Apogee Instruments S2-111 Owner's Manual - Page 16
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where 0.010 and 0.109 are the two
and 3.3905 are the two mV signals, and 0.8445 is the estimated NDVI value.
Change Sensor Address: aAb!
The change sensor address command allows the sensor address to be changed. If multiple SDI-12 devices are on
the same bus, each device will require a unique SDI-12 address. For example, two SDI-12 sensors with the factory
address of 0 requires changing the address on one of the sensors to a non-zero value in order for both sensors to
communicate properly on the same channel:
Command
Response
aAb!
b<cr><lf>
where a is the current (old) sensor address ("0-9", "A-Z"), A is an upper-case ASCII character denoting the
instruction for changing the address, b is the new sensor address to be programmed ("0-9", "A-Z"), and ! is the
standard character to execute the command. If the address change is successful, the datalogger will respond with
the new address and a <cr><lf>.
Send Identification Command: aI!
The send identification command responds with sensor vendor, model, and version data. Any measurement data
in the sensor's buffer is not disturbed:
Command
Response
"aI!"
a14Apogee S2-412vvvxx...xx<cr><lf>
where a is the sensor address ("0-9", "A-Z", "a-z", "*", "?"), 412 is the sensor model number, vvv is a three
character field specifying the sensor version number, and xx...xx is serial number.
Get/Set Alpha Command: aXALPHA! and aXALPHA+<alpha>!
The get/set alpha () command is used to get or set the estimated ratio of incoming red to NIR irradiance. This
ratio is used in the "aM2!" command to provide an estimated NDVI value from downward-facing NDVI
measurements alone (see "Reflectance and Reflectance Indices (NDVI)" section above). The alpha value can
substitute for direct measurements of incoming red and NIR irradiance when an upward-facing sensor is not
available. This value is usually in the range of 1.1 to 1.4, with lower values occurring at high solar zenith angles (sun
low in the sky) or under overcast sky and higher values occurring under clear sky at low solar zenith angles (sun
high in the sky). The default alpha value from the factory is 1.35. The alpha value and the "aM2!" command
provide an estimated NDVI value, but for the most precise NDVI measurements paired upward and downward-
facing sensors should be used to directly measure both incoming irradiance values and reflected radiance values.
Command
Response
"aXALPHA!"
a<alpha value><cr><lf>
"aXALPHA+<alpha>!"
a<cr><lf>
measurements for lower and upper wavelengths respectively, 0.2858
Watts m⁻²
Description
Change the address of the sensor (see example below)
Description
The current alpha value used for estimating NDVI is returned.
Sets the current alpha value used for estimating NDVI.
Description
The sensor serial number and other identifying values are
returned