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Simulating stepped frequency radar systems

Posted: 29 Oct 2012     Print Version  Bookmark and Share

Keywords:frequency hopping  resolution  stepped frequency radar 

In any radar receiver, the received echo signals contain the target return and background clutter. Detection of the target in an environment with background clutter requires high range and cross-range resolution in the radar system. The traditional way to accomplish this goal involves use of short duration pulse waveforms and wideband-FM pulses. However, this approach requires a complex system architecture and results in higher implementation cost due to its wideband receiver usage. Another way to achieve high range resolution, without increasing system complexity, is to employ stepped frequency radar (SFR), a scheme well known for its use in non-destructive testing and ground searching applications.

With SFR, the echoes of stepped frequency pulses are synthesised in the frequency domain to obtain wider signal bandwidth. Using frequency hopping, both high resolution and a high signal-to-clutter ratio can be received. Because of its high resolution and low cost it is today widely used in both the commercial and aerospace/defence (A/D) industry. However, it is very difficult to get an analytical solution for SFR receiver performance in the presence of background clutter caused by reflections from ground, structures, vegetation, and so on. As a result, simulation becomes more important. Using it to accurately design, verify and test SFR systems under real-world environments has become absolutely essential.

Figure 1: On the left is a pulse radar waveform. The right-most image depicts a SFR waveform.

Understanding SFR
To better understand why SFR is so advantageous, first consider the pulse radar waveform shown in figure 1 (left-most image).

Assuming the pulse width is τ and the bandwidth of the signal is ƒ0= 1/ τ, the range resolution, Rs, can be calculated as

Rs = c/(2*f0) Equation 1

where c is the velocity of the light.

As an example, assuming the pulse width τ = 0.25µs and the pulse repetition interval T = 10µs, the range resolution would be 37.5 m. For a resolution of less than 1m, from (1) the pulse duration would have to be shortened to say, T = 3.9 ns. The resulting range resolution would then be 0.58 m and instead of handling a 4MHz bandwidth the new system bandwidth would be 250 ns/3.9 ns = 64 wider than the original system bandwidth.

To achieve a high resolution at the 0.58 m without reducing the pulse duration, SFR could be employed. As shown in figure 1, SFR transmits sequences of N pulses at a fixed pulse-repetition frequency, but not at a fixed radar frequency. Unlike the pulse signal, each pulse in the sequence of a stepped frequency waveform has the same pulse width and time duration, but different carrier frequency. That frequency is given by fi = fo+N*dF, where dF is the amount of frequency increased, indicating that frequency hopping and time division are used.

Assuming the N-step stepped frequency is used, the pulse width and pulse repetition interval are still τ = 0.25µs and T = 10µs where N = 64, as from the previous example, and dF = 4MHz, the resulting range resolution bandwidth would be Rs = c/{2*f0+(N-1)*dF)} = 0.58 m. As is clearly evident from this result, SFR has a high range resolution (less than one meter ('re' for unit of measurement)). Moreover, it was achieved without having to shorten the resolution, making it preferable to pulse radar in this scenario.

Platform for designing, testing SFR
In SFR radar, clutter interferes with target detection, making it difficult to find the actual number of targets or even causing it to fail in detecting small targets. Finding a closed-form analytical solution that enables target detection to be analysed in the presence of this clutter is also difficult. Because of the significance in analysing these types of scenarios, simulation becomes critically important, as does the use of a platform solution for simulation of SFR systems under real-world environments. The platform can also be used for verification and testing of SFR systems. The simulation platform with test environment must include return signal radar cross section (RCS) and background clutter.

To better understand how such a platform might be used to design, verify and test a SFR system, a template SFR design is provided below. By customising the template SFR design for their own systems, engineers can run simulations in the platform to evaluate the design's performance. When design simulation is combined with test equipment, the simulation platform can also be used as a test platform for SFR component hardware testing. As an example, an SFR system with two target returns and ground clutter is presented in which the platform is used for both simulation and hardware test.

Simulating an SFR system
Consider the basic SFR design shown in figure 2. In the signal generator, a SFR source is followed by an RF modulator, then two target models and a clutter model are used. At the SFR receiver input, received signals include target return and clutter.

Figure 2: This example of an SFR simulation is performed using Agilent Technologies' SystemVue electronic system level design platform.

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