This document provides an overview of passive and active remote sensing systems. It defines passive sensors as those that detect natural energy emitted or reflected by an object, such as sunlight, while active sensors provide their own energy source, such as radar. Examples of different types of passive sensors are provided, such as radiometers, spectrometers, and sounders, while active sensors mentioned include radar, lidar, and scatterometers. The advantages and disadvantages of each system are discussed, with passive sensors being simpler but providing less detailed data, while active sensors can control illumination but are more complex. Examples of images from both types of sensors are also presented.

1. (passive and active remote sensing systems,
characteristics and operations)
Student Name: Copyright
Class: 4th Stage
Course Title: remote sensing
Department: Geomatics Department
College of Engineering
Salahaddin University-Erbil
Academic Year 2019-2020

2. 1
ABSTRACT
The development of remote sensing technologies increases the potential to support more
precise, efficient, and ecologically-sensitive approaches to forest resource management. One of
the primary requirements of precision forest management is accurate and detailed 3D spatial
data relating to the type and condition of forest stands and characteristics of the underlying
terrain surface. A new generation of high-resolution, active remote sensing technologies,
including airborne laser scanning (LIDAR) and interferometric synthetic aperture RADAR
(IFSAR) have the capability to provide direct, 3D measurements of forest canopy structure and
topography. remote sensing is the process of detecting and monitoring the physical
characteristics of an area by measuring its reflected and emitted radiation at a distance (typically
from satellite or aircraft). Special cameras collect remotely sensed images, which help
researchers "sense" things about the Earth. The sun provides a very convenient source of energy
for remote sensing. The sun's energy is either reflected, as it is for visible wavelengths, or
absorbed and then re-emitted, as it is for thermal infrared wavelengths. Remote sensing systems
which measure energy that is naturally available are called passive sensors. Passive sensors can
only be used to detect energy when the naturally occurring energy is available. For all reflected
energy, this can only take place during the time when the sun is illuminating the Earth. There is
no reflected energy available from the sun at night. Energy that is naturally emitted (such as
thermal infrared) can be detected day or night, as long as the amount of energy is large enough
to be recorded. Active sensors, on the other hand, provide their own energy source for
illumination. The sensor emits radiation which is directed toward the target to be investigated.
The radiation reflected from that target is detected and measured by the sensor. Active sensors
can be used for examining wavelengths that are not sufficiently provided by the sun, such as
microwaves, or to better control the way a target is illuminated. However, active systems
require the generation of a fairly large amount of energy to adequately illuminate targets.

3. 2
Table of content
ABSTRACT ............................................................................................................................. 1
Table of content........................................................................................................................ 2
INTRODUCTION ................................................................................................................... 3
BACKGROUND...................................................................................................................... 5
METHOD................................................................................................................................10
THEORY.................................................................................................................................13
Conclusion...............................................................................................................................19
REFRENCE ............................................................................................................................19

4. 3
INTRODUCTION
Remote sensing instruments are of two primary types— active and passive. Active sensors,
provide their own source of energy to illuminate the objects they observe. An active sensor
emits radiation in the direction of the target to be investigated. The sensor then detects and
measures the radiation that is reflected or backscattered from the target. Passive sensors, on the
other hand, detect natural energy (radiation) that is emitted or reflected by the object or scene
being observed. Reflected sunlight is the most common source of radiation measured by passive
sensors. A passive sensor is a device that detects and responds to some type of input from the
physical environment.
Passive sensor technologies gather target data through the detection of vibrations, light,
radiation, heat or other phenomena occurring in the subject’s environment. They contrast with
active sensors, which include transmitters that send out a signal, a light wavelength or electrons
to be bounced off the target, with data gathered by the sensor upon their reflection. Both active
and passive sensing technologies are often used to make observations and measurements from a
distance or on a scale beyond those observable to the naked eye. Sensors can also be used in
harsh environments and places inaccessible to people. Examples of passive sensor-based
technologies include: Photographic, thermal, electric field sensing, chemical, infrared and
seismic. However, as can be the case with some sensors, seismic and infrared light sensors exist
in both active and passive forms. Depending on what is being sensed these various sensors
might be mounted to a satellite, airplane, boat, submarine UAV drone, or from another
convenient point of observation such as a building top. The data gathered by remote sensing is
used for everything from cartography to resource exploration to atmospheric and chemical
measurements. Remote sensing is also one of the basic enabling technologies for the Internet of
Things (IoT), in which almost any imaginable entity can be equipped with a unique identifier
and the ability to transfer data over a network autonomously. Both types of sensors have
benefits and drawbacks. Passive sensor technologies can't be detected by observed parties as
they only sense what is in the environment rather than relying on a transmitter whose activity

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might be detected with equipment. Active sensors, however, can sometimes be used when
passive sources of observations by sensor are impossible. Passive sensors include different
types of radiometers and spectrometers.
An active sensor is a radar instrument used for measuring signals transmitted by the sensor that
were reflected, refracted or scattered by the Earth's surface or its atmosphere. Spaceborne active
sensors have a variety of applications related to meteorology and observation of the Earth's
surface and atmosphere. For example, precipitation radars measure the radar echo from rainfall
to determine the rainfall rate over the Earth's surface; and cloud profile radars measure the radar
echo return from clouds to provide a three dimensional profile of cloud reflectivity over the
Earth's surface. Spaceborne active sensors operate in the Earth Exploration-Satellite Service or
in the Space Research Service. Active sensor frequency allocations are often shared with other
radar systems, as such systems are normally compatible with the operation of the sensors.

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BACKGROUND
Most passive systems used in remote sensing applications operate in the visible, infrared,
thermal infrared, and microwave portions of the electromagnetic spectrum. Passive remote
sensors include the following:
 Accelerometer—An instrument that measures acceleration (change in velocity per unit
time). There are two general types of accelerometers. One measures translational
accelerations (changes in linear motions in one or more dimensions), and the other
measures angular accelerations (changes in rotation rate per unit time).
 Hyperspectral radiometer—An advanced multispectral sensor that detects hundreds of
very narrow spectral bands throughout the visible, near-infrared, and mid-infrared
portions of the electromagnetic spectrum. This sensor’s very high spectral resolution
facilitates fine discrimination between different targets based on their spectral response in
each of the narrow bands.
Figure 1 accelerometer
Figure 2 hyperspectral

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 Imaging radiometer—A radiometer that has a scanning capability to provide a two-
dimensional array of pixels from which an image may be produced. Scanning can be
performed mechanically or electronically by using an array of detectors.
 Radiometer—An instrument that quantitatively measures the intensity of
electromagnetic radiation in some bands within the spectrum. Usually, a radiometer is
further identified by the portion of the spectrum it covers; for example, visible, infrared,
or microwave.
 Sounder—An instrument that measures vertical distributions of atmospheric parameters
such as temperature, pressure, and composition from multispectral information.
Figure 5 atmospheric instrument sounder
Figure 4 infrared radiometer
Figure 3 imaging radiometer

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 Spectrometer—A device that is designed to detect, measure, and analyze the spectral
content of incident electromagnetic radiation. Conventional imaging spectrometers use
gratings or prisms to disperse the radiation for spectral discrimination.
 Spectroradiometer—A radiometer that measures the intensity of radiation in multiple
wavelength bands (i.e., multispectral). Many times the bands are of high-spectral
resolution, designed for remotely sensing specific geophysical parameters
The majority of active sensors operate in the microwave portion of the electromagnetic
spectrum, which makes them able to penetrate the atmosphere under most conditions. An active
technique views the target from either end of a baseline of known length. The change in
apparent view direction (parallax) is related to the absolute distance between the instrument and
target.
Figure 6 spectrometer
Figure 7 spectroradiometer-DTS140- instrument system

9. 8
 Laser altimeter—An instrument that uses LIDAR to measure the height of the platform
(spacecraft or aircraft) above the surface. The height of the platform with respect to the
mean Earth’s surface is used to determine the topography of the underlying surface.
 LIDAR—A light detection and ranging sensor that uses a laser (light amplification by
stimulated emission of radiation) radar to transmit a light pulse and a receiver with
sensitive detectors to measure the backscattered or reflected light. Distance to the object
is determined by recording the time between transmitted and backscattered pulses and by
using the speed of light to calculate the distance traveled.
Figure 8 laser altimeter
Figure 9 lider remote sensing

10. 9
 Radar—An active radio detection and ranging sensor that
provides its own source of electromagnetic energy. An active
radar sensor, whether airborne or space borne, emits
microwave radiation in a series of pulses from an antenna.
When the energy reaches the target, some of the energy is
reflected back toward the sensor. This backscattered
microwave radiation is detected, measured, and timed. The time
required for the energy to travel to the target and return back to the sensor determines the
distance or range to the target. By recording the range and magnitude of the energy
reflected from all targets as the system passes by, a two-dimensional image of the surface
can be produced.
 Ranging Instrument—A device that measures the distance between the instrument and a
target object. Radars and altimeters work by determining the time a transmitted pulse
(microwaves or light) takes to reflect from a target and return to the instrument. Another
technique employs identical microwave instruments on a pair of platforms. Signals are
transmitted from each instrument to the other, with the distance between the two
determined from the difference between the received signal phase and transmitted
(reference) phase. These are examples of active techniques. An active technique views
the target from either end of a baseline of known length. The change in apparent view
direction (parallax) is related to the absolute distance between the instrument and target.
 Scatterometer—A high-frequency microwave radar designed specifically to measure
backscattered radiation. Over ocean surfaces,
measurements of backscattered radiation in the
microwave spectral region can be used to derive
maps of surface wind speed and direction.
Figure 10 radar
Figure 11 scatterometer

11. 10
METHOD
Comparing active and passive microwave sensors
Passive microwave remote sensing Active microwave remote sensing
Sensor examples
AMSU-A & -B, MHS, ATMS, NPOESS. Jason-1and 2, Envisat ASAR, RA-2,
RADARSAT.
Measurement capabilities
Sense emitted microwave energy from
terrestrial sources.
Send and receive electromagnetic pulses of
energy.
Cloud and precipitations information
from layers.
Cloud and precipitation information from discrete
levels.
Sea surface wind vectors (WindSat,
MIS*), salinity.
Sea surface wind vectors, salinity.
Precipitations (rain rate and snowfall). Precipitation (rain rate and snowfall).
Cloud properties (microphysics, cloud top
and base).
Cloud properties (microphysics, cloud top, and
base).
Atmospheric temperature and moisture
profiling.
Snow and sea ice coverage and extent,
sea ice age.
Snow and sea ice coverage and extent, river ice
movement.
Snow cover characteristics Snow cover characteristics.
Soil moisture / surface wetness Soil moisture / surface wetness
Sea surface temperature Ocean surface topography, sea surface state, heat
storage and transfer (from radar altimeters).
Table 1

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Advantages & disadvantages of active sensors
advantages disadvantages
Better control of noise sources
Through control of injected of signal.
because both of sources and receivers are under
the surveyor’s control, he must supply both.
Therefore, filed equipment tends to be more
complex.
Because propagating filed are generally
measured, active experiments usually provided
better depth control over source of anomalous
signal.
Field operations and logistics are generally
more complex and time consuming than passive
experiments.
Many different source/receiver configurations
can be used allowing for a wide variety of
survey designs. This allows survey designers
great flexibility in customizing surveys for
particular problems.
Many different source/receiver configurations
can be used allowing for a wide variety of
survey designs. The increase in the number of
field options inevitably leads to greater survey
design coasts and potentially to increased
probability of field
Once set up, active experiments are capable of
producing vast quantities of data that can be
used to interprets subtle details of the earth’s
subsurface.
The large quantity of data obtained in many
active experiments can become overwhelming
to process and interpret.
Table 2

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Advantage & disadvantage of passive sensors
advantages Disadvantages
Surveyor need only record a naturally
occurring field; therefore, he need supply
only a sensor and a data recorder.
Less controls of noise because source of the
signal is out of the control of the surveyor.
Field operation are generally very time
officiant. Thus, passive experiments can be
run over wider areas in a more cost-effective
manner .
Because passive fields are generally the result
of integrating anomalous geologic
contributions over wide areas, identification
of the source of an anomalous reading can be
difficult.
One or two well-established field procedures
are generally used. Contractors can provide
these surveys on short notice with relatively
easily quantifiable results.
One or two well-established field procedures
is generally used. This limits the amount of
customization that can be done for specific
problems.
Interpretation of the limited set of
observations can be accomplished with
modest computational requirements quickly
and efficiently.
The data sets collected in passive experiments
are smaller than those collected in active
experiments and usually do not allow for as
detailed an interpretation.
Table 3

14. 13
THEORY
Examples of passive and active remote sensing
Now that we have a clear understanding of passive and active remote sensing, let’s see it in action
for satellite sensors. In the schematic below, you can see how the sun emits light. First, light passes
through the atmospheric window. Then, it reflects off Earth to a satellite sensor orbiting Earth.
Figure 12 active remote sensing
Active Sensors
 An active sensor is a radar instrument used for measuring signals transmitted by the
sensor that were reflected, refracted or scattered by the Earth's surface or its
atmosphere.
 Spaceborne active sensors have a variety of applications related to meteorology and
observation of the Earth's surface and atmosphere.
 Examples of passive sensor-based technologies include: Photographic, thermal, electric
field sensing, chemical, infrared and seismic.
Active Sensor: Active sensor is used for sensing environment conditions and transmitted
signals. It is an electronic sensing device which requires AC and DC source for produce output.
Figure 13

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Whereas active sensors illuminate their target. In this example, it’s a side-looking sensor that sends
its own pulse to Earth’s surface. First, it bounces off the ground. Then, it bounces again off a
building. Finally, it returns back to the sensor again. Actually, this type of backscatter is
called double bounce backscatter.
Figure 14 passive remote sensing
Passive sensor
 A passive sensor is a microwave instrument designed to receive and to measure natural
emissions produced by constituents of the Earth's surface and its atmosphere.
 The power measured by passive sensors is a function of the surface composition,
physical temperature, surface roughness, and other physical characteristics of the Earth.
 The frequency bands for passive sensor measurements are determined by fixed physical
properties (molecular resonance) of the substance being measured. These frequencies
do not change and information cannot be duplicated in other frequency bands.
 Examples of other active sensor-based technologies include: scanning electron
microscopes, LiDAR, radar, GPS, x-ray, sonar, infrared and seismic.
Figure 15

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Active remote sensing image example If you ever have a chance to see a synthetic aperture
radar image, it will look speckled like this:
Figure 16 active remote sensing (radar image)
For the untrained eye, it’s just a bunch of black and white pixels. But the reality is that there’s more
that meets the eye. For example, the 3 main types of backscatter are:
 Specular reflection
 Double-bounce
 Diffuse scattering
SPECULAR REFLECTION: Specular reflection is where dark spots are in the image. In this
case, it’s the smooth surfaces like the east-west flowing river and paved surfaces.
DOUBLE-BOUNCE: The bright white in the center is double-bounce backscatter at work. As
shown in the schematic above, it’s an urban feature like a building but it’s not entirely clear at this
scale.
DIFFUSE SCATTERING: Finally, the majority of the radar image is rough surface and diffuse
scattering. This may be from the growing vegetation in the agriculture areas.

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Passive remote sensing image example
Really, passive remote sensing can be very similar to how our eyes interpret the world. For
example, here are the Rocky Mountains in true color.
Figure 17 passive remote sensing image
But the power of passive remote sensing is to see light in the whole electromagnetic spectrum. For
example, this multispectral image can have different band combinations like color infrared.
What’s important to realize is that how it emphasizes healthy vegetation in bright red. To say the
least, there is a lot of it in this scene. While the bright white is built-up areas, the darkest shade is
water. In the east, this could be a transmission line right-of-way because how it’s constantly the
same width.
Finally, you can see the world much sharper using
the panchromatic band. If you want to pan-sharpen
an image, this is the spectral band that you use.
Figure 18 infrared color (passive remote sensing)
Figure 19 spectral band

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What are some applications for passive remote sensing?
In terms of passive remote sensing, the Landsat mission is the longest-running earth observation
program. For over 40 years, Landsat has collected and documented our changing planet.
Landsat science helps understand Earth’s climate, ecosystems and land use. For over 40 years,
the Landsat missions have been an eyewitness of our changing planet. Because of it, we have a
historical barometer where we gauge change and plan our future as a planet. Researchers have
published thousands of publications using Landsat data. Also, Digital Globe and Planet
Labs are commercial satellites with hundreds of remote sensing applications & uses.
What are some applications for active remote sensing?
Two of the key advantages of active remote sensing are:
 The capability to collect imagery night and day.
 It is unfazed by clouds and poor weather conditions.
The Shuttle Radar Topography Mission (SRTM) uses inSAR which measures
Earth’s elevation with two antennas. In only a couple days, SRTM has collected one of the most
accurate digital elevation models of Earth.
Figure 20

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Light detection and Ranging (LiDAR) is an active sensor that measures ground
height. Using light from an airplane or helicopter platform, it measures the time it takes to
bounce back to the sensor. From this, you can create Digital Surface Models which is useful in
forestry.
Active remote sensing has been used for a variety of security applications
including marine and Arctic monitoring. As shown previously, the double-bounce scattering has
provided critical information in search and rescue missions.
Active sensor examples include: The Canadian Space Agency’s RADARSAT-
1 and RADARSAT-2 and Airbus Defense & Space TerraSAR-X Radar Satellite.
APPLICACTIONS OF ACTIVE & PASSIVE SENSORS
 Radarsat: imaging characteristics, mapping of vegetation, vegetated terrain and sand
terrain.
 LISS: land use, land cover and geological aspects.
 SPOT: resources of earth and environmental aspects.
 Landsat: seasonal features like weather, atmosphere, rainfall on images.

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Conclusion
 Sensor is a device comprising of optical component or system and a detector electronic
circuity used to record.
 It is a very important component in remote sensing.
 Active sensors are more advantageous than passive sensors.
 LISS is one of the active sensors which is launched in Indian satellites.
 LISS-2 is the advanced version carried by the Indian satellite IRS-1B.
REFRENCE
 Mani, A., & Co-requisites, P. BCE093-REMOTE SENSING AND GIS.
 Deal, Jeffrey T., and Wilson Greatbatch. "MRI-compatible medical device with passive
generation of optical sensing signals." U.S. Patent No. 6,711,440. 23 Mar. 2004..
 PREAMBLE, A., & PRE-REQUISITES, B. FUNDAMENTALS OF REMOTE
SENSING AND GIS..
 Cox, Helen, Kimberle Kelly, and Laura Yetter. "Using remote sensing and geospatial
technology for climate change education." Journal of Geoscience Education 62.4
(2014): 609-620..