Abstract:
The invention is directed to a method and a combined visibility measuring and precipitation measuring instrument for determining visibility, amount of precipitation and type of precipitation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority of German Application No. 101 20 747.6, filed Apr. 25, 2001, the complete disclosure of which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   a) Field of the Invention 
   The invention is directed to a method for determining visibility, amount of precipitation and type of precipitation, and to a combined visibility and precipitation measuring instrument, particularly for distinguishing between different types of precipitation and amounts of precipitation. 
   b) Factors Relating to the Invention 
   Measurement of environmental parameters and weather parameters is growing in importance. In order to replace former visual observations with automatic detection of weather phenomena and, accordingly, in order gradually to enable a complete automation of weather stations, the availability of measurement instruments capable of determining as many of the required parameters as possible is required. 
   Atmospheric turbidity is brought about by aerosols whose type and concentration determine visibility. In this connection, there are two physical effects that are important in determining visibility with optical measuring instruments. Irradiation of a measurement volume by a light source produces light scattering on the one hand and light absorption on the other hand. 
   Both effects are detected by a transmissometer arrangement because the latter evaluates what percentage of the emitted light reaches the location of the receiver. The reception signal is influenced by scattering at the aerosols as well as by absorption by the aerosols. Both phenomena are covered under the concept of extinction, wherein the absorption may be disregarded in favor of scattering. Transmissometers measure the atmospheric extinction coefficient directly. The visibility can then be determined by definition by way of the relationship described by Koschmieder. 
   Apart from the large air volume utilized for determining visibility, the advantage of transmissometers consists in this definite relationship between transmission and visibility. The quality of the measurement performance of transmissometers is generally very highly rated. 
   Nevertheless, there are reasons in favor of using a simplified measurement concept such as that offered by scattered light measurement devices: 
   greater visibility measurement range; 
   possibility of determining the type and intensity of precipitation; 
   low initial costs; 
   reduced space requirement; the typical distance of the transmitter and scattered light receiver from the measurement volume is less than 1 m; 
   no aligning or adjusting labor is required; 
   reduced influence of contaminated device plates on measuring performance; 
   simple calibration; and 
   low maintenance costs. 
   c) Description of the Related Art 
   In scattered light measurement devices such as those described in DE-OS 21 21 088, a receiver is positioned at an appropriate angle to the light source. The light component scattered by the aerosols and precipitation particles in the measurement volume at this angle is determined. In this case, knowledge of the relationship between the optical scattering function and the atmospheric extinction coefficient is required for evaluating the reception signal. 
   Differences in the scattering behavior between the various particles cannot be taken into account adequately, or at all, in conventional scattered light measuring instruments by measuring either at a selected scattering angle or over the largest possible scattering angle area. 
   The conventional devices realize methods that are based on the assumption that the utilized scattering angles in the light wavelength used for the measurement can be taken as representative for all occurring phenomena. 
   In this connection, a forward scattering angle of approximately 30° to 40° has proven to be representative for a large number of types, mixtures and concentrations of aerosols based on various simulations of atmospheric models and practical tests and comparisons for the visible wavelength range. In general, the theory of scattering proposed by Mie applies for this order of magnitude of particles with much greater diameters than the light wavelength. However, different assessments of the measured scattering signal are also necessary in this case because there are significant differences in the scattering behavior of mist and fog. 
   There are very many greater influences when considering precipitation. Drizzle droplets and raindrops, hail, soft hail or snow pellets, and snowflakes result in entirely different scattering behavior for which it is no longer possible to carry out the same evaluations of the measured scattering signal as those for mist and fog. Further, these particles do not remain in a quasi-stationary spatial position; rather, because of their size, they undergo a movement in the direction of the ground so that the period during which they remain in the measurement volume is limited. 
   OBJECT AND SUMMARY OF THE INVENTION 
   It is the primary object of the invention to provide a method and an arrangement which ensure the ability to distinguish easily between different phenomena such as fog, rain, snow, and so on. 
   According to the invention, this object is met in an instrument for measuring visibility and precipitation containing a light transmitter, at least one receiver in forward scattering arrangement and evaluating electronics in that a first receiver is arranged opposite to the light transmitter, a second receiver is arranged in a forward scattering angle range of 20° to 40°, a third receiver is arranged in a forward scattering angle range of 70° to 120°, and a fourth receiver is arranged in a forward scattering angle range of 140° to 150°, a modulation generator is arranged in front of the light transmitter, and every receiver communicates with the evaluating electronics via a postamplifier and a synchronous rectifier. 
   Advisable constructions are characterized by the subclaims. 
   The invention will be described more fully in the following with reference to embodiment examples shown in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  shows a schematic construction of an instrument according to the invention; 
       FIG. 2  shows a measurement signal graph; 
       FIG. 3  shows a block diagram of the evaluating electronics; 
       FIG. 4  is a schematic view illustrating the first reception signal conditioning; 
       FIG. 5  is a schematic view illustrating the second reception signal conditioning; 
       FIG. 6  is a schematic view illustrating the third reception signal conditioning; 
       FIG. 7  is a schematic view illustrating the fourth reception signal conditioning; and 
       FIG. 8  shows the principle of the reception signal evaluation. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows the essential components of the arrangement according to the invention, comprising the light transmitter  1 , the four optoelectronic receivers  2 ,  3 ,  4 ,  5 , each of which is connected to the common processing unit  14  (as shown in  FIG. 3 ) via a postamplifier  6 ,  7 ,  8 ,  9  and subsequent synchronous rectifiers  10 ,  11 ,  12 ,  13 , and the modulation generator  15 . The optoelectronic receiver  2  is located opposite to the light transmitter  1  and the other optoelectronic receivers  3 ,  4  and  5  are arranged at measurement angles of 32°, 90° and 143°, respectively. The measurement volume  16 , as it is called, is formed in the intersection area of the viewing fields of the optoelectronic receivers  2 ,  3 ,  4 ,  5  and of the light bundle emitted by the light transmitter  1 . 
   The measurement volume  16  is illuminated in a defined manner at least several thousand times per second by the light transmitter  1  with a narrow light cone. The emitted light is intensity-modulated (e.g., sinusoidally) and has a defined wavelength. 
   For reasons relating to independence of the measurement from daylight and to improve the signal-to-noise ratio, the intensity of the light source is modulated by the modulation generator  15  at a frequency greater than 1 kHz. The optoelectronic receivers  2 ,  3 ,  4 ,  5  are provided with optical bandpass filters which are transparent only for the wavelength used by the light transmitter  1 . The postamplification of the received signals is carried out in a narrow band around the modulation frequency. Rectification is carried out in the subsequent synchronous rectifiers  10 ,  11 ,  12 ,  13  in rigid phase ratio to the modulation frequency and the signals are then fed to evaluating electronics  33 . 
   The spatial arrangement of the light transmitter  1  and receivers  2 ,  3 ,  4 ,  5  is selected so as to take into account the opening angles of the transmitter unit and receiver units such that the resulting measurement volume size ensures that only one particle at a time remains in the measurement volume  16  at the anticipated maximum spatial occurrence density of precipitation particles during the heaviest precipitation. Individual particle detection is made possible in this way. 
   Since these particles, because of their size, generate a sufficiently large scattering signal which can be distinguished from the aerosols that are otherwise present, it is possible to detect the presence of precipitation particles generally by a time-discrete analysis of the scattering signals. The raw signal shown in  FIG. 2  is the reception signal typically obtained by the measuring arrangement when a precipitation particle penetrates the measurement volume  16 . 
   The signal amplitudes produced by a precipitation particle can be definitely distinguished from the signal that is unrelated to precipitation. Therefore, by comparing the spontaneous signal to the mean signal value, it is possible to make conclusive judgments about whether or not a precipitation particle is present in the measurement volume  16 . 
   This information, together with the usual time-continuous reception signal evaluation, also makes possible a time-discrete handling of precipitation particles. 
   Significant measurement quantities for every detected precipitation particle are the quantity of half-waves per precipitation event and the maximum signal amplitude occurring in this connection. 
   When these steps for signal handling are carried out in all participating receivers  2 ,  3 ,  4 ,  5 , it is possible to directly evaluate absolute values and distinguish between the magnitudes of the scattering signals in a time-averaged manner as well as for every individual precipitation particle event separately, that is, a precipitation particle is detected when it penetrates into the measurement volume  16  and the dwell time in the measurement volume  16  and the signal amplitude occurring because of the particle are recorded. 
   Based on the individual particle detection described above, the present invention makes it possible to separate the precipitation-related reception signal components from the reception signal components that are not related to precipitation. 
   The raw signals obtained at different angles by means of the receivers  2 ,  3 ,  4 ,  5  are initially conditioned in the following manner according to  FIG. 3  in the evaluating electronics: 
   1. The following information and measurement quantities are derived from the raw signal obtained by the receivers  3  by means of the first reception signal conditioning  17  (according to  FIG. 4 ) and are made available for further processing: 
   the presence of a precipitation particle in the measurement volume  16  is registered, 
   the maximum value of the reception signal is determined for every individual precipitation particle penetrating the measurement volume  16  →&gt;A max (32°), 
   the mean value of the scattering signal is determined, wherein the influence of precipitation particles on this mean value is prevented →&gt;A av (32°). 
   2. By means of the second reception signal conditioning  18  (according to  FIG. 5 ), the maximum value of the reception signal for every individual precipitation particle detected at 32° is determined from the raw signal obtained by the receiver  4  →&gt;A max (90°). 
   3. By means of the third reception signal conditioning  19  (according to  FIG. 6 ), the mean value of the scattering signal is formed from the raw signal obtained by receiver  5 , wherein the influence of precipitation particles on this mean value is prevented →&gt;A av (143°). 
   4. The following measurement quantities are derived from the raw signal obtained by the receiver  2  by means of the fourth reception signal conditioning  20  (according to  FIG. 7 ): 
   minimal value of the reception signal for every individual precipitation particle penetrating the measurement volume  16  →&gt;A min (0°); 
   mean value of the reception signal, wherein the influence of precipitation particles on this mean value is likewise prevented →&gt;A av (0°); 
   The corresponding signals are obtained in the manner described in detail in the following with reference to  FIGS. 4 to 7 . 
   According to  FIG. 4 , the raw signal A obtained by the receiver  3  at 32° is supplied for averaging to the lowpass  21  and, in parallel, to the first input of the threshold switch  22 . The output of the lowpass  21  is linked to the second input of the threshold switch  22 . Due to the fact that a precipitation particle causes a reception signal that is greater than the mean signal value, it can be decided on the basis of a comparison between the mean reception signal value and the spontaneous signal (reception signal that is not averaged) whether or not a precipitation particle is actually located in the measurement volume  16 . The threshold switch  22  is used for this purpose. When a precipitation particle penetrates into the measurement volume  16 , a rectangular pulse is generated at the output of the threshold switch  22  for every modulation signal half-wave as long as the precipitation signal remains in the measurement volume  16  (signal sequence B). On the one hand, this rectangular pulse sequence is used in the processing unit  14  for determining the particle dwell time. On the other hand, the “event trigger” signal which serves as a control signal for the signal separating units  23 ,  24 ,  25  and  26  is derived therefrom. The event trigger signal indicates whether or not a precipitation particle is actually located in the measurement volume. 
   With the assistance of the event trigger signal, the signal separating unit  23  is capable of freeing the raw signal A obtained at 32° from precipitation-related components and delivers signal sequence D via its first output. This reception signal component which is not related to precipitation is subsequently averaged and, for this reason, is supplied via the lowpass  27 . Signal sequence E is formed. The precipitation-related signal component is generated at the second output of the signal separating unit  23  and is fed to the maximum value detector  28 . In this way, the maximum value of the signal amplitude caused by the precipitation particle is made available to the processing unit  14  (signal sequence F). After the precipitation particle has exited from the measurement volume  16 , this measured value is read out from the processing unit  14  and the maximum value detector  28  is reset by means of the control signal Max/Min Reset and is thus prepared for the next measurement. 
   According to  FIG. 5 , the maximum value of the precipitation-related reception signal at 90° is determined from the provided raw signal G by means of the signal separating unit  24  and the maximum value detector corresponding to the process described for the signal conditioning at 32° for every precipitation particle detected at 32° (signal sequences H and I). 
   According to  FIG. 6 , the raw signal J obtained by the receiver  5  at 143° is freed from signal components not caused by precipitation by means of the signal separating unit  25  and is conveyed after averaging through the lowpass  30  to the processing unit  14  (signal sequences K and L). 
   When a precipitation particle penetrates the measurement volume  16 , a signal break-in in the direct light measurement can be determined by means of the receiver  2 . According to  FIG. 7 , signal sequence M is generated. In order to evaluate this process using measurement technology, the minimum direct light measurement value (signal sequence P) is determined for every particle detected at 32° by means of the signal separating unit  26  and the minimum value detector  31  and is fed to the processing unit  14 . The minimum value detector  31  is reset after the particle has exited from the measurement volume  16 . Further, the reception signal (signal sequence N) that was obtained at 0° and freed from precipitation-related signal components by the-signal separating unit  26  is fed to the lowpass  32 , so that the precipitation-related mean value (signal sequence O) is subsequently made available to the processing unit  14 . 
   The following relationships and evaluation mechanisms are to be used as a basis for the following further processing and evaluation of the obtained signals. The information relates to the use of a light wavelength in the visible spectral range. The angular ranges mentioned above are shifted for other light wavelengths. In general, however, the relationships and procedures noted in the following also apply in the latter case. 
     FIG. 8  is a schematic view illustrating the reception signal evaluation: 
   1. An optimal evaluation of fog phenomena can only be carried out at a scattering angle of about 20° to 40°. In this optimal angular range for fog, an equality evaluation of mist results in an uncertainty of up to 60% for visibility determination. By parallel evaluation of the measurements (quotient formation) at 32° and 143°, conclusions can be arrived at regarding the prevailing mist phenomenon. This enables a phenomenon-dependent evaluation of the reception signal at 20° to 40°. The measurement uncertainty can accordingly be kept to values of less than 10% for all known fog and mist phenomena. 
   After averaging the conditioned scattering signals at 32° (A AV (32)) and 143° (A AV (143)), a typical device constant K 32  and K 143  is applied (unit  1 /Vm). Subsequently, the two measurements are placed in a ratio (quotient formation) in order to conclude whether there is fog or one of several mist phenomenon groups (mist  1 , mist  2  and mist  3 ). After the phenomenon category has been determined, the extinction component not related to precipitation can be calculated from the scattering signal obtained at 32° by taking into account a correspondingly adapted evaluation factor (K fog , K mist1 , K mist2  or K mist3 ) and the typical device constant K 32 . 
   In this connection, it is not mandatory that the mist phenomena be classified into mist groups. A dynamic reception signal adaptation depending on the quotient of K 32 *A AV (32) and K 143 *A AV (143) can also be applied for optimized mist evaluation.
         2. Evaluation of the signal components caused by precipitation:       

   As has already been shown, the process described above for individual particle detection enables time-discrete handling of precipitation particles which penetrate the scattered light measurement volume. Accordingly, the scattering signal amplitude and the dwell time in the measurement volume are available for each of these particles. The scattered light signal at a lateral scattering angle involves a particularly high information content about the presence of liquid precipitation, solid precipitation, or rain, snow or soft hail. When a precipitation particle is detected when measuring at 32°, the type of precipitation is decided upon simultaneously by means of the quotient of the measurement values obtained at 32° (A max (32)) and 90° (A max (90)) (after applying the typical device constants K 32  and K 90 ) and by means of the dwell time in the measurement volume. 
   At the 32-degree measurement angle, liquid precipitation events can be evaluated in a representative manner by introducing the device constant K 32  and taking into account the adaptation factor K liq precip  for liquid precipitation. This procedure makes it possible to determine the following parameters: 
   extinction through liquid precipitation; 
   particle size distribution for liquid precipitation and, therefore, distinguishing between drizzle and rain; 
   intensity of the liquid precipitation. 
   The integral of the amplitude over the dwell time of the particle event is proportional to the effective cross-sectional area of the particle and, therefore, to the particle extinction. When the individual particle extinctions are summed over a given time period, this provides the extinction component for liquid precipitation. Further, the liquid water content can be determined for every liquid precipitation particle being considered and can also be summed for a given time period. This leads to precipitation intensity information for liquid precipitation. If there is solid precipitation, no reliable conclusions can be derived from the scattered light information regarding the corresponding extinction component or precipitation intensity. However, the extinction component caused by solid precipitation and the corresponding precipitation intensity can be determined when the effective cross-sectional area of the particle is known. 
   A measurement at 0° enables direct determination of this effective cross-sectional area of the particle by evaluating the relative transmission break-in that occurs when a particle penetrates the measurement volume. The relative transmission break-in can be calculated by the quotient of the measured values A min (0) and A AV (0) and by taking into account another device constant for the direct receiver K 0 . In addition, a corresponding adaptation factor must be applied (K snow  or K soft hail ) depending on whether there is snow or soft hail. 
   This measurement is only carried out when a precipitation particle has been detected at 32° and identified as solid precipitation by the additional measurement at 90°. The integral of the amplitude over the dwell time of the detected relative signal break-in of the direct light measurement is proportional to the effective cross-sectional area of the particle and, therefore, to the particle extinction. When the individual particle extinctions are summed over a time period under consideration, the extinction component for solid precipitation is given. Further, the liquid water content can be determined for every solid precipitation particle being considered and can also be summed for a given time period. This gives the precipitation intensity information for solid precipitation. 
   The total precipitation-related extinction component can be determined by adding the partial extinctions for snow, soft hail and liquid precipitation. 
   3. Evaluation of the signal components caused by precipitation and the signal components not caused by precipitation 
   For final determination of visibility, the signal components not caused by precipitation and the signal components caused by precipitation are added together and converted to visibility by taking into account the contrast threshold. 
   While the foregoing description and drawings represent the present invention, it will be obvious to those skilled in the art that various changes may be made therein without departing from the true spirit and scope of the present invention. 
   REFERENCE NUMBERS 
   
       
         1  light transmitter 
         2  receiver 
         3  receiver 
         4  receiver 
         5  receiver 
         6  postamplifier 
         7  postamplifier 
         8  postamplifier 
         9  postamplifier 
         10  synchronous rectifier 
         11  synchronous rectifier 
         12  synchronous rectifier 
         13  synchronous rectifier 
         14  processing unit 
         15  modulation generator 
         16  measurement volume 
         17  reception signal conditioning 
         18  reception signal conditioning 
         19  reception signal conditioning 
         20  reception signal conditioning 
         21  lowpass 
         22  threshold switch
         23  signal separating unit     
         24  signal separating unit 
         25  signal separating unit 
         26  signal separating unit 
         27  lowpass 
         28  maximum value detector 
         29  maximum value detector 
         30  lowpass 
         31  minimum value detector 
         32  lowpass 
         33  evaluating electronics 
         34  reception signal conditioning unit