Spectrally Compliant Waveforms for Wideband Radar
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.
Thus 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 (that is 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 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 U.S. 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 percent 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.)
Figure 1 TRACER waveform.
Figure 2 Spectrum of desired signal.
SAR Waveform Example
The Tactical Reconnaissance and Counter-concealment Enabled Radar (TRACER) is a UHF SAR that is being developed by Lockheed Martin in Phoenix, AZ, for the U.S. Army CERDEC. For a waveform designed specifically for domestic testing purposes, TRACER 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 TRACER domestic testing waveform. 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.)
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.
For example, Figure 2 is a simulated case where a transmitter having a steep spectral roll-off 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.)
Figure 3 Matched pulse compression filter response for the notched TRACER waveform.
When a significant fraction of the waveform is notched, high pulse compression sidelobes result. This is shown in Figure 3 for the notched TRACER 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 percent broadening of the 3 dB compressed pulse width and a 2.0 dB SNR loss. These values are comparable to a weighting function (that is Hamming) that would typically be applied to a radar signal.
Figure 4 Mismatched pulse compression filter response for the notched TRACER waveform.
The Spectrally Compliant Waveform Generation Software, which is a licensed TSC product currently being used by the Army's TRACER program, has been used to support several other radar development efforts. The waveform generation software can provide the capability of a wideband system to operate in complex RF environments and to address the requirements of both U.S. and host nation spectrum management organizations.
Technology Service Corp.