

10. The radio altimeter with pre-biased local oscillator according to claim 9, characterized in that: The fractional frequency synthesizer is configured by the user to write the start frequency, end frequency and modulation rate parameters of the modulation signal, and starts the modulation of the rising and falling edges of the triangle wave under the control of the TRIG signal.

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