Satellite Imaging
Viewing
Satellites orbit the earth taking pictures of either outer space or the earth itself. Certain satellites also use electromagnetic waves to take measurements of changes in the earth’s climate and weather. NASA and other organizations have many satellites procuring information on these changes. For our purposes, we will particularly explore NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS), as its data was imperative in our research. Here’s an image of the earth taken by MODIS. The satellite takes one image of 2,330 km of the earth at a time and provides a complete image of the earth every one to two days.
You can find all the information you want including data measurements here.
Scaling
The pixels that make up these images are also geographic placeholders that take MODIS data measurements as associated values. Different satellites take readings at different resolutions. For example, one “pixel” may have the dimensions of a 5km square. This would mean that the measured value of say, temperature, is assigned to every latitude/longitude pair within that 5km2 area. MODIS collects different data at resolutions of 250m, 500m, and 1,000m. Along with differences in resolution, there are also differences in the amount of time passing between data collection. MODIS collects data on daily, 8-day, 16-day, monthly, quarterly, and yearly time-scales. A lack of uniformity in these resolutions and time-scales makes combining data products into models of climate trends difficult, and was a major complication in my research.
Reading
The final piece of the puzzle in understanding how satellites acquire data is their ability to convert electromagnetic waves into inferences on environmental features. This is done with the help of the Spectroradiometric Calibration Assembly pictured.
You can find all the information you want here.
The device uses mirrors and slits to accurately beam waves of light in three different forms to earth. These forms include visible, near infrared, and short wave infrared. The visible waves are used mainly to record accurate geographic boundaries of the areas measured. Near infrared is more easily absorbed by water than land which helps in determining boundaries, and it is also helpful in determining areas of vegetation as plants reflect this wavelength strongly and healthier plants more so than stressed ones. Finally, short wave infrared can determine accurate water measurements in both water bodies and soil. These readings are crucial in extraditing data for research on the changes in an area’s water composition over time.
Application of Satellite Imaging
Data inputs from satellites are able to indicate general water trends when incorporated into algorithms. In my research, we concentrated on their contribution to trends in evapotranspiration. Evapotranspiration is the combination of evaporation from soil and transpiration from plants. Evapotranspiration is a key component of the water cycle. The major components of this cycle are indicated in the infographic below:
Water cycles between land and the atmosphere. An area of land is exposed to both the water that falls upon it through precipitation (P) and the water added via irrigation (I). The addition of these two factors (P, I) yields the total water put into use in a given area. The total water put into use has to equal the total water expelled. Water is expelled through means of evapotranspiration (ET), runoff (Q), and groundwater storage (ΔS). Given this equality, we arrive at the above equation.
ET is the base measurement of global water used. This product is derived from surface energy balance models that input factors collected from satellites including albedo, LST, and NDVI, and then output values for ET. The below table indicates particular measurements included in each ET model:
