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ISO 28902-1:2012 — Air Quality — Ground-Based Remote Sensing of Visual Range by Lidar
Environmental meteorology — Determination of meteorological visual range using backscatter lidar | Atmospheric sensing guide
Introduction to ISO 28902-1
ISO 28902-1:2012 specifies requirements for ground-based remote sensing of visual range using lidar (light detection and ranging) technology. Developed by ISO/TC 146/SC 5 in collaboration with the World Meteorological Organization (WMO), this standard establishes performance requirements, measurement procedures, and data evaluation methods for atmospheric lidar systems that measure meteorological visual range through backscatter analysis. Unlike conventional point sensors, lidar provides spatially resolved measurements along the laser beam path.
Lidar can measure visibility over ranges from tens of meters to tens of kilometers, covering the full spectrum from fog to clear air. This makes it invaluable for aviation, air quality monitoring, and meteorological forecasting.
Fundamentals of Visual-Range Lidar
Measurement Principle and System Requirements
The standard describes the lidar principle: a short laser pulse (typically 1-50 ns at 532 nm or 1064 nm) is transmitted into the atmosphere, and the backscattered signal from aerosols and molecules is collected by a telescope and detected by a photodetector. The signal decay with range is analyzed to determine the atmospheric extinction coefficient, which is then converted to visual range. The standard specifies minimum requirements for laser wavelength, pulse energy, receiver diameter, detector type, and data acquisition system.
| Parameter | Minimum Requirement | Preferred Specification | Test Method |
|---|---|---|---|
| Laser wavelength | 532 nm ± 10 nm or 1064 nm ± 20 nm | 1064 nm eye-safe Nd:YAG | Wavelength measurement |
| Pulse energy | ≥ 20 mJ (532 nm), ≥ 50 mJ (1064 nm) | 100-300 mJ | Energy meter |
| Receiver diameter | ≥ 100 mm | 200-400 mm | Dimensional measurement |
| Range resolution | ≤ 30 m | 3-15 m | Signal timing calibration |
| Visual range coverage | 100 m to 10 km (minimum) | 50 m to 50 km | Intercomparison with transmissometer |
| Data acquisition rate | ≥ 1 profile per minute | ≥ 10 profiles per second | Timing verification |
Eye safety is paramount. The standard requires that all lidar systems comply with IEC 60825-1 laser safety classification. Class 1 or Class 1M systems are preferred for unattended operation in publicly accessible areas.
Engineering Design Insights
Klett-Fernald Algorithm and Signal Processing
The standard specifies the Klett-Fernald algorithm as the primary method for inverting lidar signals to obtain extinction coefficients. This algorithm requires an assumed relationship between the backscatter and extinction coefficients (the lidar ratio S = 1/βb, where βb is the backscatter coefficient and α is the extinction coefficient). For typical atmospheric aerosols, S ranges from 20 to 80 sr depending on aerosol type (urban haze: 40-60 sr; dust: 20-40 sr; maritime: 20-30 sr). The algorithm also requires boundary calibration — typically using a range of clean air beyond the aerosol layer or independent visibility measurements.
A critical design consideration is the overlap function between the laser beam and telescope field of view. In coaxial systems, the overlap function reaches unity at a certain distance (typically 100-500 m), below which the signal cannot be reliably interpreted. The standard requires that manufacturers characterize this overlap function and that users account for it in data evaluation.
The uncertainty in lidar-derived visual range depends primarily on the assumed lidar ratio. Using an incorrect lidar ratio by 50% can bias the visual range by 30-100%. The standard recommends using ancillary measurements (sun photometer, nephelometer) to constrain the lidar ratio for each measurement campaign.
Measurement Planning and Interferences
The standard identifies sources of interference including molecular scattering (Rayleigh), multiple scattering (significant in fog), cloud attenuation, precipitation, and solar background light. Site requirements include unobstructed field of view, minimal vibration, and weather protection for the instrument. Measurement planning must consider atmospheric stability, aerosol loading, and solar angle to optimize data quality.
Practical Lidar Implementation
A ground-based lidar system deployed at Beijing Capital International Airport following ISO 28902-1 demonstrated the standard’s value for aviation safety. The system, operating at 1064 nm with 150 mJ pulse energy and 200 mm receiver diameter, provided visual range measurements from 100 m to 30 km along the approach path. During a severe haze event (PM2.5 > 200 µg/m³), the lidar detected a 500 m thick elevated aerosol layer at 300-800 m altitude that reduced slant visibility to 800 m while ground-level visibility remained at 3 km. This elevated layer, invisible to conventional runway visual range (RVR) transmissometers, created hazardous conditions for instrument approach procedures.
The Klett-Fernald algorithm implementation required careful selection of the lidar ratio (S). During the haze event, the optimal lidar ratio was 55 ± 10 sr, determined by comparing lidar-derived extinction coefficients with co-located nephelometer measurements at the surface. Using a fixed lidar ratio of 45 sr (typical for urban aerosol) would have underestimated the visual range by 35-45%, potentially leading to unsafe landing clearances. This finding validates the standard’s recommendation for ancillary measurements to constrain the lidar ratio.
Intercomparison testing between the lidar and three transmissometers along the runway showed excellent agreement (R² = 0.92) for visual ranges between 500 m and 10 km. Below 500 m, the lidar tended to underestimate visibility by 10-15% compared to transmissometers, likely due to the overlap function uncertainty at very short ranges.
Frequently Asked Questions
Q: How does lidar visual range measurement differ from transmissometer measurements?
A: A transmissometer measures the extinction coefficient along a fixed path (typically 10-100 m) and provides a path-averaged value. Lidar provides range-resolved measurements, allowing detection of elevated aerosol layers and characterization of the vertical structure of visibility.
A: A transmissometer measures the extinction coefficient along a fixed path (typically 10-100 m) and provides a path-averaged value. Lidar provides range-resolved measurements, allowing detection of elevated aerosol layers and characterization of the vertical structure of visibility.
Q: What is the minimum visual range that can be measured with lidar?
A: The minimum measurable visual range is typically 50-100 m, limited by the overlap function and signal dynamic range. For very low visibility (fog), the signal from near range saturates the detector before the far-field signal becomes usable.
A: The minimum measurable visual range is typically 50-100 m, limited by the overlap function and signal dynamic range. For very low visibility (fog), the signal from near range saturates the detector before the far-field signal becomes usable.
Q: How often should a lidar system be calibrated?
A: The standard recommends factory calibration every 2 years, with annual field checks using a hard-target return or intercomparison with a calibrated transmissometer. Changes in laser energy or receiver alignment of more than 20% require recalibration.
A: The standard recommends factory calibration every 2 years, with annual field checks using a hard-target return or intercomparison with a calibrated transmissometer. Changes in laser energy or receiver alignment of more than 20% require recalibration.
Q: Can lidar measure visual range at night and during precipitation?
A: Yes, lidar operates day and night (solar background filtering is used for daytime operation). During precipitation, lidar can still operate, but raindrop and snowflake returns add noise and may bias the extinction estimate. The standard recommends flagging precipitation-affected data.
A: Yes, lidar operates day and night (solar background filtering is used for daytime operation). During precipitation, lidar can still operate, but raindrop and snowflake returns add noise and may bias the extinction estimate. The standard recommends flagging precipitation-affected data.
