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ISO 28902-3:2018 — Air Quality — Ground-Based Remote Sensing of Wind by Continuous-Wave Doppler Lidar
Environmental meteorology — Wind profiling using CW Doppler lidar technology | Atmospheric sensing guide
Introduction to ISO 28902-3
ISO 28902-3:2018 specifies requirements for ground-based remote sensing of wind using continuous-wave (CW) Doppler lidar. Developed by ISO/TC 146/SC 5, this standard complements ISO 28902-2 by addressing CW lidar systems that employ focus-based range discrimination rather than the pulse time-gating used in pulsed systems. CW Doppler lidar offers superior near-range resolution and lower system complexity, making it ideal for boundary layer studies, wind energy site assessment, and meteorological research at heights typically below 250 m.
CW Doppler lidar systems use a different range discrimination principle — focusing the transmitted beam at the desired measurement height. This provides excellent near-range resolution (2-5 m at 10-50 m height) but limits the maximum measurement range compared to pulsed systems.
Measurement Principles
CW Heterodyne Detection and Focus-Based Range Discrimination
The standard describes the CW lidar principle: a continuous-wave laser beam (typically 1.5 µm, eye-safe) is focused at the desired measurement height using a telescope with adjustable focus optics. The backscattered signal is collected through the same telescope and mixed with a local oscillator for heterodyne detection. The range resolution is determined by the depth of focus of the telescope — proportional to (λ · R² / D²) where λ is wavelength, R is range, and D is aperture diameter. Unlike pulsed lidar, CW lidar measures at only one height at a time, requiring sequential measurements for vertical profiles.
| Parameter | CW Doppler Lidar | Pulsed Doppler Lidar | Advantage |
|---|---|---|---|
| Range resolution | 2-50 m (range-dependent) | 10-30 m (constant) | CW better at near range |
| Maximum range | 100-300 m (typical) | 1-10 km (typical) | Pulsed better for range |
| Height coverage | 10-250 m | 30 m to several km | Pulsed better for altitude |
| System complexity | Lower (CW laser, no pulsing) | Higher (pulsed laser, timing) | CW simpler and cheaper |
| Measurement speed | Sequential heights (~10-60 s per level) | Simultaneous heights (~1 s per profile) | Pulsed faster for profiles |
| Near-range capability | Excellent (2-5 m resolution at 10 m) | Limited (overlap function issues) | CW superior near ground |
For wind energy applications, CW lidar excels at measuring wind shear and turbulence from 10 m to 200 m height — precisely the rotor sweep zone of modern wind turbines. The higher near-range resolution captures the wind profile shape with greater detail than pulsed lidar.
Engineering Design Insights
Signal Processing and Spectral Analysis for CW Systems
CW lidar signal processing differs fundamentally from pulsed systems. Since there is no range-gating, the measured signal at any instant contains contributions from all ranges along the beam, weighted by the focus function. The standard specifies that signal processing must include correction for the range-weighting function and account for the varying probe volume with range. The spectral analysis uses the same heterodyne principle as pulsed lidar, but the longer integration time per measurement (typically 0.5-5 seconds per height) allows higher velocity precision for a given signal strength.
A key engineering insight is the speed-range trade-off: the depth of focus increases quadratically with range, so range resolution degrades from 2-3 m at 10 m range to 30-80 m at 200 m range. This means CW lidar is best suited for the lower boundary layer where fine resolution is most valuable. The standard provides guidance on selecting focus heights to optimize the trade-off between profile resolution and total measurement time.
CW Doppler lidar has a significant cost advantage over pulsed systems for applications below 200 m. The simpler laser architecture (semiconductor or fiber CW laser instead of pulsed laser with amplifiers) reduces system cost by 30-50% while providing comparable or better velocity precision at these heights.
Applications and Limitations
The standard identifies primary applications: wind resource assessment for small to medium wind turbines, boundary layer research (turbulence profiles, nocturnal jets), wind shear detection for aviation (particularly low-level wind shear on approach paths), and urban wind environment studies. Known limitations include degraded performance in clean air (low aerosol loading), rain and fog interference, and the inability to measure above cloud base.
Practical CW Lidar Implementation
An urban wind environment study in central London used ISO 28902-3-compliant CW Doppler lidar to assess pedestrian-level wind comfort around new high-rise developments. The lidar was deployed at 12 locations over a 6-month period, measuring wind profiles from 10 m to 150 m height. The CW lidar’s superior near-range resolution (3 m at 10 m height, 8 m at 50 m height) was critical for capturing the high wind shear in the urban canopy layer — gradients exceeding 3 m/s per meter height were measured near street level during strong wind events.
The study also demonstrated the standard’s guidance on measurement planning for urban environments. Building-induced turbulence required longer integration times (3-5 seconds per height instead of the typical 1-2 seconds) to obtain stable mean wind estimates. The sequential height measurement approach, with profiles from 10 m to 150 m in 18 measurement heights, required approximately 5 minutes per complete profile. The standard’s requirement for site-specific limiting conditions (including buildings within the laser path, reflections from windows, and radio frequency interference) was particularly relevant for the urban deployment, where these factors reduced data availability from a typical 95% to approximately 85%.
The CW lidar’s cost advantage over pulsed systems (approximately 40% lower capital cost) made it economically feasible to deploy multiple units simultaneously at different urban locations. This multi-point measurement capability, combined with the standard’s specifications for system intercomparison, enabled characterization of spatial wind patterns across a 2 km² urban area at significantly lower cost than an equivalent network of meteorological masts.
Frequently Asked Questions
Q: What is the typical maximum height for CW Doppler lidar wind measurement?
A: For standard commercial systems, the maximum practical height is 150-250 m under typical aerosol conditions. With high aerosol loading (urban or industrial areas), ranges up to 400 m are possible. Clean air (mountain or marine environments) limits the range to 80-150 m.
A: For standard commercial systems, the maximum practical height is 150-250 m under typical aerosol conditions. With high aerosol loading (urban or industrial areas), ranges up to 400 m are possible. Clean air (mountain or marine environments) limits the range to 80-150 m.
Q: How long does a complete wind profile measurement take?
A: A typical profile from 10 m to 200 m with 10-20 measurement heights takes 2-10 minutes, depending on the integration time per height and the number of averaging cycles. Faster profiling reduces precision.
A: A typical profile from 10 m to 200 m with 10-20 measurement heights takes 2-10 minutes, depending on the integration time per height and the number of averaging cycles. Faster profiling reduces precision.
Q: Can CW lidar measure turbulence?
A: Yes, with high temporal resolution (0.1-1 s) at fixed height, CW lidar can measure velocity variance and turbulence intensity. The probe volume averaging effect must be corrected for accurate turbulence estimation.
A: Yes, with high temporal resolution (0.1-1 s) at fixed height, CW lidar can measure velocity variance and turbulence intensity. The probe volume averaging effect must be corrected for accurate turbulence estimation.
Q: How does CW lidar handle rain or fog?
A: Rain and fog produce strong backscatter signals that can improve data availability, but the Doppler spectrum may be broadened or bimodal (aerosol + hydrometeor contributions). The standard recommends spectral quality checks to filter precipitation-contaminated data.
A: Rain and fog produce strong backscatter signals that can improve data availability, but the Doppler spectrum may be broadened or bimodal (aerosol + hydrometeor contributions). The standard recommends spectral quality checks to filter precipitation-contaminated data.
