HF radar receiving system image rejection requirements

The sensitivity and consequent detection capability of an HF radar employing a superheterodyne receiving system may be compromised by spurious signals arising from inadequate receiver image rejection characteristics. The image rejection phenomenon arises due to the fact that in addition to the intended action of a mixer in translating a wanted signal from its input frequency to an intermediate frequency, the mixer will also be susceptible to mixing an unwanted signal (the image), displaced from the wanted signal by twice the intermediate frequency, to the same intermediate frequency. Unless the amplitude of the image signal is contained by an appropriate degree (the image rejection), the image signal will be processed along with valid radar returns, and constitute a source of spurious signals with the potential to degrade radar detection performance. In order to estimate a measure of adequate receiver image rejection, it was first necessary to establish a theoretical basis on which the estimation could be based. The theoretical analysis revealed that, in addition to several radar specific parameters, adequate image rejection was critically dependent on the ratio of the power at the receiver image frequency to that of the background noise spectral density at the intended radar operational frequency. The article includes a theoretical analysis and the extraction of relevant data from a synoptic data base of the HF environment. Based on a statistical analysis of the data base, it has been possible to deduce the impact on radar performance as a function of the receiver image rejection performance, and hence ascertain a measure of adequate image rejection.