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intro2radarA VISUAL INTRODUCTION TO RADAR REMOTE SENSING v2.0Iain H Woodhouse

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These animations have been created by Iain H Woodhouse using Hype. They are based on key diagrams in his book, Introduction to Microwave Remote Sensing, and are best used in conjuction with this text. The Terms and Conditions are primarily to protect the copyright, but also to allow the use of Google Analytics. Note that some Internet Browsers are not compatible with HTML5. You may need to upgrade your Browser to the latest version to see all the functionality. No technical support is offered. Google Chrome seems to work well.

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UNDER CONSTRUCTIONClick here to join mailing list, or click on button on right to go back to oblique view.

ANTENNA

LEARN ABOUT AZIMUTH GEOMETRY

IMAGING SWATH

FOOTPRINT

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FLIGHT DIRECTION

CONTINUE TO ANIMATION

NADIR

LEARN ABOUT ECHOLOCATION

LEARN ABOUT RANGE GEOMETRY

INSTRUMENT GROUND TRACK

Basic Radar Imaging GeometryThis figure shows the basic geometry for imaging radar. Put your mouse over key annotations for further information. Choose one of the buttons on the righthand side to learn more about the range geometry or the azimuth geometry.Compare with Figure 10.1 in the book.

ANTENNA The radar system has an antenna that both transmits and receives the radio- or micro-waves. The geometry shown here is to examine the way it collects information in the azimuth direction. "Azimuth" refers to the along-track direction, i.e. the direction of flight.

FLIGHT DIRECTION A radar imaging system must travel relative to the imaged surface. The image in the range direction is constructed using time delays of the radio echoes, whereas the image in the azimuth direction requires the relative motion of the instrument and target to gradually build up enough data to construct the image.

NADIR The point directly below the radar system is known as the nadir. As the instrument travels along its flight path the nadir point tracks out the ground path.

INSTRUMENT GROUND TRACK This is the line that the instrument traces along the ground as it travels along its flight path. Note that since the radar images obliquely, the ground track runs parallel to the imaged swath. The ground track lies outside of the imaging swath.

IMAGING SWATH The antenna transmits a signal obliquely. It forms a elliptical footprint on the Earth's surface. As the instrument moves along the flight path it scans this ellipical footprint across the ground. The area scanned is called the swath. Its width is determined by the vertical height of the radar antenna.For many spaceborne imaging radars the swath width can be from 10s to 100s km wide.

FOOTPRINT Each pulse from the radar illuminates an elliptical area on the ground. The size and shape of the footprint is governed by the size of the antenna and the wavelength of the radio signal. The pattern is not actually discrete, as shown, but is a smooth function. The line drawn usually equates to the point where the signal intensity (or the sensitivity, when in receive-mode) has dropped to half of the maximum intensity.

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TIME

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TRANSMIT A PULSE

RANGE

SIGNAL

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AZIMUTH

LINES OF EQUAL RANGE

GROUND RANGE

Basic range imaging

Layover

Range resolution

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Basic Range ImagingThis figure shows imaging in the range direction. One dimension of a radar image is constructed based on the time delay of echoes from the surface. The signal displayed in the top right is converted to one row of pixels. Compare with Figure 10.7 in the book.

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TIMEThis axis represents the time delay in the echo. As the time increases echos arrive from targets that are further away on the ground.

SIGNALThis axis represents the "loudness" of the echo (i.e. the signal intensity). This is measured as a function of time. Since the radio waves travel at a constant speed, time is a function of distance from antenna to target. Note that the signal strength drops off with increasng distance (time) from the atenna.This signal drop-off is proportional to distance and since we can calculate distance using the time delay, we can adjust the signal accordingly when making an image.

LINES OF EQUAL RANGEThese grey arcs represent lines of equal range from the antenna. The time delay is directly proportional to range distance. A single radar system only measures the time delay of the echos. It does not measure the direction of the echoes.

GROUND RANGEBy transmitting the pulse sideways (obliquely) the distance from the antenna is approximately the distance along the ground. That is how a map-like image can be produced with radar.

GROUND RANGEBy transmitting the pulse sideways (obliquely) the distance from the antenna is approximately the distance along the ground. That is how a map-like image can be propoduced with radar.

ANTENNA The radar system has an antenna that both transmits and receives the radio- or micro-waves. The geometry shown here collects range data to build up a picture of one "slice" along the ground. The Instrument must fly perpendicular to this image plane to build up a 2D image.

SHADOW

LayoverThis figure shows the reason for layover in a radar image. Layover is when targets (such as mountains or buildings) appear to lean over in an image. This is because one dimension of a radar image is constructed based on the time delay (equivalent to slant range distance) of the echoes. For the first pyramid in this figure, the top of the pyramid is closer to the antenna than the near-side base of the pyramid. At higher incidence angles (far range) the near-side base is closer than the top.Compare with Figure 10.7 in the book.

TIMEThis axis represents the time delay in the echo. As the time increases echos arrive from targets that are further away on the ground. Note that the echo from target B arrives first, before the echo from target A. The top of the pyramid (B) is then mapped at a ground range distance closer to nadir than point A.At far range, C and D are mapped the "right way around".

BWhen the echoes are mapped onto the ground surface (or a references surface) target B is mapped "in front of" target A since it is closer to the antenna. This is what appears as "layover" in the final image.

BWhen the echoes are mapped onto the ground surface (or a references surface) target B is mapped "in front of" target A. This is what appears as "layover" in the final image.

DTarget D is mapped on the correct side of target C, but still much closer to it than its planimetric position (i.e. where it would appear in a proper map).

SHADOWThe region behind this hill receives no signal from the antenna. When the echoes return, there is just radio "silence" during this time as there is no return signal. Areas of radar shadow appear black in the final image. At far range, C and D are mapped the "right way around".

GROUND RANGE RESOLUTION

Range ResolutionThis figure shows the effective criteria for resolving targets in the range direction. The length of the pulses (the thickness of the orange arcs) are longer in this figure so that you can see what is happening. The closer two targets are just on the limit of the range resolution: any closer together and their pulses would overlap. Because the incidence angle is larger for the second set of targets, the targets can still be resolved. Compare with Figures 9.1 and 10.2 in the book.

RANGE RESOLUTION

TIMEThis axis represents the time delay in the echo. To resolve the two targets in the time dimension, the echoes must be seperable, and not overlapping.

SIGNALThis axis represents the "loudness" of the echo (i.e. the signal intensity). This is measured as a function of time. Note that the first two echoes are very nearly over-lapping. This is the limit of the range resolution as we now see one target, not two. At the far range, where the incidence angle is higher, the two targets still have echoes that are seperable and so are resolvable.The slant range resolution is governed by the width of the pulse. The resolution on the ground has to also take into account the incidence angle.

GROUND RANGEOn the ground, the range resolution is better at the far-range (large incidence angles) than the near-range (low incidence angles). The ground range resolution is at its worst at nadir, directly below the antenna.

GROUND RANGE RESOLUTIONThe ground range resolution is the closest distance between two targets before their echoes overlap. The ground range resolution is the range resolution divided by the sine of the incidence angle. Both pairs of targets shown are the same distance apart on the ground, yet the echoes from those in the near range are not separable - their echoes overlap. The echoes from the targets in the far range are separable. Ground range resolution is different across the swath.

RANGE RESOLUTIONThe (slant) range resolution is governed only by the effective length (of time) of the pulse. This corresponds to when the echoes from two targets start to overlap.

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INSTRUMENT GROUND TRACK

This is based on Figure 10.1

FLIGHT DIRECTION The radar system has an antenna that both transmits and receives the radio- or micro-waves. The geometry shown here collects range data to build up a picture of one "slice" along the ground. The Instrument must fly perpendicular to this image plane to build up a 2D image.

INSTRUMENT GROUND TRACK The radar system has an antenna that both transmits and receives the radio- or micro-waves. The geometry shown here collects range data to build up a picture of one "slice" along the ground. The Instrument must fly perpendicular to this image plane to build up a 2D image.

IMAGING SWATH The radar system has an antenna that both transmits and receives the radio- or micro-waves. The geometry shown here collects range data to build up a picture of one "slice" along the ground. The Instrument must fly perpendicular to this image plane to build up a 2D image.

FOOTPRINT The radar system has an antenna that both transmits and receives the radio- or micro-waves. The geometry shown here collects range data to build up a picture of one "slice" along the ground. The Instrument must fly perpendicular to this image plane to build up a 2D image.

EcholocationThis figure shows the basic principle behind echolocation. A pulse is transmitted and, if there is a target in the beam, an echo is detected. Since the speed of the pulse is known, the time delay between transmission and detection gives the range distance to the target. Different types of target can scatter the energy in different directions. Three different targets types are shown. You can click on them to see the effect they have on the scattered energy.

ISOTROPIC

REDIRECTED

BACKSCATTER

REDIRECTEDThe intercepted energy is scattered unequally, with most of the energy being scattered in a direction away from the sensor.

BACKSCATTERThe intercepted energy is scattered unequally, with most of it scattered directly back to the sensor.

ISOTROPICThe intercepted energy is scattered equally in all direction.

ANTENNA The radar system has an antenna that both transmits and receives the radio- or micro-waves. The geometry shown here is for the most generic range finding device. It applies equally well to any system based on echolocation, namely radar, lidar or sonar.