DME/TACAN Multipath Impact on GNSS L5/E5a Airborne Receivers Part I: C/N0 Degradation Model

This article is the first part of a two-part manuscript addressing the DME/TACAN multipath (MP) impact on the future airborne GNSS Dual-Frequency Multi-Constellation (DFMC) GNSS L5/E5a receiver correlator input. GNSS L5/E5a signals and DME/TACAN pulsed signals share the same Aeronautical Radionavigation Service (ARNS) frequency band with consequent interference between the two systems. The current GNSS signal
C/NO
degradation model for future airborne GNSS DFMC receivers due to DME/TACAN pulsed signals is expressed at the DMFC correlator input and is standardized in different civil aviation documents. This model considers the introduction of a temporal blanker at the DMFC Radio-Frequency Front-End (RFFE) block output to mitigate the DME/TACAN pulsed signals and thus, relies on the definition of the above blanker pulse width (pweq) and the below blanker equivalent pulse width (PWeq) . However, this model did not consider the presence of DME/TACAN pulsed signals MP. Indeed, the DME/TACAN MP impact on the GNSS signal
C/NO
degradation was never inspected while it could be significant, especially at low altitude. The objective of this paper is thus to update the
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degradation model by considering DME/TACAN MP impact generated by ground scatterers. This is done by updating pweq and PWeq formulas which require the knowledge of the MP delays and Peak Envelope Powers. In particular, this paper proposes two models of the DME/TACAN MP impact, the statistical and the fixed environment models, where the main difference is the modelling of the additional phase generated by the channel environmental condition θe as an uniform variable in the statistical model and as a constant in the fixed environment model. As a consequence, the Pulse Independency Condition (PIC) is satisfied (power of the sum of received DME/TACAN pulses is equal to the sum of the received pulses power) for the statistical model, which is not always true for the fixed environment model. The two models are proposed to cover different needs. The statistical model is a low complexity model completely defined by closed-form formulas, and is proposed to be used for standardization purposes. The fixed environment model is a higher complexity model with two main purposes. First, the fixed environment model is able to provide the
C/NO
degradation for one specific set of DME/TACAN signal conditions. Second, the fixed environment model is used to inspect the limitations of the statistical model by identifying geographical zones where the received power of the DME/TACAN MP signals generated by the ground scatterers only loosely fulfills the PIC.