The defunct, obsolete LORAN navigation system is being revived with enhancements, to overcome the potential performance and availability risks of GNSS.
There’s no need to tell the readers of this FAQ about the impact of global navigation satellite systems (GNSS) on positioning, navigation, and timing (PNT) services from global, regional, or even localized perspectives. Billions of GNSS-enabled circuits and systems are daily use, which is a testament to its success and widespread utility.
Whether it’s aircraft and ship navigation, drone guidance, or people and package tracking, the various embodiments of these GNSS systems—also called SatNav systems—have truly revolutionized how we answer the eternal questions which are easy and obvious to ask, but difficult to answer: “where are you? where am I?”
Note that the US-initiated Global Positioning Systems (GPS) is often confused in casual conversion with GNSS. While GPS was the first and proved concept, the broader GNSS now consists of similar systems from different countries, such as the European Galileo, Russian GLONASS, Chinese BeiDou, Japan’s QZSS, and India’s NavIC (Figure 1). These satellite constellations broadcast positioning information to receivers which use it to calculate their absolute position.
The problem which GNSS largely solves has existed since the earliest days of human life. Navigation began with the use of dead reckoning, and directional pointing using Polaris, the North Star (northern hemisphere) or Southern Cross (southern hemisphere), and continued with the compass, sextant, and sun/star sightings, flux-gate compasses, and some form of radio guidance. Each of these has the potential to provide rough accuracy, which is good enough for some situations but not at all. Inertial navigation systems (INS) using precision gyroscope-based INS can provide extremely good results but at a high cost in size, weight, and power—until recently.
This FAQ will look at a now-obsolete radio-based positioning system known as LORAN, and why and how being revived, albeit in a very modern format called enhanced LORAN, or eLORAN.
Q: What are the basic facets of what is often simply called “navigation?”
A: A system has three considerations:
- Positioning is the ability to accurately and precisely determine location and orientation in two dimensions (or three dimensions when required).
- Navigation is the ability to determine current and desired position (relative or absolute) and apply corrections to course, orientation, and speed to attain a desired position anywhere around the world, from sub-surface to surface and from surface to space.
- Timing is the ability to acquire and maintain accurate and precise time from a standard (Coordinated Universal Time, or UTC), anywhere in the world, and within user-defined timeliness parameters.
Q: The acronym GNC also shows up in relation to navigation—what’s that?
A: GNC is short for guidance, navigation, and control and has a great deal of overlap with PNT; it depends on the user’s perspective:
- Guidance refers to the determination of the desired path of travel (“trajectory”) from the vehicle’s current location to a designated target, as well as desired changes in velocity, rotation, and acceleration for following that path.
- Navigation refers to the determination, at a given time, of the vehicle’s location and velocity (the “state vector”) as well as its attitude.
- Control refers to the manipulation of the forces, by way of steering controls, thrusters, and other techniques as needed to execute guidance commands while maintaining vehicle stability.
Q: Since GNSS has totally changed our ability to determine positioning, navigation, and timing (PNT) information, why do we care about the obsolete LORAN system? Why do we even need an alternative?
A: GNSS is wonderful, accurate, and now low-cost, lightweight, and small. But the system has some fundamental potential weaknesses, and these may make it unavailable, slightly inaccurate, or grossly in error.
Q: How so?
A: Keep in mind that a received GNSS signal is very weak (typically -90 dBm or less) since it comes from a constellation of satellites in medium Earth orbits (MEO) at an altitude of approximately 20,200 km (12,550 miles) for GPS; other GNSS satellites are at roughly the same altitude (Figure 2).
Further, received signal-to-noise ratio (SNR) is also very low, on the order of a few dB, due to the physics of the situation, the transmitted signal strength, and free-space path distance. There are three primary concerns with GNSS availability and performance: loss of signal, jamming, and spoofing.
- loss of signal occurs when the receiver is underground, in a tunnel, underwater, or in most (types of buildings; the carrier frequency of GNSS signals—above over 1 GHz—does not penetrate most barriers.
- jamming is when a non-GNSS signal of sufficient strength overwhelms the satellite signal. This can be accidental, such as when a strong, nearby EMI/RFI source is in the band (very common), or deliberate using a standard commercially available jammer which is turned on to blanket the desired signal.
- spoofing is when someone deliberately transmits false signals designed to fool the receiver, so it decodes a false position. There are two principal ways of spoofing. The first is dubbed meaconing and consists of intercepting, storing, and then rebroadcasting GNSS signals later on. The second consists of generating and transmitting modified satellite signals. Spoofing is more dangerous than loss or jamming as in those two cases, you know the signal is absent or unusable, while spoofing can unknowingly mislead with obvious negative implications.
Q: What can be done about these weaknesses?
A: There’s very little that be done about loss of signal. Received signal-to-noise ratio (SNR) is very low, on the order of just a few dB, and received signal strength is also low, on the order of -90 dBm. Both numbers are due to the physics of the situation and free-space path loss.
As for jamming, there are techniques for smart decoding of the received signal that can overcome some jamming but only up to a point. For spoofing, smart algorithms can determine if the shift in position is eliminated by seeing it is commensurate with the appreciation. For example, a GNSS receiver in a drone should not see a shift in position by several miles in just a millisecond. Of course, the spoofers know this and do not suddenly engage the full spoof; instead, they slowly and gradually spoof the receiver to shift the apparent signal off course.
None of this says that GPS is a weak system, as its capabilities are proven. However, there’s a need for a parallel system that has zero technical overlap with GNSS and so has different attributes, capabilities, and even weaknesses. This solution will be discussed in the next section.
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- S. Department of Transportation, “What is Positioning, Navigation and Timing (PNT)?”
LORAN and eLORAN
- Smithsonian, Time and Navigation, “Why Was LORAN Such a Milestone?”
- Smithsonian, Time and Navigation, “Hyperbolic Systems”
- Wikipedia, “LORAN”
- Britannica, “loran”
- Web Pages Of Jerry Proc, “LORAN-A”
- Military & Aerospace Electronics, “Public-private partnership to launch eLORAN technology to back-up and accompany GPS”
- L National Bureau of Standards, Monograph 129, “The Development of LORAN-C Navigation and Timing” (a 1972 source document, 166 pages)
- GPS World, “GPS backup demonstration projects explained”
- UrsaNav, Inc., “eLoran System Definition and Signal Specification Tutorial” (47 pages PowerPoint, technically detailed)
- UrsaNav, Inc., “About Enhanced Loran” (brief overview)
- UrsaNav, Inc., “eLoran Points of Light” (16-page facts and myths – very useful)
- UrsaNav, Inc., “Resource Vault” (links to papers and tutorials)
- UrsaNav, Inc., “eLoran” (excellent two-page overview)
- S. Coast Guard, U.S. Department of Homeland Security, and Federal Aviation Administration, U.S. Department of Transportation, “Benefit-Cost Assessment Refresh The Use of eLORAN to Mitigate GPS Vulnerability for Positioning, Navigation, and Timing Services – Final Report (2009)” (37 pages with lots of bureaucratic content but also some nuggets)
- gov, “LORAN-C Infrastructure & E-LORAN”
- Spirent Communications, “Working With the Strengths and Weaknesses of Satellite Navigation Systems”
- Bliley, “What’s The Differences Between the 5 GNSS Constellations?
- GNSS spoofing Septentrio, “OSNMA anti-spoofing technology now on PolaRx5 GNSS reference receivers” [Open Service Navigation Message Authentication]
Hyperbolas and math