The detection of gravitational waves requires a strain sensitivity at unprecedented precision. The planned space observatory LISA overcomes this extreme challenge by heterodyne laser interferometry at picometer-precision based on the exploitation of carrier phase measurements between spacecraft separated by millions of kilometers. In addition, data transmission and absolute ranging, necessary to mitigate effects of laser frequency fluctuations in post-processing, are achieved with direct-sequence spread spectrum signals. The foreseen receivers shall typically operate in a sequential phase-locked loop and delay-locked loop configuration for consecutive phase and distance measurement. Recent analysis observed code tracking delay variations, identified as ranging bias variations, as a result of this sequential arrangement. Hereafter, we present an analytical analysis of these ranging bias variations. Comparisons to numerical simulations reveal the compelling influence of the cross-correlation of the chip sequences on the ranging bias variations for a fixed modulation scheme and thus affirm the necessity of numerical analysis. In addition, a generic model for the quantisation error of a digital delay-locked loop is introduced that may be used for analysis and design of digital code tracking loops in various applications. Finally, comparison to a numerical simulation reveals that at small ranging bias variations, the code tracking error is fully described by the quantisation error, while at high ranging bias variations, this effect is negligible and the code tracking error is dominated by ranging bias variations.
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