In simple terms, remote sensing means obtaining information about an object from a distance. This can be done in various ways including from space, air, or the ground.

The earth receives energy from the sun through electromagnetic radiation. This radiation is either emitted, reflected, or absorbed by the earth.

Satellites carry instruments or sensors that measure the electromagnetic radiation reflected or emitted. These calibrated instruments can measure features of the earth surface including fire, vegetation cover, moisture content and so on.

Most of the electomagentic spectrum cannot be detected by the human eye. In fact, even satellites and sensors could only capture a small portion of the spectrum.

Different materials have different spectral signature. That means that different materials reflect different amounts of electromagnetic radiation. For instance: 

• Pigments in plants absorb wavelengths of visible light, while leaves reflect infra read which are invisible to the human eye. As plant changes through season, the reflectance properties also change
• Water absorbs longer visible wavelengths (green and red) and NIR than shorter visible wavelength
• Electromagnetic radiation passes through the atmosphere twice; and atmospheric correction is fundamental to obtain the surface reflectance characteristics.

Characteristics of Satellites

Orbits

• Geostationary satellites orbit approximately 36,000km over the equator with the same rotational period over the earth.
• Low Earth orbit could be polar (Landsat or terra) or non polar (GPM)

Energy Source

• Passive sensors examples include Landsat OLI/TIRS, Terra MODIS, GPM GMI, GRACE. They use energy emitted or reflected by earth systems
• Active Sensors: Provide their own source of energy for illumination. They often use microwaves. Examples include Lidar, Radar and Sentinel missions

Resolution of an Image

Spectral Resolution

• Spectral resolution defines the spectral bands of the sensor e.g. Visible, UV, IR, Microwave…
• It could be hyperspectral, multispectral, or panchromatic.

Spatial Resolution

• Refers to the ground cover that forms one pixel in the image. High spatial resolution means a small area is covered by each image.
• For instance Landsat has a resolution of 30 m while MODIS has a resolution of between 0.25 to 1 Km.

Temporal Resolution

• Refers to the amount of time it takes for a satellite to complete one cycle> Also called revisit time.
• For instance Landsat has a resolution of 16 days while MODIS has a resolution of 2 days.

Radiometric Resolution

• Refers to the sensors ability to discriminate differences in energy, or radiance.
• The higher the number, the more sensitive a sensor is to small differences in energy. For instance, Landsat OLI has a radiometric resolution of 16 bit while Landsat MSS is 6 bit.

Satellite data processing level refers to processing from a raw level to a polished product that visualizes information. NASA has levels 0-4 with 0 being the most difficult to use and 4 being the easiest.