Fragility and GPS - Inviting LORAN back to the party.

11.10.2013 James Wall

Isn’t it funny how in the quest for better performance, we often see a higher performing but less resilient alternative take centre stage?  Depending on your requirements, you could be very disappointed in choosing one over the other. Thinking back to my car racing days, we all lauded the increased rigidity and lower weight of composite structures over chrome-molly space frames until we had a small ‘excursion’ from the track and would need to build an entirely new tub rather than bash and weld a bit of steel. Whilst the improvements of the new material were key to better performance, it came with an element of fragility that could suddenly end a weekend’s racing.

Now consider the reliability of airline or super-tanker radio signal navigation. Hardly in the same realm, right? Actually, there is inherent fragility built into the navigation system that has proliferated over an earlier and in many ways more robust alternative. I’m talking GNSS (Global Navigation Satellite System), colloquially termed GPS from the original US Department of Defence (US DoD) system, versus its older terrestrial cousin LORAN (LOng RAnge Navigation).

GPS systems work with a constellation of satellites orbiting the earth at about 20,000km altitude. They use precise timing and a constellation position almanac (ephemeris) to allow users to pinpoint their position anywhere on the surface of the earth. There are several such systems in use - the original "GPS" (US DoD), GLONASS (Russia) and Galileo (European Union). To improve the accuracy of GNSS systems within given locales, differential signal transmitters are used to correct satellite position error against the known fixed position of a ground station receiver.

LORAN was a predecessor to GPS and used a similar principle of precise timing signals and known position transmitters. LORAN transmitters operated at lower frequencies than GPS signals (kHz as opposed to GHz), which provided lower positional accuracy amongst other things. Advances in LORAN technology have led to new ‘eLORAN’ position fixes rivalling the accuracy of unaided GPS signals. Using a network of land based stations transmitting in the high kW to low MW range, large areas of coverage can be achieved, although not the global coverage of GPS.

eLORAN stations are fixed to the same reference frame as the end user and operate in our relatively nice earthly environment, shielded from gamma rays, micro-asteroids and other space nasties. eLORAN signals can be directly received from the station (groundwave) or bounced off the ionosphere (skywave) for increased range but lower accuracy.  High power atmospheric disturbances can cause signal interference and denial of service but generally pose no hazard to the equipment.

Contrast these operational conditions to the GPS constellation. Each of the GPS constellations comprises roughly 30 satellites in medium earth orbit.  They share this space with hundreds of thousands of bits of space junk and micro-asteroids, some with closing speeds of tens of thousands of km/h. In addition to the collision risk, GPS satellites are blasted off the earth into orbit, where they are constantly subjected to high fluxes of electromagnetic and particle based radiation. A significant solar event can not only deny service to millions of users, but knock out large parts of the constellation. 

Low signal strength presents another key fragility of the GPS system. With each satellite limited to about 25W transmitter power and an equivalent isotropic radiated power (EIRP) of about 500W, the signal is in the order of -155dBW when it reaches the end user; that’s in the order of fractions of a micro-watt! All it takes is a local transmitter of modest power on a GPS frequency and you can effectively and almost undetectably deny GPS to large areas. And though illegal, such jammers are readily available online at low cost. North Korea has been using a more powerful version to deny GPS access to South Korea for periods of up to 16 days at a time, playing havoc with shipping, aviation and people looking to find a new bar on Saturday night.

Recently, the idea of falsifying (or ‘spoofing’) the GPS signal using a local transmitter to trick receivers has gained some attention. Researchers used a spoofed civilian GPS signal to force a ship off course without setting off any alarms and Iran indicated that they’d spoofed the military GPS signal to allow capture of a US drone. Pretty tough given the encryption but possible no less. Systems such as GPS-RAIM (Receiver Autonomous Integrity Monitoring) have been incorporated into critical aviation and marine systems for years and can provide some protection from cruder forms of spoofing but they do not completely resolve the issue.  Dedicated anti-spoofing techniques range from signal strength monitoring to signal direction, phasing and timing techniques.

Susceptibility to jamming and spoofing are both symptoms of the non-trivial fragility of GPS based navigation. This has driven renewed interest in eLORAN systems as another wide area means to cross check systems already in place in aviation and shipping, such as inertial and dead reckoning navigation. High power eLORAN signals are inherently harder to jam and spoof than GPS signals – you’d require a lot of planning and a conspicuous transmitter.  Immunity to jamming and spoofing are less of an issue in the middle of the Pacific Ocean but are becoming a very large issue in populated (and disputed) areas.

While neither of these two is allowed as a sole means of navigation for critical services, the reality is that increased reliance on the omnipresence of GPS signals has exposed millions of users to its fragility by design.

Design is always a trade-off and depending on your requirements, you can end up being incredibly disappointed by an expensive system that appears to solve your needs. Careful consideration and rationalisation of all the constraints you face is paramount to identifying the best possible solution to your particular situation.