FOLIAGE PENETRATION RADAR

Full text

Introduction
While significant development has been made in enhancing the surveillance
grid by employment of a variety of sensors on our borders, significant
progress is yet to be made in the field of foliage see through or foliage
penetration radar (FOPEN or FPR). This becomes even more pertinent
when we are employing sensors along our Northern borders where the tree
line becomes thin only in the snow desert sector. Majority of the geography
in the valley lends itself to heavy coniferous vegetation & thick under
growth. Such dense vegetation is used for camouflage & concealment by
anti-national elements/ rogue elements to infiltrate. There is hence a need to
understand the utility of FPR & how it can be gainfully employed. The aim of
this paper is to acquaint the readers with FPR techniques & basics of
associated technology.
Lt Col Vivek Gopal, a graduate of the National
Defence Academy, was commissioned in
December 2000. A MTech, paratrooper & certified
Project Management Associate, the officer is
presently posted as Instructor at a premier
training establishment.
FOLIAGE
PENETRATING
RADAR - USE OF
SYNTHETIC
APERTURE RADAR

2
Fig.1 - An Overview of the Electromagnetic Spectrum
Historical Perspective
The first development of FPR occurred during the Vietnam conflict, where
early systems were needed to detect & recognize ground moving targets.
Three innovations were in turn needed:-
(a) Coherent waveforms.
(b) Associated signal processing.
(c) Radar Installations on major hills.
These innovations increased the target signal-to-noise & minimized the
clutter spread that masked the small returns from personnel & vehicles.
FPR has continued to be a developing technology to provide geospatial &
military users with detection & characterization of objects under dense
foliage. Many areas of the earth are remote & inhospitable for
characterization, as well as monitoring the effects of weather, atmosphere &
geological changes on the region. Similarly, military commanders want to
know about recent construction or tactical maneuvers in an area covered by
dense foliage.
RADAR has the inherent ability to characterize a wide area, to assess
changes in fixed objects, & to detect & track moving objects. Early RADARs
were limited to detection & tracking of objects by the attenuation &
scattering of clutter between the RADAR & the features being observed.
Forests have been particularly difficult environment due to the scattering of
the waveforms & severe attenuation at microwave frequencies. Over the

3
past 40 years, the advances in wave form synthesis have led to finer &
more accurate FPRs being developed.
After many trials, it was clear from operation of the foliage penetration GMTI
(Ground Moving Target Indication) systems, that if the targets were not
moving, it was impractical to detect the important tactical objects—vehicles
& structures. A need arose to develop a method of detecting stationary
objects, & an evolving technology was the synthetic aperture RADAR
(SAR). Excellent cross-range resolution could be obtained with coherent
processing of long collections of RADAR data. Early RADARs developed for
foliage penetration were in response to military needs to find & locate
insurgents in a severe tropical environment (read by US Army for Vietnam
operations). Little quantitative data existed to characterize the clutter &
propagation losses in this environment. Based on a series of data
collections in tropical regions, trials were resorted to for homing onto better
system post-validation.
Only the GMTI RADARs were taken to the military operations in South East
Asia by the US Army. The development of SAR capability was attempted,
but the military planners could not justify the development due to several
factors. First, the resolution of FPR- SAR was limited to tens of meters.
Operational SAR systems were significantly better than this & were not
accepted due to the unreliable image recognition results. Second, the SAR
systems were large & could not be carried on aircraft that would survive in a
military environment. Finally, the state of the art in real-time signal
processing was not mature enough to meet the needs of the mil users.
Scope
The paper covers the aspects of vegetation / foliage as prevalent in our
Northern frontiers (mainly the valley) & there by acquaints the readers with
respect to FPR & its associated technology which may lend itself useful for
future acquisitions. The scope of the paper thus is as under: -
(a) Part I - Foliage/ Vegetation in the valley.
(b) Part II - Need for FPR Technology & Its Development.

4
(c) Part III - Synth Aperture Radars – Basics.
(d) Part IV - Synth Aperture Radars – Best Suited for FPR.
(e) Part V - Recommendations & Future Potential.
PART I: FOLIAGE/ VEGETATION IN NORTHERN LATITUDES
Fig.2- Google Imagery of Area
The area, the species and density of forests are directly influenced by
lithology, rock-structure, altitude, aspect of slope, insolation and
precipitation. The influence of these physical factors is quite pronounced in
the state of Jammu and Kashmir. Consequently, there is great diversity in
the natural vegetation as over 4,000 species belonging to 1500 genera are
found in the state. The districts of Udhampur (1.63%), Kupwara (8.18%) and
Rajouri (6.46%) are the areas in which forest cover is reasonably significant
while in the remaining districts little or very little area is under forest. Being
situated at higher latitudes and characterized by undulating and
mountainous topo, most of the forests of the state of Jammu and Kashmir
belong to the coniferous category. In the valley floor of Kashmir, poplar,
chinar, maple and vir (willow) are the main species of vegetation which are
deciduous in character. Where soil conditions permit, mixture of broad-
leafed deciduous trees, such as maples and oaks, grow together. The thick
undergrowth in the forests stores up rain water and allows it to flow slowly
and that is why rivers that have their sources in the forests do not run dry in
the dry seasons and check floods during the rainy season.

5
Soils. In the regions of Jammu and Kashmir the soils are loamy and there
is little clay content in them. Poor in lime but with a high content of
magnesia, the soil is treated with chemical fertilisers and enriched with
green manure and legume before cultivation. There is sufficient organic
matter and nitrogen content in the alluvium of the Kashmir valley as a result
of plant residue, crops stubble, natural vegetation and animal excretion.
PART II: NEED FOR FPR TECHNOLOGY & ITS DEVELOPMENT
The early FPR systems were developed for detecting & characterizing
objects under both foliage & through ground penetration. The latter
capability was important; as demining operations were required after military
actions in war-torn areas. In addition, for finding objects that have been
hidden, the systems’ long wavelengths & polarimetric sensing found
usefulness in characterizing land use, land cover, & terrain elevation in
many geographic areas.
Significant progress was made in the design of antennas & transmitters for
FPR. The antennas needed to have wide azimuth coverage to enable
the requisite illumination angle for achieving the desired cross-range
resolution. They also needed to have an efficient match to the transmit
waveform over a very large bandwidth to support the range resolution.
Polarization has found an important place in FPR for characterization of the
clutter & objects. Providing Ultra-wide band (UWB) polarimetric antenna
was an early challenge. The design of the transmit waveform & match to the
antenna was also important to limit the spectral transmission as controlled
by the need for frequency allocation constraints.
Every new FPR system development needs to answer the question of using
VHF or UHF of operation. Optical photographs & microwave RADARs
cannot reliably detect man-made objects that have been hidden in the
dense forest cover. Two emerging technologies that could reduce the
unreliable detection of targets under foliage are as under:-
(a) The first technology being ultra-wideband (UWB) waveforms that
would enable high-resolution SAR images at both VHF & UHF freq.

6
(b) The second technology utilises of polarization of the RADAR
signal in the FPR SAR processing.
All four panes in the figure below are of the same scene; a forested region
with several vehicles parked under the foliage & in the tree lines, but
collected with different imaging technologies.
(a) On the left is a moderate to high-resolution optical picture, but
the vehicles cannot be observed until the sensor is nadir looking.
Fig.3 - Different Frequencies Consideration for SAR
(b) The next image to the right is a typical 1-meter resolution
X-band image ofthe scene taken on the same day. Sporadic
detections were obtained, but only when the glint of targets
could be captured in the image.
(c) Neither of these two image products would satisfy the
user, especially when high area coverage rate is needed.
(d) The next two images to the right, which are UHF & VHF
SARimages, show a more optimistic ability to detect the fixed
targets. The UHF panel shows images of many of the man-
made targets but high false alarms with the foliage clutter in
the scene.

7
(e) The detection at Very High Frequencies is higher where
the foliage attenuation is significantly lower & the target
cross sections are larger than the clutter. However, there is
limited resolution (i.e., pixels ontarget) to characterize the
objects in the image.
High-resolution imagery serves two purposes:-
(a) Provide a better separation of the object scattering from the
background clutter.
(b) Provide more detail of scattering of objects for characterization.
To find a small vehicle or a buried land mine, image resolution is a major
consideration. Polarization diversity has been evolving as a significant
capability for both target detection & characterization of terrain & man-made
objects. If characterization is an important system objective, then
polarization must be factored into the system wave form & processing
approach from the start. For FPR, higher grazing angles is important for
providing less foliage loss & better signal-to-clutter ratio. So, at higher
grazing angles the target signal return has the potential to be enhanced
relative to the background clutter but with a reduction in ground plane range
resolution. For resolutions under a meter, the required bandwidth is
above 150 MHz, independent of any range side lobe weighting. Figure
below, illustrates the importance of bandwidth when compared with the
carrier frequency as well as the importance to range resolution.
Fig.4 - Effect of Bandwidth & Resolution with Angle of View

8
FPR has many of the same characteristics as high-resolution microwave
SAR, (i.e., wide bandwidth, range curvature, & fine motion compensation for
SAR collection geometries). However, to obtain the maximum image
resolution, UWB waveforms & large integration angles are required. These
extremes in data collection make increased demands not only on the
amount of processing for image formation but also on the motion
measurement & compensation to focus over the full image &ensure
geospatial accuracy.
UHF television & radio stations have always limited the ability to
communicate or sense in these RF spectrum bands. It is not possible to
avoid these interference (i.e., they are actually jamming) sources because
of the spatial & spectral density of the emissions. As a result, techniques are
to developed to remove the background interference by waveform design &
adaptive processing techniques.
PART III: SYNTHETIC APERTURE RADAR (SAR) – BASICS
SAR has been found to be the best suited technology for FPR. However, it
is pertinent to understand the basics include the working principle behind
the same. The succeeding paragraph aims to highlight the basics of this
technology. It is also note worthy that FPR can also be considered as
an extension
of remote
sensing.
Fig.5 –SAR Basics
The APERTURE is SYNTHETIC i.e. created by the movement of the RADAR
which enables greater collection of backscatter.

9
SAR obtains fine resolution for ground images through two effects: -
(a) The range resolution, similar to conventional RADARs, is
obtained primarily by the bandwidth of the waveform.
(b) Cross range resolution is obtained by a physical antenna
angular pattern & the ability to coarsely resolve objects within the
real beam. However, for fine cross-range resolution, it is necessary to
form a synthetic aperture length by flying a certain length & coherently
integrating the returns to obtain the resolution. This is especially true
for imaging from VHF & UHF RADAR.
It is pertinent to note that any RADAR will actually be capturing the
backscatter as received from objects. To give a brief idea on the back-
scatter the following is beneficial: -
Fig.6 - Employment of Frequency Bands & Backscatter Scenarios
The data flow in SAR is very complex & depends on various factors as well
as algorithms used to finally fine tune the image captured. A basic
explanation of the system which involves capture of the image & thereafter
its 2D formation as visible is as under: -

10
Fig.7 - SAR Image Processing Flow
PART IV: SAR – BEST SUITED FOR FPR
After having seen the technology involved in SAR & its applications it can be
well appreciated that SAR will be one of the best suited for FPR. An
overview of the properties in summary are as under: -
 High resolution capability.
 Weather independent.
 Day & night functionality.
 Polarization signal can be exploited.
 It can always be complimentary to optical systems in use.
 Terrain topography can be measured.
PART V: RECOMMENDATIONS & FUTURE POTENTIAL
Taking into account the applicability of the RADAR systems & its inherent
advantages, the Foliage Penetration Radar (FPR) offers immense potential
with respect to counter-terrorist operations (as also counter LWE
operations
1
) to include search & destroy missions or any tactical
reconnaissance of the area of interest. The same radars when mounted on
the rotary wing aircraft for the Army can greatly help in the wide area
monitoring & detection (we may extrapolate to UAVs & heavy drones based
on payload limitations). Use by the Border Security Force in the Northern
Sectors of our country will also be greatly benefited from such a radar
2
.
While global firms
34
have already forayed into this domain, we are still a few
notches behind. FPR-10 (refer image/brochure placed below as on
1
PTI. “Government Plans to Get Foliage Penetration Radar for Naxal Areas.” The Economic
Times, Economic Times, 26 Apr. 2017,
economictimes.indiatimes.com/news/defence/government-plans-to-get-foliage-penetration-radar-
for-naxal-areas/articleshow/58382883.cms?from=mdr.
2
Ankit Panda. “India Eyes Israeli Foliage-Penetrating Radar for Kashmir Border
Security.” Thediplomat.com, 20 Aug. 2016, thediplomat.com/2016/08/india-eyes-israeli-foliage-
penetrating-radar-for-kashmir-border-security/.
3
“FORESTER - Foliage Penetration Radar | SRC, Inc.” Srcinc.com, 2021,
www.srcinc.com/products/radar/forester-radar.html.
4
“TRACER.” Lockheed Martin, 2018, www.lockheedmartin.com/en-us/products/tracer.html.

11
manufacture’s website) developed by Elisra (Israel) in June 2016
5
has been
one such candidate in the past. To give it the necessary impetus, the
Defence India Startup Challenge (DISC) has also been launched with the
development of FPR as one of the challenges in the year 2020-21.
Fig.8 - DISC Challenge – Development of FPR
Whenever developed, the FPR will aid in Reconnaissance, Surveillance,
Tracking and Engagement. An airborne radar system will help provide
stand-off, persistent, wide-area surveillance for situational awareness of
foliage-covered areas. Collaborations with start-ups, IIsT & in-house
5
“Report: India Is Interested in Elbit’s Foliage Penetration Radar | Israel
Defense.” Israeldefense.co.il, 2021, www.israeldefense.co.il/en/content/report-india-interested-
elbits-foliage-penetration-radar.

12
educational/ training institutions will also help in the expedited delivery of
such a potent radar platform.
With the existing exploitation of the quadcopters & mini UAVs there is a
need to home on to specific FPR based sensors/ payloads which can
augment the present-day surveillance capability. In house proposals may be
invited from private sector as well as DPSUs which can either help
development such a system or aid in miniaturizing the payloads to be used
for setting up an augmented surveillance grid.
REFERENCES & BIBLIOGRAPHY
1. White Paper on FOPEN Radar for UGS Applications by Sergio Gallone.
2. EW & Radar Engineering Handbook – Naval Air Warfare Centre, California.
3. Book Titled ‘Foliage Penetration Radar’ – Mark E Davis.
4. Article in Science & Technology Magazine – Radar Counters Camouflage.
5. Brochure – Elbit Sys (IAI) – FPR 10.
6. Book Titled ‘Foliage Penetration Radar: History & Developed Technology’ – Louis
Surgent, Army Land Warfare Lab, Maryland.
7. White Paper on Radiowave Propagation Through Vegetation’ by Mir Ghoraishi &
Jun Ichi Takada.
8. Trans Nav Article - Surveillance Unattended Foliage Penetrating Radar for Border
Control and Homeland Protection by F. Amato, A. Farina, M. Fiorini & S. Gallone, Selex
ES –A Finmeccanica Company, Rome, Italy.
CERTIFICATE
The paper is author’s individual scholastic articulation. The author
certifies that the article is original in content, unpublished and it has not
been submitted for publication / web upload elsewhere and that the
facts and figures quoted are duly referenced, as needed and are
believed to be correct. The paper does not necessarily represent the
views of the CENJOWS.
Disclaimer: Views expressed are of the author and do not necessarily
reflect the views of CENJOWS.