Modern radars often require the use of wideband waveforms to perform high-resolution target imaging. In microwave systems, the bandwidth can be on the order of 1.5 GHz, while in UHF systems that typically operate between 200 and 500 MHz, the waveform bandwidth might exceed 200 MHz. A major issue in the operation of such systems is that they often overlap the spectrum used by other radars, and even the spectrum allocated for other types of systems such as communications and navigation devices.
For the radar to operate using a wideband waveform, spectral notches must be included that suppress the radiated signal by 30 dB or more at frequencies allocated to other systems. One method for a radar to generate such notches is to interrupt the sweep of a linear FM (i.e. a CHIRP) pulse. While this method can be effective, it often results in a significant loss in radiated power as the transmitter is turned off during the notching. The action of turning the transmitter on and off during can also cause significant VSWR problems. Additionally, there are systems for which a modulation such as a phase coded or noise-like modulation is required.
To address these challenges, Technology Service Corp. (TSC) has developed software for the US Army to generate constant envelope amplitude, spectrally compliant wideband waveforms. The waveform generation approach is based on constrained optimization theory. Such waveforms are currently being used in a state-of-the-art wideband UHF synthetic aperture radar (SAR). Among the capabilities of the software are the abilities to:
- Generate either constant amplitude pulses, or pulses with controlled leading edge rise times, trailing edge fall times, and pulse envelope tapers.
- Create multiple, narrow, and wide spectral notches, both within and outside the radar waveform bandwidth (notching in excess of 15% of the signal bandwidth has been demonstrated).
- Pre-distort the signal that is input to the radar’s high-power amplifier (HPA) to ensure that the requisite notches are preserved in the transmitted signal.
- Generate mismatched pulse compression filters that suppress (typically by 15 dB) the high-range sidelobes created by the spectral notching.
The software produces the digital waveform coefficients (currently done offline) that are stored in the radar’s digital arbitrary waveform generator within nominally one minute. (This time could be shortened by many orders of magnitude by re-hosting the code in a language such as C++ on an FPGA processor.)
SAR Waveform Example
A UHF SAR that is being developed by Lockheed Martin for the US Army is required to have a frequency-compliant waveform that must be designed specifically for domestic testing purposes. The SAR waveform was required to incorporate four spectrum notches. There are three in-band notches centered at 243, 332, and 410 MHz with widths of 0.5, 6.8, and 20 MHz, respectively, and one out-of-band notch centered at 452.5 MHz with a width of 5 MHz. Figure 1 shows the notched spectrum of the resulting waveform that was used for domestic testing. The Lockheed measurements have thus confirmed that all of the spectral notches had depths of at least 40 dB when measured at the HPA output. (Note: The pre-distortion techniques described below were not applied to this waveform.)
Waveform Pre-Distortion In some radar systems, the transmitter amplitude and phase characteristics can degrade the spectral notch characteristics. To prevent this from occurring, waveform pre-distortion techniques that compensate for transmitter effects have been developed. The waveform generation software uses the measured transmitter characteristics to pre-distort the signal at the HPA input in a manner that preserves the desired characteristics at the output.
Figure 2 is a simulated case where a transmitter having a steep spectral rolloff and a nonlinear phase characteristic was modeled. Figure 2a shows the spectrum of a desired constant amplitude transmit pulse. Figure 2b shows the spectrum on the pre-distorted signal that was input to the simulated transmitter. Figure 2c shows the resulting spectrum at the HPA output. As can be seen, the spectrum at the simulated transmitter output very closely resembles the ideal spectrum. The output pulse’s envelope amplitude ripple was less than 0.1 dB. Thus the pre-distortion techniques should be effective in preserving the desired pulse amplitude and spectral characteristics. (Note: Although there are no spectral notches in this example, simulated notched waveforms show similar performance.)
When a significant fraction of the waveform is notched, high-pulse-compression sidelobes result. This is shown in Figure 3 for the notched SAR waveform presented in Figure 1. To reduce the sidelobes, the software also provides a mismatched pulse compression filter (MMF). As shown in Figure 4, the MMF suppresses the high range sidelobes by nominally 15 dB. The cost for achieving this sidelobe suppression is a 58% broadening of the 3 dB compressed pulse width and a 2.0 dB SNR loss. These values are comparable to a weighting function (e.g. Hamming) that typically would be applied to a radar signal.
The Spectrally Compliant Waveform Generation Software, which is a licensed TSC product that is currently being used by the Army to support synthetic aperture radar (SAR) programs, has been used to support several other radar development efforts.
This article was written by Lee R. Moyer of Technology Service Corporation, Fairfax, VA. For more information, Click Here