Apogee Instruments S2-111 Manual do Proprietário - Página 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.
−
=
+
(version 1.4, August 10, 2016).