Sea Clutter & Surface Clutter

Introduction • are defined by the operational purpose of the radar • Target detection is degraded by Clutter Changes in propagation of radio waves Intentional/unintentional jamming

• In order to optimize radar signal processing methods the effects the surface and volume clutter have on radar detection capabilities have to be known

Radar systems detect targets by examining Courtesy of SELEX S.I. reflected energy, or returns, from objects • Along with target echoes, returns come from the sea surface, land masses, buildings, rainstorms, and other sources • Much of this clutter is far stronger than signals received from targets of interest • The main challenge to radar systems is discriminating these weaker • The IEEE Standard Radar Definitions (Std 686-1990) defines target echoes fromprocessing clutter coherent signal as echo integration, filtering, or •detection Coherent signal of the received signals and its using the amplitude processing phase referred to that of a reference oscillator or to the techniques signal. are used to this transmitted •

Clutter environment • Target detection in radar systems is based on the amplitude and phase characteristics of the return echo signal • Doppler effect provides means for detecting a moving target in the presence of clutter whose amplitude exceeds that of the target • Methods are MTI- (moving target indication) and Doppler processing

CLUTTERS  Clutter refers to radio frequency (RF) echoes returned from targets which are uninteresting to the radar operators.  Such targets include natural objects such as ground, sea, precipitation (such as rain, snow or hail), sand storms, animals (especially birds), atmospheric turbulence, and other atmospheric effects, such as ionosphere reflections, meteor trails, and three body scatter spike.  Clutter may also be returned from man-made objects such as buildings and, intentionally, by radar countermeasures such as chaff.

 Clutter may also originate from multipath echoes from valid targets due to ground reflection, atmospheric ducting or ionospheric reflection/refraction.  The basic types of clutter can be summarized as follows: Surface Clutter Sea Clutter

Surface Clutter • Ground or sea returns are typical surface clutter. • Returns from geographical land masses are generally stationary, however, the effect of wind on trees etc means that the target can introduce a Doppler Shift to the radar return. • Doppler shift : to remove unwanted signals in the signal processing part of a radar system. • Clutter returned from the sea generally also has movement associated with the waves.

Sea Clutter • Sea-clutter are disturbing radar-echoes of sea wave crests. This clutter gets also a Doppler- speed by the wind. Eg: the scenario „moves away”, i.e. changes with time, while for ground clutter it stays the same. Therefore, in practice, Sea-clutter is very difficult to control without some loss in detection. • The radial speed of the waves is very small, is „cleaned” by the MTI system very clearly.

Radar Detection Problem The radar scenario involves a transmitter and a receiver, at the same location (monostatic configuration), equipped with an array of sensors, a target at a certain “distance” from the array (range) in the far zone, and a narrowband signal that travels the roundtrip between the radar and the target. The received signals will always contain a component due to receiver noise and may contain components due to both desired targets and undesired interference (jamming and clutter).

What is the clutter? Clutter refers to radio frequency (RF) echoes returned from

targets

which

are

uninteresting

to

the

radar

operators and interfere with the observation of useful signals. Such targets include natural objects such as ground, sea, precipitations (rain, snow or hail), sand storms, animals (especially birds), atmospheric turbulence, and other atmospheric effects, such as ionosphere reflections and meteor trails. Clutter may also be returned from man-made objects such as buildings and, intentionally, by radar countermeasures such as chaff.

Radar clutter  Radar clutter is defined as unwanted echoes, typically from the ground, sea, rain or other atmospheric phenomena.  These unwanted returns may affect the radar performance and can even obscure the target of interest.  Hence clutter returns must be taken into in deg a radar system.

Towards this goal, a clutter model assumption is necessary! The function of the clutter model is to define a consistent theory whereby a physical model results in an analytical model which can be used for radar design and performance analysis.

Clutter reflectivity A perfectly smooth and flat conducting surface acts as a mirror, producing a coherent forward reflection, with the angle of incidence equal to the angle of reflection. If the surface has some roughness, the forward scatter component is reduced by diffuse, non-coherent scattering in other directions. radar

i

For monostatic radar, clutter is the diffuse backscatter in the direction towards the radar

r

specular reflection i=r smooth surface specular component

diffuse component

rough surface

Radar Equation and propagation factor F •

For monostatic radar, received power Pr from a target with RCS  is 2 2 PG  Pr  t 3 4  F 4  4  R

• • • •

Pt = transmit power G = antenna gain R = distance of target from antenna F = the pattern-propagation factor, the ratio of field strength at a point to that which would be present if free-space propagation had occurred A clutter measurement provides either F4 or oF4. Even so, normally the data are reported as being  or  o.

Sea clutter: Dependence on grazing angle

At near vertical incidence, the backscatter is quasispecular and varies inversely with surface roughness with a maximum at vertical incidence for a perfectly smooth surface. At medium grazing angles the reflectivity shows a lower dependence on grazing angle (plateau region).

0 dB

10 log O

Below some critical angle (~ 10º, depending on the roughness) the reflectivity reduces rapidly with smaller grazing angles (interference region, where propagation is strongly affected by multipath scattering

0o

c

GRAZING ANGLE 

o

90o

Empirical model for sea clutter 0 GIT model: Wind speed dependence HH POL, 10 GHz Cross-wave direction, 2 m signif. wave height, winds 3, 5, 10, 20 m/s oF4 increases with wind speed Critical angle unchanged, because wave height assumed fixed.

Empirical model for sea clutter  0

GIT model: Dependence on significant wave height h1/3 HH POL, 10 GHz Cross-wave direction, 10 m/s wind speed, h1/3 = 0.5, 2, and 6 m In plateau region, oF4 is independent of h1/3 (for fixed wind speed) oF4 increases with h1/3 (multipath reduces critical angle) at angles < 1o

Empirical model for sea clutter  0

GIT model: Comparison between HH and VV POL, 10 GHz Wind speed/wave height in equilibrium oF4 increases with wind speed and h1/3 HH/VV ratio increases with increased surface roughness and reduced grazing angle HH>VV at small angles under rough conditions at 1.25 and 10 GHz

Radar clutter modeling •In the quest for better performance, the resolution capabilities of radar systems have been improved •For detection performance, the belief originally was that a higher resolution radar system would intercept less clutter than a lower resolution system, thereby increasing detection performance • However, as resolution has increased, the clutter statistics have no longer been observed to be Gaussian, and the detection performance has not improved directly •The radar system is now plagued by target-like “spikes” that give rise to non-Gaussian observations •These spikes are ed by the detector as targets at a much higher false alarm rate (FAR) than the system is designed to tolerate •The reason for the poor performance can be traced to the fact that the traditional radar detector is designed to operate against Gaussian noise •New clutter models and new detection strategies are required EGO Workshop 2012 - October 15-17, to reduce the effects of the spikes and to improve detection

Sea clutter PSD The relative motion of the sea surface with respect to the radar causes an intrinsic Doppler shift of the return from individual scatterers. Because the motion of the scattering elements have varying directions and speeds the total echo contains a spectrum of Doppler frequencies. Two effects are of interest: • the spectral shape and width • the mean Doppler shift of the entire spectrum.

Sea clutter PSD The spectrum of sea clutter is sometimes assumed to have Gaussian shape. An approximate relationship between the -3dB bandwidth f of the spectrum and sea state S (Douglas scale) has been derived by Nathanson:

f  3.6 f 0( GHz ) S The standard deviation of the Gaussian spectrum is related to f by the expression:

 f  0.42f Recently more complex and realistic models have been proposed for sea clutter PSD. We are going to analyze them later on.

Sea clutter: stationary or nonstationary process?

Large-scale and small-scale components Bragg waves: C0 Wind drift: Dw Long waves: VOR

Current: VC

Bragg scattering: the return signals from scatterers with wavelength B reinforce each other since they are in phase

0 B  2cos  0

Large-scale structure changes the distance between the antenna and the patch, tilts and advects the small-scale structure

2cos  0 fD  (C0  VOR  Dw  Vc ) 0

fD 

g 2 B

C0 phase velocity of Bragg waves VOR orbital velocity: periodic Dw wind drift Vc current velocity

Ground clutter: range spectral analysis The range PSD is almost flat. The correlation time of ~100 ns (~ one pulse length). provides frequency content over the complete Nyquist frequency range. The assumption usually made in adaptive radar detection of independence of the data from different range cells seems to be reasonable in the Wolseley data. Owing to the heterogeneity of the spatial scattering ensemble in open farmland terrain (strong discrete sources dispersed over a weakly scattering medium), the returned signal from the scanning antenna largely decorrelates from one spatial cell to the next, whether the variation is in the range direction or in the azimuth