Stepped Chirp Lidar Cloud Scatter Noise Removal
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Solution Overview
Problem
Continuous wave (CW) lidar systems used for CO2 spectroscopic analysis face challenges in accurately measuring atmospheric CO2 concentrations due to cloud scattering noise, which is indistinguishable from ground-scattered signals, leading to underestimation of column density.
Innovation Solution
A stepped chirp lidar system is introduced, using synchronized stepped chirp waveforms for online and offline signals, along with a pseudo-random noise modulated signal, to separate and remove cloud scatter noise, allowing for accurate assessment of CO2 column density by processing the returned signals with a lock-in amplifier and regression analysis.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a continuous wave (CW) lidar system is used for CO2 spectroscopic analysis, then the system lifetime is improved due to more reliable technology, but cloud scattering noise cannot be easily separated from ground scattered signal leading to measurement precision degradation
Solution Approach 1:
The patent applies periodic action by using pulsed modulation of the CW laser at specific frequencies (e.g., 10-100 kHz). The laser is modulated to create periodic on-off cycles, allowing the system to distinguish between cloud scatter (which occurs during pulse periods) and ground scatter (which occurs at predictable intervals). This periodic modulation enables frequency-domain separation of noise components while maintaining CW system reliability.
Solution Approach 2:
The patent implements dynamics by introducing time-varying modulation schemes where the laser frequency is swept or chirped over time. The modulation frequency and pulse width are dynamically adjusted to optimize the separation between cloud scatter and ground scatter signals. This dynamic approach allows the system to adapt to different atmospheric conditions while maintaining measurement precision.
2Measurement precision
If a pulsed laser system is used, then cloud scatter noise can be more easily separated from ground scattered signal improving measurement precision, but system reliability deteriorates due to less reliable pulsed laser technology
Solution Approach 1:
The patent applies universality by using a CW laser that can perform both continuous operation (providing reliability) and pulsed modulation (enabling cloud scatter separation). The CW laser is modulated with pseudo-random binary sequences (PRBS) or other coding schemes, allowing it to function as both a continuous source and a pulsed system. This multi-functionality resolves the contradiction by combining the advantages of both CW and pulsed systems in a single platform.
3Quantity of substance
If cloud scatter noise is present in CO2 retrieval, then the light path length through clouds is shorter than ground scattered light, but this leads to underestimation of column density due to noise contamination
Solution Approach 1:
The patent introduces an intermediary approach by using correlation detection with known modulation codes (e.g., PRBS sequences). The received signal is correlated with the transmitted code to extract the ground scatter component, while the cloud scatter (which lacks the coded modulation) appears as noise that can be filtered out. This intermediary correlation process separates the useful signal from noise based on their different temporal characteristics.
Solution Approach 2:
The patent implements feedback by using the detected cloud scatter noise level to adjust the retrieval algorithm. The system continuously monitors the noise characteristics and adjusts the weighting between different path length contributions in the inversion algorithm. This feedback mechanism ensures that the shorter cloud path length does not lead to systematic underestimation of the total column density.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system effectively removes cloud scatter noise, providing accurate CO2 column density measurements with improved signal-to-noise ratio, enabling precise characterization of atmospheric CO2 concentrations even in the presence of cloud layers.
Implementation Method 1
An online laser transmits the online signal and an offline laser transmits the offline signal
Implementation Method 2
A modulator is included for modulating the online signal with a first stepped chirp waveform and modulating the offline signal with a second stepped chirp waveform
Implementation Method 3
A lock-in amplifier for (a) multiplying a returned signal with the first stepped chirp waveform to obtain a detected online signal, and (b) multiplying the returned signal with the second stepped chirp waveform to obtain a detected offline signal
Implementation Method 4
The lidar system relies on the ratio between light scattered from the ground at two slightly different wavelengths around the 1.57 μm is CO2 absorption line
Implementation Method 5
The processor is configured to assess the column of gas by performing regression between the detected online and offline signals and the simulated online and offline signals
Data Source
AI summary
A system assesses a column of gas in an atmosphere. The system includes an optical transmitter for transmitting (a) an online signal tuned to an online wavelength of gas, (b) an offline signal tuned to an offline wavelength of the gas, and (c) an optical signal for providing altimetry data. A modulator is included for modulating the online signal with a first stepped chirp waveform and modulating the offline signal with a second stepped chirp waveform. A processor assesses the column of gas, after receiving a returned online signal, a returned offline signal and a returned optical signal. A lock-in amplifier is included for (a) multiplying a returned signal with the first stepped chirp waveform to obtain a detected online signal, and (b) multiplying the returned signal with the second stepped chirp waveform to obtain a detected offline signal. The processor is configured to remove ground reflections from the optical signal to obtain scattered noise, and assess the column of gas based on the scattered noise. The processor is also configured to (a) multiply an estimated returned online signal with the first stepped chirp waveform to obtain a simulated cloud only online signal, and (b) multiply an estimated returned offline signal with the second stepped chirp waveform to obtain a simulated cloud only offline signal. In addition, the processor is configured to assess the column of gas by performing regression between the detected online and offline signals and the simulated online and offline signals.