Chapter 6: Matched Filter and the Radar Ambiguity Function

Chapter 6 Matched Filter and the Radar Ambiguity Function

6.1. The Matched Filter SNR The most unique characteristic of the matched filter is that it produces the maximum achievable instantaneous SNR at its output when a signal plus additive white noise are present at the input. The noise does not need to be Gauss- ian. The peak instantaneous SNR at the receiver output can be achieved by matching the radar receiver transfer function to the received signal. We will show that the peak instantaneous signal power divided by the average noise power at the output of a matched filter is equal to twice the input signal energy divided by the input noise power, regardless of the waveform used by the radar. This is the reason why matched filters are often referred to as optimum filters in the SNR sense. Note that the peak power used in the derivation of the radar equation (SNR) represents the average signal power over the duration of the pulse, not the peak instantaneous signal power as in the case of a matched filter. In practice, it is sometimes difficult to achieve perfect matched filtering. In such cases, sub-optimum filters may be used. Due to this mismatch, degra- dation in the output SNR occurs. () Consider a radar system that uses a finite duration energy signal si t . Denote the pulse width as τ' , and assume that a matched filter receiver is utilized. The main question that we need to answer is: What is the impulse, or frequency, response of the filter that maximizes the instantaneous SNR at the () output of the receiver when a delayed version of the signal si t plus additive white noise is at the input? The matched filter input signal can then be represented by

() ()() xt Csi tt– 1 += ni t (6.1)

213 where Ct is a constant, 1 is an unknown time delay proportional to the target () range, and ni t is input white noise. Since the input noise is white, its corresponding autocorrelation and Power Spectral Density (PSD) functions are given, respectively, by

N0 Rn ()t = ------δ()t (6.2) i 2

N0 Sn ()ω = ------(6.3) i 2 () () where N0 is a constant. Denote so t and no t as the signal and noise filter outputs. More precisely, we can define

() ()() yt = Cso tt– 1 + no t (6.4) where () ()• () so t = si t ht (6.5) () ()• () no t = ni t ht (6.6) The operator (• ) indicates convolution, and ht() is the filter impulse response (the filter is assumed to be linear time invariant). () Let Rh t denote the filter autocorrelation function. It follows that the output noise autocorrelation and PSD functions are

N0 N0 Rn ()t ==Rn ()t • R ()t ------δ()t • R ()t =------R ()t (6.7) o i h 2 h 2 h

2 N0 2 Sn ()ω ==Sn ()ω H()ω ------H()ω (6.8) o i 2 where H()ω is the Fourier transform for the filter impulse response, ht() . The () total average output noise power is equal to Rno t evaluated at t 0= . More precisely,

∞

N0 2 Rn ()0 = ------hu() ud (6.9) o 2 ∫ –∞ ()2 The output signal power evaluated at time t is Cso tt– 1 , and by using Eq. (6.5) we get

–∞ A general expression for the output SNR at time t can be written as

()2 Cso tt– 1 SNR() t = ------(6.11) () Rno 0 Substituting Eqs. (6.9) and (6.10) into Eq. (6.11) yields

∞ 2 2 ()() C ∫ si tt1 –– u hu ud () –∞ SNR t = ------∞ - (6.12) N -----0- hu()2 ud 2 ∫ –∞ The Schwartz inequality states that

∞ 2 ∞ ∞ 2 2 ∫ Px()Qx()xd ≤ ∫ Px() xd ∫ Qx() xd (6.13)

–∞ –∞ –∞ where the equality applies only when PkQ= ∗ , where k is a constant and can be assumed to be unity. Then by applying Eq. (6.13) on the numerator of Eq. (6.12), we get

∞ ∞ 2 ()2 ()2 C ∫ si tt1 –– u ud ∫ hu ud ()≤ –∞ –∞ SNR t ------∞ (6.14) N -----0- hu()2 ud 2 ∫ –∞ ∞ 2 ()2 2C ∫ si tt1 –– u ud = ------–∞ N0

© 2000 by Chapman & Hall/CRC Eq. (6.14) tells us that the peak instantaneous SNR occurs when equality is ∗ achieved (i.e., from Eq. (6.13) hks= i ). More precisely, if we assume that equality occurs at tt= 0 , and that k 1= , then

() ∗() hu = si t0 – t1 – u (6.15) and the maximum instantaneous SNR is

∞ 2 ()2 2C ∫ si t0 t1 –– u ud () –∞ SNR t0 = ------(6.16) N0 Eq. (6.16) can be simplified using Parseval’s theorem,

∞ 2 ()2 EC= ∫ si t0 t1 –– u ud (6.17)

–∞ where E denotes the energy of the input signal; consequently we can write the output peak instantaneous SNR as

() 2E SNR t0 = ------(6.18) N0 Thus, we can draw the conclusion that the peak instantaneous SNR depends only on the signal energy and input noise power, and is independent of the waveform utilized by the radar. Finally, we can define the impulse response for the matched filter from Eq. (6.15). If we desire the peak to occur at t0 = t1 , we get the non-causal matched filter impulse response,

() ∗() hnc t = si –t (6.19) Alternatively, the causal impulse response is

() ∗()τ hc t = si – t (6.20) τ where in this case, the peak occurs at t0 t1 += . It follows that the Fourier () () transforms of hnc t and hc t are given, respectively, by

()ω ∗()ω Hnc = Si (6.21)

()ω ∗()ω –jωτ Hc = Si e (6.22)

© 2000 by Chapman & Hall/CRC ()ω () ()ω where Si is the Fourier transform of si t . Thus, the moduli of H and ()ω Si are identical; however, the phase responses are opposite of each other. Example 6.1: Compute the maximum instantaneous SNR at the output of a linear filter whose impulse response is matched to the signal xt()= exp()–t2 ⁄ 2T . Solution: The signal energy is

∞ ∞ ()2 ⁄ Ext==∫ ()2 td ∫ e –t T td =πTJoules

–∞ –∞ It follows that the maximum instantaneous SNR is

πT ------SNR = ⁄ N0 2 ⁄ where N0 2 is the input noise power spectrum density.

6.2. The Replica () Again, consider a radar system that uses a finite duration energy signal si t , and assume that a matched filter receiver is utilized. The input signal is given in Eq. (6.1) and is repeated here as Eq. (6.23),

() ()() xt Csi tt– 1 += ni t (6.23) The matched filter output yt() can be expressed by the convolution integral between the filter’s impulse response and xt() ,

∞

yt()= ∫ xu()ht()– u ud (6.24)

–∞ Substituting Eq. (6.20) into Eq. (6.24) yields

∞ () ()∗()τ ()τ yt ==∫ xusi – t + u ud Rxsi t – (6.25)

–∞ ()τ () ()τ where Rxsi t – is a cross-correlation between xt and si – t . Therefore, the matched filter output can be computed from the cross-correlation between the radar received signal and a delayed replica of the transmitted waveform. If the input signal is the same as the transmitted signal, the output of the matched

© 2000 by Chapman & Hall/CRC filter would be the autocorrelation function of the received (or transmitted) signal. In practice, replicas of the transmitted waveforms are normally computed and stored in memory for use by the radar signal processor when needed.

6.3. Matched Filter Response to LFM Waveforms In order to develop a general expression for the matched filter output when an LFM waveform is utilized, we will consider the case when the radar is tracking a closing target with velocity v . The transmitted signal is

µ 2 j2π f t + ---t t 0 2 s ()t = Rect --- e (6.26) 1 τ'

The received signal is then given by

s ()t = s ()t – ∆()t (6.27) r1 1 2v ∆()t = t – ------()tt– (6.28) 0 c 0 where t0 is the time corresponding to the target initial detection range, and c is the speed of light. Using Eq. (6.28) we can rewrite Eq. (6.27) as

() 2v() ()γ() s t ==s1 tt– 0 + ------tt– 0 s1 tt– 0 (6.29) r1 c and v γ 12+= -- (6.30) c is the scaling coefficient. Substituting Eq. (6.26) into Eq. (6.29) yields

γ()π γ()πµγ2()2 tt– 0 j2 f0 tt– 0 j tt– 0 s ()t = Rect ------e e (6.31) r1 τ' () which is the analytical signal representation for sr t . The complex envelope () 1 ()π of the signal sr t is obtained by multiplying Eq. (6.31) by exp –j2 f0t . 1 () Denote the complex envelope by sr t , then after some manipulation we get

π γ()π ()γ ()πµγ2()2 –j2 f0 t0 tt– 0 j2 f0 – 1 tt– 0 j tt– 0 s ()t = e Rect ------e e (6.32) r τ' The Doppler shift due to the target motion is

j2πf ()tt– ()≈ d 0 () sr t e st– t0 (6.35) where

π ()–j2 f0t () st– t0 = e s1 tt– 0 (6.36) () s1 t is given in Eq. (6.26). The matched filter response is given by the convolution integral

∞ () () () so t = ∫ husr tu– du (6.37)

–∞ For a non-causal matched filter the impulse response hu() is equal to s∗()–t ; it follows that

∞ () ∗()() so t = ∫ s –u sr tu– ud (6.38)

–∞ Substituting Eq. (6.36) into Eq. (6.38), and performing some algebraic manipu- lations, we get

∞ j2πf ()tut+ – () ∗() d 0 () so t = ∫ s u e st+ u– t0 ud (6.39)

–∞ Finally, making the change of variable t' tu+= yields

∞ j2πf ()t' – t () ∗()()d 0 so t = ∫ s t' – t st' – t0 e dt' (6.40)

–∞

It is customary to set t0 0= , and it follows that ∞ j2πf t' () ()∗()d so tf; d = ∫ st' s t' – t e dt' (6.41)

–∞

© 2000 by Chapman & Hall/CRC () where we used the notation so tf; d to indicate that the output is a function of both time and Doppler frequency. The two-dimensional (2-D) correlation function for the signal st() is obtained from the matched filter response by replacing t by –τ , then

∞ j2πf t' χτ() ()∗()τ d ;fd = ∫ st' s t' + e dt' (6.42)

–∞

6.4. The Radar Ambiguity Function The radar ambiguity function represents the output of the matched filter, and it describes the interference caused by range and/or Doppler of a target when compared to a reference target of equal RCS. The ambiguity function evalu- ()τ, (), ated at fd = 00 is equal to the matched filter output that is matched perfectly to the signal reflected from the target of interest. In other words, returns from the nominal target are located at the origin of the ambiguity func- τ tion. Thus, the ambiguity function at nonzero and fd represents returns from some range and Doppler different from those for the nominal target. The radar ambiguity function is normally used by radar designers as a means of studying different waveforms. It can provide insight about how different radar waveforms may be suitable for the various radar applications. It is also used to determine the range and Doppler resolutions for a specific radar waveform. The three-dimensional (3-D) plot of the ambiguity function versus frequency and time delay is called the radar ambiguity diagram. The radar ambiguity function for the signal st() is defined as the modulus squared of its χτ()2 2-D correlation function, i.e., ;fd . More precisely,

∞ 2 j2πf t χτ()2 ()∗()τ d ;fd = ∫ sts t + e td (6.43)

–∞ ()τ, (), In this notation, the target of interest is located at fd = 00 , and the ambiguity diagram is centered at the same point. Note that some authors define χτ() χτ() the ambiguity function as ;fd . In this book, ;fd is called the uncertainty function. Denote Est as the energy of the signal () ,

∞

2 Est= ∫ () td (6.44)

–∞ We will now list the properties for the radar ambiguity function:

{}χχτ()2 ()2 ()2 max ;fd ==00; 2E (6.45)

χτ()2 ≤ χ()2 ;fd 00; (6.46) 2) The ambiguity function is symmetric,

χτ()2 χτ()2 ;fd = – ;–fd (6.47) 3) The total volume under the ambiguity function is constant,

χτ()2 τ ()2 ∫∫ ;fd dfd d = 2E (6.48) 4) If the function Sf() is the Fourier transform of the signal st() , then by using Parseval’s theorem we get

2 χτ()2 ∗() ()–j2πfτ ;fd = ∫S f Sf– fd e fd (6.49)

6.5. Examples of the Ambiguity Function The ideal radar ambiguity function is represented by a spike of infinitesimal width that peaks at the origin and is zero everywhere else, as illustrated in Fig. 6.1. An ideal ambiguity function provides perfect resolution between neigh- boring targets regardless of how close they may be with respect to each other. Unfortunately, an ideal ambiguity function cannot physically exist. This is because the ambiguity function must have finite peak value equal to ()2E 2 and a finite volume also equal to ()2E 2 . Clearly, the ideal ambiguity function cannot meet those two requirements.

6.5.1. Single Pulse Ambiguity Function Consider the normalized rectangular pulse st() defined by

() 1 t st = ------Rect--- (6.50) τ' τ' From Eq. (6.42) we have

∞ j2πf t χτ() ()∗()τ d ;fd = ∫ sts t + e td (6.51)

–∞

τ ()00,

Figure 6.1. Ideal ambiguity function.

Substituting Eq. (6.50) into Eq. (6.51) and performing the integration yield,

()π ()τ τ 2 2 τ sin fd ' – χτ() ------ττ≤ ;fd = 1 – τ π ()τ τ ' (6.52) ' fd ' – MATLAB Function “single_pulse_ambg.m” The function “single_pulse_ambg.m” implements Eq. (6.52). It is given in Listing 6.1 in Section 6.7. The syntax is as follows: single_pulse_ambg [taup] taup is the pulse width. Fig. 6.2 (a-d) shows 3-D and contour plots of single pulse uncertainty and ambiguity functions. These plots can be reproduced using MATLAB program “fig6_2.m” given in Listing 6.2 in Section 6.7. The ambiguity function cut along the time delay axis τ is obtained by setting fd 0= . More precisely,

2 2 τ χτ()0; = 1 – ----- ττ≤ ' (6.53) τ' Note that the time autocorrelation function of the signal st() is equal to χτ()0; . Similarly, the cut along the Doppler axis is

2 sinπτ'f 2 χ()0;f = ------d (6.54) d πτ 'fd Figs. 6.3 and 6.4, respectively, show the plots of the uncertainty function cuts defined by Eqs. (6.53) and (6.54). Since the zero Doppler cut along the time delay axis extends between –τ' and τ' , then, close targets would be unambiguous if they are at least τ' seconds apart.

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Doppler - Hz -0.5

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-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 Delay - seconds

Figure 6.2b. Contour plot corresponding to Fig. 6.2a.

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0.5

Doppler - Hz - Doppler -0.5

-1

-1.5

-2

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 Delay - seconds

Figure 6.2d. Contour plot corresponding to Fig. 6.2c.

© 2000 by Chapman & Hall/CRC The zero time cut along the Doppler frequency axis has a ()sinx ⁄ x 2 shape. ∞ ∞ ⁄± τ It extends from – to . The first null occurs at fd = 1 ' . Hence, it is possible to detect two targets that are shifted by 1 ⁄ τ' , without any ambiguity. We conclude that a single pulse range and Doppler resolutions are limited by the pulse width τ' . Fine range resolution requires that a very short pulse be used. Unfortunately, using very short pulses requires very large operating bandwidths, and may limit the radar average transmitted power to impractical values.

amplitude

–τ' τ' τ Figure 6.3. Zero Doppler uncertainty function cut along the time delay axis.

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0.4 Uncertainty - Volts - Uncertainty 0.3

0.2

0.1

0 -5 -4 -3 -2 -1 0 1 2 3 4 5 Frequency - Hz Figure 6.4. Uncertainty function of a single frequency pulse (zero delay). This plot can be reproduced using MATLAB program “Fig6_4.m” given in Listing 6.3 in Section 6.7.

2 () 1 t jπµt st = ------Rect--- e (6.55) τ' τ' In order to compute the ambiguity function for the LFM complex envelope, we will first consider the case when 0 ≤≤ττ′ . In this case the integration limits are from –τ′ ⁄τ′2 to ()τ⁄ 2 – . Substituting Eq. (6.55) into Eq. (6.51) yields

∞

1 t t + τ jπµt2 –jπµ()t + τ′ 2 χτ()---- ---- ------;fd = τ′ ∫ Rectτ′ Rectτ′ e e dt (6.56) –∞ It follows that

τ' --- – τ 2 πµτ2 –j πµτ() e –j2 – fd t χτ();f = ------e dt (6.57) d τ' ∫ –τ' ------2 We will leave the rest of the integration process to the reader. Finishing the integration process in Eq. (6.57) yields

τ sin πτ'()µτ + f 1 – --- πτ τ d τ' χτ() j fd  ≤≤ττ′ ;fd =0e 1 – ------(6.58) τ' τ πτ'()µτ + f 1 – --- d τ'

Similar analysis for the case when –τ′≤≤ τ 0 can be carried out, where in this case the integration limits are from ()τ–τ′ ⁄ 2 – to τ' ⁄ 2 . The same result can be obtained by using the symmetry property of the ambiguity function χτ()χτ, (), χτ() (– –fd = fd ). It follows that an expression for ;fd that is valid for any τ is given by

τ πτ ()µτ  sin' + fd 1 – ----- jπτf τ τ' χτ() d ττ≤ ;fd = e 1 – ------' (6.59) τ' τ πτ'()µτ + f 1 – ----- d τ' and the LFM ambiguity function is

© 2000 by Chapman & Hall/CRC τ 2 sin πτ'()µτ + f 1 – ----- τ d τ' χτ()2  ττ≤ ;fd = 1 – ------' (6.60) τ' τ πτ'()µτ + f 1 – ----- d τ' Again the time autocorrelation function is equal to χτ(), 0 . The reader can verify that the ambiguity function for a down-chirp LFM waveform is given by

τ 2 sin πτ'()µτ – f 1 – ----- τ d τ' χτ()2  ττ≤ ;fd = 1 – ------' (6.61) τ' τ πτ'()µτ – f 1 – ----- d τ' MATLAB Function “lfm_ambg.m” The function “lfm_ambg.m” implements Eqs. (6.60) and (6.61). It is given in Listing 6.4 in Section 6.7. The syntax is as follows: lfm_ambg [taup, b, up_down] where

Symbol Description Units Status taup pulse width seconds input b bandwidth Hz input up_down up_down = 1 for up chirp none input up_down = -1 for down chirp

Fig. 6.5 (a-d) shows 3-D and contour plots for the LFM uncertainty and ambiguity functions for

taup b up_down 1101

These plots can be reproduced using MATLAB program “fig6_5.m” given in Listing 6.5 in Section 6.7.This function generates 3-D and contour plots of an LFM ambiguity function. The up-chirp ambiguity function cut along the time delay axis τ is

τ 2 sin πµττ' 1 – ----- τ τ' χτ()2  ττ≤ 0; = 1 – ------' (6.62) τ' τ πµττ' 1 – ----- τ'

Doppler - Hz -2

-4

-6

-8

-10 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 Delay - seconds

Figure 6.5b. Contour plot corresponding to Fig. 6.5a.

Doppler - Hz - Doppler -2

-4

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-8

-10 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 Delay - seconds

Figure 6.5d. Contour plot corresponding to Fig. 6.5c.

© 2000 by Chapman & Hall/CRC Fig. 6.6 shows a plot for a cut in the uncertainty function corresponding to Eq. (6.62). Note that the LFM ambiguity function cut along the Doppler frequency axis is similar to that of the single pulse. This should not be surprising since the pulse shape has not changed (we only added frequency modulation). However, the cut along the time delay axis changes significantly. It is now much narrower compared to the unmodulated pulse cut. In this case, the first null occurs at

τ ⁄≈ n1 1 B (6.63) which indicates that the effective pulse width (compressed pulse width) of the matched filter output is completely determined by the radar bandwidth. It follows that the LFM ambiguity function cut along the time delay axis is narrower than that of the unmodulated pulse by a factor

τ' ξ ==------τ'B (6.64) ()1 ⁄ B ξ is referred to as the compression ratio (also called time-bandwidth product and compression gain). All three names can be used interchangeably to mean the same. As indicated by Eq. (6.64) the compression ratio also increases as the radar bandwidth is increased.

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0 -1.5 -1 -0.5 0 0.5 1 1.5 Delay - seconds

Figure 6.6. Zero Doppler Ambiguity function of an LFM pulse (τ′ 1= , b = 20 ). This plot can be reproduced using MATLAB program “fig6_6.m” given in Listing 6.6 in Section 6.7.

© 2000 by Chapman & Hall/CRC Example 6.2: Compute the range resolution before and after pulse compression corresponding to an LFM waveform with the following specifications: Bandwidth B = 1GHz ; and pulse width τ' = 10ms . Solution: The range resolution before pulse compression is

–3 8 cτ' 10× 10 310×× 6 ∆R ==------=1.5× 10 meters uncomp 2 2 Using Eq. (6.63) yields

τ 1 n1 ==------1 ns 110× 9

cτ 310× 8 110×× –9 ∆R ==------n1------=15 cm . comp 2 2

6.5.3. Coherent Pulse Train Ambiguity Function Fig. 6.7 shows a plot of coherent pulse train. The pulse width is denoted as τ' and the PRI is TN . The number of pulses in the train is ; hence, the train’s length is ()N – 1 Tst seconds. A normalized individual pulse () is defined by

() 1 t s1 t = ------Rect--- (6.65) τ' τ' When coherency is maintained between the consecutive pulses, then an expression for the normalized train is

N – 1 () 1 () st = ------∑ s1 tiT– (6.66) N i 0= The output of the matched filter is

∞ j2πf t χτ() ()∗()τ d ;fd = ∫ sts t + e td (6.67)

–∞ Substituting Eq. (6.66) into Eq. (6.67) and interchanging the summations and integration yield,

N – 1 N – 1 ∞ π 1 j2 fd t χτ();f = ---- s ()tiT– s ∗()tjT–– τ e td (6.68) d N ∑ ∑ ∫ 1 1 i 0= j 0= –∞

T ()N – 1 T

Figure 6.7. Coherent pulse train. N=5.

Making the change of variable t1 = tiT– yields

N – 1 N – 1 ∞ π π 1 j2 fd iT j2 fdt1 χτ();f = ---- e s ()t s ∗()t – []τ – ()ij– T e dt (6.69) d N ∑ ∑ ∫ 1 1 1 1 1 i 0= j 0= –∞ The integral inside Eq. (6.69) represents the output of the matched filter for a χ single pulse, and is denoted by 1 . It follows that

N – 1 N – 1 π 1 j2 fdiT χτ();f = ---- e χ []τ – ()ij– T;f (6.70) d N ∑ ∑ 1 d i 0= j 0= When the relation qij= – is used, then the following relation is true1:

N N 0 N – 1 – q N – 1 N – 1 – q ∑ ∑ ∑ ∑ += ∑ ∑ (6.71) () i 0= m 0= qN= – – 1 i 0= for j= i– q q 1= j 0= for i j+= q Using Eq. (6.71) into Eq. (6.70) gives

0 N – 1 – q j2πf iT χτ()1 χ ()τ d ;fd = ---- ∑ 1 – qT;fd ∑ e (6.72) N  qN= –()– 1 i 0= N – 1 N – 1 – q j2πf qT j2πf jT 1 d χ ()τ d + ---- ∑ e 1 – qT;fd ∑ e N  q 1= j 0=

1. Rihaczek, A. W., Principles of High Resolution Radar, Artech House, 1996.

N – 1 – q Nq– j 1 – z z = ------(6.73) ∑ 1 – z j 0= yield

N – 1 – q j2πf iT []jπf ()N – 1 – qT sin[]πf ()N – 1 – qT d d ------d - ∑ e = e ()π (6.74) sin fdT i 0= Using Eq. (6.74) into Eq. (6.72) yields two complementary sums for positive and negative q . Both sums can be combined as

N – 1 []π ()[]π () 1 j fd N – 1 + q T sin f NqT– χτ()---- χ ()τ ------d - ;fd = ∑ 1 – qT;fd e ()π (6.75) N sin fdT qN= –()– 1 Finally, the ambiguity function associated with the coherent pulse train is computed as the modulus square of Eq. (6.75). For τ′ T ⁄< 2 , the ambiguity function reduces to

N – 1 1 sin[]πf ()NqT– χτ();f = ---- χ ()τ – qT;f ------d - (6.76) d N ∑ 1 d ()π sin fdT qN= –()– 1 Thus, the ambiguity function for a coherent pulse train is the superposition of the individual pulse’s ambiguity functions. The ambiguity function cuts along the time delay and Doppler axes are, respectively, given by

N – 1 2 2 q τ – qT χτ()0; = 1 – ----- 1 – ------; τ – qT < τ′ (6.77) ∑ N τ′ qN= –()– 1

()π τ′ ()π 2 2 1 sin f sin f NT χ() ------d ------d 0;fd = π τ′ ()π (6.78) N fd sin fdT MATLAB Function “train_ambg.m” The function “train_ambg.m” implements Eq. (6.76). It is given in Listing 6.7 in Section 6.7. The syntax is as follows: train_ambg [taup, n, pri]

Symbol Description Units Status taup pulse width seconds input n number of pulses in train none input pri pulse repetition interval seconds input

Fig. 6.8 (a-d) shows typical outputs of this function, for

taup n pri 0.2 5 1

given in Listing 6.8 in 6.8 in Listing given

” fig6_8a.m “ Section 6.7. 6.7. Section train. Pulse width is 0.2 seconds; and PRI is 1 second, N=5. This plot can be be can plot This N=5. 1 second, is PRI and seconds; 0.2 is width Pulse train. program MATLAB using reproduced Figure 6.8a. Three-dimensional ambiguity plot for a five pulse equal amplitude coherent

160

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80 D oppler - H z

20 40 60 80 100 120 140 160 Delay - seconds

Figure 6.8b. Contour plot corresponding to Fig. 6.8a.

delay

2τ′ T Figure 6.8c. Zero Doppler cut corresponding to Fig. 6.8a.

0 1 ⁄ T frequency

Figure 6.8d. Zero delay cut corresponding to Fig. 6.8a.

6.6. Ambiguity Diagram Contours Plots of the ambiguity function are called ambiguity diagrams. For a given waveform, the corresponding ambiguity diagram is normally used to determine the waveform properties such as the target resolution capability, measurements (time and frequency) accuracy and its response to clutter. Three-dimensional ambiguity diagrams are difficult to plot and interpret. This is the reason why contour plots of the 3-D ambiguity diagram are often used to study the charac- teristics of a waveform. An ambiguity contour is a 2-D plot (frequency/time) of a plane intersecting the 3-D ambiguity diagram that corresponds to some threshold value. The resultant plots are ellipses. It is customary to display the ambiguity contour plots that correspond to one half of the peak autocorrelation value. Fig. 6.9 shows a sketch of typical ambiguity contour plots associated with a gated CW pulse. It indicates that narrow pulses provide better range accuracy than long pulses. Alternatively, the Doppler accuracy is better for a wider pulse than it is for a short one. This trade-off between range and Doppler measurements comes from the uncertainty associated with the time-bandwidth product of a single sinusoidal pulse, where the product of uncertainty in time (range) and uncertainty in frequency (Doppler) cannot be much smaller than unity. Note that an exact plot for Fig. 6.9 can be obtained using the function “single_pulse_ambg.m” and the MATLAB command contour.

time time 1 ⁄ τ′ 1 ⁄ τ′

τ′ τ′ long pulse short pulse

Figure 6.9. Ambiguity contour plot associated with a sinusoid modulated gated CW pulse. See Fig. 6.2.

Multiple ellipses in an ambiguity contour plot indicate the presence of multiple targets. Thus, it seems that one may improve the radar resolution by increasing the ambiguity diagram threshold value. This is illustrated in Fig. 6.10. However, in practice this is not possible for two reasons. First, in the presence of noise we lack knowledge of the peak correlation value; and second, targets in general will have different amplitudes. Now consider the case of a coherent pulse train described in Fig. 6.7. For a pulse train, range accuracy is still determined by the pulse width, the same way as in the case of a single pulse, while Doppler accuracy is determined by the train length. Thus, time and frequency measurements can be made indepen- dently of each other. However, additional peaks appear in the ambiguity diagram which may cause range and Doppler uncertainties. This is illustrated in Fig. 6.11.

frequency frequency

time time

low threshold value high threshold value

Figure 6.10. Effect of threshold value on resolution.

time

Figure 6.11. Ambiguity contour plot corresponding to Fig. 6.7. For an exact plot see Fig. 6.8b.

As one would expect, high PRF pulse trains (i.e., small T ) lead to extreme uncertainty in range, while low PRF pulse trains have extreme ambiguity in Doppler, as shown in Fig. 6.12. Medium PRF pulse trains have moderate ambiguity in both range and Doppler, which can be overcome by using multiple PRFs, as illustrated in Fig. 6.13 for two medium PRFs. Note that the two diagrams (in Fig. 6.13) agree only in one location (center of the plot) which corresponds to the true target location. It is possible to avoid ambiguities caused by pulse trains and still have rea- sonable independent control on both range and Doppler accuracies by using a single modulated pulse with a time-bandwidth product that is much larger than unity. Figure 6.14 shows the ambiguity contour plot associated with an LFM waveform. In this case, τ′ is the pulse width and B is the pulse bandwidth. In this case, exact plots can be obtained using the function “lfm_ambg.m”.

time time

high PRF low PRF

Figure 6.12. Uncertainty associated with low and high PRFs.

frequency frequency

tim e time

Figure 6.13. Uncertainty of two medium PRFs.

time 1 ⁄ τ′

1 ⁄ B Figure 6.14. Ambiguity contour plot associated with an up-chirp LFM waveform. For an exact plot see Fig. 6.5b.

6.7. MATLAB Listings This section presents listings for all MATLAB programs/functions used in this chapter. The user is strongly advised to rerun the MATLAB programs in order to enhance their understanding of this chapter’s material.

Listing 6.1. MATLAB Function “single_pulse_ambg.m” function x = single_pulse_ambg (taup) colormap (gray(1)) eps = 0.000001; i = 0; taumax = 1.1 * taup; taumin = -taumax; for tau = taumin:.05:taumax i = i + 1; j = 0; for fd = -5/taup:.05:5/taup j = j + 1; val1 = 1. - abs(tau) / taup; val2 = pi * taup * (1.0 - abs(tau) / taup) * fd; x(j,i) = abs( val1 * sin(val2+eps)/(val2+eps)); end end

Listing 6.2. MATLAB Program “fig6_2.m” clear all eps = 0.000001;

© 2000 by Chapman & Hall/CRC taup = 2.; taumin = -1.1 * taup; taumax = -taumin; x = single_pulse_ambg(taup); taux = taumin:.05:taumax; fdy = -5/taup:.05:5/taup; figure(1) mesh(taux,fdy,x); xlabel ('Delay - seconds') ylabel ('Doppler - Hz') zlabel ('Ambiguity function') figure(2) contour(taux,fdy,x); xlabel ('Delay - seconds') ylabel ('Doppler - Hz') y = x.^2; figure(3) mesh(taux,fdy,y); xlabel ('Delay - seconds') ylabel ('Doppler - Hz') zlabel ('Ambiguity function') figure(4) contour(taux,fdy,y); xlabel ('Delay - seconds') ylabel ('Doppler - Hz')

Listing 6.3. MATLAB Program “fig6_4.m” clear all eps = 0.0001; taup = 2.; fd = -10./taup:.05:10./taup; uncer = abs( sinc(taup .* fd)); ambg = uncer.^2; plot(fd, ambg) xlabel ('Frequency - Hz') ylabel ('Ambiguity - Volts') grid figure(2) plot (fd, uncer); xlabel ('Frequency - Hz') ylabel ('Uncertainty - Volts') grid

Listing 6.4. MATLAB Function “lfm_ambg.m” function x = lfm_ambg(taup, b, up_down) eps = 0.000001;

© 2000 by Chapman & Hall/CRC i = 0; mu = up_down * b / 2. / taup; for tau = -1.1*taup:.05:1.1*taup i = i + 1; j = 0; for fd = -b:.05:b j = j + 1; val1 = 1. - abs(tau) / taup; val2 = pi * taup * (1.0 - abs(tau) / taup); val3 = (fd + mu * tau); val = val2 * val3; x(j,i) = abs( val1 * (sin(val+eps)/(val+eps))).^2; end end

Listing 6.5. MATLAB Program “fig6_5.m” clear all eps = 0.0001; taup = 1.; b =10.; up_down = 1.; x = lfm_ambg(taup, b, up_down); taux = -1.1*taup:.05:1.1*taup; fdy = -b:.05:b; figure(1) mesh(taux,fdy,x) xlabel ('Delay - seconds') ylabel ('Doppler - Hz') zlabel ('Ambiguity function') figure(2) contour(taux,fdy,x) xlabel ('Delay - seconds') ylabel ('Doppler - Hz') y = sqrt(x); figure(3) mesh(taux,fdy,y) xlabel ('Delay - seconds') ylabel ('Doppler - Hz') zlabel ('Uncertainty function') figure(4) contour(taux,fdy,y) xlabel ('Delay - seconds') ylabel ('Doppler - Hz')

Listing 6.6. MATLAB Program “fig6_6.m” clear all

© 2000 by Chapman & Hall/CRC taup = 1; b =20.; up_down = 1.; taux = -1.5*taup:.01:1.5*taup; fd = 0.; mu = up_down * b / 2. / taup; ii = 0.; for tau = -1.5*taup:.01:1.5*taup ii = ii + 1; val1 = 1. - abs(tau) / taup; val2 = pi * taup * (1.0 - abs(tau) / taup); val3 = (fd + mu * tau); val = val2 * val3; x(ii) = abs( val1 * (sin(val+eps)/(val+eps))); end figure(1) plot(taux,x) grid xlabel ('Delay - seconds') ylabel ('Uncertaunty') figure(2) plot(taux,x.^2) grid xlabel ('Delay - seconds') ylabel ('Ambiguity')

Listing 6.7. MATLAB Function “train_ambg.m” function x = train_ambg (taup, n, pri) if( taup > pri / 2.) 'ERROR. Pulse width must be less than the PRI/2.' break end gap = pri - 2.*taup; eps = 0.000001; b = 1. / taup; ii = 0.; for q = -(n-1):1:n-1 tauo = q - taup ; index = -1.; for tau1 = tauo:0.0533:tauo+gap+2.*taup index = index + 1; tau = -taup + index*.0533; ii = ii + 1; j = 0.; for fd = -b:.0533:b j = j + 1; if (abs(tau) <= taup)

© 2000 by Chapman & Hall/CRC val1 = 1. -abs(tau) / taup; val2 = pi * taup * fd * (1.0 - abs(tau) / taup); val3 = abs(val1 * sin(val2+eps) /(val2+eps)); val4 = abs((sin(pi*fd*(n-abs(q))*pri+eps))/(sin(pi*fd*pri+eps))); x(j,ii)= val3 * val4 / n; else x(j,ii) = 0.; end end end end

Listing 6.8. MATLAB Program “fig6_8a.m” clear all taup =0.2; pri=1; n=5; x = train_ambg (taup, n, pri); figure(1) mesh(x) xlabel ('Delay - seconds') ylabel ('Doppler - Hz') zlabel ('Ambiguity function') figure(2) contour(x); xlabel ('Delay - seconds') ylabel ('Doppler - Hz')

Problems {}() , , {}() ,, 6.1. Define xI n = 111– and xQ n = 11– 1 . (a) Compute the discrete correlations: R , R , R , and R . (b) A certain radar transmits xI xQ xIxQ xQxI () () π () π the signal st = xI t cos2 f0t – xQ t sin2 f0t . Assume that the autocorre- () () () π () π lation st is equal to yt = yI t cos2 f0t – yQ t sin2 f0t . Compute and () () sketch yI t and yQ t . 6.2. Compute the frequency response for the filter matched to the signal

–t2 (a) xt()= exp ------; (b) xt()= ut()exp()– αt , 2T where α is a positive constant. 6.3. Repeat Example 6.1 for xt()= ut()exp()– αt .

© 2000 by Chapman & Hall/CRC 6.4. Derive Eq. (6.43). 6.5. Prove the properties of the radar ambiguity function. 6.6. Starting with Eq. (6.61) derive Eq. (6.62). 6.7. A radar system uses LFM waveforms. The received signal is of the () ()τ () τ form sr t As t – += nt , where is a time delay that depends on range, () ()⁄ τ′ ()π ψ() ψ() π 2 ⁄ τ′ st = Rect t cos 2 f0t – t , and t = – Bt . Assume that the radar bandwidth is B = 5MHz , and the pulse width is τ′ = 5µs . (a) Give the quadrature components of the matched filter response that matched to st() . (b) Write an expression for the output of the matched filter. (c) Compute the increase in SNR produced by the matched filter. 6.8. (a) Write an expression for the ambiguity function of an LFM waveform, where τ′ =326.4µs , and the compression ratio is . (b) Give an expression for the matched filter impulse response. 6.9. Repeat Example 6.2 for B = 25, , and 10GHz . 6.10. (a) Write an expression for the ambiguity function of a LFM signal with bandwidth B = 10MHz , pulse width τ′ = 1µs , and wavelength λ = 1cm . (b) Plot the zero Doppler cut of the ambiguity function. (c) Assume ⁄ a target moving towards the radar with radial velocity vr = 100ms . What is the Doppler shift associated with this target? (d) Plot the ambiguity function for the Doppler cut in part (c). (e) Assume that three pulses are transmitted with PRF fr = 2000Hz . Repeat part b. 6.11. (a) Give an expression for the ambiguity function for a pulse train consisting of 4 pulses, where the pulse width is τ′ = 1µs and the pulse repetition interval is T = 10µs . Assume a wavelength of λ = 1cm . (b) Sketch the ambiguity function contour. 6.12. Hyperbolic frequency modulation (HFM) is better than LFM for high radial velocities. The HFM phase is ω2 µ α 0 t ψ () ------h h t = µ ln1 + ω h 0 µ α where h is an HFM coefficient and is a constant. (a) Give an expression τ′ for the instantaneous frequency of a HFM pulse of duration h . (b) Show that µ HFM can be approximated by LFM. Express the LFM coefficient l in terms µ τ′ of h and in terms of B and . 6.13. Consider a Sonar system with range resolution ∆R = 4cm . (a) A sinusoidal pulse at frequency f0 = 100KHz is transmitted. What is the pulse width, and what is the bandwidth? (b) By using an up-chirp LFM, centered at

© 2000 by Chapman & Hall/CRC f0 , one can increase the pulse width for the same range resolution. If you want to increase the transmitted energy by a factor of 20, give an expression for the transmitted pulse. (c) Give an expression for the causal filter matched to the LFM pulse in part b. 6.14. A pulse train yt() is given by

2 yt()= ∑ wn()xt()– nτ′

n 0= where xt()= exp()–t2 ⁄ 2 is a single pulse of duration τ′ and the weighting {}() {},, sequence is wn = 0.510.7 . Find and sketch the correlations Rx , Rw , and Ry . () ()2 ⁄ π 6.15. Repeat the previous problem for xt = exp–t 2 cos 2 f0t . 6.16. Modify the function “train_ambg.m” to accommodate the case τ′ = T . 6.17. Using the MATLAB functions presented in this chapter, generate the exact plots that correspond to Figs. 6.13 and 6.14. 6.18. Using the function “lfm_ambg.m” reproduce Fig. 6.6b for a down- chirp LFM pulse.