INTERPRETATION OF THE FLUCTUATING ECHO FROM RANDOMLY DISTRIBUTED SCATTERERS. PART I

Abstract

The radar echo from randomly distributed scatterers fluctuates in intensity about a mean equal to the sum of the intensities contributed by each particle. If k independent intensity values are averaged, the r.m.s. deviation of the fluctuation is reduced by a factor √k. Counting the fraction of signals falling above one or more thresholds is a practically simpler, though somewhat less rapid, alternative to averaging. As a further alternative, one can superimpose many independent sweeps of amplitude or signal level on an A-scope, and so observe the most probable value.In range interval l, a single sweep contains 2l/h independent data, where h is the pulse length. The intensity averaged under the continuous trace is slightly more accurate than the average of the independent data.Pulses sent through the same scatterers yield identical returns until the scatterers have had time to reshuffle themselves. Successive pulses give independent echoes, however, if their frequencies differ by about 1/τ, τ being the pulse duration. Echoes from the same pulse received at separate antennas are similarly independent.Several (k) independent echo values are helpful in measuring steady echoes against a noise background. For fluctuating echoes, k must be large, regardless of the relative strengths of echo and background. Large values of k in addition materially enhance the detectability of fluctuating echoes weak compared to the noise.