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======== 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 = AR<sup>b</sup> 
                        * 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|<sup>2</sup> Z)/r<sup>2</sup>

      *|K|<sup>2</sup> 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 mm<sup>6</sup> m<sup>-3</sup>.
      * 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.