Aerial Photography and Remote Sensing – Study Outline

microwave, radar

You should be able to:

1. describe the place of microwaves in the electromagnetic spectrum

2. give a simple description of how radar works

3. describe the factors that determine how the microwaves interact with the surface

4. list some benefits and problems with radar images

A. Microwaves

1. are the part of the spectrum that comes after infra red, with wavelengths of about 1 – 100 cm .

2. their benefits include

a. very small natural thermal emissions

b. the ability to penetrate clouds and some dry soils

B. Radar

1. radar is an active remote sensing technology, meaning we supply the “light” to illuminate the image

2. the basic process is to generate a very short pulse of microwaves, which goes out, bounces off the surface and returns to the antenna. The time it takes to make the trip and the strength of the return signal determine the distance and nature of the surface.

3. the resolution gets better

a. with a shorter pulse

b. with a shorter wavelength

c. with a longer antenna.

d. closer to the antenna

4. Interaction with surface

a. radar is scattered from rough surface, resulting in a moderate return signal

b. radar is reflected from smooth usually resulting in a weak return sign

c. reflected back to the antenna by a smooth, right angle corner (e.g. building) for the strongest return signal

d. rough vs. smooth depends on wavelength and angle

i. the longer the wavelength to smoother to surface looks (inch diameters rocks can look smooth to radar)

ii. the smaller the angle of the beam away from horizontal, the smoother the surface

e. the return signal also depends on an electrical property of the surface, it’s dielectric constant, which is low for most earth materials (rocks and soil), moderate for metal and high for water

i. this means that cultural feathers with metal in them and many corner reflectors often stand out

ii. shorter wavelengths interact strongly with water and therefore with plants.

iii. longer wavelengths have lower resolution but can penetrate plant canopies and even a short distance through dry soil.

5. Radar advantages

a. night operation

b. ability to see 100+ km to the side from an airplane

c. suppression of detail makes underlying topography easier to see

d. ability to see through clouds

e. very accurate distance measurements

6. Radar image problems

a. layover, where nearby tall features are strongly tilted toward the antenna

b. speckle

c. fading with distance from the antenna

C. Microwave radiometer

1. while thermal microwave emissions are small they are measurable.

2. measuring temperature with microwaves has the advantage better transmission through the atmosphere

3. sophisticated signal analysis can separate out the contribution from different layers of the atmosphere, so that a temperature profile, from the ground to the sensor, can be displayed