PPI Interference Prediction

Abstract

Summary form only given. The interference prediction model is a major tool in the effort to obtain electromagnetic compatibility in the crowded radio frequency spectrum. Work has progressed in the prediction modeling of the transmitter, antenna, propagation, and receiver, but additional expressions for the degrading effect of interference upon information output to the operator are required to complete the analysis. The paper introduces an analytical approach to Obtaining a quantitative prediction of the interference degradation to a radar PPI display. Determining the signal output from the receiving system due to environment has been accomplished by various approaches and is reported on elsewhere. For the purpose of this investigation, a description of the receiver output in terms of the pulse density distribution by received power level i s assumed. The task remaining and the subject of the study performed, is the relationship between the pulse signal distribution input to the display and the resulting interference to the PPI. The transfer characteristic of the Cathode Ray Tube (CRT) when excited with pulse signals is utilized to determine the relationship of input pulse power level to light output on the display. The proportionality of the display spot intensity to the logarithm of pulse power is thereby established. where intensity is defined in photometric terms and means total light output from a source. It is important in this case to define intensity. to distinguish between it and brightness or which are intensity per unit area. The CRT response to pulse exc itation. whereby the luminance levels off while the intensity continues to increase with higher amplitude pulses. makes this distinction necessary. The overall display intensity in the case of random pulse interference will be the cumulative intensity of the individual pulse excitations. In the report. the relative scope interference level is logically related to the clutter intensity resulting from the pulse interference signals. At this point the means for expressing PPI interference due to a pulse signal environment in a relative sense has been established. To make the concept useful, relative levels must be tied to a normal usage frame of reference. To accomplish this, photographs of operating PPl' s in a n RFI environment were classified by comparison with the five conditions o f PPI interference employed by the Air Force in their routine interference reporting regulation. Measured data on the pulse signal enviromnent producing the PPI interference were utilized to compute the relative Interference Index according to the analytical relationships derived. The range of index values in each condition category then established the correspondence of interfering signals to resulting PPI condition. Scope conditions, computed from measured data on the pulse signal environment, agreed with photographs of the PPI in 64% of the available sam.ples and did not differ by more than one scope condition in any of the samples. A second comparison, wherein the pulse signal conditions input to the PPI were first predicted from knowledge of the operating environment, produced nearly comparable re sults. Determination by laboratory simulation means of the relationship between scope condition and degradation of detection range appears as a logical extension of this study.