Remote Sensing: A VERY Brief Overview Some basic concepts:

1. Remote Sensing: A VERY Brief Overview • Some basic concepts: • All natural surfaces emit photons (energy) when they are above absolute zero. • This emission follows Stefan-Boltzmann Law: • WB= T4 • The spectral distribution is given by Planck’s Law • WB= f( T), such thathigher emitting T, smaller 

2. Electric field Direction Magnetic field The Electromagnetic Wave

3. The Electromagnetic spectrum for a black body A gray body emits a fixed proportion of the blackbody radiation at that temperature: WB= T4 , where  is the emissivity of the surface. Object T WB Sun 6000OK 105 ly/min Earth 287OK 0.56 ly/min Snow 273OK 0.46 ly/min

4. Reflection, scattering, absorption As photons enter an object, some are reflected from the surface, some pass through the object, and some are absorbed. Those photons that are reflected from the surface or refracted through a particle are said to be scattered. Photons may also originate from a surface, a process called emission. Emitted photons are subject to the same physical laws of reflection, refraction, and absorption to which incident photons are bound. Photons are absorbed in objects by several processes. The variety of absorption processes and their wavelength dependence allows us to derive information about the chemistry of an object from its reflected or emitted energy.

6. The principles of a Radiometer A radiometer can in general be described with 4 parameters: Spectral range, which is important to cover enough diagnostic spectral absorptions to solve a desired problem. (There is some confusion in literature about the names of the spectral ranges. Depending on the field of interest different ranges are used for the terms near infrared (NIR), shortwave infrared (SWIR), middle infrared (MIR) and thermal infrared. Spectral bandwidth, which is the width of an individual spectral channel in the spectrometer. Spectral sampling. The shape of the bandpass profile. Ideally each radiometer channel rejects all energy except that from within a given narrow wavelength range. The most common bandpass in radiometers is a Gaussian profile. The width of the bandpass is usually defined as the width in wavelength at the 50% response level of the function. Signal-to-noise ratio (S/N). A radiometer must measure the spectrum with enough precision to record details in the spectrum.

7. Other sensors LIDAR LIDAR stands for LIght Detection And Ranging. It is a radiometer system that uses a light beam to obtain measurements of speed, altitude, direction and range of a target.

8. Other sensors/systems RADAR It is a radiometer system that emits an energy beam (in the microwave frequency) to obtain measurements of speed, altitude, direction and range of a target. SOUNDERS A sounder is a special kind of radiometer which measures changes of atmospheric temperature with height, and changes in water vapor content of the air at various levels.

9. Where do we observe from space? In view of the properties of the atmosphere, traditionally two windows are used: 1.optical window, wavelengths from 300 - 14000 nm 2.microwave window, wavelengths from 1 mm - 1 m. The optical window is often subdivided into a reflective optical part (300-3000 nm) and a thermal infrared part (3000 – 14000 nm)

10. Water Vapor Window

11. Orbits Altitude and inclination Orbital altitude, the height above the surface of the earth; inclination is defined as the angle between the earth's equatorial plane and the satellite's orbital plane. Repeat Cycle A repeat cycle is defined as the number of days before the satellite passes over exactly the same orbital path relative to the earth's surface. Descending Node Crossing (DNC) The point in the orbit path of a satellite, where the orbit crosses the earth equator and the spacecraft moves from North to South. Low Earth Orbit (LEO) A satellite orbit with a typical altitude range of several hundred kilometres is called LEO.

12. Orbits (cont.) Geosynchronous Orbits An orbit is geosynchronous if it has a fixed position relative to the surface of the earth Geostationary Orbits A geostationary satellite is a geosynchronous satellite without an inclination angle. Sun-synchronous Orbits For near-earth satellite orbits (lower than 1000 km) sun-synchronous orbits are polar, and the inclination and altitude is such that a satellite passes over a given site always at the same local time. Nadir Mode In Nadir Mode, the line of sight of a sensor points from the orbit downward to the earth’s surface or atmosphere. An alternative measurement mode of spaceborne remote sensing instruments is the Limb Mode.

13. Soil Moisture Algorithm Results OklahomaH Polarization DMSP Special Sensor Microwave/Imager (SSM/I) • Shown here are 1.4 GHz results obtained using an aircraft sensor and 19 GHz satellite data • The difference between the sensitivity of the two instruments is quite apparent • Standard error of estimates:1.4 GHz = 2.6% and 19 GHz = 5.3%.

15. July 4-6 Aqua AMSR Three Day Composite Brightness Temperature

16. July 4-6 Aqua AMSR Three Day Composite Brightness Temperature