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SRS Sensors
6
Understanding Data Outputs
6.1
Using Decagon's Em50 series data loggers
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.
6.1.1
Up Looking Sensor Outputs
For any hemispherical sensor oriented in the up looking position,
outputs will include the calibrated spectral irradiance (W m
and α, where α is the ratio of 630 nm to 800 nm for NDVI sensors
and 570 nm to 532 nm for PRI sensors. See Equation 4 for further
details on α.
6.1.2
Down Looking Sensor Outputs
When hemispherical or field stop sensors are mounted in a down-
looking orientation, outputs include the calibrated spectral radiance
2
1
(W m
nm
up looking and down looking sensors of the same variety (e.g., up
looking hemispherical NDVI and down looking field stop NDVI) are
connected to the same data logger then α from the up looking sensor
is combined with the spectral radiance values from the down looking
sensor to calculate the vegetation index, using Equation 4. In the
event that only down looking sensors are connected to a data logger,
then either the default or the user-specified static α value is used
to calculate the vegetation index. Based on observations collected
near Pullman, WA (46 45'0"N, 117 09'6"W), default α values have
been set to 0.98 and 1.86 for PRI and NDVI, respectively. Note that
actual values of α will change depending on atmospheric conditions
and sun angle, so users are encouraged to measure α with an up look-
ing hemispherical sensor. In the event that nearby up looking and
6 UNDERSTANDING DATA OUTPUTS
1
sr
) of each band and either NDVI or PRI. If both
25
2
1
nm
)
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