With the growing proliferation of a large variety of transmitters around the world, there should be little question that the noise floor for GNSS receivers will continue to increase along with the threat of disabling in-band radio-frequency interference (RFI). RFI poses a serious threat to the reliable operation of GNSS receivers when the received RFI power level is high enough to render the GNSS receiver inoperable. There are usually no visible external signs of anything being out of order, so the user initially assumes the receiver has experienced an internal failure.
Without the sophistication of built-in RFI situational awareness in the GNSS receiver design, the determination of the presence and seriousness of in-band RFI problems is an extremely inefficient and frustrating process. A simple jamming-to-noise-power (J/N) monitor can be a low-cost built-in feature of the GNSS receiver if a J/N meter design is preplanned in the original GNSS receiver front-end component design, layout, and implementation. A retrofit to an existing design is usually impractical. An additional justification for including a J/N meter in the initial GNSS receiver design is the significant performance advantages obtained through RFI situational adaptation especially during initial search.
J/N meter design
Figure 1 is a high-level functional block diagram of a GNSS receiver RF front end that contains a J/N meter that is obtained as a byproduct of the digital gain control part of the digital automatic gain control (AGC) design. Figure 2 (p. 30) is an expanded functional block diagram of the digital gain control function shown in Figure 1. The J/N meter design shown in Figure 1 is implemented in the last intermediate frequency (IF) stage of the GNSS receiver RF front-end architecture - that is, prior to signal detection by the multiple digital receiver channels (not shown in the figure). At the last receiver IF stage, the GNSS spread spectrum received signal power (SIF) from each satellite in view is well below the thermal noise power (NIF). This assumes the receiver is located on or near the surface of the Earth and the antenna gain is in the region of near unity gain with respect to an ideal isotropic circularly polarized antenna (0 dBic).
For a typical unjammed wideband (WB) C/A-code receiver example, SIF is typically 30 dB below NIF at the last IF stage; that is, the C/A-code signal power is a thousand times lower in power than the thermal noise power just before the signal detection process takes place in the digital receiver channels. A key point to remember is that all GNSS spread spectrum signals are designed so they can be readily acquired and tracked even though (S/N)IF is considerably negative. When in-band RFI jamming power (JIF) is present at IF, the effective noise level is increased so that the receiver may be unable to acquire and track the GNSS signals in view. Since the undetected signal is always below the noise level (whether or not JIF is present), it is impossible to obtain a measure of the composite signal-to-noise power ratio at IF.