JED - November 2010 - (Page 48)

EW 101 EW Against Modern Radars – Part 13 Pulse Doppler Radar AMBIGUITIES continued 48 The Journal of Electronic Defense | November 2010 As discussed in the August 2000 “EW 101,” the maximum unambiguous range of a radar is the distance for which a transmitted pulse can make a roundtrip at the speed of light before the next Transmitted Signal pulse is transmitted (see Figure 1). RU = (PRI/2) x c Where: RU is the unambiguous range in meters Received PRI is the pulse repetition Signal interval in seconds c is the speed of light (3 x 108 Time for signal Time for reflection to reach target to reach radar m/sec) Time For example, if the PRI is 100 sec, the unambiguous range is 15 km. The Figure 1: The maximum unambiguous range is the range at which the radar pulse can make a higher the pulse repetition frequency round-trip to the target at the speed of light before another pulse is transmitted. (PRF), the shorter the PRI, hence the shorter the unambiguous range. If the the design engagement. The lower the PRF, the greater the PRF is quite high, there will be many range ambiguities. frequency ambiguity. A PRF of 1000 pps will have many amThe Doppler shifted frequency of the return signal falls into biguous responses less than 33.3 kHz, while a PRF of 300 kpps a Doppler filter in the pulse Doppler (PD) radar’s processor. will be totally unambiguous within the frequency range of The maximum Doppler frequency shift is: the processing matrix. ∆F = (vR/c) x 2F As shown in Figure 3, the range is ambiguous if the PRI is less than the round-trip time to the maximum target range of Where: ∆F is the Doppler shift in kHz the processing matrix, and the frequency is ambiguous if the vR is the rate of change of range in m/sec PRF is less than the maximum Doppler shift in the matrix (i.e., F is the radar operating frequency in kHz the frequency of the highest Doppler filter). For example, if a 10-GHz radar were designed to handle an engagement with a maximum range rate of 500 meters/second (a little over mach 1.5): ∆F = (500 m/sec / 3 x 108 m/sec) x 2 x 107 kHz = 33.3 kHz The spectrum of a pulsed signal has spectral lines spaced at frequency increments equal to the PRF as shown in Figure 2. If the PRF is low, for example 1000 pulses per second (pps), the spectral lines are only 1 kHz apart. If the PRF is high, for example 300 kpps, the spectral lines are 300 kHz apart. Each of these lines will also be Doppler shifted, and will cause frequency responses in the processing matrix (i.e., frequency ambiguities) if they are less than the Figure 2: In the frequency domain, a pulse signal has spectral lines separated by a frequency maximum Doppler frequency shift for equal to the PRF. Range Measurement

Table of Contents for the Digital Edition of JED - November 2010

JED - November 2010
Table of Contents
The View From Here
Calendar
From the President
Letters
The Monitor
Washington Report
World Report
Protecting Helicopters: Why ASE is About to Change the Game
The Rise... and Further Rise of FPGAs in EW
Physics of the Cyber-EMS Problem – Why We Have the Language Wrong
EW 101
AOC News
Index of Advertisers
JED Quick Look

JED - November 2010

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