RANDOMIZATION OF ALTERNATING CODES - IMPROVING INCOHERENT-SCATTER MEASUREMENTS BY REDUCING CORRELATIONS OF GATED AUTOCORRELATION FUNCTION ESTIMATES

Incoherent scatter radar autocorrelation function estimates produced b y alternating codes are found to contain significant correlations betw een different lags and different heights. While no correlations are ca used by the background thermal noise, the special structure of the alt ernating code sequences causes clutter or the self-noise contribution of the codes themselves to introduce maximal positive correlations bet ween the different lag estimates. Because of this, postintegration of adjacent heights does not improve the results as much as could be expe cted. For a high signal-to-noise ratio, alternating code results are w orse than estimates produced by similar random codes. In this paper we show that we can correct the situation by randomizing the alternating codes. This is accomplished by multiplying each alternating code sequ ence in the alternating code cycle by a fixed random-looking sequence. This multiplying sequence can be changed and randomly chosen at the e nd of each alternating code cycle. The multiplying sequence can also b e kept constant with almost equally good results. If this is done, the special structure of the alternating code sequences actually causes t he correlations to become less than the correlations produced by rando m codes. The improvement in accuracy is discussed in various situation s. The reasons for the appearance of maximal positive correlations wit h alternating codes are explained in a mathematical appendix.