On August 4, 1998 NASA's Advanced Thermal and Land Applications Sensor (ATLAS) was flown over experimental fields in southwestern Georgia. The flights were designed to acquire high resolution (~2.5 meter) imagery over the farm at least twice within 30 minutes. Our experiment was to study the energy budget of the crops and demonstrate a relationship between multi-temporal thermal imagery and crop yield. As often happens with research, the results were unexpected.

Because there is overlap between some of the flight lines, several of the fields were actually imaged 4 times. Panel 1 (above) shows one such field and the GMT time stamps. The data are from the Thermal IR (Band 13 of the sensor). The first feature most people note is the streaking from lower left to upper right. The streaking changes between images and there are zones which on average are brighter or darker. The signal is essentially pure variation in temperature of the crop, soybeans.

Panel 2 (above) shows the same views in the Near-IR (Band 6 of the sensor). All of these images were processed to use the full dynamic range within the single field. Clearly the Near-IR reflectivity is varying rapidly within 3-4 minutes. What is especially significant is that this Near-IR reflectance variation is highly correlated with the emitted, thermal signal. This is illustrated by panel 3 which shows scattergrams for Near-IR and visible (Band 3) versus Thermal-IR for each data set. Even the visible bands in at least on case are correlated with the thermal and the spatial distribution of the thermal signal is changing radically within minutes!

We have considered several possible explanations for these observations. For example artifacts of the scanner, artifacts of the crop rows, artifacts of look angle or some combination of these. Multiple points prove these are not factors:

1. The same patterns are equally visible in many of the fields at the farm.
2. The track of the scanner's sweep is oblique to the average trend of the streaks.
3. Various fields have differing row orientations.
4. The crops involved include soybeans, cotton and peanuts.
5. The 4 images are taken from just two views, as the aircraft flew nearly identical tracks on the repeat flights.

We think this is a response to wind. Wind is the only phenomena which can change this rapidly and on such a small spatial scale and affect a variety of crops several kilometers apart. The Albany, GA airport, which is less than 16 kilometers north of the fields provided the following data.

10:47 Z - calm, less than 3 knots
11:47 Z - calm, less than 3 knots
12:47 Z - calm, less than 3 knots
13:50 Z - 60 degrees at 5 knots
14:50 Z - 60 degrees at 9 knots

The direction is in agreement with the observed streaking in the thermal images. Ground observation by a meteorologist supporting the data acquisition at the experimental farm at time of over flight reports clear skies, no clouds.

As yet we do not have a detailed explanation for the mechanisms involved. Probable factors include the incidence angle of irradiance on the leaves, the elastic behavior of plant stems, the coupling of leaf with air temperatures.

The significance of the observations is that this is a previously unrecognized source of significant variation in visible and Near-IR imagery.

paul.meyer@msfc.nasa.gov

Responsible Official: Dr. Steven J. Goodman (steven.goodman@nasa.gov)
Technical Contact: Dr. Doug Rickman (doug.rickman@msfc.nasa.gov)
Page Curator: Diane Samuelson (diane.samuelson@msfc.nasa.gov)