FMCW Radar Altimeter Interference Prevention via Dynamic Frequency
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Solution Overview
Problem
Radar altimeters face significant challenges in preventing mutual interference between identical devices, leading to erroneous altitude readings or loss of signal lock, which is particularly dangerous in safety-critical applications like final approach guidance or terrain avoidance.
Innovation Solution
The implementation of a pseudorandom noise sequence generator to vary the start and stop frequencies of frequency-modulated continuous wave (FMCW) ramps in radar altimeters, using a field-programmable gate array (FPGA) to generate integer values that index frequency tables, ensuring constant range resolution and simplified signal processing while minimizing interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple radar altimeters of the same type are installed on the same aircraft for redundancy, then system reliability is improved, but mutual interference between devices occurs causing erroneous altitude readings or loss of signal lock
Solution Approach 1:
The patent applies dynamics by making the FMCW ramp parameters variable rather than fixed. Specifically, the ramp start frequency, stop frequency, and/or ramp rate are dynamically adjusted across different ramp cycles using pseudorandom selection from predefined sets of values. This dynamic variation ensures that while multiple altimeters operate simultaneously, their frequency-time characteristics differ, preventing coherent interference and false signal tracking, thus maintaining system reliability without mutual interference
Solution Approach 2:
The patent implements parameter changes by modifying key FMCW ramp parameters including start frequency, stop frequency, and ramp rate. These parameters are selected from predefined sets using pseudorandom noise sequences, ensuring that each altimeter or each ramp cycle uses different parameter combinations. This parameter diversification allows multiple altimeters to coexist without causing harmful interference, as the frequency sweeps no longer overlap in identical patterns
2Object-affected harmful factors
If sweep rate is made sufficiently different from other altimeter products, then susceptibility to interference from different type products is reduced, but this approach is insufficient for preventing mutual interference between altimeters of the same type
Solution Approach 1:
The patent extends the static sweep rate differentiation approach by making all FMCW ramp parameters dynamic. Instead of simply using different fixed sweep rates, the system dynamically varies start frequency, stop frequency, and ramp rate across ramp cycles using pseudorandom selection. This provides both intra-type and inter-type interference mitigation, as the varying parameters ensure no two altimeters (regardless of type) will have identical frequency-time characteristics
Solution Approach 2:
The patent generalizes the parameter differentiation approach by applying parameter changes across multiple dimensions: start frequency, stop frequency, and ramp rate. These parameters are selected from predefined sets using pseudorandom noise sequences, creating a comprehensive parameter space that prevents interference from both same-type and different-type altimeters. This multi-parameter approach is more versatile than simple sweep rate differentiation
Data Source
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AI summary
Systems and methods of operating an interference prevention system in a radar altimeter are provided. A method includes generating integer values with at least one pseudorandom noise sequence generator. The generated integer values are used as indexes to select at least one of start frequencies and stop frequencies from at least one frequency table for frequency modulated continuous wave (FMCW) ramps for a generated radar signal. The selected at least one of the start frequencies and the stop frequencies is provided to a frequency synthesizer. The at least one of start frequencies and the stop frequencies are used in generating transmit frames of the radar signal with the frequency synthesizer.