MIT-LL_Intro to radar systems_lecture 3_Propagation Effects v2.pdf
- 1. Propagation-1 RJG 7/31/2008 MIT Lincoln Laboratory Introduction to Radar Systems Propagation Effects
- 2. MIT Lincoln Laboratory Propagation-3 RJG 7/31/2008 Radar Block Diagram Transmitter Pulse Compression Recording Receiver Tracking & Parameter Estimation Console / Display Antenna Propagation Medium Propagation Medium Target Cross Section Doppler Processing A / D Waveform Generator Detection Signal Processor Main Computer “The Soup”
- 3. MIT Lincoln Laboratory Propagation-4 RJG 7/31/2008 Radar Classes • Ground based • Sea based • Airborne Nearly all radar systems operate through the atmosphere and near the Earth’s surface Nearly all radar systems operate through the atmosphere and near the Earth’s surface AEGIS Patriot AWACS Courtesy of Raytheon. Used with permission. Courtesy of U.S. Air Force. Courtesy of U.S. Navy.
- 4. MIT Lincoln Laboratory Propagation-5 RJG 7/31/2008 Propagation Effects on Radar Performance • Atmospheric attenuation • Reflection off of Earth’s surface • Over-the-horizon diffraction • Atmospheric refraction Radar beams can be attenuated, reflected and bent by the environment Radar beams can be attenuated, reflected and bent by the environment
- 5. MIT Lincoln Laboratory Propagation-6 RJG 7/31/2008 What’s in the Soup? • Atmospheric parameters vary with altitude – Air density and humidity – Rain rate – Fog/cloud water content – Index of refraction • Atmospheric parameters vary with altitude – Air density and humidity – Rain rate – Fog/cloud water content – Index of refraction MIT Lincoln Laboratory 02101-061 • Earth’s surface – Surface material (water vs land) – Surface roughness (waves, mountains) – Earth’s curvature • Earth’s surface – Surface material (water vs land) – Surface roughness (waves, mountains) – Earth’s curvature
- 6. MIT Lincoln Laboratory Propagation-7 RJG 7/31/2008 Outline • Atmospheric attenuation • Reflection from the Earth’s surface • Over-the-horizon diffraction • Atmospheric refraction
- 7. MIT Lincoln Laboratory Propagation-8 RJG 7/31/2008 Atmospheric Attenuation at Sea Level Radar power absorbed by water vapor and oxygen Attenuation is a loss of power characterized by L in radar range equation High frequencies are not well suited for long-range low-altitude surveillance High frequencies are not well suited for long-range low-altitude surveillance 1 10 100 0.01 0.1 1 10 100 Radar Frequency (GHz) Specific Attenuation (dB/km) 1 L S C X W Ka Ku 10 100 1000 10000 Total 50 km 2-Way Attenuation (dB) 1/10 power left 1/10000000000 power left
- 8. MIT Lincoln Laboratory Propagation-9 RJG 7/31/2008 Attenuation in Rain and Fog Radar performance at high frequencies is highly weather dependent Radar performance at high frequencies is highly weather dependent Figure by MIT OCW.
- 9. MIT Lincoln Laboratory Propagation-10 RJG 7/31/2008 Outline • Atmospheric attenuation • Reflection from the Earth’s surface • Over-the-horizon diffraction • Atmospheric refraction
- 10. MIT Lincoln Laboratory Propagation-11 RJG 7/31/2008 Interference Basics • Two waves can interfere constructively or destructively • Resulting field strength depends only on relative amplitude and phase of the two waves – Radar voltage can range from 0-2 times single wave – Radar power is proportional to (voltage)2 for 0-4 times the power – Interference operates both on outbound and return trips for 0-16 times the power Destructive Interference Constructive Interference Wave 1 Wave2 Sum of Waves 1 + 2
- 11. MIT Lincoln Laboratory Propagation-12 RJG 7/31/2008 Propagation over a Plane Earth Reflection from the Earth’s surface results in interference of the direct radar signal with the signal reflected off of the surface Surface reflection coefficient ( Γ ) determines relative signal amplitudes Dependent on: surface material, roughness, polarization, frequency Close to 1 for smooth ocean, close to 0 for rough land Relative phase determined by path length difference and phase shift on reflection Dependent on: height, range and frequency Radar Direct path Multipath Target
- 12. MIT Lincoln Laboratory Propagation-13 RJG 7/31/2008 Multipath Alters Radar Detection Range • Multipath causes elevation coverage to be broken up into a lobed structure • A target located at the maximum of a lobe will be detected as far as twice the free-space detection range • At other angles the detection range will be less than free space and in a null no echo signal will be received Reflection Coefficient Γ=-1 Γ=-0.3 Γ=0 Target Range Target Altitude Radar Coverage
- 13. MIT Lincoln Laboratory Propagation-14 RJG 7/31/2008 Multipath is Frequency Dependent Lobing density increases with increased radar frequency Lobing density increases with increased radar frequency Reflection Coefficient Γ=-1 Γ=-0.3 Range Altitude Range Radar Coverage 0 0.5 1 1.5 2 0 0.5 1 1.5 2 0 0.5 1 1.5 2 0 0.5 1 1.5 2 Frequency 1 2 x Frequency 1 x x 1 lobe over distance x : 2 lobes over distance x :
- 14. MIT Lincoln Laboratory Propagation-15 RJG 7/31/2008 Outline • Atmospheric attenuation • Reflection from the Earth’s surface • Over-the-horizon diffraction • Atmospheric refraction
- 15. MIT Lincoln Laboratory Propagation-16 RJG 7/31/2008 Tsunami Diffracting around Peninsula Diffraction • Radar waves are diffracted around the curved Earth just as ocean waves are bent by an obstacle • Web references for excellent water wave photographic examples: – http://upload.wikimedia.org/wikipedia/commons/b/b5/Water_diffraction.jpg – http://yhspatriot.yorktown.arlington.k12.va.us/~ckaldahl/wave.gif • The ability of radar to propagate beyond the horizon depends upon frequency and radar height Courtesy of NOAA / PMEL / Center for Tsunami Research. See animation at http://nctr.pmel.noaa.gov/animations/Aonae.all.mpg
- 16. MIT Lincoln Laboratory Propagation-17 RJG 7/31/2008 Propagation Over Round Earth • Interference region – Located within line of sight radar • Diffraction region – Below radar line of sight – Signals are severely attenuated Interference Region Tangent Ray Diffraction Region Earth Radar
- 17. MIT Lincoln Laboratory Propagation-18 RJG 7/31/2008 Combined Diffraction and Multipath vs Radar Frequency • Low altitude multipath detection: favors higher frequencies • Diffraction detection: – Favors lower frequencies – Is tough at any frequency 100 ft altitude at 60 km Geometric Horizon X-Band Lowest multipath lobe is plotted 100 ft altitude 60 dB L-band loss 80 dB X-band loss L-Band
- 18. MIT Lincoln Laboratory Propagation-19 RJG 7/31/2008 Outline • Atmospheric attenuation • Reflection from the Earth’s surface • Over-the-horizon diffraction • Atmospheric refraction
- 19. MIT Lincoln Laboratory Propagation-20 RJG 7/31/2008 Refraction of Radar Beams Radar rays bend downwards due to decreasing index of refraction of air with altitude Same effect as refraction of light beam shining from water into air Figure by MIT OCW.
- 20. MIT Lincoln Laboratory Propagation-21 RJG 7/31/2008 Earth’s Radius Modified to Account for Refraction Effects Atmospheric refraction is accounted for by replacing the actual Earth radius a, in calculations, by an equivalent earth radius ka and assuming straight line propagation 4/3 is a typical value for k Average propagation is referred to as a “4/3 Earth” Average propagation is referred to as a “4/3 Earth” Figure by MIT OCW.
- 21. MIT Lincoln Laboratory Propagation-22 RJG 7/31/2008 Anomalous Propagation • Occurs when k not equal to 4/3 • Categorized as: superrefraction, subrefraction and ducting – Superrefraction extends the radar horizon – Subrefraction limits the radar horizon – Ducting traps radar energy near the Earth’s surface 4/3 Earth Radius 4/3 Earth Radius Subrefraction Superrefraction Ducting
- 22. MIT Lincoln Laboratory Propagation-23 RJG 7/31/2008 No Surface Duct Surface Duct Ducting Effects on Target Detection Ducting extends low-altitude detection ranges but can cause unexpected holes in radar coverage Ducting extends low-altitude detection ranges but can cause unexpected holes in radar coverage Target seen Target not seen
- 23. MIT Lincoln Laboratory Propagation-24 RJG 7/31/2008 Ducted Clutter from New England Ducting conditions can extend horizon to extreme ranges Ducting conditions can extend horizon to extreme ranges 50 km range rings PPI Display
- 24. MIT Lincoln Laboratory Propagation-25 RJG 7/31/2008 Radar Propagation Effects Summary 1 10 100 0.01 0.1 1 10 100 Radar Frequency (GHz) Specific Attenuation (dB/km) L S C X W Ka Ku Atmospheric Attenuation Multipath and Diffraction Multipath Reflection Refraction (Ducting) 4/3 Earth Radius Reflection Coefficient Γ=-1 Γ=-0.3 Γ= 0 Range Altitude Target seen Target not seen Geometric Horizon X-Band L-Band 60 dB L-band loss 80 dB X-band loss
- 25. MIT Lincoln Laboratory Propagation-26 RJG 7/31/2008 References • Skolnik, M., Introduction to Radar Systems, New York, McGraw-Hill, 3rd Edition, 2001 • Skolnik, M., Radar Handbook, New York, McGraw-Hill, 2rd Edition, 1990