Abstract
Abstract. A fast method for theoretically comparing the posteriori variances produced by different phase code sequences in incoherent scatter radar (ISR) experiments is introduced. Alternating codes of types 1 and 2 are known to be optimal for selected range resolutions, but the code sets are inconveniently long for many purposes like ground clutter estimation and in cases where coherent echoes from lower ionospheric layers are to be analyzed in addition to standard F-layer spectra. The method is used in practice for searching binary code quads that have estimation accuracy almost equal to that of much longer alternating code sets. Though the code sequences can consist of as few as four different transmission envelopes, the lag profile estimation variances are near to the theoretical minimum. Thus the short code sequence is equally good as a full cycle of alternating codes with the same pulse length and bit length. The short code groups cannot be directly decoded, but the decoding is done in connection with more computationally expensive lag profile inversion in data analysis. The actual code searches as well as the analysis and real data results from the found short code searches are explained in other papers sent to the same issue of this journal. We also discuss interesting subtle differences found between the different alternating codes by this method. We assume that thermal noise dominates the incoherent scatter signal.
Highlights
In incoherent scatter radar measurements the desired range resolution is often much better than the available pulse length of the radar
In order to maximize the statistics of the results special coding methods like alternating codes have been developed to allow the radar to be used at full duty cycle and still produce range resolutions much shorter than the pulse length used (Lehtinen and Haggstrom, 1987)
This paper describes a fast method for doing this, a method that is not necessarily as accurate as slower methods, but is good enough to allow good code sets to be separated from the vast number of bad code sets
Summary
– Long pulses are modulated by a binary phase pattern, which gives range resolution approximately equal to the baud length of this pattern. The actual (accurate) method for decoding the codes is to apply linear statistical inversion to the lag profiles measurements, which represent the desired unknown plasma scattering lag profiles convolved with the range ambiguity functions corresponding to the different lagged products of the codes in the code group under study. We discuss the possibility of doing inversion analysis of echo amplitude data to solve for the scattering amplitudes instead of correlation properties of the target (Vierinen et al, 2008b). This way of analysis is advantageous with very narrow targets like meteor heads and narrow ionospheric layers. This paper is purely a generalization of the classical way of coding and decoding by summation – with no need for inversion methods
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