Global navigation satellite system (GNSS) receivers are one of the vital tools that military aircraft depend on during flights. However, these receivers work poorly when subjected to strong radio frequency (RF) interference. Once these receivers stop working, the surveillance system will no longer detect the aircraft’s position accurately — and this could lead to a number of possible dangers.
Strong RFI largely affects GNSS’ three navigation phases: the front-end, the acquisition stage, and the tracking stage.
In the Front-end
During the initial phase of navigation, the receivers filter the incoming signals, demodulating them before performing the analog-to-digital conversion (ADC). When the RF interference is present, a component of the receiver, called the automatic gain control (AGC), will have to squeeze in or saturate the incoming signal to match the high dynamic range of the ADC. In turn, the amplitude of the useful signal will go lower.
Acquisition Stage
When the RF interference is not too strong, it won’t drive the ADC to full saturation and the GNSS receiver may still complete the acquisition phase. Unfortunately, if there’s wideband interference (RF band wider than the wanted signal), the acquisition search space won’t be able to perform its task. The interference will cause the noise floor to increase, masking the correct correlation peak involved in the acquisition process.
Tracking Stage
RF interference has a direct impact on the quality of navigation measurements. It modifies the variance of the time-of-arrival (TOA) and the shape of the S-curve, which are essential in tracking the position of an aircraft.
Since GNSS receivers are susceptible to radio frequency interference, there is a need to simulate these devices before the actual flight. To make a dynamic, controlled testing, it is essential to utilize testing systems that produce radio frequency signals and simulate interference, as well. After all, it is important that all army and government aircraft has efficient navigation tools before operating in the airspace.
If you want to learn more about GNSS and the testing systems for it, don’t hesitate to call us.
