48 The Concept of Microwave Remote Sensing, its Geometry, and its Applications

A brief history of Remote Sensing

• The beginning of remote sensing starts from capturing aerial photography with the help of balloons, kites, pigeons, and gliders. In late 1908 after the development of operational aircraft, the first functional aircraft was used for aerial photography.

• During World War I and II the aerial photographs were used as a military perspective for identifying the enemy’s location on the ground. In the period of 1920s and 1930s

• Aerial photography is the standard source of gathering information about the ground and preparing the thematic as well as topographic maps. The use of black & white, color, and infrared imageries in remote sensing were widely used from the time period of the 1950s until the year 1960s.

The use of aerial photography has some merits which make it very efficient for lots of applications because:-

• Aerial photography is very helpful for mapping the features which are located in a very remote location on the ground.

• With the help of stereo-captured imageries, 3D analysis is possible.

• Near-infrared aerial imageries provide spectral information beyond human vision.

There were also some demerits in the eerily aerial remote sensing imageries such as:-

• Early aerial photographs highly depend on the weather conditions

• Aerial photographs were normally recorded in an analog format and were not calibrated, which precludes quantitative analysis.

Introduction: Microwave Remote Sensing

The term remote sensing was firstly introduced by Evelyn Pruitt of the US Office of Naval Research in the 1950s. The traditional form of aerial photography evolved into remote sensing in about 1960. As per the sabins (1987), remote sensing as methods that employ electromagnetic energy to detect, record, and measure the characteristics of a target, such as the Earth’s surface. (The Basic Concept of Remote Sensing).

The infrared portion of EMS is considered to be from 0.7 – 1,000 μm. Most commonly the main aspect of remote sensing is electromagnetic radiation. After late 1960 after the launch of the Sputnik USSR satellite the use of satellite technology in the field of remote sensing begins. The first remote sensing-based satellite was Landsat which was launched in 1972 (47 Satellite Remote Sensing and Landsat 9). Since the present time, the modernist form of remote sensing is Microwave and Lidar Remote Sensing.

Concept of Microwave Remote Sensing (RADAR)

The term radar stands for “radio detection and ranging. In simple words, we may understand by the term microwave remote sensing is that the use of microwave radiation for gathering information about the surface of the earth without physically contacting it. In the case of microwave remote sensing, the process entails transmitting short bursts, or pulses, of microwave energy in the direction of interest and recording the strength and origin of “echoes” or “reflections” received from objects within the system’s field of view.

The system of radar is generally based on the transmission of long-wavelength microwaves (e.g., 3 – 25 cm) through the atmosphere and then recording the amount of energy backscattered from the terrain. The present form of radar as we know was investigated by A. H. Taylor and L. C. Young in the late 1920s. some examples of microwave sensor satellites are ERS-1 and 2, RADARSAT-1 and 2, ENVISAT ASAR, Sentinel-1, and Risat-1.

Shorter wavelengths (band X) were used by the Shuttle Radar Topographic Mission (SRTM), TerraSAR-X, and COnstellation of small Satellites for the Mediterranean basin Observation (COSMO-SkyMed), while longer wavelengths (bands L or P) are used by JERS-1 and ALOS-PALSAR. A radar remote sensing system uses its electromagnetic energy in microwave bands to “illuminate” the terrain and detects the energy returning from the terrain, with the transmitter and the receiver in the same location. The way electromagnetic waves propagate through a material can be described by a radar equation. Neglecting the path losses, the radar equation may be written as follows (Fung and Ulaby 1983):

Pr = stands for received pulse (energy) at polarization,

Pt = defined as the transmitted energy at polarization t,

Gt = stands for the gain of the transmitting antenna in the direction of the target at polarization t (target),

R = distance between radar and target t,

σrt  = radar cross-section

Ar = it is the effective receiving area of the receiving antenna aperture at polarization r.

The following points denote some of the advantages of radar remote sensing:-

• Penetration of cloud cover in certain microwave frequencies

• Synoptic views of large areas for mapping at 1:10,000 to 1:400,000 Satellite coverage of cloud-shrouded countries is possible.

• Capacity to detect and obtain information during both day & night

• Good for thermal and moisture analysis related study

• Microwave energy penetrates vegetation, sand, and surface layers of snow.

• The microwave sensors are based on their own illumination which helps for controlling the sensor

• The radar system uses the principle of polarization for gathering information about the surface of the earth (HH, VV, HV, VH)

• It Can measure ocean wave properties, even from orbital altitudes.

• Provides good quality stereoscopic viewing and radargrammetry.

Concept of Polarization in Radar system Unpolarized energy vibrates in all perpendicular directions to the path of travel. Polarized energy is sent and received by radar antennas. This signifies that the energy pulse is filtered such that its electrical wave vibrations are limited to a single plane perpendicular to the direction of propagation. The energy (pulse) is generated by the antenna may be vertical or horizontal as shown in the following figure (figure 3). The sent pulse of electromagnetic energy interacts with the landscape, and some portion of it is backscattered at the speed of light toward the vehicle of sensing or spacecraft, where it must be filtered once again. It is recorded whether the antenna takes the backscattered radiation. The radar can record many types of backscattered polarised radiation. It is possible, for example, to VV = sends and receive both in vertical energy HH = sends and receive both horizontal energies HV = sends horizontal energy and receive vertically VH = sends vertical and receive horizontal energy Now we may understand the term polarization is that it is basically a principle of transmitting and recording energy which is used in radar as well as lidar and sonar systems for detecting energy. Figure 1. different forms of polarization (VV and HH) Figure 2. How the antenna of microwave remote sensing works Figure 3. Radar polarization. Many imaging radars can transmit and receive signals in both horizontally and vertically polarized modes. By comparing the like-polarized and crosspolarized images, analysts can learn about characteristics of the terrain surface. From NASAJPL. P45541, SIR C/X SAR, October 1994.

Radar Wavelength

Airborne imaging radars have frequently used C-, K-, and X-bands. The choice of a specific microwave band has several implications for the nature of the radar image. The following table represents the different wavelengths used in radar systems for different applications.

Band	Wavelength
P-band	107–77 cm
L-band	30–15 cm
S-band	15–7.5 cm
C-band	7.5–3.75 cm
X-band	3.75–2.40 cm
Ku-band	2.40–1.67 cm
K-band	1.67–1.18 cm
Ka-band	1.18–0.75 cm

Geometry of Radar System (how Radar record Datasets)

The geometric components of a radar system are following:-

Azimuth Direction (A radar system, the Sensor’s position is mounted beneath and parallel to the aircraft fuselage as mentation in figure 1). The straight-line travel of an aircraft is known as azimuth flight direction.  

Range Direction, The range or look direction for any radar image is the direction of the radar illumination that is at right angles to the direction the aircraft or spacecraft is traveling. Look direction usually has a significant impact on feature interpretation.

Depression Angle, the depression angle is denoted by the symbol of (γ). It is defined as the angle between a horizontal plane extending out from the aircraft fuselage and the electromagnetic pulse of energy from the antenna along the radar line-of-sight to a specific point on the ground (figure (1)).

Look Angle, the look angle is represented by the symbol of (φ), an angle between the vertical from the antenna to the ground and the radar line of sight.

Incident Angle, is represented by the sign by (θ), an angle between the radar pulse of energy and a line perpendicular to the Earth’s surface where it makes contact.

Polarization (detailed discusses above in the note paragraph).

Application of Microwave Remote Sensing

1. Microwave radiometry gives useful information about sea ice coverage. Unlike visible and infrared satellite images, it can be used in all seasons and at all times of day, independent of clouds. It can show the ice edge with a high degree of accuracy.

2. Application of microwave remote sensing in the prediction of the atmospheric boundary layer. The atmospheric temperature profiles are derived with 5 mm (54.5 GHz) radiometer angle-scanning observations. Due to the fact that microwave radiometer could monitor the atmospheric temperature profile continuously and make the initialization of numerical model any time, it helps improve the accuracy in prediction of the evolution of atmospheric boundary layer;

3. monitoring atmospheric temperature, humidity, and water content in clouds. The Advanced Microwave Sounding Unit-A (AMSU-A) is a 15-channel microwave sounder designed to obtain temperature profiles in the upper atmosphere and to provide a cloud-filtering capability for the AIRS infrared measurements, for increased accuracy in troposphere temperature profiles.

4. Satellite remote sensing of atmosphere and cloud. The TIROS-N TOVS satellite data are used to obtain atmospheric temperature profiles.

5. For improvement of the accuracy of rainfall measurement, a radiometer-radar system (λ=3.2 cm) has been developed. The variation of rainfall distribution and area-rainfall may be obtained by its measurements, which may be helpful for hydrological prediction.

6. microwave remote sensing of soil moisture, The response of soil moisture to a microwave remote sensing system from the ground surface is impacted by factors such as land cover, plant density, and soil temperature. the texture of the soil, which makes the retrieval process more hard complex. Several studies have been conducted. Out to look at the link between emission and as well as backscatter, soil moisture, and vegetation criteria for several research fields

7. The use of radar in forestry demonstrates that SAR systems are capable of identifying different types of (tropical) forest cover utilizing multi-temporal and multi-frequency SAR data. Some studies have shown that synthetic aperture radar (SAR) may be used to determine above-ground standing biomass. remote sensing with Synthetic Aperture Radar (SAR) to collect biophysical information from forest sites

8. The extraction of URBAN FOOTPRINT is one of the important application areas of microwave remote sensing.

9. The microwave remote sensing datasets also helpful for the studies of Glacier Inventory

10. SNOW AND GLACIER, Glacier Inventory, Snow Cover Monitoring, Monitoring of Glaciers for Retreat/Advance and Glacier Mass Balance Estimation.