Microwave Radiometry - Imaging Technologies and … · 2010-08-25 · Microwave Radiometry - Imaging Technologies and Applications M. Peichl, ... dish antenna with a Gaussian edge

Abstract— Radiometry is the passive measurement of the

electromagnetic radiation which is either emanated or reflected

from all materials. The energy that is received by a radiometer

from an object is due to the combination of the apparent

temperature of the object, the apparent temperatures reflected

by the object, and the apparent temperatures transmitted

through the object. Potential application areas of microwave

radiometry include geo-science, climatology, agriculture,

pollution and disaster control, detection, reconnaissance,

surveillance, and status registration in general. Passive RF

sensors can be operated on ground, airborne and in space,

requiring sophisticated techniques and technologies to save

volume and mass. In parallel, increased spatial resolution and

sensitivity, wide fields of view, and real-time capabilities are

demanded challenges. This paper gives a brief introduction to the

physical background of microwave radiometry and illustrates the

mostly considered imaging principles. Typical examples from

current practice and basic experimental measurements for future

applications are shown.

Index Terms—Microwave radiometry, fundamentals, imaging

principles, applications.

I. INTRODUCTION

ICROWAVE radiometry addresses the domain of

passive measurement of the natural thermally caused

electromagnetic radiation of matter at a physical temperature

above 0K. In the case of Earth observation significant

contrasts can be observed between reflective and absorbing

materials due to the impact of reflected sky radiation of

cosmic origin. For Earth observation, an approximate

brightness temperature range from 3K to more than 300K can

be observed. In the microwave region the spatial two-

dimensional brightness temperature distribution can be used as

a daytime and almost weather independent indicator for many

different physical phenomena. Hence, interesting application

areas incorporate geo science, climatology, agriculture,


surveillance, and status registration in general. Since a few

years security applications like personnel screening and the

monitoring of critical infrastructures are also of major interest.

Many of those applications require high spatial and

M. Peichl is with the Deutsches Zentrum fuer Luft- und Raumfahrt (DLR),

Microwaves and Radar Institute (HR), Oberpaffenhofen, 82234 Wessling,

Germany (phone: +49-8153-282390; fax: +49-8153-281135; e-mail:

[email protected]).

All the other authors are also with HR, DLR, Oberpfaffenhofen, Germany.

radiometric resolution, high precision, large fields of view,

and high frame rates.

Today three radiometric imaging methods are mainly

considered. A first more classical one is based on a

linescanner approach, a second more innovative method called

aperture synthesis uses interferometric techniques, and a third

principle uses a focal plane array and a focusing aperture as in

many optical systems. All methods have certain benefits

concerning performance, expense, and costs. Hence, they are

used depending on the requirements and the application.

II. BASIC PRINCIPLES OF MICROWAVE RADIOMETRY

A. Brightness temperature

In the microwave region, we mostly measure the noise

power instead of voltage or current and therefore a microwave

radiometer can be characterized as a low-noise highly

amplifying frequency-selective power meter. At microwave

frequencies the noise power is proportional to the physical

temperature of either an electronic resistor following

Nyquist’s law, or of thermally radiating matter following the

Rayleigh-Jeans law, where the resistor can be considered as a

lossless receiving antenna in a thermal equilibrium. In the

second case the received radiation has not to be generated

exclusively by the observed matter due to emission as in the

case of a blackbody, but it can be composed of additional

partly reflected and partly translucent radiation generated in

the same manner elsewhere. Consequently the observed

temperature is apparent and does in general not correspond to

the physical temperature T0. It is called the brightness

temperature TB and its composition is illustrated strongly

simplified in fig. 1.

TE

TE

TG

TG

T0

T0

TB = eT0 + rTE + tTG

T0, e, r, t

Observed object

Radiometer

Environment

Background

Energy conservation:

e + r + t = 1 e = a

e Emissivitya Absorptivity r Reflectivity t Transmissivity

Fig. 1. Composition of the observable brightness temperature.

Microwave Radiometry - Imaging Technologies

and Applications

M. Peichl, S. Dill, M. Jirousek, and H. Süß

M

75 Proceedings of WFMN07, Chemnitz, Germany

WFMN07_II_C1, pp. 75-83 http://archiv.tu-chemnitz.de/pub/2007/0210/

The chemical and physical properties of matter like the

permittivity, surface roughness, and shape of an object, and

the measurement parameters like frequency, polarization, and

viewing angle are mathematically incorporated in the

quantities of emissivity e, reflectivity r and transmissivity t.

For many materials and especially in the millimetre-wave

(MMW) region the penetration depth of electromagnetic

waves is very low and their transmissivity is close to zero.

Differences in emissivity and consequently in reflectivity

allow the discrimination of objects among one another and

from the background.

B. Sky brightness temperature

In the case of Earth observation significant contrasts can be

observed between reflective and absorbing materials due to

the impact of reflected sky radiation of cosmic origin. The

space outside the terrestrial atmosphere acts like a blackbody

radiator having a physical temperature of about 3K. This cold

appearance can be observed more or less strongly depending

on the reflective properties of an object and the frequency

dependent absorption and self-emission of the atmosphere.

The main MMW windows for sufficient atmospheric

penetration and low sky brightness temperatures as illustrated

in fig. 2 are at frequencies around 35, 94, 140 and 220GHz

and offer corresponding available bandwidths of about 16, 23,

26 and 50GHz. Out of those windows, the frequency selective

absorption and scattering of energy by the atmospheric

molecules takes place and hampers the application of MMW

imaging. However, those areas are preferred in atmospheric

research.

50 100 150 200 250 300Frequency [GHz]

0

50

100

150

200

250

300

Bri

gh

tne

ss

te

mp

era

ture

[K

] = 60°

= 30°

= 0°

H2O

H2O O2

O2

Fig. 2. Calculated observable brightness temperature of the sky for different

observation angles using the US standard atmosphere model [1].

Note that the sky brightness temperature increases with

increasing observation angle due to the longer path of the

cosmic radiation through the atmosphere.

C. Antenna temperature

The observable brightness temperature of a scene depends

on the direction of observation. It can be measured by an

antenna connected to a low-noise receiver. Such a device is

called a microwave radiometer. The actual measured

brightness temperature is given by the antenna temperature

TA( ‘, ‘), which is the true brightness temperature TB( , )

of the actual pixel convolved with the antenna power pattern

P( , ), which can be a real or a synthetic beam. The

convolution represents the movement of the antenna beam in

spherical coordinates , , for which d = sin d d is

valid.

dPTT B

A

A )','(),(1

)','( (1)

Note that the antenna beam degrades the true brightness

temperature due to its finite beam width. Furthermore the

receiver adds its own noise to the observable noise power of

the scene, and the available observation time is limited

depending on the application. As a consequence the

radiometer can only measure with a finite sensitivity. Both

degrading impacts are briefly discussed next.

D. Spatial resolution

The angular resolution in degrees is determined by the

ratio of the wavelength and the aperture diameter D of the

imaging antenna, which essentially corresponds to its half-

power beam width. The constant kA depends on the field

distribution in the aperture plane of the antenna (e.g. 72° for a

dish antenna with a Gaussian edge taper down to -10 dB).

Dk A

(2)

Note that for a given wavelength the antenna size has to be

increased in order to improve the spatial resolution. There is

only very small margin by controlling the aperture field

distribution.

E. Sensitivity

The sensitivity or temperature resolution T of a single

measurement is determined by the antenna temperature TA, the

receiver noise temperature TE, the radiometer bandwidth f,

and the integration time .

f

TTkT EA

R

(3)

The constant kR depends on the receiver type and the

imaging principle used. For classical single-receiver scanning

systems it is 1 for a total-power receiver and 2 for a Dicke

receiver, where the latter is used to compensate for gain drifts

[1]. For a high sensitivity the antenna and receiver noise

temperatures have to be low and the bandwidth and the

integration time have to be large. However, the application

mostly limits the margin for the bandwidth and the integration

time due to technical or observational constraints. The antenna

temperature is given by the scene, and the receiver noise

temperature depends on the availability of low-noise receiver

components. Typically the latter gets worse with increasing

frequency band to be used for the observation.



III. IMAGING TECHNOLOGIES

Today, in principle three radiometric imaging methods are

considered. The first more classical one is based on a line

scanner approach and is relatively easy to implement. It has

strong limitations concerning real-time imaging, the spatial

resolution, and the field of view (FOV). The second more

innovative method called aperture synthesis uses

interferometric techniques. It offers higher resolution, larger

FOVs, and real-time capability at the cost of much more

expense. The third principle uses a focal plane array and a

focusing aperture as in many optical systems. Thus real-time

imaging is possible, but for present technologies high

resolution systems with larger FOVs are associated with a

high expense. For aperture synthesis and focal plane arrays the

data processing is also more complicated. However, both

techniques are subject of actual research.

A. Line scanner

Basically a line scanner radiometer system operates by

moving a real aperture antenna beam across the desired FOV,

and measuring the incident noise power by a high-amplifying

low-noise receiver as shown in fig. 3. Thus the whole two-

dimensional image is sampled consecutively in time pixel by

pixel.

Fig. 3. Operational principle of a line scanner system for airborne imaging.

Typically a line scanner operates with a single receiver.

Hence, the image generation requires two mechanical or

electronic movements of the real-aperture antenna beam. As a

consequence, a high-resolution image requires a short dwell

time of the antenna beam on each resolution cell, leading to a

rather small integration time and a corresponding decreased

temperature resolution. A low receiver noise figure and a large

bandwidth can improve this situation to some extent. Typical

sensitivities for airborne systems are around 1…2 K.

Mechanical movements are limited by a maximum size and

mass of the antenna structure due to inertia problems. Even

highly sophisticated mechanics nowadays do not allow an

angular resolution well below 0.5° at millimeter-waves like W

band, for instance. Electronic beam steering by real aperture

antennas, as in the case of a phased array, suffers mostly from

the very high number of single elements. Higher signal

attenuation causes a decreased temperature resolution. Single-

receiver line-scanner systems do not allow proper real-time

imaging.

B. Aperture synthesis

In contrast to the line scanner imaging method an aperture

synthesis radiometer operates in the spatial frequency domain

[2, 3]. The basic idea of aperture synthesis as shown in fig. 4

is to replace the large real-aperture antenna of a radiometer by

a thinned array of single small-aperture antennas and to

correlate the input signals in pairs. For an adequate placement

of the single antennas this procedure measures the relative

amplitudes and phases of the incoming radiation in

dependence of the baseline distances, the spatial frequencies,

with a minimum of redundancy. From a complete set of

spatial frequencies, called the visibility function, a

corresponding synthesized beam of the array can be

constructed in the spatial domain. Thus the brightness

temperature distribution to be determined is the convolution of

the original one with the synthesized beam and can therefore

be found by an inverse Fourier transform of the measured

spatial frequency spectrum.

Fig. 4. Airborne imaging arrangement for an aperture synthesis system

consisting of a multitude of two-element interferometers.

The initial field of view is given by the antenna pattern of

the single antenna elements. All pixels of an image are

determined at the same time without any mechanical

movement. This offers the capability of real-time operation as

in the case of an optical camera. Furthermore the

mathematical generation of the image from the spatial

frequency measurements allows a flexible operation on the

image reconstruction process using methods of digital signal

processing. In particular, an aperture synthesis system can be

adjusted for far-field as well as near-field imaging without

changing the array geometry, because the focusing can be

done purely mathematical [3]. Aperture synthesis can be

applied in one or in two dimensions. Hybrid systems

combining it with a real-aperture approach can be used in

order to save costs and expense [4].

C. Focal plane array

A focal plane array system [5] basically operates similar to

a line scanner in the spatial domain. Instead of the mechanical

or electronic steering of the antenna beam, a set of single



receivers is placed in the focal plane of a focusing aperture to

generate simultaneously a bunch of antenna beams, each one

pointing to another direction. A schematic of an airborne one-

dimensional focal plane array system is shown in fig. 5.

Fig. 5. Operational principle of a focal-plane-array system illustrated for a

one-dimensional airborne application.

The offset of each antenna within the focal plane with

respect to the focal point generates a squint for each antenna

beam. Thus the number of single receivers and their

displacement determine the actual number of image pixels and

the sampling pattern on a scene. In order to fulfill the

sampling theorem and to acquire a large field of view

simultaneously, a very large number of receivers in a very

compact arrangement would be required. Thus sometimes a

combination of a focal plane array together with some less

expensive mechanical movement of the whole imaging system

is used [6].

IV. SYSTEMS AND APPLICATIONS

Microwave radiometers are widely used since many

decades in space-borne, airborne, or ground-based

applications. Only a sample of missions and activities can and

shall be illustrated here.

A. Space-borne systems

In the 1990s the European Space Agency (ESA) developed

the MIMR (Multi-frequency Imaging Microwave Radiometer)

satellite. The instrument was designed to observe numerous

atmospheric and oceanic parameters, including precipitation,

soil moisture, global ice and snow cover, sea surface

temperature and wind speed, atmospheric cloud water, and

water vapor [7]. From an 824 km orbit MIMR should operate

at six frequencies, each with horizontal and vertical

polarization: 6.8, 10.65, 18.7, 23.8, 36.5, and 90 GHz. The

corresponding ground resolution should run from about 60 km

to 5 km, and the corresponding sensitivity should be in the

order of 0.25 K to 1 K. MIMR was designed to have a cross-

track swath of 1,400 km at a constant incidence angle of 50

degrees, which provided a 3-day global coverage of the Earth.

The imaging principle followed a conical scan using a dish

antenna with various feeds as illustrated in fig. 6.

Fig. 6. Imaging principle of the MIMR space-borne radiometer system

(Source: ESA-ESTEC, The Netherlands).

MIMR was designed upon the successful design of the

Special Sensor Microwave/Imager (SSM/I) [8]. The SSM/I

system consists of several satellites launched since 1987

measuring at 85, 37, 22, and 19GHz. The primary mission of

SSM/I was to support the American Department of Defense

operations, but the satellite system is also used for many

civilian geo-science applications. Other space-borne conical-

scan radiometer systems are TMI, GMI, SSMIS, Coriolis

WindSat, AMSR, and AMSR-E. Fig. 7 shows a satellite

brightness temperature image from Coriolis WindSat of the

typhoon Songda.

Fig. 7. Coriolis WindSat 37 GHz horizontal polarization brightness

temperature product shown for Typhoon Songda on September 4, 2004

(Source: Naval Research Lab, USA).

Aquarius is a NASA/CONAE satellite mission to measure

global Sea Surface Salinity (SSS) [9]. The Aquarius science

goals are to observe and model the processes that relate

salinity variations to climatic changes in the global cycling of



water and to understand how these variations influence the

general ocean circulation. Aquarius uses a 2.5 m reflector

antenna and three feeds for three footprints in a push-broom

configuration as shown in fig. 8. The corresponding incidence

angles and ground resolutions are 29°, 38°, 45°, and 76 x 94

km, 84 x 120 km, 96 x 156 km. A sensitivity of about 1 K for

a 6 s integration time is achieved.

Fig. 8. Artist view of the Aquarius satellite measuring with three antenna

beams simultaneously in a push-broom configuration (Source: NASA, USA).

The fully polarimetric radiometer operates in L band at

1413 MHz with a <27 MHz bandwidth, which is a protected

band for radio-astronomical measurements of the galactic

atomic hydrogen line. Additionally and L-band scatterometer

at 1.26 GHz is added for Faraday rotation correction and

retrieval algorithm improvement. The system shall be

launched in spring 2009 and have a three-year lifetime. The

swath width will be 407 km in a 657 km sun-synchronous

orbit. Fig. 9 shows a global salinity map indicating the

variability of the salinity level for the different oceans.

Fig. 9. Global map of the sea surface salinity distribution (Source: ESR,

USA).

The SMOS (Soil Moisture and Ocean Salinity) mission [10]

currently prepared by ESA has the objective to observe two

crucial variables, soil moisture over land and ocean salinity

over sea. Both variables are used in predictive atmospheric,

oceanographic and hydrologic models and may be important

for extreme event forecasting. The SMOS radiometer system

is based on an innovative two-dimensional L-band

(1.413GHz) Y-shaped aperture synthesis design as shown in

fig. 10.

Fig. 10. Artist view of the SMOS satellite using two-dimensional aperture

synthesis imaging techniques. The antenna spacing is 0.875 wavelengths

(Source: ESA-ESTEC, The Netherlands).

Global observation is done with a revisit time of 3 days in

H and V polarization. The spatial resolution will be better than

50km and the incidence angle coverage is ranging from 0°-

50°. A sun-synchronous orbit was proposed to ensure the

requisite sampling, and at the same time to minimize signal

perturbing effects such as Faraday rotation, Sun glint, and

thermal differences in soils and vegetation. A 3 x 6 Y

configuration (3 segments per arm with 6 radiometers each)

with additional 18 radiometers on the central hub, thus a total

of 69 receivers coupled to patch antennas, was selected as the

baseline. The snapshot sensitivity is expected to be around 2.4

K, which will be improved for salinity measurements by a

spatial and temporal averaging of the ground resolution to a

grid of about 200 km pixel size. The circular orbit has an

altitude of 756 km and the instrument has a tilt angle of 32.5°

with respect to nadir. The spatial resolution is about 35 km in

the center of the swath and about 50 km at the edge of the

swath. The launch is planned for spring 2008.

Both missions, SMOS and Aquarius, operate exactly at the

same frequency band and have an operational life-time

overlap of about two years. Hence a cross-correlation of the

measurements in order to check the absolute accuracy will be

possible. However, the differences in the imaging geometry,

the spatial resolution, and the sensitivity have to be

considered.

ESA and NASA-JPL are currently investigating large

aperture synthesis instruments operating from a geo-stationary

orbit, i.e. in a distance of about 36,000 km from the Earth’s



surface. GeoSTAR (Geostationary Synthetic Thinned

Aperture Radiometer) [11] headed by NASA-JPL shall be a

stationary Y-shaped instrument to provide high-resolution

microwave images at bands around 53 and 183 GHz for

monitoring rapidly evolving and dynamic atmospheric

phenomena such as hurricanes and thunderstorms, or

measuring temperature, water vapor, and rain from space due

to the ability to penetrate cloud cover. GAS (Geostationary

Atmospheric Sounder) [12] funded by ESA has similar goals

using frequency bands around 53, 118, 183, and 380 GHz. In

contrast to GeoSTAR it shall utilize a highly thinned Y-

shaped array, rotating around its central axis in order to reduce

the required number of receivers and to compensate for spatial

under-sampling.

B. Airborne systems

Airborne radiometer systems offer a broad spectrum of

applications for military and civil use. In the military area

millimeter-wave systems can be used for short- to mid-range

observations of a few hundred of metres or a few kilometres

in surveillance, reconnaissance, and target tracking on seekers

or unmanned areal vehicle (UAV) platforms. Here the

advantage of radiometers is their passive and therefore covert

operation, a quasi-optical and therefore easy-to-interpret

appearance of images, and the capability of nadir looking

especially for mountainous and urban areas. DLR has

constructed an airborne line scanner using an oscillating dish

antenna, mainly used at W band around 90 GHz with an

angular resolution of about 1° and 1.8K sensitivity at 100 µs

integration time. Fig. 11 shows an image of an airport scene

with some parking aircrafts measured from a 100 m altitude.

Note the quasi-optical appearance of the radiometric image,

which allows a clear attribution for reconnaissance even for

bad weather conditions.

Fig. 11. Video image and measured 90 GHz brightness temperature map of an

airport area using an airborne line scanner system.

Another application of imaging radiometry is the use in

pollution control like oil spill detection [13]. Here the layered

structure of an oil spill on the sea surface shows a reflectivity

and emissivity corresponding to the oil spill thickness and the

observation frequency. A further application is the use as a

possible indicator for vegetation type and status. From passive

images different types of farmlands can be discriminated

against each other and groups of trees or forests. Using more

than one frequency can allow a more detailed analysis of field

types and their temporal ripeness. The high sensitivity of

radiometric measurements on physical temperatures as well as

on water contents allows additionally the mission of

radiometer systems for safety monitoring in the case of forest

fires or floods. Furthermore, the prevention of disasters can be

achieved by a periodic surveillance of sensitive areas as in the

case of protected zones above pipelines, for instance.

Soil moisture and ocean salinity are also measured from

aircraft, in many cases as a support for the design and

operation of satellite missions. So ESA is also funding several

aperture synthesis demonstrators like MIRAS [14] and HUT-

2D [15], on one hand as learning platforms for image

reconstruction and retrieval algorithm development, on the

other hand for acquiring true data. MIRAS has a similar Y

shape and similar receivers and correlators as the SMOS

instrument, but it consists only of 13 elements synthesizing an

antenna aperture of about 1.1 m diameter and a beam width of

about 9° for constant tapering. Fig. 12 shows some

photographs of the array structure using patch antennas and

the installation on the aircraft.

Fig. 12. Photographs of the MIRAS demonstrator – the array configuration

(left) and the system mounted on the backside of the aircraft (right) (Source:

ESA-ESTEC, The Netherlands).

HUT-2D is a U-shaped array of about 1.95 m x 1.8 m size

using 36 receivers coupled to patch antennas with a spacing of

0.7 wavelengths. Hence a synthesized beam width between 5°

and 10° can be achieved depending on the tapering used, and

the sensitivity is about 2.8 K at a 250 ms integration time.

Here also the center frequency and bandwidth are chosen

similar to SMOS. Fig. 13 shows a photograph of the

instrument and an imaging result of a test flight over land.

The American ESTAR system [4] was the first airborne

instrument using aperture synthesis at L band for measuring

soil moisture. Here aperture synthesis was used across track

and a real aperture was used along track with respect to the

flight direction. This imaging principle is also used for the

LRR-X system built by NASA and the University of

Michigan, synthesizing an aperture of about 1 m x 1 m at X

band (10.7 GHz) using 12 receivers connected to slotted

waveguide antennas. A nadir angular resolution of 1.5° and a

sensitivity of 0.3 K are achieved. The purpose of this

radiometer is the conduction of high quality rainfall

measurements in order to support the Global Precipitation



Measurement (GPM) mission [16]. Fig. 14 shows a

measurement result prior and after a rainfall event and the

system as mounted on the aircraft.

Fig. 13. Photograph of the HUT-2D instrument as mounted below the aircraft.

The lower images show the measurement result (left) for a flight over land

with changing texture (right) (Source: HUT, Finland).

Fig. 14. Measurement results of the LRR-X instrument prior to and after a

rainfall event. The lower left image is a corresponding optical image of the

scene. The photograph on the right shows the instrument as mounted on the

aircraft (Source: University of Michigan, USA).

C. Ground-based systems

Modern ground-based applications of imaging radiometry

are represented by surveillance, hidden object detection, or

long-term data acquisition for geo-science applications [17].

The contamination by landmines is increasingly recognized as

an inhumane burden on countries ravaged by war, many of

these poor. Conventional techniques, like metal detectors or

trained dogs are too inefficient to solve this problem. Also,

plastic materials are increasing being used to avoid detection.

International terrorism has reached a level where adequate

countermeasures to protect the population have to be provided

by the authorities. Similarly, the improved surveillance and

protection of sensitive infrastructures, like for instance nuclear

power plants, airports, or mass events attracts increased

attention. Hence, research on these new challenges had been

animated through various national and international

organizations, and so we also consider passive microwave

sensors to improve the situation [18].

For buried object detection like landmines the lower

microwave region has to be used due to penetration depth

constraints. Within the HOPE (Handheld Operational

dEmining system) project DLR developed a multi-spectral

radiometer system in the range of 1.5 to 7 GHz. Imaging

measurements can be carried out in bands of 50 MHz width

by moving manually or automatically a broadband antenna

close to the surface of the area of interest. An imaging result

performed on a buried object scenario using landmine

dummies and false targets is shown in fig. 15 [19].

Fig. 15. Photographs showing the HOPE radiometer in operation and the

scenario with the objects on the surface. Measurements for all objects buried

have been carried out at various frequencies using a regular raster scan.

The surveillance of critical infrastructures requires a 24

hour operation in all weather conditions on large areas. The

imaging principle of the DLR ground-based experimental

imager ABOSCA is shown in fig. 16. The main goal was the

capability to image a full hemisphere and have a high

flexibility concerning modifications. A rotating parabolic

mirror provides an image line, and the azimuth movement of

the whole unit delivers the second image dimension.

Fig. 16. Photograph of the ABOSCA ground-based line scanner system.



The system is operated at 90GHz, 37GHz and 9.6GHz with

about 0.6°, 1.5°, and 5.8° of angular far-field resolution. The

measurement duration for the complete hemisphere is less

than 5 minutes at W band and even less for the other bands,

and the sensitivity is in the order of 0.1 K. An image of a large

front court area is shown in fig. 17. Note again the quasi-

optical character of the radiometer image simplifying the

interpretation. Many fine details can be observed due to the

high sensitivity and angular resolution. Note the mirror images

of various objects in the concrete area. The system was also

successfully operated for through-wall imaging and change

detection experiments [18, 20].

Fig. 17. Photograph and 90 GHz image of a large front court area. Both

images represent a panoramic view for 180° in azimuth.

In order to investigate the suitable resolution and

penetration requirements with respect to concealed weapons

or explosives detection, another line-scanner system was

developed [18]. Here the intention is to generate images of

persons at a close distance of a few meters with a few

centimetres of spatial resolution. Hence, the system is a near-

field imaging device. A fixed parabolic mirror receives the

radiation from a rotating deflection plate. The vertical

movement of the whole unit delivers the second image

dimension. Fig. 18 shows some imaging results of persons

carrying various hidden objects under their clothing.

Fig. 18. Imaging examples of persons carrying various optically non-visible

objects under their clothing, measured at 90 GHz.

The persons have been located in about 2.5 m distance in

front of the instrument measuring again at a frequency band

around 90 GHz. A spatial resolution of about 2 cm was

achieved at this distance. For comparison the optical images

of each identical situation are also illustrated. In the upper left

image a metallic knife wrapped in a newspaper can be

recognized. In the upper right example a handgun in a bag and

a mobile phone in the shirt pocket are visible. Note that no

target at all can be detected in the optical images.

Finally it should be noted that huge efforts are presently

undertaken by many groups in order to develop real-time

high-resolution sufficiently sensitive radiometer systems for

security applications. Many imaging mechanisms are

investigated, and even the sub-millimetre and terahertz

frequency ranges are explored, while here the focus is more

on active systems for sufficient contrast [21].

V. CONCLUSION

Passive imaging technologies have a wide field of

applications in the whole microwave and millimeter-wave

region from about 1 GHz to 300 GHz and above. The main

application areas are geo science, climatology, agriculture,


surveillance, status registration, and safety and security in

general. Many of those applications require high spatial and

radiometric resolution, high precision, large fields of view,

quasi real-time imaging, and high frame rates. Consequently

the requirements for the instruments are high and the design is

a challenge. The rapid developments in receiver and computer

electronics of the past ten to twenty years allow the advanced

design and construction of more complex and more powerful

measurement systems, and widen the application areas of each

technology. A detailed but obviously incomplete overview on

existing techniques and technologies was given and illustrated

with examples believed to be representative.

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