**1 Introduction**
This paper presents a novel type of frequency-modulated continuous wave (FM CW) altimeter designed for high-precision altitude measurement in various applications. The system is built on an FPGA/SCM hardware platform, offering strong versatility and the ability to perform field software updates. By leveraging advanced software algorithms, it achieves high search and tracking capabilities, along with STC (sensitivity time control) and AGC (automatic gain control). This flexibility allows for easy modification of signal processing algorithms and control software, enabling the system to adapt to different operational scenarios.
The altimeter employs a constant beat structure, which enhances its resistance to interference and enables spectrum front tracking. It has a wide altitude range of up to 1500 meters and maintains high accuracy—within 1 meter at low altitudes. In addition to measuring relative height between the aircraft and the ground or sea surface, it can also estimate parameters such as surface roughness and ocean wave height. These features make it widely applicable in areas like automatic landing systems, navigation, and terrain matching.
Radio altimeters are typically divided into two categories: frequency-modulated 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 an LFMCW (linear frequency modulated continuous wave) altimeter based on an FPGA/MCU architecture. It offers high precision, a simple design, excellent reliability, and cost-effectiveness, making it a promising solution for modern avionics systems.
**2 Working Principle**
The fundamental working 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 through the transmitting antenna. After reflecting off the ground, it is received by the receiving antenna with a time delay proportional to the aircraft’s altitude. The received signal is mixed with the transmitted one, producing a beat frequency (fb), which is then filtered, amplified, and processed.
The beat signal is analyzed by a tracking discriminator, which determines if it falls within the tracking band. If so, a lock signal is generated, and the control unit adjusts the modulation slope to maintain a constant beat frequency. This creates a closed-loop system that continuously tracks the altitude. The aircraft's height is calculated based on the slope of the triangular wave and the maximum frequency deviation (Δf).
Key performance specifications include:
- Operating frequency: C-band
- System type: LFMCW
- Altitude range: 0–1500 m
- Distance resolution: 1 m
- Height data interface: RS422, 9600 baud rate
This altimeter uses a transceiver antenna separation configuration, operates with triangle wave frequency modulation, and employs spectrum leading edge tracking. Its working principle is illustrated in Figure 1.
Figure 1: Schematic diagram of the chirp altimeter. The triangular wave generator produces a signal with a fixed amplitude but variable slope, controlled by a voltage. During operation, the slope adjusts automatically to maintain a constant beat frequency. The tracking discriminator checks if the beat signal is within the tracking band, and if so, locks onto it. When no lock is detected, the system enters a search mode, scanning the full altitude range until a signal is found.
The antennas used are microstrip integrated panel types, with a spacing of at least 1 meter, ensuring transceiver isolation above 70 dB. The antenna has a 3dB bandwidth of 300 MHz, a sidelobe level below -12 dB, a standing wave ratio of S=2, and an efficiency of around 80%. The total size is less than 15 cm × 15 cm.
The transceiver component uses a self-difference structure, generating a zero intermediate frequency beat signal proportional to the aircraft's altitude. The VCO has a modulation bandwidth of up to 200 MHz, with a linearity better than 1.2%. The receive gain is 30 dB, and the noise figure is 3.5 dB. The video display unit amplifies the beat signal with a total gain of at least 80 dB and a gain control range of 90 dB.
The frequency selective filter is a custom mechanical filter with a center frequency of 225 kHz and a bandwidth of 30 kHz. The main amplifier uses AD's video amplifier, integrating two modules that can be used separately or together to increase gain and expand dynamic range. Each module can provide up to 54.4 dB of gain, with a control range of 48.4 dB.
**3 Signal Processing Components**
**3.1 Hardware Design**
The signal processing component handles functions such as ground height search/tracking, AGC, and STC. The circuit block diagram is shown in Figure 2. The core of the system consists of one FPGA and one MCU, with most signal processing tasks performed via software algorithms.
Figure 2: Signal processing component circuit block diagram. The FPGA and MCU control the output state based on the lock threshold decision circuit. The VCO frequency is adjusted according to a specific algorithm to ensure the beat signal remains 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. If the signal becomes saturated, 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 ensures effective AGC performance.
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