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Radar Performance Factors

Radar Performance Factors. Objectives. Define the following radar terms and interpret/apply their relationships that effect radar performance. - Duty Cycle (DC) - Directional Gain (Gdir) - Pulse Width (PW) - Power Gain (G) - Pulse Repetition Time (PRT) - Peak Power (Ppk)

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Radar Performance Factors

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  1. Radar Performance Factors

  2. Objectives • Define the following radar terms and interpret/apply their relationships that effect radar performance. - Duty Cycle (DC) - Directional Gain (Gdir) - Pulse Width (PW) - Power Gain (G) - Pulse Repetition Time (PRT) - Peak Power (Ppk) - Average Power (Pave) - Minimum Range (Rmin) - Bandwidth (BW) - Maximum Unambiguous Range (Runamb) • Angular resolution (cross range resolution) (Rcross) - Number of returns per sweep (circular scan) - Pulse Compression Ratio (PCR) - Threshold level (TL) - Receiver sensitivity - Transmitted power • Calculate Power Gain and Effective Antenna Aperture. • List the factors that determine radar cross section.

  3. AN/SPS-55 TypeI-band (8 to 10 GHz) surface search and navigation radar. SpecificationsAntennaRotation rate: 16 rpmPolarization: circular or linearHorizontal beamwidth (3 dB): 1.5ºVertical beamwidth (3 dB): 20ºGain: 31 dB TransmitterFrequency: I-band (9.05-10 GHz). A G-band (5.45-5.825 GHz) version is also availablePeak power: 130 kWPRF/pulse-width: 750 pps/1 µs; 2,250 pps/0.12 µs ReceiverType: low noise Bandwidth: 1.2 MHz (long pulse); 10 MHz (short pulse)Receiver processors: linear logarithmic, FTC, variable sensitivity time control

  4. Carrier Frequency • Frequency emitted by the antenna • Determines antenna size and directivity of beam. • Lower Frequency • longer range • bigger antenna required (L≈λ/2) • more power required (same E field over longer distance) • Higher Frequency • Support high resolution features • better ability to resolve targets • smaller antenna (L ≈ λ /2) • greater attenuation losses. (scattering & absorption)

  5. Pulse Shape Xmit on Xmit off • Determines range accuracy and min/max range. • Desire pulse with vertical leading and trailing edge. • Crisp leading edge – improves range accuracy • Crisp trailing edge – improves min range • Crisp edges – improve range resolution • But perfect square wave requires infinite bandwidth Min Range Range Accuracy Trailing edge Leading edge time  Range Accuracy Rmin (decreases) Range Resolution (smaller)

  6. Pulse Width • Interval of time between leading edge of pulse and trailing edge of pulse. • Usually measured in microseconds (m). • Usually measured at half-power points. • Determines Rmin and range resolution. Short pulse width Long pulse width • Reduces max radar range • Better range resolution • Shorter Min detectable range • Increases max radar range • Degrades range resolution • Longer min detection range

  7. Pulse Compression • Signal processing technique. • allows use of wide pulses to increase range while maintaining the higher resolution of short pulses. • Increases frequency of the wave within the pulse. • Allows for good range resolution while packing enough power to provide a large maximum range.

  8. Pulse Compression

  9. Typical Pulse Widths

  10. Pulse Repetition Frequency (PRF) • Number of pulses transmitted per second • Expressed in Hz or PPS • Higher PRF • More hits per sweep • Higher probability of detection • Max theoretical range decreases • Unambiguous range decreases • Fire Control Radars • Use high PRF for high update rate • Search Radars • Use low PRF • Compensate thru slower scan or wider beam  PRF Scan Rate Runambig

  11. Runambig • Unambiguous Return – Return of echo prior to transmittal of next pulse (within unambiguous range). • Ambiguous Return – • Eclipsed Return – echo from first pulse appears as second pulse is transmitted. • Second Time Around Return – echo from first pulse arrives after second pulse sent.

  12. Typical PRFs

  13. Beamwidth (BW) • Measure of angular extent of the most powerful portion (main lobe) • ½ power points (-3dB) q = kl/L k= 0.88 linear 1.02 circular Frequency increases Wavelength decreases Beamwidth decreases Length of antenna increases Beamwidth decreases

  14. Beamwidth Vs. Accuracy

  15. Radiation Pattern Square Antenna

  16. Radiation Pattern Circular Antenna

  17. Scan Rate • Scan rate (Ω) • How fast antenna is rotated (user controlled) • Too fast…miss min number of returns • Too slow…incomplete radar coverage • Scan Rate & Beam Width & PRF together affect number of returns processed by the receiver

  18. Scan Rate & Beam Width

  19. Transmitter Power • High peak power is desirable to achieve maximum ranges. • But low power supports being covert. • Sometimes power is a design parameter • Sometimes power is a design constraint

  20. Radar Cross Section • Major factors which determine RCS • Target’s size • Shape • Material • Aspect angle (relative to radar) • radar frequency • polarization • RCS () measured in m2

  21. RCS WW-II B-26

  22. Signal Reception • The weaker the signal the receiver can process (Smin), the greater the effective range.

  23. Receiver Bandwidth • The frequency range the receiver can process. • Receiver must process many frequencies. • Pulse train is series of sine waves that approximate a square wave shape. • Frequency shifts occur from Doppler Effects. • Reduce bandwidth? • Increases the Signal-to-Noise ratio (good). • Returned signal frequency may be outside of bandwidth. (bad).

  24. Receiver Sensitivity • Smallest return signal discernible against noise background. • An important factor for determination of maximum radar range. • Smin = Minimum Signal for Detection (W) • MDS = Minimum Discernable Signal (dBm)  Sensitivity  Smin  MDS  Rmax • Large Smin / MDS NOT desirable. • Sensitivity would decrease. • Detection ability would decrease. • Max Range would decrease

  25. Signal-to-Noise Ratio • Ability to recognize target in background noise. • Noise is always present - external and internal. • At some range, noise will be greater than target return. • Noise or Smin sets lowest limit of the radar sensitivity. • Whichever (Noise or Smin) has the higher value. • Threshold Level used to suppress noise. • The higher SNR - the better!

  26. Radar Range Equation 1 1 4pR2 4pR2 s Pt G Ae Smin • s – Radar Cross Section 1/4 PtGAes R = (4p)2Smin

  27. Radar Range Equation 1 1 4pR2 4pR2 s Pt G Ae Smin • Pt – Power at the transmitter • Large EM Field • Corona Effect • Ionizes air • Arcing • Ionized air conducts 1/4 PtGAes R = (4p)2Smin

  28. Radar Range Equation 1 1 4pR2 4pR2 s Pt G Ae Smin • G – Antenna Power Gain • Product of • r – Antenna Efficiency • Gdir – Directive Gain 1/4 PtGAes R = (4p)2Smin

  29. Radar Range Equation 1 1 4pR2 4pR2 s Pt G Ae Smin • Ae – Effective Antenna • Product of • r – Antenna Efficiency • A – Antenna Area 1/4 PtGAes R = (4p)2Smin

  30. Radar Range Equation 1 1 4pR2 4pR2 s Pt G Ae Smin • Spherical Spreading • To and From Target • Represents spreading of omni-directional antenna 1/4 PtGAes R = (4p)2Smin

  31. Radar Range Equation 1 1 4pR2 4pR2 s Pt G Ae Smin • Smin – Minimum Sensitivity • Lowest energy level receiver is capable of detecting • Lower Smin – Better the range capability 1/4 PtGAes R = (4p)2 Smin

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