Development and Evaluation of a Range-Gated Step-Frequency Radar
Project Award Date: 12-15-1999
The Radar Systems and Remote Sensing Laboratory is currently working to develop and test a range-gated, step-frequency (RGSF) radar. The primary objectives are to design and build radar operating over the frequency range from 10 to a 1,000 MHz in different modes, and to test its performance in measuring properties of soils that may be similar to Martian soils.
Radars operating at different frequencies and in different modes, such as impulse, step-frequency, and frequency-modulated, continuous-wave (FM-CW), have been widely used for sub-surface, remote-sensing applications. Impulse radar is the most common technique used today for geophysical applications. An impulse radar sends a short RF pulse with three to five cycles of carrier bandwidth through a wideband transmitting antenna. The received pulse containing one to five damped RF oscillations is sampled and stored for further processing. Impulse radar obtains high resolution using a narrow pulse and sensitivity to detect targets at longer ranges by transmitting a pulse with very high-peak power. Higher sensitivity can also be obtained by employing repetitive-trace coherent integration.
A FM-CW radar transmits a wideband, swept-frequency signal. The received signal is mixed with a sample of the transmitted signal to generate a beat signal whose frequency is proportional to the target range, and whose amplitude is proportional to the target scattering characteristics.
In step-frequency radar, the transmitter carrier frequency is stepped over the desired bandwidth at a predetermined interval, and the amplitude and phase of the received signal are measured at each frequency step. The amplitude and phase data are transformed to obtain a signal that is a function of time or range. Step-frequency radar range resolution is indirectly proportional to the bandwidth over which the carrier frequency is stepped, and the unambiguous range is indirectly proportional to the step size.
The main disadvantage of FM-CW and step-frequency radar is the range sidelobes associated with the use of a band-limited transmit signal. The range sidelobes of strong signals from a surface or near-surface target will limit the performance of the system for target detection at long ranges. The minimum detectable signal in a FM-CW or step-frequency radar is rarely limited by receiver thermal noise, but by range sidelobes introduced by antenna leakage or near-surface targets. For step-frequency radar, the effect of these unwanted echoes can be reduced by employing range gating and/or by weighting the received signal for reducing range sidelobes. Range gating improves the sensitivity of short-range radar by suppressing unwanted reflections including antenna leakage and strong signals from near-surface targets. However, very fast switches are required for implementing range gating in short-range applications.
At the University of Kansas, we developed the concept of range-gated, step-frequency radar by combining FM-CW and step-frequency techniques to implement range gating, with filters for short-range applications (United States Patent 5,867,117, Gogineni, et al., February 2, 1999). The system we are developing will operate in a simple FM-CW radar mode; a range-gated, step-frequency radar mode; and a stepped-pulse radar mode to evaluate its performance in sub-surface remote sensing of Mars-type soils.
Primary Sponsor(s): NASA Jet Propulsion Laboratory