Joint parameter estimation and Cramer-Rao bound analysis in ground-based forward scatter radar
© Zeng et al; licensee Springer. 2012
Received: 15 May 2011
Accepted: 11 April 2012
Published: 11 April 2012
Forward scatter radar (FSR) has potential applications such as target detection, classification, and recognition. The success of these issues depends on the accuracy of parameter estimation. Many parameter estimation methods for air-based FSR have been given, but cannot directly be applied in the ground-based ones for the different system functions. The received signal in ground-based FSR depends on the target's electrical size and trajectory, which are unknown a priori. It is impossible to construct an optimal reception with accurate reference function in practical situations. An adaptive method of parameter estimation is therefore proposed in this article, which includes the construction of reference function and the two-dimensional parameter estimation. Furthermore, the Crammer-Rao bounds of estimation accuracy are obtained via the analytical derivation, which can be in turn utilized to determine the estimation step in the algorithm. Finally, the effectiveness of the algorithm is shown using both simulated and experimental data.
No matter in the modern or future electronic warfare, radar is continuously playing a key role. Historically, bistatic radar is the earliest radar system applied in the warfare field. Only after the substantiation of a pulse method of radar and the development of antenna switches, more convenient monostatic radars were developed and therefore impeded the development of bistatic radar [1, 2]. However, in the modern electronic warfare, due to the advent of the fast development of technology, new threats such as electronic interference, low altitude penetration, stealth weapon, and anti-radiation missiles have to be faced and dealt with. The conventional monostatic radar system has been distressed into dilemma. Radar community revisited the potential of the bistatic system due to its potential to deal with the above issues, and then numerous of hardware and software for the bistatic radars have been proposed, investigated, constructed and built up.
Forward scatter radar (FSR) is an extreme bistatic radar configuration, which is generally defined with the bistatic angle around 170°-180° , i.e., the target is sensed close to the radar baseline. The basic principles of FSR can be found in Willis  and Chernyak [3, 4]. The main difference compared with the conventional radars (both monostatic and bistatic) lies in its target scattering nature. In the forward scattering (FS) area, the radar utilizes the diffraction effects rather than the reflection effects of the electromagnetic wave to capture the target's information, and therefore brings some winning properties compared with conventional ones. The target can be considered as a secondary antenna which has the silhouette of the target. The gain of this secondary antenna is directly related to the target FS Cross-Section (CS) and is independent from the target material . Therefore, such systems are robust to stealth technology and benefit for target detection. Another peculiarity is that the received signal has little phase and amplitude fluctuation. All FS EM wave components are traveling toward the receiving antenna in phase, and the signal could coherently be integrated over a long period of time, which will in return introduces an excellent frequency resolution. As a result, FSR has unique potential in automatic target recognition . In 2000, the great Shadow Inverse Synthetic Aperture theory was proposed by Chapurskiy and Sablin , which makes significant contribution to target classification and recognition based on FSR system.
Till now, most existing publications dedicated to FSR studies focus on the airborne issues. In the air-based FSR system, the velocity of the air target is high , and the length of the baseline is far larger than the target size , furthermore the clutter background is relatively small . Therefore, the instantaneous Doppler frequency and angle of arrival can effectively be obtained via Doppler filter and multi-beam antenna, and then based on the nonlinear relationship between target coordinate and Doppler frequency as well as angle of arrival, the approximate coordinate estimates are obtained with numerical solution [8–10], but the methods of air targets detection and tracking cannot be used in the ground-based FSR due to the different system behaviors and target motion characteristics.
The ground-based network of FS Micro Radars (FSMR) has been considered for target detection and automatic recognition in [3, 11–16]. It has been demonstrated by means of both analytical and experimental studies that ground targets crossing the baseline can reliably be detected. While in these practical ground-based FSR systems, the sensors are positioned nearly on the ground; the operational range is restricted to hundreds of meters due to the local horizon and landscape peculiarity as well as power budget constraints. All these make it impossible to use the mono-pulse antenna or the array antenna to sense the target. So, the angle of arrival cannot be achieved directly. Moreover, the velocity of the ground target is much lower than the air target, the Doppler frequency will not change enough to allow the Doppler filter extract the Doppler shift (the velocity of the target). The conventional signal processing method for the air target detection and parameter estimation cannot be applied in the ground-based system.
In this article, we consider the method of target detection and parameter estimation in the ground-based FSR system. The radars work under the single carrier Continuous Wave mode and have no capability of range resolving. This means that this radar is only able to detect moving targets. Because the direct signature (from transmitter to receiver directly) is far stronger than the moving target signature, using the envelope detector or phase detector, we can extract the signature of moving target (beat frequency). This signature will occur in the background of clutter including foliage and vegetation, thermal and atmospheric and possibly industrial noise and so forth. The latter could be viewed as Additive White Gaussian Noise (AWGN). Therefore, we will only consider the case of target detection against AWGN under the assumption that a target is following a nearly linear trajectory. Furthermore, numerous observations of ground targets confirm that the linear trajectory assumption is correct for practical cases. Moreover, foliage clutter can be approximated as a narrowband Gaussian process [17–21], and the statistic characteristics in FSMR have empirically been estimated via the experimental data [18–20], thus the optimal detection procedure against clutter background in FSMR includes a whitening filter. Hence, the assumption that the target is detected against AWGN is also true for clutter presence.
This article is organized as follows: The signal model will be presented in Section 2. The signal processing algorithm, including signal-to-noise ratio (SNR) maximization and the joint parameter estimation, will be proposed in Section 3. The Crammer-Rao bounds of our method as well as the simulation and experimental results are given in Section 4. Finally, the conclusion is drawn in Section 5.
2. Signal model of ground moving target in FSR
In this section, the signal model of FSR ground moving target will be given. Figure 1 shows the topology of the FSR configuration for moving target. The transmitting and receiving antennas have an equal elevation h A . The target has the size of l * h, v is the target motion speed, O is the crossing point at the baseline, R t and R r are the range of T-Target (transmitter to target) and R-Target (receiver to target), respectively, d T and d R are the baseline range of TO and OR correspondingly. On the transmitter side, the azimuth and elevation diffraction angle are α h and α v , and similarly on the receiver side, we have β h and β v . In short, the relative position of the transmitter, target, and the receiver thoroughly determines the topology of the FSR configuration.
According to (3), the received target signal consists of two parts, one is the target magnitude U tg (t), and the other is the target Doppler signal sin(2π f d (t)·t). Compared with the Doppler signal, the magnitude will not change much during the observation time, and it is largely determined by the propagation mechanism and target scattering model rather than the motion parameter of the target. More details about the characteristic of magnitude can be looked up in [17–21]. On the contrary, the Doppler signal changes much more rapidly with the moving of the target. It is mainly determined by the motion parameter, and is the focus in our algorithm. So, a detailed analysis about the Doppler signal will be presented here.
In (7), we can see that the Doppler frequency depends on the target velocity, the wavelength of the transmitted signal, and the target horizontal diffraction angle. When the target crosses the baseline, namely that α h = β h = 0°, the Doppler frequency in (7) equals to zero. Using this point, we can estimate the target crossing moment in the time-frequency domain. To see how the Doppler signal and the target magnitude behave in the real case, simulation and experimental results are given below.
Figure 3a-c shows the comparison of the simulated signal and experimental signal at different carrier frequencies. In Figure 3a, the simulated signal has nearly the same Doppler oscillation with the experimental signal, which verifies the correctness of (7). The higher the carrier frequency is, the higher the Doppler oscillation is. As Figure 3a-c illustrates, the target magnitude behaves more like a window function without much variation. The fast oscillating phase signal has the signature of a double-sided chirp form. The phase signal obviously shows more close relationship with the target motion parameter and could certainly provide more accurate estimation. In the following discussion, we will approximate the target magnitude with a rectangular window function, which is just used to determine the observation interval.
3. General signal processing algorithm
In this section, a joint parameter estimation method will be given, which is an improvement of the method given in . In the classical processing algorithm proposed in , three different assumptions have been made, namely that the target's motion direction is normal to the baseline, the target crosses the baseline at the midpoint, and the motion of the target is uniformly linear without acceleration. For the velocity direction angle assumption, it is reasonable because the velocity estimation algorithm is very robust to the velocity direction angle. For the no-acceleration assumption, it is very reasonable as well because the interested ground target (human or car or tank) has very small or no acceleration over the observation time (only a few seconds), the small acceleration only results in the asymmetry of compression side-lobe, does not affect the velocity estimation (shown in ). For the midpoint assumption, in the proper operation area (d T /L around 0.5), we can obtain the correct velocity estimation and good compression results; outside this operation area, the performance of velocity estimation and target compression deteriorates a lot. However, in practical case, it is impossible to guarantee that the target crosses the baseline at midpoint. Moreover, the target crossing position affects the Doppler signal a lot and then causes the deviation of velocity estimation. Therefore, we need a new algorithm to adapt this case, what is to say, we want to estimate the target crossing position and velocity at the same time.
From (11), because of the high nonlinearity, we cannot obtain the analytical expression of and , we can only obtain the numerical solution from the block diagram in Figure 4. But this is a time-consuming process.
where L is the baseline length, dT 0is the starting range from target to transmitter, M and P are the channel numbers of the filter bank.
The operation structure shown in Figure 5 is an equivalent implementation of (11). The corresponding parameters (v m ,d Tp ) with the maximum output are the final estimated results. The values of Δv,Δd in (13) are very important parameters in our algorithm. If the intervals are too long, the estimated results will deviate from the real values a lot and reduce the estimation accuracy. On the other hand, if the intervals are too short, it will dramatically increase the computation cost. Therefore, selecting appropriate incremental intervals is very important, and the Crammer-Rao bounds of the estimation are naturally good references. Using the theoretically best accuracy, i.e., the Crammer-Rao bound, the estimated step can guarantee a good estimation as well as an appropriate computation cost.
4. The Crammer-Rao bound
From Figure 8, we can see that, when the SNR is 15 dB, the square root of CRLB for the velocity estimation is less than 0.02 m/s, for the crossing point estimation it is less than 0.5 m. So, the estimated step should be set as 0.02 m/s for velocity estimation and 0.5 m for crossing point estimation (where the target model is human with a velocity of 1 m/s, the baseline length is 50 m). If so, the estimation accuracy of the relative velocity can reach 2%, which can well satisfy the requirement for target recognition.
where α Max is the maximal diffraction angle. We can see that velocity step depends on the carrier frequency, baseline distance, and target velocity, where wavelength and baseline distance are known in advance, the diffraction angle are also preset beforehand and approximately determined by the target FS patterns. Here, we assume, as an example, that baseline distance is equal to 50 m, α Max is 30° (for a human target) and for typical velocity (for a human, 1 m/s) and carrier frequency (151 MHz), the velocity step should be chosen as 0.015 m/s at most.
The simulated and experimental results show that the classical processing algorithm cannot obtain the correct the velocity estimation because of the mismatched target crossing position. Our new algorithm cannot only estimate the velocity, but also estimate the target crossing position correctly, although there is no range resolution in the baseline direction. In addition, the new algorithm extend the operation area, that is to say no matter where is the target crossing position, we can always obtain good estimations of the v and d T , contrary to the classical processing algorithm only works in the operation area around d T /L = 0.5 (crossing near the midpoint).
We should point out that there is an ambiguous crossing position in Figure 8. This is because the reference signal is symmetrical when the constructed reference signal has the distance d T to the transmitter or to the receiver. If we use the netted FSR, it is not difficult to solve the ambiguity problem of crossing position.
In this article, we have presented a signal processing method for the ground-based FSR system, including the target detection and the joint parameter estimation. It extended the operation area of the classical method. The estimated parameters are very important for further requirements such as target classification and recognition. The Crammer-Rao bounds for the accuracy of the estimations have also been given and be used to determine the estimated step. The algorithm was tested both on simulated and experimental data, all of the results verify the effectiveness of our algorithm.
This study was supported by the National Natural Sciences Foundation of China (Grant Nos. 61172177, 61032009, 61120106004), and partially supported by the Electro-Magnetic Remote Sensing Defence Technology Centre, established by the UK Ministry of Defence, under Project No. 1-27.
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