Microwave Sensing Systems Uses sensors that operate in the microwave portion of the EM Spectrum

Microwave Spectrum includes wavelengths within the approximate range of 1 mm to 1m.

Scanners and radiometers can operate outside the visible and photographic wavelength regions. Note: scale is logarithmic.

● Electromagnetic Spectrum

Visible (VIS) : 0.4 µm to 0.7 µm (400 to 700 nanometers)

Infrared (IR): 0.7 µm to 14 µm Microwave : 1 mm to 1 m

In the visible, features are observed by virtue of reflected solar energy. By contrast, in the infrared portion, sensing of emitted energy predominates. The dividing line between sensing reflected and emitted radiation is the wavelength of ~3 µm.

Radar is an acronym for Radio Detection and Ranging. Radar is an active sensor systems. It generates its own illumination as an outgoing signal that interacts with the target such that some of the signal is returned as backscatter that is picked up by the same antenna that emitted the radar beam.

Radar operates in part of the microwave region of the electromagnetic spectrum, specifically in the frequency interval from 40,000 to 300 megahertz (MHz). The latter frequency extends into the higher frequencies of the broadcast-radio region.

Radar

Water vapor, dust, smoke, airborne pollutants and other small particles close in magnitude to visible and infrared (VIR) wavelengths cause interference in the path between target and sensor. VIR wavelengths can be dispersed or blocked before they reach the sensors. Since microwaves are longer, they are not as affected by these types of small particle matter. Therefore, sensors transmitting and/or receiving microwaves are able to 'see' through haze, cloud, light rain, snow, smoke, and pollution. As a result, radar images can yield valuable information that is not available in VIR images.

Atmospheric Penetration

categories of Remote sensing radars

Real aperture and Synthetic Aperture Radars (SAR).

Side-Looking Airborne Radar (SLAR)

Radar altimeter

Microwave scatterometer

Real aperture radars transmit and receive microwave signals with a fixed length antenna. They are limited in their ability to produce resolutions fine enough for most remote sensing applications, simply because it is difficult to transport a very long antenna.

To solve this problem synthetic aperture radars (SAR) were developed. SARs have physically shorter antennas, which simulate or synthesize very long antennas. This is accomplished through modified data recording and signal processing techniques.

Side-Looking Airborne Radar (SLAR)

A radar pulse is transmitted off-nadir by an antenna fixed below an aircraft to image large ground areas adjacent to the flight line. The echoes are processed to produce an amplitude/time video signal which is then recorded as an image line, with brighter pixels indicating higher energy returns. The oblique look angle used to acquire radar imagery results in characteristic geometric distortions such as radar shadows and layover effects.

The propagation of one radar pulse is shown using solid lines to indicate the wavefront locations at time intervals 1 to 12. The reflected waves or echoes are shown by dashed lines beginning at time 7 for the house and 9 for the tree. These return signals reach the antennae at times 13 and 17 respectively.

The antennae response graph shows a strong echo for the house at time 13 and a weaker echo for the tree at time 17. The strength of the echo depends on the way an object reflects radio waves.

Pixel size in SLAR imagery is determined by the time duration of the pulse and the beamwidth of the SLAR antennae.

Radar altimeter

This is a non-imaging radar which detects the EM backscattering of a surface from a narrow pulse with near normal incidence. By directing the pulse onto the Earth's surface from a nadir position (that is, directly overhead) the distance being measured is the altitude of the scanning platform above the surface.

Radar altimeter

Over oceans, radar altimeters are used to determine significant wave height, wind speed and mesoscale topography. Data from this non-imaging device can also be plotted in image format, such as to map sea surface topography, if the required data volume is available. Other applications include mapping surface topography and type of ice masses and sea/ice boundaries.

Microwave scatterometer This device measures the microwave scattering or reflective

properties of surfaces. It is also non-imaging, and is specifically designed to measure backscattering. The surface is scanned in two or more directions, usually by multiple sensors.

The primary application of microwave scatterometers is to measure wind vectors (that is, speed and direction) over the ocean surface. Although radar scatterometers are non-imaging, their data may be used to construct global wind

maps when collected over a sufficiently large area.

Scatterometer

A scatterometer is a microwave radar sensor used to measure the reflection or scattering effect produced while scanning the surface of the earth from an aircraft or a satellite.

Two Generic Types of Sensors

• A "passive" system generally consists of an array of small sensors or detectors which record the amount of electro-magnetic radiation reflected and/or emitted from the Earth's surface.

• An "active" system propagates its own electro-magnetic radiation and measures the intensity of the return signal.

Examples of Passive Sensors

• Advanced Very High Resolution Radiometer (AVHRR) Sea Surface Temperature

• Sea-viewing Wide Field-of-View Sensor (SeaWiFS) Ocean Color

Why Sea-Surface-Temperature?

• Sea surface temperature (SST) is a key parameter for energy transfer between the oceans and the atmosphere and AVHRR-derived SST can deliver important data to meteorological and climatological models.

• AVHRR imagery is also useful for meteorological applications (for example, providing detailed imagery of cloud systems), and land applications such as vegetation mapping and monitoring.

Advantages of passive microwave remote Advantages of passive microwave remote sensingsensing

• Sees through clouds at lower frequencies• Long heritage, various instruments since 1978• Emissivity sensitive to state of surface, particularly

moisture– Soil moisture

– Snow-water equivalent

– (water is ~80× as absorptive as ice at these frequencies, whereas in visible through infrared, water and ice have similar absorption coefficients)

• But, because of small amount of energy emitted, pixel size must be large

Microwave Remote Sensing: Principles and Applications.

• Advantages– Day/night coverage.– All weather except

during periods of heavy rain.

– Complementary information to that in optical and IR regions.

• Disadvantages– Data are difficult to

interpret.– Coarse resolution

except for SAR.

Infared Satellite Imagery

• These images were obtained with an AVHRR sensor (Advanced Very High Resolution Radiometer) carried on a NOAA Polar Orbiting Environmental Satellite (POES).

• Measures the amount of thermal infared radiation given off by the surface of the ocean.

• The amount of thermal infared radiation given off by an object is related to its temperature (dying embers give off less radiation than a hot fire and a person gives off even less).

• Thus by measuring the amount of radiation given off by the ocean we can calculate its temperature.

Infared Satellite Imagery

• These images were obtained with an AVHRR sensor (Advanced Very High Resolution Radiometer) carried on a NOAA Polar Orbiting Environmental Satellite (POES).

• Measures the amount of thermal infared radiation given off by the surface of the ocean.

• The amount of thermal infared radiation given off by an object is related to its temperature (dying embers give off less radiation than a hot fire and a person gives off even less).

• Thus by measuring the amount of radiation given off by the ocean we can calculate its temperature.