Lecture 3 SIGNAL PROCESSING

Transcription

1 Lecture 3 SIGNAL PROCESSING

2 Pulse Width t Pulse Train Spectrum of Pulse Train Spacing between Spectral Lines =PRF -1/t 1/t

3 -PRF/2 PRF/2 Maximum Doppler shift giving unambiguous results should be with in one-half of PRF

4 Example: If a radar operates at a PRF of 1250 pps and has an illumination frequency of 3 GHz, find the maximum target velocity giving an unambiguous Doppler shift. Solution: Maximum Doppler shift giving unambiguous results should be with in one-half of PRF f f v d R PRF 2 vr 2 ft c 31.25m d max / s Hz

5 High PRF implies Smaller unambiguous range and Larger unambiguous Doppler. Low PRF implies Larger unambiguous Range and Smaller unambiguous Doppler. From previous discussion we can conclude that, a target unambiguous in range may be ambiguous in Doppler. How we can resolve Range and Doppler ambiguities? We will discuss in Chapter 6.

6 Signal Definition: Signals are electrical quantities (voltages and currents) which convey information. examples include: Echoes of the wave from desired targets. Echoes from undesired targets like clutter. Signals produced within radar itself, noise and spillover. Signals transmitted by hostile sources with the intent of interfering with target echoes. (known as jamming) Signals from friendly sources.

7 A typical signal in a pulsed radar is shown below

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9 The main goal is to improve the signal to interference ratio: S/I = P R / P I P R = target echo component of the composite signal (watts) P I = the interfering component of the composite signal (watts) TYPES: Frequency: Rate at which the signal vector rotates Phase: reference. Time offset between the signal and an internal

10 Radar usually illuminate the target for several pulses and collect a series of echoes as shown

11 Signal processing attempts to separate composites into their components by exploiting differences between the components.

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13 The effectiveness of SP in separating signals from interference is described by a parameter called Process Gain. Process Gain: ratio of the signal to interference ratio out of the process and the signal to interference ratio into the process. S Io Gp S Gp = Process Gain I S/I 0 = Signal to interference ratio out of the signal process S/Ii = Input signal to interference ratio. i

14 Ability to detect targets: Detect presence of targets in interference. Radar Cross section and RCS variations and fluctuations Transmitter energy output. Antenna Gain Effective capture area of antenna Receiver s sensitivity Signal processor s Process Gain.

15 Measure of the radar s ability to separate desired targets from echoes from land, sea and weather. Clutter occurring at times other than the target echo can be separated by time discrimination. TIME GATING.

16 Clutter discrimination by two processes: RESOLUTION limits the amount of clutter present simultaneously with target echo. DOPPLER or spectrum analysis is then used to separate echoes signals by their velocities.

17 Doppler Clutter Processing: Moving Target Indication (MTI) identify targets with Doppler shifts different from those of clutter. MTD radars can estimate the velocity of targets. Countermeasures immunity: ECM attempts to hide targets by presenting interfering signals of greater amplitude than the target echoes. EMI Immunity: Electromagnetic interference sources include communication transmitters, other radars, and friendly jammers.

18 Primary and Secondary Radars. Monostatic/Bistatic Classification by primary radar mission. Classification by Frequency Band. Classification by waveform and sample rate. Classification by special mission and mode.

19 Primary Radars: Those where the radar s transmitter illuminates the target and the echo from the illumination is used to extract information. Secondary Radars: Those where the primary radar s illumination or a separate interrogation signal triggers an active response by the target. It an be used with cooperative targets.

20 Primary and Secondary Radars. Monostatic/Bistatic Classification by primary radar mission. Classification by Frequency Band. Classification by waveform and sample rate. Classification by special mission and mode.

21 Those systems where antenna is shared by transmitter and receiver or where separate transmit and receive antenna are in essentially the same location, are monstatic.

22 Primary and Secondary Radars. Monostatic/Bistatic Classification by primary radar mission. Classification by Frequency Band. Classification by waveform and sample rate. Classification by special mission and mode.

23 Search radars and modes. Surface Search. Air Search. 2-D Search. 3-D Search. Tracking radars and modes. Track while Scan(TWS).

24 Primary and Secondary Radars. Monostatic/Bistatic Classification by primary radar mission. Classification by Frequency Band. Classification by waveform and sample rate. Classification by special mission and mode.

25 Radar Systems operate in most frequency bands, refer to handout for the frequency range within: HF 3 30 MHz VHF MHz UHF MHz L 1 2 GHz S 2 4 GHz C 4 8 GHz X 8 12 GHz Ku GHz K GHz Ka GHz V GHz W GHz mm GHz

26 Classification by Frequency Band. Lower Frequencies: To view events beyond the horizon. VHF and UHF: Very long range early warning radars. L-Band: Long range military and air traffic control search radar. S-Band: Medium range ground based and shipboard search radars. Recent addition is Next generation Doppler weather radar. C-band: Moderate range search and fire control with reduced antenna size. X-Band: Military airborne multimode radars with limited antenna size. K-Band: Very short range for terrain avoidance and terrain following radars. Infra Red Band: Optical ranging and imaging.

27 Primary and Secondary Radars. Monostatic/Bistatic Classification by primary radar mission. Classification by Frequency Band. Classification by waveform and sample rate. Classification by special mission and mode.

28 Classification by waveform. Unmodulated CW: continuous sinusoidal wave for Doppler measurement. Modulated CW: Time variant modulation e.g. LFM and Phase modulation. Gated CW pulse: Unmodulated sinusoid gated for PRF. Complex waveform pulse: Gated but with time variant modulation to increase bandwidth. Classification by PRF class. Low PRF: Unambiguous in range and ambiguous in Doppler. High PRF: Ambiguous in range and unambiguous in Doppler. Medium PRF: Simultaneously recover both range and Doppler for large number of targets.

29 Primary and Secondary Radars. Monostatic/Bistatic Classification by primary radar mission. Classification by Frequency Band. Classification by waveform and sample rate. Classification by special mission and mode.

30 Surface-to-surface missions. Surface-to-air missions. Air-to-air missions. Air-to-surface missions. Terrain Following and Terrain Avoidance. Ground moving target search. Sea surface search. Multi-mode system.