Impact of a Civil Aviation GNSS Receiver Temporal Blanker in the Presence of RFI DME/TACAN Signals

In the L5/E5a band, the predominant radio frequency interferences (RFI) that impact the global navigation satellite system (GNSS) are the Distance Measurement Equipment/TACtical Air Navigation (DME/TACAN) pulsed signals. To mitigate its impact, airborne dual-frequency multiconstellation GNSS receivers must implement a temporal blanker (TB), a device setting to zero the I/Q received signal samples having an instantaneous power envelope above a certain threshold. The presence of RFI and TB decreases the effective carrier-to-noise ratio ${\bm{C}}/{{{\bm{N}}}_{0,\mathbf{eff}}}$, whose determination is critical to guarantee the GNSS-based safety-of-life service in civil aviation. To accurately determine ${\bm{C}}/{{{\bm{N}}}_{0,\mathbf{eff}}}$, the exact model of the normalized power spectrum density (PSD) of each RFI signal at the TB output, or postblanker RFI signal, is required. However, the postblanker RFI signal PSD is difficult to calculate since a TB is a strongly nonlinear device, and its behavior is aircraft location and flight altitude (L&A) dependent. To address this problem, in this work, a new model of a postblanker DME/TACAN signal is proposed from the multiplication of the before-blanker RFI signal by two artificial signals: autoblanking, which is the triggering of the blanker by the DME/TACAN signal itself, and blanking-by-others, which is the triggering of the blanker by other DME/TACAN beacons signal. The proposed model allows the mathematical determination of a mean postblanker DME/TACAN signal PSD and is L&A customized. In comparison, the previous reference or other proposed PSD models were either not-mathematically justified approximations or did not consider the TB effect (simplification); and neither were L&A customized. The proposed postblanker DME/TACAN signal PSD expression is verified through simulations with differences smaller than 0.11 dB. Comparisons with reference and simplified models are conducted in terms of PSD, spectral separation coefficient (SSC), and ${{{\bm{R}}}_{\bm{I}}}$ (linked to ${\bm{C}}/{{{\bm{N}}}_{0,\mathbf{eff}}}$). Previous reference model is shown to be a loose approximation, and simplified models present less accurate PSD and SSC results but a small ${{{\bm{R}}}_{\bm{I}}}$ underestimation. Finally, the radio frequency front-end block impact, including TB, on the DME/TACAN RFI signals is thoroughly modeled in this work to provide a clear tutorial on the ${{{\bm{R}}}_{\bm{I}}}$ calculation from the link budget received characteristics of the DME/TACAN signals.