Mapping of overspread targets in radar astronomy

The problem of delay‐Doppler mapping of overspread targets, i.e., targets where the product of delay extent θt and Doppler width Bt is greater than unity is discussed. For a transmitter modulation waveform which is believed to be representative of continuous wave modulation, namely, a random Gaussian noise modulation, the effect of the “self‐noise” and the additive noise on the output signal‐to‐noise ratio is evaluated, and expressions are derived which allows the signal‐to‐noise ratio to be predicted and the quality of the mapping assessed for various types of targets and modulation waveform properties. The problem of the self‐noise is shown to be a serious problem, particularly for targets where the power returned varies greatly with range. A scheme involving a combination of random noise modulation and polarization modulation is developed which will greatly improve the capability to map this type of overspread targets. The particular case of the mapping of Mars with an existing facility is dealt with in detail, and it is shown that a limited amount of mapping with a resolution corresponding to 100 by 100 points on the planet is possible in certain regions.