Apogee Instruments S2-411 소유자 매뉴얼 - 페이지 13

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While paired upward-looking and downward-looking sensors provide the most accurate measurements of NDVI,

radiance measurements from downward-looking sensors can also be used to approximate NDVI without

calculating reflectance from paired upward-looking and downward-looking sensors. The equation has a similar

form to the NDVI equation above, but uses radiances (R) in the calculation:

where  is the ratio of red irradiance to NIR irradiance (the values measured by an upward-looking sensor). Data

from multiple solar spectra indicate  ranges from about 1.1 to 1.4, with lower values occurring at high solar zenith

angles or under overcast sky and higher values occurring under clear sky at low solar zenith angles. Approximations

of NDVI from the equation above are least sensitive to the value of  when the difference between NIR and Red

radiances are relatively large (e.g., measurements over green vegetation) and most sensitive to the value of 

when the difference between NIR and Red radiances are relatively small (e.g., measurements over senesced

vegetation or soil).

Sensor Calibration

All Apogee SDI-12 NDVI sensor models have sensor-specific calibration coefficients determined during the custom

calibration process. Coefficients are programmed into sensor microcontrollers at the factory.

SDI-12 Interface

The following is a brief explanation of the serial digital interface SDI-12 protocol instructions used in Apogee S2-

400 series NDVI sensors. For questions on the implementation of this protocol, please refer to the official version

of the SDI-12 protocol:

http://www.sdi-12.org/specification.php

Overview

During normal communication, the data recorder sends a packet of data to the sensor that consists of an address

and a command. Then, the sensor sends a response. In the following descriptions, SDI-12 commands and

responses are enclosed in quotes. The SDI-12 address and the command/response terminators are defined as

follows:

Sensors come from the factory with the address of "0" for use in single sensor systems. Address "1 to 9" and "A

to Z," or "a to z," can be used for additional sensors connected to the same SDI-12 bus.

"!" is the last character of a command instruction. In order to be compliant with SDI-12 protocol, all commands

must be terminated with a "!". SDI-12 language supports a variety of commands. Supported commands for the

Apogee Instruments S2-400 series two-band radiometers are listed in the following table ("a" is the sensor

address. The following ASCII Characters are valid addresses: "0-9" or "A-Z"). Please note that SDI-12 commands are

case-sensitive.

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(version 1.4, August 10, 2016).