======== Lufft R2S (Radar Rain Sensor) ========= {{:atmos:535:projects:R2S_image.png}} * 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: -40°C to +70°C * Relative humidity: 0 to 100% RH * Operating temperature: -40°C to +60°C * Relative humidity: 0 to 100% RH ==== Installation Procedures ==== {{:atmos:535:projects:Installation.png}} * 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. {{:atmos:535:projects:Connections.png}} * 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: < 100 mA (heating off) * 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/Hz ^ | 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 ==== {{:atmos:535:projects:Configuration.png}} * 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 = ARb * 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|2 Z)/r2 *|K|2 is the refractive index factor of the target * 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 mm6 m-3. * 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, the attenuation is rather small while at 3 cm it is quite significant. * 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 ^ Wavelength ^ | 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, established the STWRC Road Weather Field Research Facility (RWFRF) in late 2006. * The purpose of this site is to understand better the physics associated with the interactions between the atmosphere and the roadway. {{:atmos:535:projects:Precipitation.png}} * 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://stwrc.und.edu/timeplot/rwfrf.html * Pavement Condition Model Validation * Lufft R2S Observed Precipitation * RWFRF Camera Images ==== Maintenance/Cost ==== * 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. http://stwrc.und.edu/. 2008.