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2. 2 CLUTTER ANALYSIS

3. 3 Table of Contents 1.Introduction to Clutter Analysis 2.Surface Clutter 3.Volume Clutter 4.Clutter Statistical Model 5.Summary 6.Reference

4. 4 1.Introduction to Clutter Analysis

5. 5 Introduction Clutter: Unwanted radar returns that cause interference to normal radar operations. Parasitic returns that enter the radar through the antenna’s main lobe are called main lobe clutter; otherwise they are called side-lobe clutter. Two major clutter categories: surface clutter and volume clutter.

6. 6 Introduction Clutter echoes are random and thermal noise-like signals since the individual scatterer has random phases and amplitudes. In many cases, the clutter signal level is much higher than the receiver noise level. The ability to detect targets, which embeds in high clutter background, depends on the Signal-to-Clutter Ratio (SCR) rather than the Signal-to-Noise Ratio (SNR).

7. 7 Clutter Radar Cross Section (RCS) Clutter RCS: the equivalent RCS attributed to reflections from a clutter area, A c. The average clutter RCS can be written as, where  o (m 2 / m 2 ), is the clutter scattering coefficient. Thus, where  t, is the target’s RCS.

8. 8 2.Surface Clutter

9. 9 Surface Clutter Surface clutter includes land and sea clutter such as trees, vegetation, ground terrain, man-made structures, and sea surface. It also refer as area clutter.  c depends on radar parameters, grazing angle and surface roughness Typically,  c is larger for smaller radar wavelength

10. 10 Geometry of grazing angle: Dependency of  on Grazing Angle gg 0dB gg >60 o Critical angle High grazing angle region Low grazing angle region Plateau region  dB

11. 11 Clutter Regions Low grazing angle region: Diffused clutter — due to rough surface. Flat/Plateau region:  o is almost constant, minimal dependency of  o on the grazing angle. High grazing angle region: Direct reflection from smooth surface, diffuse clutter components disappear. Smooth surface have larger  o than rough surfaces, opposite of the low grazing angle region.

12. 12 Critical Angle Critical angle: The angle below, which a surface is considered to be smooth, and above, which a surface is considered to be rough. Raylegh criteria: The surface is considered to be smooth if gg gg Smooth path Rough path Smooth surface level h rms

13. 13 Critical Angle The “rough path” is longer that the “smooth path” by a distance 2h rms sin  g. The phase differential  g can be written as, Critical angle  gc is computed when  g =  (first null).

14. 14 Critical Angle Therefore, For sea clutter, the rms surface height can be approximated as where S state, is the sea state. The sea state is affected by the wave height, period, length, particle velocity, and wind velocity.

15. 15 Sea State Classification Douglas Sea Scale Sea StateWave height (m)Descriptive term 00Calm, glassy 10-0.1Calm, rippled 20.1-0.5Smooth, wavelets 30.6-1.2Slight 41.2-2.4Moderate 52.4-4.0Rough 64.0-6.0Very rough 76.0-9.0High 89.0-14Very high 9>14Phenomenal

16. 16 SCR for Area Clutter The size of footprint, A c is dependent on the grazing angle,  g, and the antenna 3dB beamwidth,  3dB. Therefore, the SCR for area clutter is given by gg R  3dB R  3dB R  3dB /sin  g AcAc

17. 17 Sea Clutter The echo from the surface of the sea is dependent upon: 1.Sea parameters –wave height, period, length (sea state) –wind speed, the length of time and the distance (fetch) over which the wind has been blowing –direction of the waves relative to that of the radar beam –whether the sea is building up or is decreasing –the presence of swell as well as sea waves –the presence of contaminants that might affect the surface tension 2.Radar parameters –frequency, polarization –grazing angle –the size of the area under observation

18. 18 Detection of Targets in Sea Clutter To reduce sea clutter, the radar can employ: –Lower frequency –Polarization perpendicular to sea wave –Better resolution by shorter pulse width (P t should be high enough) and narrower  3dB (but not too narrow) To detect moving targets (aircraft, ships) in the presence of sea clutter: –Employ MTI radar with moderate resolution (RCS of ships is relatively large) To detect small targets (buoys, land-sea boundaries, non-moving objects): –Employ high resolution radar

19. 19 Example of Sea Clutter Detection LFM source Use built-in RADAR_SEA_CLUTTER and RADAR_CLUTTER_H models in SystemVue. Sample of Sea Clutter Echo

20. 20 Land Clutter Examples: desert, forests, vegetation, bare soil, cultivated fields, mountains, swaps, cities, roads, and lakes — all have different scattering characteristics, making land clutter more complicated than sea clutter. Use MTI to detect moving target for high land clutter. Sometimes, land clutter can be “useful information”: 1.Altimeters — large clutter energy is desired since the “clutter” is the target. 2.The detection of terrain features — such as hills and mountains for terrain avoidance or terrain following. 3.Mapping or imaging radars that utilize high resolution.

21. 21 3.Volume Clutter

22. 22 Volume Clutter Volume clutter has larger extents and includes rain, chaff, birds, and insects.  is defined as RCS per unit resolution volume, V W. It can be computed by the sum of all individual scatterers RCS within the volume (m 2 /m 3 ) where N is the total number of scatterers within the resolution volume The total RCS of a single resolution volume is

23. 23 Volume Clutter A resolution volume is given as where  a and  e are the antenna beam width in azimuth and elevation, respectively;  is the pulse width in seconds, c is speed of light, and R is range. R aa ee Geometry of a Resolution Volume The SCR for volume clutter is then given by

24. 24 Types of Volume Clutter Birds, insects, and other flying particles –referred as angel echo or biological clutter –average RCS for individual birds or insects, –where W b is the individual bird or insect weight in grams Rain droplets –modeled as perfect small spheres: –can be suppressed by co-polarizing the radar transmit and receive antennas Chaff –consists of a large number of small dipole reflectors (made of aluminum foil or fiber glass) that have large RCS values

25. 25 Angel Echoes Obtained from regions of the atmosphere where no apparent reflecting sources seem to exist. Dot angels: point targets due to birds and insects. Distributed angels: due to inhomogeneities of the refractive index of the atmosphere. Birds are the most prominent source of angels: –The RCS of a bird with size of 0.01 m 2 at 100 m = The RCS of a target with size of 100 m 2 at 1k m –8 birds per square mile can completely blank a PPI screen

26. 26 Angel Echoes Mitigation Since angels (birds) are moving clutter to an MTI radar, they are difficult to be removed by doppler filtering. Sensitivity time control (STC) –receiver gain is programmed such that its gain reduces at short range and increases at far range. The received signal from a target of constant cross section remains unchanged with range. operate at low frequency (such as UHF) since  t  f 4.

27. 27 4.Clutter Statistical Model

28. 28 Clutter Statistical Model Type of PDF depends on the nature of clutter (sea, land, volume), the radar operating frequency, and the grazing angle. Sea/land clutter (composed of many small scatterers)  Rayleigh PDF Land clutter at low grazing angles  log-normal PDF Other clutters at low grazing angles (< 5  ) for frequencies between 1 and 10 GHz  Weibull PDF

29. 29 Summary Clutter: Unwanted radar returns that cause interference to normal radar operations. The ability to detect targets, which embeds in high clutter background, depends on the Signal-to-Clutter Ratio (SCR) rather than the Signal-to-Noise Ratio (SNR). Surface clutter includes both land and sea clutter. Volume clutter has larger extents and includes rain, chaff, birds, and insects. Clutter can be statistically described by a probability distribution function (PDF).

30. 30 References “Introduction to radar systems”, Merrill l. Skolnik, Boston : McGraw Hill, 2001. “Radar Signal Analysis and Processing Using MATLAB”, Mahafza, Bassem R., Chapman and Hall/CRC, 2008. “Microwave Remote Sensing: Active and Passive, Volume II: Radar Remote Sensing and Surface Scattering and Emission Theory”, F.T. Ulaby, R.K. Moore, A.K. Fung, Artech House, 1986. “Radar Handbook”, Merrill Skolnik, McGraw Hill Professional, 2008.