Lecture 16 – Active Microwave Remote Sensing 2 December 2008

Lecture 16 – Active Microwave Remote Sensing2 December2008

Recommended Readings • Chapter 7 in Campbell

Active microwave systems operate at wavelengths (3 to 70 cm) that are not influenced by the atmosphere, e.g., these wavelengths have 100% transmission • Figure 1-18 from Elachi, C., Introduction to the Physics and Techniques of Remote Sensing, 413 pp., John Wiley & Sons, New York, 1987.

Lecture 16 Topics • Definition of RADAR • Measurements made by a RADAR - Range to the target, Azimuth resolution, Range resolution, Intensity of the returned pulse, • Microwave or Radar backscatter • Factors controlling microwave backscatter • Surface roughness • Bragg scattering • Variations in dielectric constant • Spaceborne Radar Systems and Applications • Altimeters • Scatterometers • Synthetic Aperture Radar (SAR)

The basic concept of RADAR was discovered by scientists working at the Naval Research Laboratory who were investigating using microwave EM energy as a source for radio transmissions

RADAR – Radio Detection and Ranging • Concept behind radars discovered in 1923 • RADARs systems invented in the 1930s • A high powered, radio transmitter/receiver system was developed that would transmit a signal that was reflected from a distant object, and then detected by the receiver • Thus, RADAR’s initial function was to detect and determine the range to a target

Microwave Transmitter / Receiver Target Antenna Microwave EM energy pulse transmitted by the radar Microwave EM energy pulse reflected from a target that will be detected by the radar

Key Components of a Radar System • Microwave Transmitter – electronic device used to generate the microwave EM energy transmitted by the radar • Microwave Receiver – electronic device used to detect the microwave pulse that is reflected by the area being imaged by the radar • Antenna – electronic component used through which microwave pulses are transmitted and received

Band Frequency Wavelength (most common) X 8 to 12 GHz 2.5 to 4.0 cm (3.0 cm) C 4 to 8 GHz 4 to 8 cm (6.0) L 1 to 2 GHz 15 to 30 cm (24.0) P 0.3 to 1 GHz 30 to 100 cm (65 cm) Common Radar Bands

Radar systems control the polarization of both the transmitted and received microwave EM energy Figure 9.6 from Jensen

Radar System Designation Radar systems typically have a 3 letter designation to describe the frequency-polarization of operation: • First letter denotes the radar frequency and wavelength (e.g., X,C, L,P – see slide 10) • The second letter denotes the polarization of transmitted EM waves (H for horizontal, V for vertical) • The third letter denotes the polarization of the received EM waves (H for horizontal, V for vertical) For example, an C-VH radar is one that transmits EM radiation at a C-band wavelength (between 4 and 8 cm), it transmits horizontally polarized EM energy and it receives vertically polarized EM energy

Lecture 16 Topics • Definition of RADAR • Measurements made by a RADAR - Range to the target, Azimuth resolution, Range resolution, Intensity of the returned pulse • Microwave or Radar backscatter • Factors controlling microwave backscatter • Surface roughness • Bragg scattering • Variations in dielectric constant • Spaceborne Radar Systems and Applications • Altimeters • Scatterometers • Synthetic Aperture Radar (SAR)

Measurements made with a simple radar • Range to the target • Range resolution • Azimuth resolution • Intensity of the returned pulse

Microwave Transmitter / Receiver Target Antenna Microwave EM energy pulse transmitted by the radar Microwave EM energy pulse reflected from a target that will be detected by the radar

Microwave Transmitter / Receiver Target 1. Transmitted pulse travels to the target Antenna 2. The target reflects the pulse, and the reflected pulse travels back to the microwave antenna / receiver, where it is DETECTED 3. The radar measures the time (t) between when the pulse was transmitted and when the reflected signal reaches the receiver – The time it takes the pulse to travel from the radar to the target and back is used to estimate the RANGE

Radar range - R The distance, R, from the antenna to the target is calculated as R = ct / 2 where c is the speed of light (3 x 10-8 m sec -1) t is the time between the transmission of the pulse and its reception by the radar antenna

Pulse Duration (p) p Radars send out pulses of EM energy, e.g., a burst of energy that lasts for a very short time period, the pulse duration

Pulse Duration (p) and Pulse Length Radar systems transmit microwave pulses with of specific durations - p The pulse length of the system defines the range resolution (r) of the radar

Antenna Beamwidth -  Microwave Transmitter / Receiver Antenna  The microwave energy transmitted by a radar is focused into a beam, with an angular dimension, 

Antenna Beamwidth -  If the length of the antenna is L, and the microwave wavelength is , then  =  / L

Azimuth Resolution - a Microwave Transmitter / Receiver Antenna  a is the azimuth resolution of the radar, e.g., the distance 2 targets have to be separated in order to be distinguished by the radar The direction parallel to the antenna length is called the azimuth dimension

Azimuth Resolution - aReal Aperture Radar Microwave Transmitter / Receiver Antenna a R  a =  R

Synthetic aperture radar (SAR) is a specific type of imaging radar system – A SAR operates by continuous transmitting and receiving pulses reflected from a target the entire time the target is within the beamwidth of the system

The pulses transmitted and received by a SAR are linearly swept in frequency, e.g., the frequency of the pulse is lower at the beginning of the pulse than at the end A single target results in thousands of pulses that are detected and recorded by the SAR These pulses are specially processed using fourier transforms to recreate a single point on the image representing the imaged target

Azimuth resolution for a Synthetic Aperture Radar By collecting data over a very long time, a SAR creates an synthetically long antenna or aperture (Ls) – hence the term synthetic aperature radar As a result, the azimuth resolution of a SAR is independent of range to the target: a = L /2 where L is the actual length of the antenna Ls Slides 26 to 27 are for background only – know material on this slide

The Radar Equation

Microwave (Radar) Backscatter When microwave EM energy transmitted by a RADAR system reaches the earth surface, some is absorbed by the surface and the remainder is reflected in multiple directions In microwave remote sensing, surface reflection is referred to as scattering of microwave EM energy The microwave EM energy that is scattered in the Radar’s direction of transmission is the only EM energy that is detected by the radar – this EM energy is referred to as microwave or radar backscatter

Radar cross section - σ Radar cross section is the area of a theoretical, perfect reflector of EM energy (e.g., a metal sphere) that would reflect the same amount of energy back to the radar as the actual target resulting in the microwave EM energy To determine the radar cross section for a detected microwave signature, engineers build targets with known cross section and use these to calibrate radar image intensity values The units for radar cross section is m2

Radar scattering coefficient - σ° The radar scattering coefficient is used to describe the radar intensity per unit area of the image pixel σ° = σ / A where A is the area of the pixel

The Decibel unit (dB) • Radar scattering coefficient is typically described using decibels, where σ° (dB) = 10 log (σ°)

Surface Reflectance or Scattering • Specular reflection or scattering • Diffuse reflection or scattering

Specular Reflection or Scattering • Occurs from very smooth surfaces, where the height of features on the surface << wavelength of the incoming EM radiation

Diffuse Reflectors or Scatterers • Most surfaces are not smooth, and reflect incoming EM radiation in a variety of directions • These are called diffuse reflectors or scatterers

Radar backscattering is dependent on the relative height or roughness of the surface Figures from http://pds.jpl.nasa.gov/ mgddf/chap5/f5-4f.gif

Microwave scattering as a function of surface roughness is wavelength dependent

Microwave scattering is dependent on incidence angle As incidence angle increases, radar backscatter decreases for all surface roughnesses Figure from http://pds.jpl.nasa.gov/ mgddf/chap5/f5-4f.gif

Variation in MW backscatter from a rough surface (grass field) as a function of wavelength – As the wavelength gets longer, the backscattering coefficient drops

Microwave Scattering from a Water Surface – Bragg Scattering Water has a dielectric constant of 80 • All scattering from water bodies in the Microwave region of the EM Spectrum is from surface scattering as no EM energy penetrates the water surface

Small surface or capillary waves present on a water surface – these waves are generated by wind  = 3 cm  = 24 cm

Smooth area – no wind

L-band airborne SAR Image of ship and its wake from previous slide Why do you have backscatter at L-band from an ocean surface?

Bragg Scattering from Water Surfaces • Wind creates small waves on the ocean surface (capillary waves) which in the absence of wind will continue to propagate • If wind continues, waves will grow in size and increase in wavelength and height to become ultra-gravity waves and eventually gravity waves • A water surface affected by wind will have a spectrum of surface waves, e.g., multiple wavelengths and heights

Bragg Scattering from Water Surfaces • Microwave EM energy has been shown through wave tank experiments to constructively interfere or resonate with surface capillary and ultra-gravity waves • This phenomenon is known as Bragg Scattering

Lecture 16 – Active Microwave Remote Sensing 2 December 2008