Resilient PNT - FAQ Part 2

The increasing reliance on GNSS signals for positioning, navigation and timing applications across different industries has raised questions about GNSS vulnerabilities and how to address them. We sat down with John Fischer, Spectracom’s chief technology officer, to talk to him about Resilient PNT (also known as Assured PNT and Alternative PNT) and its implications for today’s GPS/GNSS systems.  This continues from Part 1 of John Fischer’s Resilient PNT FAQ.

Q: What do you mean by “Signals of Opportunity?”

Signals of opportunity – such as WiFi, cellular, broadcast TV – are signals that were not designed specifically for navigation but can be used for it. This is not a new concept as in the early days of aviation, AM radio transmitters were used a navigation beacons. s. STL – Satellite Time and Location – is not a signal of opportunity. It was designed specifically for navigation and timing. To learn more about STL, go here and here.

Other alternative navigation techniques include vision systems which recognize landmarks, map matching systems, celestial navigation, and radar, lidar and sonar systems. Also, traditional radio navigation signals such as eLoran and aviation signals such as VOR, TACAN, DME and ILS are still in use.

Q: What is “Crowdsource Navigation?”

Almost everything is connected to a network. Crowdsource Navigation is when you don’t know where you are, but you are communicating with other nodes that know their locations. By measuring your distance/proximity to those other nodes, you can infer your own position. For example, on a wireless network it is very easy to measure range by measuring the time delays of the data sent back and forth or from signal strength. With several range samples to several other nodes, you can converge onto a solution for your own position. This concept is growing in popularity.

Q: I only have GNSS timing equipment. No P or N. Should I protect my equipment against jamming and spoofing?

If you have an atomic clock, you can afford to lose GPS for days and flywheel through this. This adds a level of resiliency. But alternative external references can also be utilized – for example, NTP, PTP, IRIG. These are backup references. I recommend, however, that you use a holdover oscillator.

Q: Beyond having a holdover oscillator, should I do anything else to protect my timing equipment against jamming and spoofing threats?

It depends. If your system goes down, how critical is not having precise time? Does your world come to an end? You must consider the cost and the availability of other backup systems. It boils down to the criticality of your application. A recent UK study estimated that if GPS were out for a week or more, it would cost companies around £12 billion. So if your equipment is mission-critical, you must protect against catastrophic events. If it’s not, you may be able to get by using other backup references.

Q: What are the pros and cons of eLORAN?

eLORAN stations are terrestrial, high power and low frequency, so it is ideally diverse vs. GPS, which is space based, low power, and at microwave frequency – literally at the other end of the spectrum. You want diversity. Whatever is bad for one of the systems is unlikely to be bad for the other system, as each requires different types of jammers. So eLORAN is the perfect complement to GPS.

Q: I have an IRIG, NTP, PTP backup reference for my timing equipment. Is that good enough to protect me against potential jamming and spoofing?

One must consider what is the ultimate source of timing for those back-up systems. IRIG, NTP and PTP are just ways of distributing timing – for them to have UTC traceability, they need to be connected to a timing source, and often it is through GPS. If you are only concerned about localized jamming or spoofing, then a geographically distance source for these other references may be OK.

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John Fischer
John Fischer

For more than 15 years, John has been part of Orolia, where he works with global navigation satellite systems (GNSS), wireless, positioning navigation and timing (PNT),and specialized systems for our customers. Prior to joining Orolia, he specialized in wireless telecom as a founding member of two startups: Aria Wireless in 1990 and Clearwire Technologies in 1997. At Clearwire, he served as Chief Technology Officer in creating wireless broadband equipment for Internet connectivity. Early in his career, John worked as a systems engineer in radar, EW and command and control systems at Sierra Research and Comptek Research. He holds Masters and Bachelor of Science degrees in electrical engineering and computing engineering from the State University of New York at Buffalo.