Observations from which of these satellites can be used to calculate NDVI?Options: a. GRACE b. GPM c. Terra d. SNPP*1 pointGRACE and GPMGPM, Terra and SNPPTerra and SNPPSNPP only

The Normalized Difference Vegetation Index (NDVI) is a measure of the amount and health of vegetation on land. It is calculated from the visible and near-infrared light reflected by vegetation. Healthy vegetation absorbs most of the visible light that hits it, and reflects a large portion of the near-infrared light. Unhealthy or sparse vegetation reflects more visible light and less near-infrared light.

Now, let's look at the satellites mentioned in the options:

a. GRACE (Gravity Recovery and Climate Experiment) - This satellite is used to map Earth's gravity field and is not equipped to measure reflected light from vegetation.

b. GPM (Global Precipitation Measurement) - This satellite is used to measure rain and snowfall and does not have the capability to calculate NDVI.

c. Terra - This satellite carries the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument which can provide data to calculate NDVI.

d. SNPP (Suomi National Polar-orbiting Partnership) - This satellite carries the VIIRS (Visible Infrared Imaging Radiometer Suite) instrument which can also provide data to calculate NDVI.

So, the answer to the question "Observations from which of these satellites can be used to calculate NDVI?" is "Terra and SNPP".