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Decagon Devices SRS Operator's Manual

Spectral reflectance sensor
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SRS
Spectral Reflectance Sensor
Operator's Manual
Decagon Devices, Inc.
Version: January 15, 2014 — 12:16:45

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  • Page 1 Spectral Reflectance Sensor Operator’s Manual Decagon Devices, Inc. Version: January 15, 2014 — 12:16:45...
  • Page 2 SRS Sensors Decagon Devices, Inc. 2365 NE Hopkins Court Pullman WA 99163 Phone: 509-332-5600 Fax: 509-332-5158 Website: www.decagon.com Email: support@decagon.com or sales@decagon.com Trademarks c 2007-2013 Decagon Devices, Inc. All Rights Reserved...

  • Page 3: Table Of Contents

    Down Looking Sensors ....14 4 Connecting the SRS 4.1 Connecting to Decagon Data Logger ..19 4.2 3.5 mm Stereo Plug Wiring .
  • Page 4 CONTENTS SRS Sensors 8 Troubleshooting 8.1 Data Logger ......28 8.2 Sensors ......28 8.3 Calibration .

  • Page 5: Introduction

    Introduction Thank you for choosing Decagon’s Spectral Reflectance Sensor (SRS). We designed the SRS for continuous monitoring of Normalized Differ- ence Vegetation Index (NDVI) and/or the Photochemical Reflectance Index (PRI) of plant canopies. We intend it to be low cost, easily and quickly deployable, and capable of reliable operation over years.

  • Page 6: About This Manual

    1 INTRODUCTION SRS Sensors About This Manual Please read these instructions before operating your sensor to ensure that it performs to its full potential. Warranty The sensor has a 30-day satisfaction guarantee and a one-year war- ranty on parts and labor. Your warranty is automatically validated upon receipt of the instrument.

  • Page 7: About Srs

    2 ABOUT SRS About SRS Overview The SRS are two-band radiometers that measure either incident or reflected radiation in wavelengths appropriate for calculating the Normalized Difference Vegetation Index (NDVI) or the Photochemi- cal Reflectance Index (PRI). Sensors are manufactured in four differ- ent versions: NDVI-hemispherical, NDVI-field stop, PRI-hemispherical...

  • Page 8: Specifications

    2 ABOUT SRS SRS Sensors The SRS is a digital sensor. Its outputs follow the SDI-12 stan- dard. The SRS is best suited for use with Decagon’s Em50, Em50R, and Em50G data loggers. However, customers can use the SRS with other loggers, such as those from Campbell Scientific.
  • Page 9 SRS Sensors 2 ABOUT SRS Other Features: SDI-12 digital sensor, compatible with Decagon’s EM50 family and CSI loggers In-sensor storage of calibration values Four versions NDVI-hemispherical NDVI-field stop PRI-hemispherical PRI-field stop NDVI or PRI sensors with Teflon cosine correcting heads NDVI or PRI sensors with 20 degree field stops sealed...

  • Page 10: Theory

    3 THEORY SRS Sensors Theory Decagon designed the SRS instruments to measure the NDVI and PRI vegetation indices from plant canopies. We caution users that NDVI and PRI are measurements of electromagnetic radiation re- flected from canopy surfaces, and therefore provide indirect or cor- relative associations with several canopy variables of interest and should not be treated as direct measurements of these variables.
  • Page 11 SRS Sensors 3 THEORY of photosynthetic vegetation in the field of view of a radiometer. See Royo and Dolors (2011) for an extensive introduction to using spectral indices for plant canopy measurements. Figure 3: Reflectance spectra for bare soil (Soil) and a healthy wheat crop at various stages of development: heading (H), anthesis (A), milk-grain stage (M), and post maturity (PM).

  • Page 12: Fractional Interception Of Photosynthetically Active Radiation

    3 THEORY SRS Sensors correlate well with green LAI, although the relationship is specific for each crop or natural canopy. For example, Aparicio et al. (2002) studied NDVI versus LAI in more than twenty different durum wheat genotypes in seven experiments over two years and found the rela- tionship shown in Figure 4.
  • Page 13 SRS Sensors 3 THEORY Robinson et al. (2012) suggest changes in LAI are difficult to detect when LAI is much greater than 3 m . This should not be surpris- ing considering the spectral measurement being made. NDVI mea- surements rely on reflected light from leaf surfaces. As the canopy fills and upper leaves begin to cover lower leaves, the leaf area will...

  • Page 14: Canopy Phenology

    Two of these variables are the focus of Ryu et al. (2010), who used a simple two-band LED-based sensor, similar to the SRS-NDVI, to measure canopy phenology and associated changes in photosynthesis in an annual grassland over a four year period. Ryu et al (2010).

  • Page 15: Photochemical Reflectance Index (Pri)

    PAR sensor or pyranometer. Considering the near linear relation- ship between NDVI and fractional interception noted above, a simple two-band spectral reflectance sensor like the SRS-NDVI can provide an estimate of f . The light use efficiency term ( ) remains to be quantified in order to make accurate predictions of GPP.

  • Page 16: Sun-Sensor-Surface Geometry Considerations

    3 THEORY SRS Sensors will absorb more radiation as a consequence of saturation of chloro- phyll centers. A normalized difference of reflectance at 570 nm that remains unchanged despite changes in light saturation to 531 nm will indicate the level of xanthophyll absorbance and the efficiency of plant light use.
  • Page 17 SRS Sensors 3 THEORY measurements acquired under different sun-sensor-surface configu- rations, it is necessary to first calculate a bidirectional reflectance distribution function (BRDF). Once you have empirically derived a BRDF model from the measurements and canopy-specific param- eters, you can use it to reduce variations that arise from changes in sun-sensor-surface geometry across diurnal time series.

  • Page 18: Calculating Percent Reflectance From Paired Up And Down Looking Sensors

    3 THEORY SRS Sensors Figure 7: Another daily variation in NDVI measurements 1 m above a sub-alpine meadow Calculating Percent Reflectance from Paired Up and Down Looking Sensors Equation 1 shows that NDVI is the ratio of the difference to the sum of NIR and red reflectances.
  • Page 19 field stop SRS to measure incident irradiance. To measure incident irradiance with a down facing sensor, place a reflectance standard within the field of view of the field stop sensor, making sure that the...
  • Page 20 3 THEORY SRS Sensors References Aparicio, N., Villegas, D., Casadesus, J., Araus, J.L., and Royo, C., (2000). Spectral vegetation indices as nondestructive tools for determining durum wheat yield. Agronomy Journal, 92: 83-91. Aparicio, N.; Villegas, D.; Araus, J.L.; Casadess, J.; Royo, C., (2002).
  • Page 21 SRS Sensors 3 THEORY NDVI and PRI over vegetated canopies. Agricultural & Forest Me- teorology, 150: 489-496. Garrity, S. R., Eitel, J. U. H., Vierling, L. A., (2011). Disentan- gling the relationships between plant pigments and the photochemi- cal reflectance index reveals a new approach for remote estimation of carotenoid content.
  • Page 22 3 THEORY SRS Sensors ting diodes (LEDs), to monitor ecosystem metabolism, structure and function. Agricultural & Forest Meteorology, 150: 1597-1606. Sims, D. A., Gamon, J. A., (2002). Relationships between leaf pig- ment content and spectral reflectance across a wide range of species, leaf structures and developmental stages.

  • Page 23: Connecting The Srs

    To update your software to the latest versions, please visit Decagon’s support site at http://www.decagon.com/support/. To use the SRS with your Em50 series data logger, simply plug the stereo plug into one of the five ports on the data logger and use either O Utility, or DataTrac 3 software (see respective manuals) to configure that port for the SRS and set the measurement interval.

  • Page 24: 3.5 Mm Stereo Plug Wiring

    1,440 minutes, including periods during the night. To avoid such errors, we recommend you log data from the SRS sensors more frequently, even if you are not using all logged data. If customers require logging intervals shorter than one minute, then they must use a Campbell Scientific or similar logger capable of...

  • Page 25: Pigtail End Wiring

    Note: The acceptable range of excitation voltages is from 3.6 to 15 VDC. If you wish to read the SRS with the Campbell Scientific Data Loggers, you will need to power the sensors off of a 12 V or switched 12 V port.
  • Page 26 4 CONNECTING THE SRS SRS Sensors tions. First, you can clip off the plug on the sensor cable, strip and tin the wires, and wire it directly into the data logger. This has the advantage of creating a direct connection with no chance of the sen- sor becoming unplugged;...

  • Page 27: Communication

    The SDI-12 protocol requires that each sensor have a unique ad- dress. The SRS comes from the factory with an SDI-12 address of 0. To add more than one SDI-12 sensor to a system, the sensor ad- dress must change.
  • Page 28 0.03 mA quiescent) and will allow the sensor array to be reset if a problem arises. Reading the SRS in SDI-12 mode using a CSI data logger requires a function call. An example program from Edlog and CRBasic can be...

  • Page 29: Understanding Data Outputs

    Each SRS sensor generates multiple outputs when connected to Decagon’s Em50 data logger. The exact outputs will in part depend on how many and what type of SRS sensors are attached to the data logger. All SRS sensors are equipped with an internal tilt sensor. The ori- entation of the SRS, and therefore the tilt sensor, will determine the output from each sensor.

  • Page 30: Using Other Data Loggers

    0 and 2, with 0 indicating an indeterminate orientation, 1 indicating a down facing orientation, and 2 indicating an up facing orientation. Additional information about using the SRS with non- Decagon data loggers can be accessed at www.decagon.com/srs. For additional information about connecting your SRS to non-Decagon data loggers, see Section 4.3.

  • Page 31: Installing The Srs

    7 INSTALLING THE SRS Installing the SRS Attaching and Leveling The SRS comes with a variety of mounting hardware, allowing it to be mounted on posts, rebar, poles, tripods, etc. The mounting hardware allows for vertical adjustment and orientation on the pole and allows for sensor tilt.

  • Page 32: Data Logger

    8 TROUBLESHOOTING SRS Sensors Troubleshooting Any problem with the SRS will most likely manifest as failed com- munication or erroneous readings. Before contacting Decagon about the sensor, please check these troubleshooting steps. Data Logger 1. Check to make sure the connections to the data logger are both correct and secure.
  • Page 33 Model Number: Year of First Manufacture: 2013 This is to certify that the SRS Spectral Reflectance Sensor, manu- factured by Decagon Devices, Inc., a corporation based in Pullman, Washington, USA meets or exceeds the standards for CE compliance as per the Council Directives noted above. All instruments are built at the factory at Decagon and pertinent testing documentation is freely available for verification.
  • Page 34 Index Calibration, 28 CE Compliance, 29 Cleaning, 27 Connecting Sensors Non-Decagon Logger, 20 Connecting the Sensors, 19 Contact Information, 1 Customer Support, 1 Declaration of Conformity, 29 Email, 1 Fax, ii Fractional Interception, 8 Installation, 27 Leaf Area Index(LAI), 6 Measurement Angle, 12 NDVI, 6 Phenology, 10...

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