atmos:535:projects:lufft_r2s_precipitation_sensor
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atmos:535:projects:lufft_r2s_precipitation_sensor [2008/12/13 17:56] – mphillips | atmos:535:projects:lufft_r2s_precipitation_sensor [2020/01/29 17:25] (current) – external edit 127.0.0.1 | ||
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+ | ======== Lufft R2S (Radar Rain Sensor) ========= | ||
+ | |||
+ | {{: | ||
+ | * Dimensions: (diameter x length) 90mm x 220mm | ||
+ | * Weight: approximately 4.5 kilograms | ||
+ | * Black aluminum sensor housing | ||
+ | * Heated plastic dome | ||
+ | * Easy installation | ||
+ | |||
+ | ==== Features ==== | ||
+ | |||
+ | |||
+ | |||
+ | * Maintenance free precipitation sensor | ||
+ | * Differentiation between rain / snow | ||
+ | * Determination of quantity (1mm, 0.1mm, 0.01mm) | ||
+ | * Interface: RS485 and 2 digital outputs | ||
+ | * Can be configured for replacing tipping bucket systems | ||
+ | |||
+ | ==== Storage and Operating Conditions ==== | ||
+ | |||
+ | * Storage temperature: | ||
+ | * Relative humidity: 0 to 100% RH | ||
+ | * Operating temperature: | ||
+ | * Relative humidity: 0 to 100% RH | ||
+ | |||
+ | ==== Installation Procedures ==== | ||
+ | |||
+ | {{: | ||
+ | * Sensor can be provided with shield if required distances not met. | ||
+ | |||
+ | ==== Connections ==== | ||
+ | |||
+ | * There is an 8-pole screw-in connector on the underside of the device. | ||
+ | * This serves to connect the power supply and the interfaces using the associated connection cable. | ||
+ | |||
+ | {{: | ||
+ | |||
+ | * 1 negative power supply | ||
+ | * 2 positive power supply | ||
+ | * 3 RS485_A | ||
+ | * 4 RS485_B | ||
+ | * 5 not assigned | ||
+ | * 6 Uout1 | ||
+ | * 7 GND reference potential for the digital outputs | ||
+ | * 8 Uout2 | ||
+ | |||
+ | |||
+ | ==== Interface ==== | ||
+ | |||
+ | * Data bits: 8 | ||
+ | * Stop bit: 1 | ||
+ | * Parity: none | ||
+ | * Settable baud rates: 1200, 2400, 4800, 9600, 14400, 19200, 28800, 57600 | ||
+ | * 19200 is factory setting and baud rate for firmware update | ||
+ | |||
+ | |||
+ | ==== Electrical Data ==== | ||
+ | |||
+ | * Power supply: 20 to 30 VDC; typically 24 VDC | ||
+ | * Power consumption: | ||
+ | * Heating duty 30VA | ||
+ | * Protection class: III (SELV) | ||
+ | |||
+ | ==== Digital Outputs ==== | ||
+ | |||
+ | * The Uout1 and Uout2 digital outputs are short-circuit proof high side switches (12V) with integrated pull-down resistors. | ||
+ | * Possible configurations for Uout1 are: | ||
+ | * Tipping bucket simulation with 1mm, 0.1mm, or .01mm resolution | ||
+ | * Length of the output pulse for this simulation is typically 50 milliseconds | ||
+ | |||
+ | * Type of precipitation is transmitted on output Uout2 in the form of different frequencies. | ||
+ | |||
+ | ^ Precipitation Type ^ Frequency/ | ||
+ | | Dry | 0 | | ||
+ | | Rain | 10 | | ||
+ | | Snow | 20 | | ||
+ | | Sleet | 30 | | ||
+ | | Freezing Rain | 40 | | ||
+ | | Hail | 50 | | ||
+ | |||
+ | * If accumulated precipitation quantity is greater than 0.01mm, the frequency signal is transmitted for 2 minutes. | ||
+ | * The output of the frequency signal is maintained if a precipitation quantity greater than or equal to 0.01mm is measured within 2 minutes. | ||
+ | |||
+ | ==== Configuration ==== | ||
+ | |||
+ | {{: | ||
+ | * Factory setting | ||
+ | * Device ID: 1 | ||
+ | * Baud rate: 19200 | ||
+ | * RS485 protocol: binary | ||
+ | * ID must be changed if several R2S devices are operated in a UMB network. | ||
+ | |||
+ | * **Drizzle detection**: | ||
+ | * When activated, measurement takes place with greater sensitivity. | ||
+ | * Identifies water droplets with a diameter of 0.3mm. | ||
+ | * The only disadvantage is that the high sensitivity may cause a slightly higher water quantity to be measured. | ||
+ | |||
+ | * **Hail detection**: | ||
+ | * If hail detection is activated, the side shield must be installed in all cases. | ||
+ | * The measurement signal reacts to movements (e.g. trucks) of up to 72 km/h | ||
+ | * Movements are interpreted as precipitation. | ||
+ | * Since the fall speed is identical, very large water droplets may be interpreted as small hail | ||
+ | |||
+ | * **Evaporation per day**: | ||
+ | * Simulate the natural evaporation of a tipping bucket | ||
+ | * A defined value is deducted from the rainfall quantity every minute. | ||
+ | * This is set at 0.24mm per day in the delivered condition. | ||
+ | | ||
+ | * **Rainfall, Snowfall, and Hail correction factor**: | ||
+ | * The water quantity is assessed with this factor. | ||
+ | |||
+ | * The three particle types (rain, snow and hail) are added together and the precipitation type is assessed every minute. | ||
+ | * Requests 1-4 are carried out. | ||
+ | * They only take place if previous conditions are unfulfilled. | ||
+ | * __The factors cannot be changed__. | ||
+ | |||
+ | 1) Number of hail particles per minute > 40% (Hail factor) | ||
+ | * Precipitation type = Hail | ||
+ | |||
+ | 2) Number of rain particles > 90% (Freezing Rain factor) and ambient temperature <= 0°C | ||
+ | * Precipitation type = Freezing rain | ||
+ | |||
+ | 3) Number of rain particles >20 % (Sleet factor) and ambient temperature in the range from –5°C to 4°C: | ||
+ | * Precipitation type = Sleet | ||
+ | |||
+ | 4) Number of rain particles > 50% (Rain factor) : | ||
+ | * Precipitation type = Rain | ||
+ | |||
+ | **If none of the 4 conditions is met but particles were measured, the precipitation type is snow.** | ||
+ | |||
+ | * Adjustment values: | ||
+ | * The range of the rain sensor measurement spectrum is from 130Hz (drizzle) to 1600 Hz (heavy rain). | ||
+ | * This range is divided into 23 zones, which can be individually corrected with factors from 0.1 to 10. | ||
+ | |||
+ | ==== Theory ==== | ||
+ | |||
+ | * Doppler radars measure velocity by estimating the frequency-shift produced by an ensemble of moving targets. | ||
+ | * Doppler radars also provide information about the total power returned and about the spectrum width of the precipitation particles within the pulse volume. | ||
+ | * The reflected signal is the result of the energy from the transmitted pulse interacting with precipitation (snow, ice pellets, hail, and rain) particles. | ||
+ | * A small portion of the power is then returned to the radar and analyzed to determine an estimate of the rain or snow rate. | ||
+ | * The relationship between the size and power return is highly non-linear. | ||
+ | * An example is a very small, spherical drop. | ||
+ | * If you double the size of a the drop, you increase the reflected power return by a factor of 64. | ||
+ | * If you triple the size of the drop, you increase the reflected power return by a factor of 729. | ||
+ | * Polarimetric radars are designed to eliminate this problem. | ||
+ | |||
+ | **Precipitation Rates** | ||
+ | |||
+ | * Example of two scenarios with identical rain rates: | ||
+ | * Rain water concentrated in a very small number of large drops. | ||
+ | * Rain water concentrated in a very large number of small drops. | ||
+ | * Reflected power returned to the radar is heavily weighted towards the largest drops. | ||
+ | * If only using the returned power to estimate rain rate, you might end up with either a significant overestimation or a significant underestimation of the rain rate. | ||
+ | * Radar power returned from irregular shaped mixtures of precipitation types can get quite complicated. | ||
+ | * The rainfall rate (R) is a product of the mass content and the fall velocity in a radar volume. | ||
+ | * Precipitation rate depends on particle size distributions. | ||
+ | * The natural variability in drop-size distributions is an important source of uncertainty in radar measurements of precipitation. | ||
+ | |||
+ | * Precipitation is usually measured by using the Z-R relation: | ||
+ | * Z = AR< | ||
+ | * A and b are constants. | ||
+ | * This relationship is not unique. | ||
+ | * Many empirical relations have been developed. | ||
+ | * Typical values for the index and exponent are A = 200, b = 1.60 | ||
+ | |||
+ | **Radar Equation** | ||
+ | |||
+ | * Pr = (C |K|< | ||
+ | |||
+ | *|K|< | ||
+ | * r is the slant range from the radar to the target (meters) | ||
+ | * Z is the radar reflectivity factor (usually taken as the equivalent reflectivity factor Ze when the target characteristics are not well known), in mm< | ||
+ | * C is the radar constant. | ||
+ | |||
+ | **Attenuation** | ||
+ | |||
+ | * Attenuation by hydrometeors can result from both absorption and scattering. | ||
+ | * It is dependent on the shape, size, number and composition of the particles. | ||
+ | * Attenuation is dependent on wavelength. | ||
+ | * At 10 cm wavelengths, | ||
+ | * Wavelengths below 5 cm are not recommended for good precipitation measurement except for short-range applications. | ||
+ | * The attenuation is dependent on water mass of the target. | ||
+ | * Ice particles attenuate much less than liquid particles. | ||
+ | * Snow or ice particles (or a hailstone) can grow to a size much larger than a raindrop. | ||
+ | |||
+ | **Radar Wavelength** | ||
+ | |||
+ | ^ Band ^ Frequency | ||
+ | | UHF |300-1000 MHz | 1-0.3 m | | ||
+ | | L | 1-2 GHz | 30-15 cm | | ||
+ | | S | 2-4 GHz | 15-8 cm | | ||
+ | | C | 4-8 GHz | 8-4 cm | | ||
+ | | X | 8-12 GHz | 4-2.5 cm | | ||
+ | | K | 18-27 GHz | 1.2 – 0.75 cm | | ||
+ | |||
+ | * The larger the wavelength, the greater the cost of the radar system. | ||
+ | * This is due both to an increase in the amount of material and to the difficulty in meeting tolerances over a greater size. | ||
+ | * Bands of weather radar interest include S, C, X and K. | ||
+ | * The sensitivity or ability of the radar to detect a target is strongly dependent on the wavelength. | ||
+ | * For the same antenna, the target detectability increases with decreasing wavelength. | ||
+ | * The shorter wavelengths provide better sensitivity. | ||
+ | * The disadvantage is that the smaller wavelengths have much larger attenuation. | ||
+ | |||
+ | ==== Field Work ==== | ||
+ | |||
+ | * The Surface Transportation Weather Research Center (STWRC), along with the North Dakota Department of Transportation, | ||
+ | * The purpose of this site is to understand better the physics associated with the interactions between the atmosphere and the roadway. | ||
+ | |||
+ | {{: | ||
+ | |||
+ | * All observations are presented in 1-minute intervals. | ||
+ | * Evaluates the performance of the precipitation sensor. | ||
+ | * Provides monitoring of precipitation events. | ||
+ | |||
+ | * **Road Weather Field Research Topics Online:** | ||
+ | * Overview of the facility | ||
+ | * Field Site Data | ||
+ | * 24 hour Observations | ||
+ | * http:// | ||
+ | * Pavement Condition Model Validation | ||
+ | * Lufft R2S Observed Precipitation | ||
+ | * RWFRF Camera Images | ||
+ | |||
+ | ==== Maintenance/ | ||
+ | |||
+ | * Service and maintenance is carried out by a trained specialist. | ||
+ | * The recommended service interval is 12 months. | ||
+ | * The device must be disconnected from the power supply. | ||
+ | | ||
+ | * Estimated cost for Lufft R2S system: | ||
+ | * $3000-4000 | ||
+ | |||
+ | ==== Calibration ==== | ||
+ | |||
+ | * The device is calibrated in the factory. | ||
+ | * The recommended calibration interval is 24 months. | ||
+ | * An onsite calibration service is available on enquiry. | ||
+ | * Testing of the signal processor with known artificially generated signals. | ||
+ | * Doppler calibration includes verification and adjustment of phase stability using fixed targets or artificial signals. | ||
+ | * The presence or absence of echoes from fixed ground targets may also serve as a crude check of transmitter or receiver performance. | ||
+ | |||
+ | ==== References ==== | ||
+ | |||
+ | * Lufft R2S Precipitation Sensor Manual. 2006. | ||
+ | * __RADAR for Meteorologists.__ Rhinehart. 2004. | ||
+ | * Surface Transportation Weather Research Center. | ||
+ | |||
+ | |||