Analysis of Direct Conversion Front-End Distortions for High-Fidelity Satnav Integrity Monitoring Applications

Safety critical satnav-based applications such as aviation employ continuous signal quality monitoring (SQM) for integrity purposes. In general, these SQM techniques monitor the correlation function using a handful of points. This research aims to look at how the front-end architecture (e.g., real vs. complex sampling) impacts and causes distortions on the correlation function, and how those in turn impact the range measurements in terms of differential code biases (DCB). The majority of today’s low-cost and low-power radio frequency (RF) hardware employs the direct conversion approach. In this architecture, after some necessary amount of amplification and filtering is performed at RF, the signal is directly converted to baseband using homodyne mixing. The resulting quadrature signal is further amplified and filtered at baseband, and finally sampled by a pair of analog-to-digital converters (ADCs). The unavoidable gain and phase imbalances present in the quadrature analog processing between mixer and ADC in a direct conversion frontend can lead to significant distortions in the correlation function for a SQM system. In contrast, an intermediate frequency (IF) sampling front-end employs a single analog processing chain, and conversion to baseband is performed digitally – thus guaranteeing no quadrature gain and phase distortions. However, IF sampling front-ends in general have significantly larger size, weight, power and cost (SWaP-C), and typically cannot be sourced as commercial-off-the-shelf (COTS) equipment for use in satellite navigation applications. This paper explores the distortions caused by direct conversion and IF sampling front-end hardware in the context of the correlation function in order to better understand their suitability for integrity monitoring applications. Correlation function-based metrics to characterize gain and phase distortions will be presented. Results show significant differences between direct conversion and IF sampling front-ends.