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ISO 28902-2:2017 — Air Quality — Ground-Based Remote Sensing of Wind by Heterodyne Pulsed Doppler Lidar
Environmental meteorology — Wind velocity measurement using pulsed Doppler lidar | Atmospheric sensing guide
Introduction to ISO 28902-2
ISO 28902-2:2017 specifies requirements for ground-based remote sensing of wind using heterodyne pulsed Doppler lidar. Developed by ISO/TC 146/SC 5, this standard establishes system specifications, testing procedures, and measurement protocols for pulsed Doppler lidar systems that measure wind velocity by detecting the Doppler frequency shift of backscattered laser radiation from atmospheric aerosols. These systems are widely used in wind energy resource assessment, aviation wind shear detection, and atmospheric research.
Heterodyne pulsed Doppler lidar can measure wind profiles from near ground up to several kilometers with range gates as fine as 10-30 m, providing unprecedented detail for wind resource assessment and atmospheric boundary layer studies.
Measurement Principles
Heterodyne Detection and Spectral Analysis
The standard describes the heterodyne detection principle: a pulsed laser beam (typically 1.5 µm, eye-safe wavelength) is transmitted into the atmosphere, and the backscattered signal is mixed with a local oscillator beam on a photodetector. The resulting beating signal at the difference frequency is digitized and analyzed using Fourier transform to extract the Doppler shift, which is proportional to the radial wind velocity. The standard specifies spectral analysis requirements including windowing functions, peak detection algorithms, and signal-to-noise ratio thresholds.
| Parameter | Specification | Testing Method | Acceptance Criterion |
|---|---|---|---|
| Wavelength | 1.5 µm band (eye-safe) | Wavemeter measurement | ± 0.1 nm |
| Radial velocity accuracy | ≤ 0.1 m/s (10-min average) | Intercomparison with sonic anemometer | Bias ≤ 0.1 m/s, std ≤ 0.5 m/s |
| Range resolution | ≤ 30 m | Hard target return test | FWHM ≤ 30 m |
| Maximum range | ≥ 1 km (typical aerosol) | Signal-to-noise measurement | SNR ≥ -3 dB at max range |
| Data availability | ≥ 90% under typical conditions | Statistical analysis over 1 month | ≥ 90% valid measurements |
| Pointing accuracy | ≤ 0.1° (absolute) | Sun sighting or target survey | ≤ 0.1° |
The carrier-to-noise ratio (CNR) is the primary quality metric for Doppler lidar measurements. The standard recommends a minimum CNR threshold of -22 dB for reliable wind measurement. Data below this threshold should be flagged as low quality.
Engineering Design Insights
System Performance and Trade-offs
The standard describes the critical trade-offs in Doppler lidar design: range resolution vs. maximum range (determined by pulse length and pulse energy), temporal resolution vs. velocity precision (more accumulated pulses improve precision but reduce temporal resolution), and spatial coverage vs. system cost. The figure of merit (FOM) combines laser pulse energy, telescope area, system optical efficiency, and detector quantum efficiency into a single performance metric. A FOM of 10⁶ mJ·m² typically corresponds to a maximum range of 2-3 km under moderate aerosol loading.
For wind energy applications, the standard recommends a minimum measurement height of 200 m (hub height of modern turbines), with range gates spaced at 20 m intervals. The measurement uncertainty at hub height should not exceed 0.5 m/s for 10-minute averages. The standard provides guidance on the velocity-azimuth display (VAD) scanning technique for retrieving horizontal wind speed and direction from radial velocity measurements.
Modern pulsed Doppler lidars achieve 0.1 m/s velocity precision with 30 m range resolution at 2 km range using 100-shot averaging at 10 kHz pulse repetition frequency. The key enabling technology is the 1.5 µm fiber laser, which provides high pulse energy, excellent beam quality, and reliable eye-safe operation.
Testing and Validation
The standard requires intercomparison testing with reference instrumentation (sonic anemometers at multiple heights for vertical profiling, or cup anemometers for horizontal measurements). The intercomparison must cover the full operating range of the lidar, with statistical analysis of bias, standard deviation, and correlation coefficient. The standard also defines testing procedures for maximum operational range validation using hard-target returns and SNR measurement.
Practical Wind Energy Applications
A wind resource assessment campaign for a proposed wind farm in Inner Mongolia used an ISO 28902-2-compliant pulsed Doppler lidar to measure wind profiles from 40 m to 250 m height over a 12-month period. The lidar, operating at 1.5 µm with 50 µJ pulse energy and 10 kHz repetition rate, provided data availability exceeding 95% across all seasons. The range gates were configured at 20 m intervals with 30 m range resolution, providing 11 measurement heights. Comparison with a 100 m meteorological mast equipped with cup anemometers and wind vanes showed excellent agreement: the lidar-measured 10-minute mean wind speeds had a bias of less than 0.1 m/s and a standard deviation of 0.3 m/s at the 100 m hub height.
The campaign revealed significant diurnal wind shear patterns that were not detectable with the mast alone. Nighttime conditions frequently produced high shear (shear exponent α = 0.25-0.40) with low turbulence intensity, while daytime conditions showed lower shear (α = 0.10-0.15) with higher turbulence. This diurnal variation in the wind profile had an 8-12% impact on annual energy production estimates depending on the turbine model, highlighting the value of the lidar’s range-resolved profiling capability.
The standard’s intercomparison testing protocol proved valuable for identifying a systematic bias in one lidar unit. During the campaign, a routine intercomparison with the meteorological mast revealed a 0.15 m/s bias that was traced to a misalignment in the lidar’s pointing system (0.08° error). Correction reduced the bias to below 0.05 m/s, demonstrating the importance of the standard’s requirement for annual pointing accuracy verification.
Frequently Asked Questions
Q: What is the difference between pulsed and continuous-wave (CW) Doppler lidar?
A: Pulsed lidar provides range-resolved measurements by time-gating the return signal, allowing simultaneous wind measurements at multiple heights. CW lidar uses focus-based range discrimination, measuring at one height at a time. Pulsed lidar generally has longer range but lower near-range resolution.
A: Pulsed lidar provides range-resolved measurements by time-gating the return signal, allowing simultaneous wind measurements at multiple heights. CW lidar uses focus-based range discrimination, measuring at one height at a time. Pulsed lidar generally has longer range but lower near-range resolution.
Q: How does aerosol concentration affect lidar performance?
A: In clean air conditions (low aerosol loading), the backscatter signal is weak, reducing the maximum range. In polluted or dusty conditions, the signal is stronger but multiple scattering and attenuation may limit performance. The standard recommends seasonal performance characterization.
A: In clean air conditions (low aerosol loading), the backscatter signal is weak, reducing the maximum range. In polluted or dusty conditions, the signal is stronger but multiple scattering and attenuation may limit performance. The standard recommends seasonal performance characterization.
Q: What maintenance is required for pulsed Doppler lidar?
A: Routine maintenance includes window cleaning, laser energy verification, alignment checks, and calibration of the pointing system. The fiber laser typically operates for 20,000-50,000 hours before requiring replacement.
A: Routine maintenance includes window cleaning, laser energy verification, alignment checks, and calibration of the pointing system. The fiber laser typically operates for 20,000-50,000 hours before requiring replacement.
Q: Can Doppler lidar measure vertical wind speed?
A: Yes, by pointing the lidar vertically (zenith mode), the vertical wind component is directly measured. However, vertical wind speeds are typically an order of magnitude smaller than horizontal winds, requiring higher velocity precision and longer averaging times.
A: Yes, by pointing the lidar vertically (zenith mode), the vertical wind component is directly measured. However, vertical wind speeds are typically an order of magnitude smaller than horizontal winds, requiring higher velocity precision and longer averaging times.
