Dynamic Test System of Radio Altimeter Based on Virtual Instrument Technology

O Introduction
The radio altimeter is an important measuring element of cruise anti-ship missiles, and its performance determines the control quality of the missile's longitudinal trajectory. For missiles flying at ultra-low altitude and skimming the sea, the spectrum structure of the radio altimeter's reflected echo is relatively complex. When the missile moves, there is relative movement between each point, which makes the Doppler frequency of each reflection point different, and it is possible to form a beat (called the secondary Doppler effect) at the receiver, resulting in the spectrum broadening of the synthetic echo signal; the fluctuation of the echo signal amplitude can also form a modulation component of the spectrum. The cross-modulation of signal clutter will distort the spectrum purity and waveform of the beat signal, resulting in counter counting errors, thereby causing the altimeter to output an incorrect altitude, and in severe cases, causing the missile to enter the water prematurely.
Starting from the analysis and research of the working principle of the radio altimeter, this paper completes the overall design of the radio altimeter dynamic test system based on virtual instruments. This scheme can fully simulate the dynamic working process of the radio altimeter and detect altimeter failures in advance.

1 Principle of radio altimeter
There are two working modes of radio altimeter: pulse and frequency modulated continuous wave. At present, foreign countries basically use pulse mode, while domestic countries basically use frequency modulated continuous wave mode. There are three modulation modes of frequency modulated continuous wave: sine wave modulation, triangle wave modulation, and sawtooth wave modulation. At present, the modulation mode used by radio altimeters used on domestic anti-ship missiles is sawtooth wave modulation.
Figure 1 illustrates the basic principle of radar altimeter working in sawtooth wave modulation mode to obtain beat signal, assuming that the transmitting signal is sawtooth frequency modulated continuous wave. Tm is the frequency modulation period, and its value is much larger than the target echo delay Tr at the maximum effective distance. B is the frequency modulation bandwidth, fo is the center frequency, k=B/T is the frequency modulation slope, and the target movement is not considered, that is, the radial velocity v=0 of the target. Figure 1(a) shows the frequency-time relationship of the signal. The solid line represents the transmitted signal and the dotted line represents the echo signal. The waveform is similar to the transmitted signal, except that there is a time delay of Td=2H/c relative to the transmitted signal. This time delay is proportional to the actual height of the missile relative to the ground/sea surface. By measuring the time delay, the purpose of measuring the height can be achieved.

The transmission signal is mixed with the echo signal to obtain the beat signal. Figure 1(b) shows the frequency-time relationship of the beat signal fb. It can be seen from the figure that the beat signal has two constant frequency segments AB and BC. The frequency of the BC segment is B-fb and the time width is Td. The AB segment is the effective segment with a time width of Tm-Td, which accounts for the main component in the beat signal. It can be obtained from Figure 1(b) that the beat signal frequency in the effective segment is:

That is, the beat signal frequency within the effective period is proportional to the echo delay (or the distance to the target), so the distance to the target can be obtained by only obtaining the beat signal frequency within the effective period.
In the radar altimeter, the echo signal comes from the entire sea surface illuminated by the antenna beam. It is usually assumed that this surface contains a large number of independent random scatterers, and the scattering center of each scatterer is at a different distance from the radar altimeter transmitting antenna. In this way, the total echo signal is equivalent to the synthesis of a large number of echoes with different delays and weighted in amplitude by the scattering coefficient σ and the antenna pattern.
The beat signal output (after low-pass filtering) formed by the i-th scatterer echo is: After analysis, it can be obtained that in the case of a surface target, when the sawtooth wave is modulated, the i-th scattering unit echo spectrum is: [page]

2 Overall design
The altimeter dynamic test system can complete the dynamic detection of the altimeter height measurement process and the static detection of sensitivity. It consists of an adapter, a computer, a data acquisition card, an I/Q modulator, a fixed attenuator, a variable attenuator, a ~115 V/400 Hz power supply, a 27 V DC power supply and related test software. The system structure is shown in Figure 2.

The adapter is used to complete the docking with the altimeter, realize the signal excitation, conditioning and matching of the altimeter to be measured, receive the power supply voltage, beat signal and altitude signal from the altimeter, and provide ~115 V/400 Hz or DC power supply.
The altitude simulator is used to simulate the continuous change of the missile altitude. The I/Q modulator with a suitable working range is selected by referring to the central working frequency of the radio altimeter frequency modulation continuous wave. According to the sawtooth wave modulation law and the altitude trajectory to be simulated, the frequency of the sinusoidal excitation function input to the I/Q modulator is reasonably controlled by software, and the microwave signal from the altimeter transmitter is modulated to generate a beat frequency to simulate the constant altitude and variable altitude trajectory, and the transmitter frequency is disturbed to expand its spectrum, and the clutter signal is simulated to dynamically test the altimeter.
The microwave attenuator is used to control the change in energy. The two-stage attenuator is connected in series in the transmitting and receiving channels to match the altimeter transmitter, I/Q modulator and receiver. The variable attenuator is used in conjunction with the fixed attenuator to measure the altimeter status signal through the data acquisition system to detect the system sensitivity.
The computer and software are responsible for coordinating the work of the system, controlling the working status of the data acquisition system and the generator function. The data acquisition system collects the waveform and height voltage value of the beat signal in real time and sends it to the computer for waveform analysis and spectrum processing. The computer controls the generator function to generate the orthogonal functions of y=AIsinωt and y==AQsinωt as the input of the I/Q modulator.

3 Dynamic test system based on PCI-6229 data acquisition card
3.1 Brief introduction of PCI-6229 data acquisition module
PCI-6229 is a new M series DAQ card launched by NI, with A/D, D/A, I/O, Count/Timers functions, 32 single-ended analog input channels, 16 differential channels, 16-bit resolution, maximum sampling rate of 250 KS/s, input FIFO buffer for 4 095 samples, data transmission supports DMA, interrupt, and programmable modes; there are 4 analog output channels; there are 48 I/O channels, 32 of which have waveform output capability, and the software can read 32-bit long port status at a time; there are 2 timers/counters with a resolution of 32 bits and an internal clock frequency of 80 MHz.
3.2 System hardware composition
The test system hardware consists of two parts: transmitting and receiving. The computer transmits the control signal to the variable attenuator and the orthogonal function to the I/Q modulator through the AO channel of the data acquisition system. Then the computer collects the altitude voltage, beat signal, transmission signal, reception signal and working status of the radio altimeter through the AI ​​channel for analysis and processing. The structural block diagram is shown in Figure 3, where the I/Q modulator uses ADI's ADL5375 with an operating frequency of 400MHz to 6 GHz. The peripheral circuit diagram is shown in Figure 4.

4 Test system software design
4.1 Acquisition card settings and measurement task configuration
First, install the data acquisition card on the computer, right-click on the Data Neighborhood icon in MAX and select Create New…, select Taditional NI-DAQ virtual in the directory and press the Next button, then you can configure a channel to read the input signal. After pressing the Next button, a data acquisition card property setting window will appear. In this window, you can set analog input, analog output, digital I/O, etc. according to the use of the board, and then name the task.
4.2 Generation of I/Q modulator signal
When testing the altitude response of the altimeter, it is necessary to send a sinusoidal signal with a certain frequency range to the I/Q modulator. Use the SinePattern function in LabWindows/CVI to output the sinusoidal signal.
4.3 Sampling and reading of test signals
The signal first passes through the preamplifier and then enters the computer through the AI ​​channel of the data acquisition card, where it is stored and then displayed. In order to keep the sampling speed and display from conflicting, the interrupt method is used to read the sampling data. To implement the interrupt processing method, two main tasks are required: one is to write an interrupt processing program, and the other is to register this program with the system. [page]

In the interrupt service program, use the DAQmxReadAnalogF64() function to read the data in the buffer on the acquisition card. The user must apply for a buffer of sufficient size before starting the acquisition task, use a global pointer to point to the buffer, select the data connection as the channel connection mode, and control the selection of different channel data through the memcpy[] function, then process the data, and release the buffer after the task ends.
4.4 Waveform storage and reading
After acquisition, the data can be saved in real time as a binary file (compatible with missile telemetry data). The fopen function is used to call the text data file, the fwrite function is used to write the data, the fread function is used to read the data, and the fclose function is used to close it. Select the Graph control and call back the data read into the buffer through the plotwaveform function.
4.5 Frequency domain analysis of test signals
In LabWindows/CVI, the Fourier function is used to perform Fourier transform on the waveform array, and the output real and imaginary arrays obtained by the Fourier transform are converted into polar coordinates through ToPolarlD. The inverse Fourier transform of the waveform array is realized by InvFFT.
4.6 Software composition
4.6.1 Altitude response test
The altitude response test is used to complete the setting of altitude trajectory parameters (duration of different trajectory stages), the selection of bullet type and the display of altitude response curve, as shown in Figures 5 and 6.

4.6.2 Signal Analysis
The signal analysis completes the playback and comparison of the transmitted signal, received signal, and beat signal waveforms, and performs Fourier transform on the beat signal and analyzes its spectrum to determine whether the altimeter has a "high setting" fault. The interface is shown in Figure 7.

4.6.3 Sensitivity test
The altimeter sensitivity test requires setting the set altitude and the attenuation control law to complete the altimeter search/tracking sensitivity test at different altitudes, see Figure 9.

5 Conclusion
Virtual instrument technology has quickly occupied the market with its advantages of high cost performance and strong openness, becoming a new economic point for test instruments. The core of virtual instruments is software, which makes virtual instruments have technical characteristics that are very different from traditional test instruments, and realizes the openness of instruments that test instrument manufacturers and users dream of. This paper studies and designs the third generation of automatic test equipment based on virtual instrument technology for the radio altimeter test example. However, the system only realizes the automatic measurement of basic parameters, and a complete automatic test system needs to add more automation levels, such as remote control, update and upgrade, fault diagnosis and other functions. In this regard, in-depth research is needed to make the test system more complete.