**1 Introduction**
This paper presents a novel frequency-modulated continuous wave (FM CW) altimeter designed for high-precision altitude measurement. The key innovations include the use of an FPGA/SCM hardware platform, which offers strong versatility and the ability to perform field software upgrades. This system enables advanced signal processing through software algorithms, such as high search capability, high tracking performance, STC (sensitivity time control), and AGC (automatic gain control). By modifying the signal processing algorithms and control software, the altimeter can support multiple functions, making it suitable for a wide range of applications.
The altimeter employs a constant beat structure, which enhances its anti-interference capability and allows for spectrum front tracking. It has a maximum range of 1500 meters and achieves an accuracy of 1 meter at low altitudes. In addition to measuring the relative height between the aircraft and the ground or sea surface, this device can also assess parameters like surface roughness and ocean wave height. As a result, it is widely used in areas such as automatic landing systems, navigation, and terrain matching.
Radio altimeters are typically divided into two categories: frequency modulation continuous wave (FM CW) systems and pulse systems. FM CW systems are ideal for low-altitude operations up to 1500 meters, while pulse systems are more suitable for medium and high-altitude applications. This paper introduces a linear FM CW altimeter based on an FPGA/MCU architecture, offering high precision, a simple design, excellent reliability, and cost-effectiveness.
**2 Working Principle**
The basic principle of the linear frequency modulation continuous wave altimeter involves using a triangular wave modulated microwave oscillator. The transmitted signal is amplitude-modulated and sent toward the ground. Upon reflection, it is received with a time delay proportional to the aircraft’s altitude. After mixing, the beat frequency $ f_b $ is generated, which is then filtered, amplified, and processed by a gain-controlled amplifier before being sent to a tracking discriminator.
The tracking discriminator determines whether the beat signal falls within the tracking band. If so, it outputs a lock signal, allowing the system to maintain a stable measurement. The control unit adjusts the slope of the triangular wave to ensure the beat signal remains within the tracking range. This creates a closed-loop system where the aircraft's altitude is determined by the slope of the triangular wave and the maximum frequency deviation $ \Delta f $.
Key performance specifications include:
- Operating frequency: C-band
- System type: LFMCW (Linear Frequency Modulated Continuous Wave)
- Altitude range: 0–1500 m
- Distance resolution: 1 m
- Height data interface: RS422, 9600 baud rate
This altimeter operates using a separated transceiver antenna configuration, triangle wave frequency modulation, spectrum leading edge tracking, and a constant difference structure. Its working principle is illustrated in Figure 1.
**Figure 1: Schematic diagram of the chirp altimeter**
In the schematic, the triangular wave generator produces a constant amplitude signal whose slope is controlled by a voltage. During operation, the slope of the modulated signal is adjusted automatically to keep the beat frequency constant. As the altitude increases, the slope increases, and as the altitude decreases, the slope reduces. The tracking discriminator checks if the beat frequency is within the tracking band. If it is, a lock signal is output. The discriminator has a center frequency of 225 kHz and a narrow bandwidth of about 30 kHz, improving noise immunity.
When no lock signal is detected, the system enters a search mode, scanning the slope of the triangular wave from the lowest to highest altitude. Once the correct slope is found, the system locks onto the signal and continues tracking.
The transceiver uses a microstrip integrated panel antenna with a spacing of at least 1 meter, ensuring a transceiver isolation of over 70 dB. The antenna has a 3 dB bandwidth of 300 MHz, a sidelobe level of no more than -12 dB, a standing wave ratio of S=2, and an efficiency of approximately 80%. The total size is no larger than 15 cm × 15 cm.
The transceiver component uses a self-difference structure to produce a zero intermediate frequency beat signal, which is proportional to the ground height. The VCO has a modulation bandwidth of up to 200 MHz and a linearity better than 1.2%. The receive gain is 30 dB, and the noise figure is 3.5 dB. The video display unit performs frequency-selective amplification, with a total gain of at least 80 dB and a gain control range of no less than 90 dB.
A custom mechanical filter centered at 225 kHz with a 30 kHz bandwidth is used for frequency selection. The main amplifier utilizes AD's video amplifier AD*, which integrates two amplifier modules. These modules can be used separately or together to increase gain and expand the dynamic range. Each module can achieve a maximum gain of 54.4 dB, with a gain control range of 48.4 dB.
**3 Signal Processing Components**
**3.1 Hardware Design**
The signal processing component is responsible for ground height search/tracking, AGC, STC, and other functions. The circuit block diagram is shown in Figure 2. At the core of the system is an FPGA and an MCU, with most signal processing functions implemented via software algorithms.
**Figure 2: Signal processing component circuit block diagram**
During ground level search and tracking, the FPGA and MCU adjust the VCO frequency according to a specific algorithm, ensuring that the beat signal falls within the 225 kHz tracking band. During the search phase, the VGC voltage varies logarithmically with altitude, enabling STC. During tracking, the VGC voltage is controlled by the saturation threshold decision circuit. When saturation occurs, the VGC voltage is reduced until the beat signal intensity drops below the threshold, minimizing the impact of ground echo strength on measurement accuracy. This process ensures effective AGC functionality.
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