PWS100 Present Weather Sensor - Campbell Scientific

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Section 8. Functional Description 8.6 Algorithm Description 8.6.1 Detecting and Classifying Precipitation The <strong>PWS100</strong> has a structured detection volume consisting of four sheets of light each 0.4 mm in depth with 0.4 mm spacing. The area of detection is approximately 40 cm 2 as defined by the overlap of the two detectors each of which is 20° off of the light sheet propagation axis. Detection of precipitation is carried out in real-time. Time and frequency domain analysis of the signals from the two sensor heads is carried out to ensure all particles are detected. Time domain analysis allows quick stripping of larger particles while the frequency domain analysis picks out smaller particles using a slightly more computationally intensive routine. Using a series of thresholds and checks precipitation signals from particles as small as 0.1 mm diameter passing through the area of detection are identified while any sources of noise or particles not passing through the defined area are discounted. A routine then strips the signal from the incoming data for each real particle passing through the defined area in order to minimize the data that is to be further processed. When a particle passes through the four light sheets it provides a signal characteristic of the particle type and provides information on the velocity of the particle, by means of auto correlation, and size, by means of cross correlation of the two channels of data. Particles for which cross correlation could be poor (e.g., snow) may have their size measured by the analysis of particle transit time or signal amplitude (in the same way that present weather sensors without structured detection volumes measure particle size). Due to the Gaussian nature of the light sheet intensity there is more error involved in these transit time or signal amplitude measurements than those using the cross correlation which is independent of the signal intensity. The structure of the particle is to some degree given by the analysis of the signal peak to signal pedestal ratio. The signal pedestal is found through stripping higher frequencies from the particle signal (i.e., those produced by the four light sheets) and is higher for particles with higher crystallinity and therefore more scattering sites. Particles of water have almost no pedestal compared to the peak signal value whereas snow can have large pedestals compared to the peak signal value. Analysis of separate snow and rain/drizzle events has shown that there is very little overlap in the signal peak to pedestal peak ratio for the solid and liquid particles, though drizzle values extend further towards the snow values and snow grains extend further towards the liquid precipitation values, while the larger rain and snowflakes remain almost entirely separate. This is shown graphically in Figure 8-6 which represents two separate events one comprising snowflakes and snow grains (particle count on the left vertical axis) and one representing a mixed drizzle and rain event (particle count on the right vertical axis). This particular differentiator, only obtainable because the <strong>PWS100</strong> has a structured detection volume, is extremely useful in the fuzzy logic processing as described below and further in Section 8.6.4.1. 8-6
Section 8. Functional Description Signal to Pedestal Ratio Analysis Particle Count (Snow) 600 500 400 300 200 100 Snowflakes Snow Grains Drizzle Rain 100 90 80 70 60 50 40 30 20 10 Particle Count (Liquid) 0 0 1 1.5 2 2.5 3 3.5 4 4.5 5 Signal to Pedestal Ratio FIGURE 8-6. Signal to pedestal ratio values for different precipitation types. The above data was collected during the sensor development. Results from production instruments will differ. In particular the optics do not allow accurate measurement of the pedestal ratio for true drizzle (particles
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PWS100 Present Weather Sensor Revis
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Assistance Products may not be retu
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PWS100 Table of Contents 6.4 Ground
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PWS100 Table of Contents 8.3 Additi
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PWS100 Table of Contents List of Ta
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Section 1. Product Overview 1-2
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Section 2. Cautionary Statements If
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Section 3. Initial Inspection 3-2
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Section 4. Overview 4-2
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Section 5. Specifications 5.3 Optic
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Section 5. Specifications The parti
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Section 6. Installation In order to
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Section 6. Installation Bracket Tab
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Section 6. Installation FIGURE 6-4.
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Section 6. Installation PG9 CABLE G
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Section 7. Operation 7.3 Terminal M
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Section 7. Operation NOTE The data
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Page 48 and 49: Section 7. Operation NOTE The PWS10
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Page 76 and 77: Section 7. Operation 7.5.3 Viewing
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Page 80 and 81: Section 7. Operation Select the Xmo
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Page 84 and 85: Section 7. Operation 'Main Program
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Page 104 and 105: Section 10. Troubleshooting 10.2.3
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Page 108 and 109: Appendix A. PWS100 Output Codes Clo
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Page 116 and 117: Appendix A. PWS100 Output Codes A-1
Page 118 and 119: Appendix B. Wiring FIGURE B-2. DSP
Page 120 and 121: Appendix C. Cable Selection WIRING
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Page 124 and 125: Appendix D. Software Flowchart D-2
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Page 130 and 131: Appendix E. Menu System Map E-2
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PWS100 Present Weather Sensor - Campbell Scientific

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