Oklahoma State University

Pocket Sensor Guidelines

1)    The sensor comes pre-calibrated to a Greenseeker sensor. (the manual below would indirectly suggest that the sensor might not be calibrated)
2)    The sensor can be calibrated to an “NDVI “ sensor (not just a  Greenseeker)
3)    The existing “Greenseeker calibration must be turned to FALSE (G, F) before re-calibrating

One thing that needs to be enforced is charging the battery of the pocket sensor every night.
Also, before calibration, the sensors need to be fully charged.
This will ensure the sensor is operating at it's best.

Charging:

·         Attach the provided USB cable attachment to the front side of the pocket sensor, then plug in the USB cable directly into a USB port on a computer.  Charge for 3-4 hours.

·         Battery life is approximately 6-10 hours, depending on operation

·         A “batt” symbol will appear in the upper left-hand corner of the LCD screen if a low-battery condition occurs.

Operation

·         Care must be taken to keep the pocket sensor level while sensing, at a height of 24” to 30” (60-75 cm) above the crop canopy.  The target is circular with a dimension shown in Figure 1.

·         To operate, place the pocket sensor at the appropriate height above the canopy, then  push and hold the red button.

·         While holding the red button, walk slowly down the treatment (or strip) that you want to sense.  You will notice the NDVI value being displayed on the screen changing once per second during operation.

·         When you are done sensing, release the red button and note the last number displayed on the screen.  This is the average of all readings that were taken while the button was depressed.

·         Write down or remember this number quickly, because it will only be displayed for a few seconds before it disappears.  Ensuing readings once the red button is pressed again start anew. 

·         The pocket sensor does not have the capability of storing data, so you must record the NDVI values that you need after each operation.

Figure 1.  Target dimension for different sensing distances

Storage

·         Store in the cardboard box provided.  Be very cautious about placing the pocket sensor face down on any surface, as this could scratch the LED lenses.

Error Messages

·         An error message may be displayed on the screen if something is wrong.  The following messages may appear, with the correct description:

o  E01 – calculated NDVI value is less than 0, which is invalid

o  E02 – calculated NDVI value is greater than 1, which is invalid

Hours of Operation

·         The pocket sensor should provide the same readings day or night, as this is an active sensor.  You will not want to use the sensor if it is raining or in extreme heat or cold.

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This should already have been accomplished, but if not, re-calibration procedures follow

Calibration Procedure for Pocket Sensor:

1.      Before the Pocket Sensor can be calibrated, any existing calibration must be set to 0.  Do this by:

A.     Plug the pocket sensor into a USB port on a computer, and follow the Installation instructions to download the driver onto the pocket sensor.  The correct driver to download is the Silicon Labs CP210x USB to UART Bridge.  This can be downloaded from http://www.silabs.com/products/mcu/pages/usbtouartbridgevcpdrivers.aspx and select the correct computer operating system.

B.      A terminal program must be downloaded onto the computer to communicate with the pocket sensor.  Go to https://sites.google.com/site/terminalbpp/ and download the terminal program available on that website.

C.       With the pocket sensor plugged into the computer, open the terminal program.  At the top of the page, make sure the settings are as follows:

a.      Baud rate:  19,200

b.      Data bits: 8

c.       Parity:  none

d.      Stop bits:  1

e.      Handshaking:  none

D.     Select the correct COM port by looking at the Device Manager on your computer to determine where the Silicon Labs CP210x USB to UART Bridge is located.

E.      Hit the “Connect” button at the top-left corner of the screen.

F.       In the bottom portion of the screen, press “p” to print calibration numbers and the menu options.

a.      --Caution!!!     DO NOT press “f” while on the menu page, as this will restore the memory to the default settings, and will render the sensor unusable until reset by an OSU Engineer!!!!

G.     Press “g” to set the coefficients for the calibration equation.

H.     Once “g” is entered, the terminal will display “Enter CalNum.u8Flag.(T/F)……” press “f”.

I.        The formula is in the form of:  y = Ax² + Bx + C.  The values A, B, and C need to display 0 for calibration

-- “Enter CalNum.fpA….”    Enter 0 for A, then press Enter
-- “Enter CalNum.fpB….”    Enter 0 for B, then press Enter
-- “Enter CalNum.fpC….”    Enter 0 for C, then press Enter

J.        In the terminal program select “Disconnect” the sensor has now been set to its original calibration, and can be calibrate with the GreenSeeker.

2.      Calibration must be done with a Greenseeker.  Take at least 70-100 readings with both the Pocket Sensor and the Greenseeker over different types of surfaces (bare soil, different shades of vegetation, etc.).

Note:  Make sure that the areas sensed by both the Greenseeker and Pocket  sensor are exactly the same.  Pick an area about 24” (60 cm) square, that is flat and homogenous.

3.      Enter the NDVI values of both Greenseeker and Pocket sensor into Microsoft Excel.  Place the Pocket sensor values in the left column (for the independent, x-axis) and the Greenseeker values in the right column (for the dependent, y-axis). 

4.      Plot these values as a scatter plot.  Make sure that Pocket sensor values are on the x-axis and Greenseeker values are on the y-axis.

5.      On the scatter plot, right-click on the mouse over the points, and select “Add Trendline”.

6.      On the Trendline menu, select “Polynomial” on the Regression Type, and “2” on the Order.  Also check the boxes for “Display Equation on chart” and “Display R-squared value on chart”.

7.      The Excel chart will now display the calibration equation that needs to be entered into the pocket sensor for calibration.

Entering the Calibration Equation into the Pocket Sensor:

1.      Connect the Pocket Sensor to the computer and open the terminal program as outlined in previous section.

2.      Press “p” to print calibration numbers and menu options.

--Caution!!!     DO NOT press “f” while on the menu page, as this will restore the memory to the default settings, and will render the sensor unusable until reset by an OSU Engineer!!!!

3.      Press “g” to set the coefficients for the calibration equation.

4.      Once “g” is entered into the terminal, enter the values for the calibration equation.

5.      Press “t” for True when it states “Enter CalNum.u8Flag.(T/F)……”

6.      The formula is in the form of:  y = Ax² + Bx + C.  The values A, B, and C will then be entered into the terminal program:
-- “Enter CalNum.fpA….”    Enter value for A, then press Enter
-- “Enter CalNum.fpB….”    Enter value for B, then press Enter
-- “Enter CalNum.fpC….”    Enter value for C, then press Enter

                       Be careful to note the sign (+ or -) for the Ax² value.

7.      The Pocket Sensor is now calibrated and ready for use.  Press “Disconnect” in the upper-left corner of screen to disconnect the device.

Note:  While the Pocket sensor is plugged into the terminal program, it is possible to capture individual sensor readings.  To do this, plug the sensor into the computer and connect to the terminal program, and press “p” to display the sensor values.

Experiences with the PocketSensor:

           During calibration of the Pocket sensor at the CIMMYT Research Station in El Batan, Mexico, some considerations were:  Wheat is planted on raised beds, and at the time of sensing the wheat was headed out (Reproductive Stage).  The height of the wheat and the distance between beds allowed the GreenSeeker to read soil background while the Pocket sensor was only able to read the rows of wheat, the soil was to far away.  It is imperative that both the GreenSeeker footprint and Pocket sensor be over the same targeted area to get an accurate calibration.  Note:  This may not be an issue when the wheat is vegetative and the soil is still within range of the Pocket sensor.