GNSS RFI Part 1: Intentional vs. Unintentional Radio Frequency Interference
Reliance on global navigation satellite system (GNSS) receivers for positioning, navigation, and timing (PNT) has become so widespread that our economy, banking, communications, and, in many instances, safety depend on it. Yet GNSS signals are very weak and travel through a crowded and polluted radio frequency (RF) environment. RF pollution is called RF interference (RFI), as it interferes with the signal that we are trying to receive.
Unintentional RFI vs. Intentional RFI
Unintentional RFI is partly due to the rapid growth of the wireless telecommunication sector. Often, accidental GNSS interference is due to a nearby radio device broadcasting at a frequency that lies within the passband of one of the GNSS frequencies. This is particularly a problem in parts of the world where radio frequency transmissions are not strictly regulated.
There is also intentional GNSS interference in the form of jamming and spoofing: jamming may knock out GNSS receivers, while spoofing is when fake satellite signals are broadcast to fool receivers into believing that they are somewhere other than where they are, or at a different point in time. Jamming is most often caused by “personal privacy devices” (which are cheap and relatively easy to obtain) that deliberately interfere with GPS signal reception.
Growing Impact of Unintentional Interference, Jamming, and Spoofing
Malicious jamming and spoofing are both evolving fast, as hackers increasingly gain access to low-cost equipment. Spoofing was once extremely hard to do, but it is now easy to build a GNSS spoofer from open-source software and low-cost components.
Recently, a year-long project aboard a commercial cargo ship collected tens of thousands of snapshots of RFI in the GNSS band on a round-trip passage from Spain to Korea. It detected the most RFI in busy port areas, less along coasts, and even found some in the open ocean.
Whether unintentional or intentional, RFI can severely degrade GNSS performance. At GNSS technical conferences, the conversation now often turns to GNSS jamming, spoofing, and denial and how to mitigate these threats. The European Union has made anti-spoofing mandatory for the European Railway Traffic Management System.
RFI Mitigation Techniques, and What Users Can Do
While all GNSS receiver manufacturers agree that RFI and spoofing are serious challenges, their approaches to dealing with them differ. Most rely on some combination of monitoring, detection, and filtering. Receiver solutions include monitoring cycle slips and using analog to digital converters, correlators, and notch filters. Antenna solutions include the use of array antennas. Additional approaches include shielding, fusing data from inertial measuring units (IMUs) and other third-party sensors, advances in processing algorithms, and using the new Galileo authentication service. Trimble’s latest GNSS receivers implement hardware- and software-based techniques to detect and mitigate spoofing.
Two of the most common sources of significant GNSS interference for aircraft are: antennas for the Iridium satellite-based voice and data network, often found at large airports and military bases; and active fleet tracking antennas installed on aircraft. To address the former problem, Applanix built an Iridium filter into the primary antenna that it sells for airborne platforms. For the latter, Applanix receivers rely on inertial navigation to mitigate the effects of cycle slips by constraining the search area, as well as on post-processing to smooth out the error peaks that occur just before they reacquire a fix.
No matter the application, it is important for users who incorporate inertial navigation into their project to ask of their supplier what their system is doing to protect against RFI. Any company with knowledgeable in-field experience will help their customers explore how each project’s unique environmental conditions can create RFI vulnerabilities, and which mitigation techniques work best on a case by case basis.
Next: GNSS Radio Frequency Interference Pt 2
Having successfully distinguished between intentional and unintentional forms of interference, we now tackle how Iridium filters, inertial navigation, and post-processing all do their part to combat unintentional RFI in Part 2 of our GNSS RFI posts.