Remote Sensing Passive sensors Active remote sensing Energy Source or Illumination: Radiation and the Atmosphere: Interaction with the Target:

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1 P a g e 1 Remote Sensing Remote Sensing is the collection of information relating to objects without being in physical contact with them. Thus our eyes and ears are remote sensors, and the same is true for cameras and microphones and for many instruments used for all kinds of applications. Remote sensing is the process of acquiring data/information about objects/substances not in direct contact with the sensor, by gathering its inputs using electromagnetic radiation or acoustical waves that emanate from the targets of interest. An aerial photograph is a common example of a remotely sensed product. There are two main types of remote sensing: Passive remote sensing and Active remote sensing. a. Passive sensors detect natural radiation that is emitted or reflected by the object or surrounding area being observed. Reflected sunlight is the most common source of radiation measured by passive sensors. Examples of passive remote sensors include film photography, infrared, charge-coupled devices and radiometers. b. Active remote sensing emits energy in order to scan objects and areas whereupon a sensor then detects and measures the radiation that is reflected or backscattered from the target. RADAR is an example of active remote sensing where the time delay between emission and return is measured, establishing the location, height, speeds and direction of an object. Remote sensing makes it possible to collect data on dangerous or inaccessible areas. Remote sensing applications include monitoring deforestation in areas such as the Amazon Basin, the effects of climate change on glaciers and Arctic and Antarctic regions, and depth sounding of coastal and ocean depths. Military collection during the cold war made use of stand off collection of data about dangerous border areas. Remote sensing also replaces costly and slow data collection on the ground, ensuring in the process that areas or objects are not disturbed. Remote sensing is done from satellites or airplane or on the ground. To repeat the essence of the definition above, remote sensing uses instruments that house sensors to view the spectral, spatial and radiometric relations of observable objects and materials at a distance. Most sensing modes are based on sampling of photons corresponding frequency in the electromagnetic (EM) spectrum. In much of remote sensing, the process involves an interaction between incident radiation and the targets of interest. This is exemplified by the use of imaging systems where the following seven elements are involved. a. Energy Source or Illumination: The first requirement for remote sensing is to have an energy source which illuminates or provides electromagnetic energy to the target of interest. b. Radiation and the Atmosphere: As the energy travels from its source to the target, it will come in contact with and interact with the atmosphere it passes through. This interaction may take place a second time as the energy travels from the target to the sensor. c. Interaction with the Target: Once the energy makes its way to the target through the atmosphere, it interacts with the target depending on the properties of both the target and the radiation. d. Recording of Energy by the Sensor: After the energy has been scattered by, or emitted from the target, we require a sensor (remote - not in contact with the target) to collect and record the electromagnetic radiation. e. Transmission, Reception and Processing: The energy recorded by the sensor has to be transmitted, often in electronic form, to a receiving and processing station where the data are processed. f. Interpretation and Analysis: The processed image is interpreted, visually and/or digitally or electronically, to extract information about the target, which was illuminated. g. Application: The final element of the remote sensing process is achieved when we apply the information we have been able to extract from the imagery about the target in order to better understand it, reveal some new information, or assist in solving a particular problem. It must be noted however that remote sensing also involves the sensing of emitted energy and the use of nonemitted sensors.

2 P a g e 2 Types of Remote Sensing System a. Visual remote sensing system: The human visual system is an example of a remote sensing system in the general sense. The sensors in this example are the two types of photosensitive cells, known as the cones and the rods, at the retina of the eyes. The cones are responsible for colour vision. There are three types of cones, each being sensitive to one of the red, green and blue regions of the visible spectrum. Thus, it is not coincidental that the modern computer display monitors make use of the same three primary colours to generate a multitude of colours for displaying colour images. The cones are insensitive under low light illumination condition, when their jobs are taken over by the rods. The rods are sensitive only to the total light intensity. Hence, everything appears in shades of grey when there is insufficient light. As the objects/events being observed are located far away from the eyes, the information needs a carrier to travel from the object to the eyes. In this case, the information carrier is the visible light, a part of the electromagnetic spectrum. The objects reflect/scatter the ambient light falling onto them. Part of the scattered light is intercepted by the eyes, forming an image on the retina after passing through the optical system of the eyes. The signals generated at the retina are carried via the nerve fibres to the brain, the central processing unit (CPU) of the visual system. These signals are processed and interpreted at the brain, with the aid of previous experiences. The visual system is an example of a "Passive Remote Sensing" system which depends on an external source of energy to operate. This system won't work in darkness. b. Optical Remote Sensing: In Optical Remote Sensing, optical sensors detect radiation reflected or scattered from the target, forming images. The wavelength region usually extends from the visible and near infrared to the short-wave infrared SWIR. Different materials such as water, soil, vegetation, buildings and roads reflect visible and infrared light in different ways. The interpretations of optical images require the knowledge of the spectral reflectance signatures of the various materials of the target. c. Infrared Remote Sensing: Infrared remote sensing makes use of infrared sensors to detect infrared radiation emitted from the target. The middle-wave infrared (MWIR) and long-wave infrared (LWIR) are within the thermal infrared region. These radiations are emitted from warm objects such as the Earth's surface. They are used in satellite remote sensing for measurements of the land and sea surface temperature. Thermal infrared remote sensing is also often used for detection of forest fires, volcanoes, oil fires and human presence. d. Microwave Remote Sensing: The active sensors emit pulses of microwave radiation to illuminate the areas to be imaged. Images of the target are formed by measuring the microwave energy scattered by the target back to the sensors. The images can thus be acquired day and night. Microwaves have an additional advantage as they can penetrate clouds. Images can be acquired even when there are clouds covering the earth surface. Electromagnetic radiation in the microwave wavelength region is used in remote sensing to provide useful information about the Earth's atmosphere, land and ocean. When microwaves strike a surface, the proportion of energy scattered back to the sensor depends on many factors: i) Physical factors such as the dielectric constant of the surface materials which also depends strongly on the moisture content. ii) Geometric factors such as surface roughness, slopes, orientation of the objects relative to the radar beam direction. iii) The types of landcover (soil, vegetation or man-made objects). iv) Microwave frequency, polarisation and incident angle. e. Radar Remote Sensing: Using radar, geographers can effectively map out the terrain of a territory. Radar works by sending out radio signals and then receives them when they bounce back from the target. By measuring the amount of time it takes for the signals to return, it is possible to create a very accurate topographic map. An important advantage to using radar is that it can penetrate thick clouds

3 P a g e 3 and moisture. This allows scientists to accurately map areas such as rain forests, which are otherwise too obscured by clouds and rain. Imaging radar systems are versatile sources of remotely sensed images, providing daynight, all-weather imaging capability. Radar images are used to map landforms and geologic structure, soil types, vegetation and crops, and ice and oil slicks on the ocean surface. f. Satellite Remote Sensing: The remote sensing satellites are equipped with sensors looking down to the earth. They are the "eyes in the sky" constantly observing the earth as they go round in predictable orbits. Orbital platforms collect and transmit data from different parts of the electromagnetic spectrum, which in conjunction with larger scale aerial or ground-based sensing and analysis provides researchers with enough information to monitor trends. Other uses include different areas of the earth sciences such as natural resource management, agricultural fields such as land usage and conservation, and national security and overhead, ground-based and stand-off collection on border areas. Satellite sensors record the intensity of electromagnetic radiation (sunlight) reflected from the earth at different wavelengths. Energy that is not reflected by an object is absorbed. Each object has its own unique spectrum. Remote sensing relies on the fact that particular features of the landscape such as bush, crop, salt-affected land and water reflect light differently in different wavelengths. Grass looks green, for example, because it reflects green light and absorbs other visible wavelengths. This can be seen as a peak in the green band in the reflectance spectrum for green grass above. The spectrum also shows that grass reflects even more strongly in the infrared part of the spectrum. While this can't be detected by the human eye, it can be detected by an infrared sensor. Instruments mounted on satellites detect and record the energy that has been reflected. The detectors are sensitive to particular ranges of wavelengths, called 'bands'. The satellite systems are characterised by the bands at which they measure the reflected energy. The satellite detectors measure the intensity of the reflected energy and record it. g. Airborne Remote Sensing: In airborne remote sensing, downward or sideward looking sensors are mounted on an aircraft to obtain images of the earth's surface. An advantage of airborne remote sensing, compared to satellite remote sensing, is the capability of offering very high spatial resolution images. The disadvantages are low coverage area and high cost per unit area of ground coverage. It is not costeffective to map a large area using an airborne remote sensing system. Airborne remote sensing missions are often carried out as one-time operations, whereas earth observation satellites offer the possibility of continuous monitoring of the earth. h. Acoustic and near-acoustic remote sensing: In this type of remote sensing, sound is used as a tool. i) Sonar: Sound Navigation and Ranging is a technique that uses sound propagation to navigate, communicate with or detect objects on or under the surface of the water. It s of two types: Passive sonar: In this type listening for the sound made by another object (a vessel, a whale etc) is done. Active sonar: In this type pulses of sounds are emitted and echoes are listened. It is used for detecting, ranging and measurements of underwater objects and terrain. ii) Seismograms taken at different locations can locate and measure earthquakes (after they occur) by comparing the relative intensity and precise timing. Image Resolution The quality of remote sensing data consists of its spectral, radiometric, spatial and temporal resolutions. a. Spatial Resolution: Spatial resolution refers to the size of the smallest object that can be resolved on the ground. In a digital image, the resolution is limited by the pixel size, i.e. the smallest resolvable object cannot be smaller than the pixel size. The intrinsic resolution of an imaging system is determined primarily by the instantaneous field of view of the sensor, which is a measure of the ground area viewed by a single detector element in a given instant in time. However this intrinsic resolution can often be degraded by other factors which introduce blurring of the image, such as improper focusing, atmospheric scattering and target motion. The pixel size is determined by the sampling distance. A

4 P a g e 4 "High Resolution" image refers to one with a small resolution size. Fine details can be seen in a high resolution image. On the other hand, a "Low Resolution" image is one with a large resolution size, i.e. only coarse features can be observed in the image. b. Radiometric Resolution: Radiometric Resolution refers to the smallest change in intensity level that can be detected by the sensing system. The intrinsic radiometric resolution of a sensing system depends on the signal to noise ratio of the detector. In a digital image, the radiometric resolution is limited by the number of discrete quantization levels used to digitize the continuous intensity value. c. Spectral resolution: The ability to resolve the various frequency bands is called the spectral resolution. d. Temporal resolution: This refers to the precision of a measurement with respect to time. Applications a. Meteorology: Study of atmospheric temperature, pressure, water vapour, and wind velocity. b. Oceanography: Measuring sea surface temperature, mapping ocean currents, and wave energy spectra and depth sounding of coastal and ocean depths. c. Glaciology: Measuring ice cap volumes, ice stream velocity, and sea ice distribution. d. Geology: Identification of rock type, mapping faults and structure. e. Agriculture: Monitoring the biomass of land vegetation. f. Forest: Monitoring the health of crops, mapping soil moisture. g. Disaster warning and assessment: Monitoring of floods and landslides, monitoring volcanic activity, assessing damage zones from natural disasters. h. Planning applications: Mapping ecological zones, monitoring deforestation, monitoring urban land use. i. Oil and mineral exploration: Locating natural oil seeps and slicks, mapping geological structures, monitoring oil field subsidence. j. Military: Developing precise maps for planning, monitoring military infrastructure, monitoring ship and troop movements. k. Forensics: Forensic investigations concern locating, identifying, collecting and cataloging evidence for the purpose of presenting it in court. One aspect of forensic investigations concerns locating clandestine evidence which is often concealed in the subsurface. Often such resultant searches lead to excavations that destroy evidence. Remote sensing methods, being non destructive can be applied for better results. This will save time and cost in the search for physical evidence.

5 P a g e 5 Photoelectric Sensor A photoelectric sensor is another type of position sensing device. Photoelectric sensors use a modulated light beam that is either broken or reflected by the target. The control consists of an emitter (light source), a receiver to detect the emitted light, and associated electronics that evaluate and amplify the detected signal and send it to the load. Typical light sources are LEDs and Laser. The common detectors are photo transistors, photodiodes, photomultipliers, photovoltaic cells and charge coupled devices (CCDs). Modulated light increases the sensing range while reducing the effect of ambient light. Modulated light is pulsed at a specific frequency between 5 and 30 KHz. The photoelectric sensor is able to distinguish the modulated light from ambient light. Light sources used by these sensors range in the light spectrum from visible green to invisible infrared. Light-emitting diode (LED) sources are typically used. Types of Photoelectric Sensors a. Through Beam Sensor: Separate emitter and receiver units are required for a through beam sensor. The units are aligned in a way that the greatest possible amount of pulsed light from the transmitter reaches the receiver. An object (target) placed in the path of the light beam blocks the light to the receiver, causing the receiver s output to change state. When the target no longer blocks the light path the receiver s output returns to its normal state. Thru-beam is suitable for detection of opaque or reflective objects. It cannot be used to detect transparent objects. In addition, vibration can cause alignment problems. The high excess gain of thrubeam sensors make them suitable for environments with airborne contaminants. The maximum sensing range is about 300 feet. b. Retroreflective Scan: Reflective or retroreflective scan are two names for the same technique. The emitter and receiver are in one unit. Light from the emitter is transmitted in a straight line to a reflector and returns to the receiver. A normal or a cornercube reflector can be used. When a target blocks the light path the output of the sensor changes state. When the target no longer blocks the light path the sensor returns to its normal state. The maximum sensing range is about 30 feet. Polarized Retroreflective Scan: A variation of retroreflective scan is polarized retroreflective scan. Polarizing filters are placed in front of the emitter and receiver lenses. The polarizing filter projects the emitter s beam in one plane only. This light is

6 P a g e 6 said to be polarized. A corner-cube reflector must be used to rotate the light reflected back to the receiver. The polarizing filter on the receiver allows rotated light to pass through to the receiver. In comparison to retroreflective scan, polarized retroreflective scan works well when trying to detect shiny objects. c. Diffuse Scan: The emitter and receiver are in one unit. Light from the emitter strikes the target and the reflected light is diffused from the surface at all angles. If the receiver receives enough reflected light the output will switch states. When no light is reflected back to the receiver the output returns to its original state. In diffuse scanning the emitter is placed perpendicular to the target. The receiver will be at some angle in order to receive some of the scattered (diffuse) reflection. Only a small amount of light will reach the receiver, therefore, this technique has an effective range of about 40. Diffuse Scan with Background Suppression: Diffuse scan with background suppression is used to detect objects up to a certain distance. Objects beyond the specified distance are ignored. Background suppression is accomplished with a position sensor detector (PSD). Reflected light from the target hits the PSD at different angles, depending on the distance of the target. The greater the distance the narrower the angle of the reflected light. Operating Modes There are two operating modes, dark operate (DO) and light operate (LO). Dark operate is an operating mode in which the load is energized when light from the emitter is absent from the receiver. Light operate is an operating mode in which the load is energized when light from the emitter reaches the receiver. Dark Operate Mode Light Operate Mode

7 P a g e 7 Timer The heart of every digital system is the system clock or the timer. Since all logic operations in a synchronous machine occur in synchronism with a clock, the system clock becomes the basic timing unit. The system clock must provide a periodic waveform that can be used as a synchronizing signal. 555 Timer IC The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation, and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element. Derivatives provide up to four timing circuits in one package. It is a 8 pin IC. The pin configuration is as shown in figure. Pin Name Function Ground reference voltage, low level (0 V) The OUT pin goes high and a timing interval starts when this input falls below 1/2 of CTRL voltage (which is typically 1/3 of V CC, when CTRL is open). This output is driven to approximately 1.7 V below + V CC or GND. A timing interval may be reset by driving this input to GND, but the timing does not begin again until RESET rises above approximately 0.7 volts. Overrides TRIG which overrides THR. Provides "control" access to the internal voltage divider (by default, 2/3 V CC ). The timing (OUT high) interval ends when the voltage at THR is greater than that at CTRL (2/3 V CC if CTRL is open). Open collector output which may discharge a capacitor between intervals. In phase with output. Positive supply voltage, which is usually between 3 and 15 V depending on the variation. 555 as Astable Multivibrator IC 555 is basically a switching circuit which has two distinct levels. When the IC is connected in the way as shown in the figure, neither of the output levels is stable. As a result the circuit continuously switches between the two unstable states. The circuit oscillates and the output is a periodic rectangular waveform. Since neither output state is stable, the circuit is said to be astable and is known as free running or astable multivibrator. The frequency of oscillation and the duty cycle of the multivibrator are controlled by the two resistors, and, and the timing capacitor,. The timing capacitor is charged towards through resistors and. The charging time during which the output is high is given as, The timing capacitor then discharges to ground through the resistor. The discharging time during which the output is low is given as, The time period of the resulting clock waveform is hence, The frequency of oscillation is hence given as,

8 P a g e as Monostable Multivibrator The circuit shown in the figure produces one stable state and one quasi stable state. This type of multivibrator is called the monostable multivibrator. In its stable state, the timing capacitor is completely discharged through pin number. Hence, the stable state of this multivibrator is a low output state. When a negative pulse at the pin number i.e. the trigger pin, the output switches to the high quasi stable state. The time period of the quasi stable state,, is determined by the resistor and the capacitor. It s value is given as, After the time, the output of the multivibrator switches back to its stable state i.e. the low state. The circuit remains in this state until it receives another trigger pulse. The typical waveforms are as shown in the figure. Solar Cell A solar cell also known as photovoltaic cell) is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. It is a form of photoelectric cell in which the electrical characteristics like current, voltage, or resistance vary when light is incident upon it. When exposed to light, it can generate and support an electric current without being attached to any external voltage source, but do require an external load for power consumption. Basic operational principles Solar cell is based on the photovoltaic effect. In general, the photovoltaic effect means the generation of a potential difference at the junction of two different materials in response to visible or other radiation. The basic processes behind the photovoltaic effect are, a. Generation of the charge carriers due to the absorption of photons in the materials that form a junction. b. Subsequent separation of the photo generated charge carriers in the junction. c. Collection of the photo generated charge carriers at the terminals of the junction. The solar cell works in three steps, a. Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon. b. Electrons (negatively charged) are excited from their current molecular/atomic orbital. Once excited the electron can either dissipate the energy, and return to its orbital or travel through the cell until it reaches an electrode. Current starts flowing through the material to cancel the potential and this electricity is captured. Due to the special composition of solar cells, the electrons are only allowed to move in a single direction. c. An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity. PN Junction Solar Cell In a PN junction solar cell, sunlight is directly converted into electric current. The basic schematic with the symbol is as shown in the figure. The metallic conductor connected to the P type material and thickness of the P type material is such that maximum number of photons reaches the junction. A photon may collide with a valence electron and impart sufficient energy to leave the parent atom. The result is the formation of electron hole pairs which facilitates the generation of electric current. The commonly used materials to construct solar cells are silicon and selenium.

9 P a g e 9 Counters A counter is one of the most versatile subsystems in a digital system. A counter driven by a clock can be used to count the number of clock cycles. Since the clock pulses occur at known intervals, the counter can be used as an instrument for measuring time and therefore period or frequency. Asynchronous Counter (Ripple Counter) UP Counter Negatively edge triggered flip flops are connected in the way as shown in figure. All the and inputs of the flip flops are connected to. Hence all the flip flops are in the toggle mode. The first flip flop is connected to a square wave clock. Hence, the flip flop will change its state, i.e. toggle, with a negative transition of the clock at its clock input. All the inputs of the flip flops are connected to the clock input of the subsequent flip flop. The output is taken from the outputs with being the Least Significant Bit (LSB) and being the Most Significant Bit (MSB). Initially the output of the counter is. In the end of first clock cycle, the first flip flop toggles and it state changes to. But the flip flops being negatively edge triggered, the state of the second flip flop does not change and the new output of the counter is. In the end of the second clock cycle, the first flip flop again toggles to and this being a trigger for the second flip flop, it also toggles to. But the third and fourth flip flop do not toggle and the output is now,. In the end of the third clock cycle, the first flip flop again toggles to thus prohibiting the second flip flop and the subsequent flip flops to toggle. The output is now,. In the end of the fourth clock cycle, the first flip toggles to and triggers the second flip flop to toggle to as well. The third flip flop toggles and the subsequent output is. This continues till the 15 th clock pulse obtaining the various digital outputs as shown in the truth table. As the states go from to, this counter counts in the upward direction and hence is called the bit Up Counter. At the end of 16 th clock pulse, the counter resets back to. The timing diagram of the counter is as shown below.

10 P a g e 10 Down Counter Negatively edge triggered flip flops are connected in the way as shown in figure. All the and inputs of the flip flops are connected to. Hence all the flip flops are in the toggle mode. The first flip flop is connected to a square wave clock. Hence, the flip flop will change its state, i.e. toggle, with a negative transition of the clock at its clock input. All the inputs of the flip flops are connected to the clock input of the subsequent flip flop. The output is taken from the outputs with being the Least Significant Bit (LSB) and being the Most Significant Bit (MSB). Initially the output of the counter is preset to. In the end of first clock cycle, the first flip flop toggles and it state changes to. The output changes from to. But the flip flops being negatively edge triggered, the state of the second flip flop does not change and the new output of the counter is. In the end of the second clock cycle, the first flip flop again toggles to and to and this being a trigger for the second flip flop, it also toggles to. But the third and fourth flip flop do not toggle and the output is now,. In the end of the third clock cycle, the first flip flop again toggles to and to thus prohibiting the second flip flop and the subsequent flip flops to toggle. The output is now,. In the end of the fourth clock cycle, the first flip toggles to and to and triggers the second flip flop to toggle to and its to as well. The third flip flop toggles and the subsequent output is. This continues till the 15 th clock pulse obtaining the various digital outputs as shown in the truth table. As the states go from to, this counter counts in the downward direction and hence is called the bit Down Counter. At the end of 16 th clock pulse, the counter resets back to. The timing diagram of the counter is as shown below. In both the counters, the triggers move through the flip flops like a ripple in water. The overall propagation delay will be sum of the individual delays of the flip flops. When multiple flip flops need to toggle in a binary count sequence, they will not toggle at exactly the same time. Let us consider a transition from to i.e. decimal to in a Up Counter. The flip flops will change their states one by one and hence the output will change in the following sequence,.

11 P a g e 11 This is known as the ripple effect. Although this will happen very quickly. And hence will be of no consequence in most of the electronic circuits. But if this circuit is used to drive a multiplexer or index a memory pointer in a microprocessor, the false states so produced will have grave consequences. Hence, the need to provide clock pulses to all the flip flops at once and subsequently reduce the ripple effect. Such a counter is called a Synchronous Counter. Synchronous Up Counter A typical synchronous Up Counter is as shown in the figure. In this circuit the same system clock is applied to all the clock inputs of the flip flops. From the truth table of a Up counter, it is clear that a flip flop will toggle every time all previous flip flops are in the state and the clock makes a negative transition. (In the count down mode, flip flops toggle when the previous flip flops are in state). The and inputs of the first flip flop is connected to. Thus it is in toggle mode and it changes state when the clock provides a negatively edged signal. The and inputs of the second flip flop is connected to output of the first flip flop. Thus, this flip flop will toggle only when is in state and the clock provides a negatively edged signal. The and inputs of the third flip flop is connected to an AND gate. The inputs of the AND gate are the outputs of the first and the second flip flop i.e. and. Thus, this flip flop will toggle only when and both are in state and the clock provides a negatively edged signal. The and inputs of the fourth flip flop is connected to an AND gate. The inputs of the AND gate are the outputs of the first, second and the third flip flop i.e., and. Thus, this flip flop will toggle only when, and all are in state and the clock provides a negatively edged signal. Initially the output of the counter is. In the end of first clock cycle, the first flip flop toggles and it state changes to. But the flip flops being negatively edge triggered, the state of the second flip flop does not change and the new output of the counter is. In the end of the second clock cycle, the first flip flop again toggles to and this being a trigger for the second flip flop, it also toggles to. But the third flip flop do not toggle as output of the AND gate is (. Similarly, the fourth flip flop also does not toggle and the output is now,. In the end of the third clock cycle, the first flip flop again toggles to thus prohibiting the second flip flop to toggle. The output of the first AND gate is and the third flip flop does not toggle. Similarly, the fourth flip flop also does not toggle and the output is now,. In the end of the fourth clock cycle, the first flip toggles to and triggers the second flip flop to toggle to as well. The output of the first AND gate is. As the state changes from to, third flip flop toggles and the subsequent output is.