Admittedly- selling the idea of a back-up system for our highly vaunted Global Positioning System lacks much in the way of ‘sex appeal’ – or- apparently- anything in the way of urgency. Putting it mildly- the idea faces significant challenges.

Dave Higdon  |  01st July 2010
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Dave Higdon
Dave Higdon

Dave Higdon is a highly respected, NBAA Gold Wing award-winning aviation journalist who has covered all...

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Backing Up The Future:
GPS dependence puts NextGen on thin ice.

 Admittedly- selling the idea of a back-up system for our highly vaunted Global Positioning System lacks much in the way of ‘sex appeal’ – or- apparently- anything in the way of urgency. Putting it mildly- the idea faces significant challenges.

But according to many security experts- engineers and technicians around the globe- failing to provide a second system for area-navigation service (and its related services) leaves users and society at-large vulnerable to numerous potential problems – both natural and man-made.

As the United States accelerates development and movement toward the Next Generation Air Traffic Management System (NextGen)- concerns remain about the wisdom of depending on a vulnerable satellite system – particularly given recent events.

Firstly there’s the idea of total dependence on one system for navigation and approach guidance. Total dependence on GPS would mark the first time U.S. airspace depended wholly on one source since the advent of the Non-Directional Beacon. For decades the airspace offered multiple navigation aids behind the NDB: the four-way radio range- the VOR network- and in the first realization of an open area-nav system- the Loran C system.

Once considered the best option for backing up GPS- Loran lacked any of the satellites-in-space imagery of GPS. The sterile- isolated range of towers and transmitter buildings scattered around the country certainly didn’t scream their sophistication to the level of a constellation of about 28 orbiting GPS satellites.

Instead- the biggest technological leaps in Loran occurred largely out of sight- taking the form of either in-the-transmitter-building changes in how the low-frequency range broadcast its code- or in the shrunken size and outsized sophistication of the in-plane receivers decoding the broadcasted code. Satellites have Space as their home – and their increasing accuracy capabilities coupled with the shrinking size and expanding functionality of the navigators themselves trumped the decidedly staid- utilitarian functionality of the Loran chains.

Sadly- the Loran C network – including a Mid-Continent chain erected and activated in the late 1980s – fell silent earlier this year; victims of budget constraints and a lack of urgency about the need to preserve a backup to GPS.

The potential for losing GPS for any reason is officially acknowledged – but considered a ‘long-shot prospect’. With the view that GPS is increasingly reliable- the FAA developed the Wide Area Augmentation System (WAAS) to refine satellite accuracy to a level capable of supporting precision-approach procedures- and departure and arrival procedures dependent on en-route accuracy to within a fraction of a mile – under 1-100 feet laterally.

The system hummed along fat- dumb and happy- growing in acceptance and in the number of procedures and aircraft capable of using the WAAS-based guidance. The world was good. Then in May of this year- the unforeseen happened.


One of only two satellites providing WAAS corrections to GPS signals drifted out of orbit- out of use and out of service. In the span of a few days- a multibillion-dollar network of navigation satellites and accuracy-dependent procedures reverted to a single failure point for WAAS enhancement.

That WAAS enhancement is absolutely critical to the future of NextGen – as well as those thousands of GPS-based WAAS-enhanced approach procedures for instrument flight. No WAAS- no LPV- RNP or VNAV- or other GPS-based procedures. The advances brought by WAAS led to the creation of instrument approach procedures that provide thousands of runway ends with bad-weather access at an accuracy level comparable to the gold-stand Instrument Landing System (ILS).

In fact- since the FAA went live with WAAS a few years ago- the agency has created and activated enough WAAS-based approaches to exceed the total of ILS approaches in use – with the number of WAAS-based approaches continuing to grow at a rate of about 500 per year.

Those 500 approaches- at an FAA-estimated cost of $50-000 apiece- cost less than putting in 15 to 20 new ILS systems. That means for the same money- the FAA can install 15 to 20 new ground units to serve 15 to 20 new runway ends with ILS service…or- give 500 runway ends new instrument procedures with literally no cost in ground hardware – aside- maybe- from some lighting or signage. It’s this efficiency that stands at risk without a relatively low cost back-up area navigation backbone capable of taking over for GPS. “But”- many claim- “such a loss of GPS is highly unlikely.”

Well- the loss of half the nation’s WAAS-satellites – satellites upon which all of those slick- new RNAV- VNAV- LNAV and LPV approaches depend – fit the assessment of “highly unlikely”. That is- the assessment fit until one of the two satellites left its station – leaving the WAAS GPS system balanced on a single failure point for an estimated 18 months. That’s the time expected to pass before restoration of the WAAS backbone to two satellites once again. “Where one failure can occur”- critics claim- “so can another.”

Quite right too: with one satellite suffering an unforeseen- unexpected failure- how far fetched do other failure scenarios now seem? More to the point- how damaging would other failures – no matter how unlikely – be? And how costly- relatively speaking- would investing in a back-up system prove (relative to the costs of a disruption to GPS)?


Within aviation we know GPS best and most for its wondrous navigation capabilities. Available unrestricted around-the-world GPS- and its overseas counterparts like GLONAS and Galileo- provide fast- accurate positioning to any device capable of receiving signals from enough satellites to calculate a fix. Satellite navigation works in the service of: civil and military needs; in aviation and maritime commerce; recreational endeavors such as hiking and biking in the wilderness.

Increasingly even the humblest new automobiles sport GPS navigators with color displays to mark the way to drivers’ destinations. Trucking and shipping companies use GPS to track everything from delivery vehicles to the packages they carry. Parents can now use smart-phones their children carry to monitor and track their offspring using embedded GPS navigators.

Increasingly- as most of you know- aviation authorities embrace refined GPS like WAAS for precision navigation in instrument meteorological conditions – a breakthrough of significance for its lower costs and greater flexibility than ground-based guidance equipment talking to airborne receivers.

Beyond the navigation realm- however- technologies underpinning GPS play a role in many other areas – many of them critical to today’s high-technology civilization. The most-critical of those technologies involves timing and control.

To provide their navigation functions- GPS employs precise atomic clock systems accurate to fractions of a fraction of a second- a function as necessary to receivers’ ability to calculate position as the chronograph’s invention was to the success of longitudinal-navigation calculations.

Those clocking and timing functions underpin technologies developed to serve the Internet itself- to help telephone networks function- to manage banking transactions. Power grids- those mechanical arteries that deliver the lifeblood of electricity to a power-hungry society- also depend on GPS timing and clocking.

The loss of global navigation satellite service in even a small local area could disrupt commerce- banking- transportation- communications – even the basic delivery of electrical power. And failures not only can happen- they do – and have – happened.


New Year’s Day- 2004- the failure of an atomic clock on a GPS satellite introduced position errors to navigators in Europe- resulting in an error that started at about six miles and grew to about 25 miles before the system detected the failure and sent a signal to receivers to exclude those from the failed bird.

Almost three years later the most-powerful solar disruption ever recorded occurred- and the huge flare arrived with such power that the electronic noise it generated knocked out of service WAAS satellites – for more than 15 minutes. Regular GPS satellite service on the sunlit side of the planet went down- as well.

The above examples are two of the natural threats to GPS integrity. Manmade threats also exist- however. They include commercial and homemade- cheap- simple low-power signal jammers that simply overpower satellite signals – drowning out the receivers’ ability to ‘hear’ the satellite signals. More sophisticated units can actually transmit a signal coded falsely – in order to fool a navigator or a timing counter and send a vessel- vehicle or airplane widely off course.

The power required is only a couple of Watts – and the device can be small- portable and battery powered. Even an accidental man-made problem can have devastating potential- though.

In January 2007- for example- GPS functionality suddenly vanished for about two hours over large sections of the San Diego area in Southern California. The culprit turned out to be an unintended consequence of a U.S. Navy jamming training exercise. Nonetheless- according to reports- victims of this accidental jamming included the loss of more than 100 towers supporting cell-phone and other mobile communications- as well as a hospital’s paging system- which went non-functional. Navigation service in the impacted area was- of course- vanished during the disruption.

Now imagine being somewhere in the en route or terminal environment when your GPS navigator stops ‘hearing’ the satellites for several minutes – regardless of whether the cause was human or natural. Or think about the chaos possible if a GPS problem resulted in a failed power grid. Of course- it’s not as if these issues haven’t been studied and flogged.

The U.S. Department of Transportation issued its Volpe Report in 2001 under the title- ‘Vulnerability Assessment of the Transportation Infrastructure Relying on GPS.’ The report served to highlight the potential problems of both unintentional and intentional interference- natural and manmade. And because of the man-made potential- the report concluded that GPS would be a tempting target for hostile action against the U.S. – or other countries that use GPS.

The incidents highlighted here all occurred in the years after the release of the Volpe Report; uses and users are many times higher today than in 2001- while jamming equipment gets easier to acquire. Yet the message seems as lost in the background noise as satellite signals in a solar storm.

As the report noted- radio interference- unintended and intended- can be reduced and some protections improved- but the threat can never be eliminated completely. Hence the interest in a back-up system capable of fulfilling both the full-spectrum area-navigation needs and the precision-timing functions on which society has become dependent.

Sadly for the space-oriented- no solution exists that depends on satellites. The solution is far more down-to-Earth.


The following are back-up options or alternatives: Loran; Loran A; Loran C; or Enhanced Loran (eLoran).

Think of these stages in the development of the original long-range navigation network as the parallel to the development of the Global Navigation Satellite Systems (GNSS). The GNSS networks started with a few relatively crude satellites. The small number meant that that full navigation capability occurred over any point of the planet for only a few hours at a time every few hours.

As the constellations grew and the electronics and software evolved- the service improved; accuracy grew and the constellations expanded to a number guaranteeing that any point on the planet enjoyed full-strength sat-nav any moment of any day with at least four birds always in view of the receivers. So it went with Loran.

Developed during World War II as the LRN – Loomis Radio Navigation (after inventor Alfred Lee Loomis) – Loran employs very low-frequency- high-power radio signals to broadcast a coordinated timing code identified as coming from a known station location. The receiver determines the time difference between the signals to determine a distance from those stations – and then makes another comparison with another pair of stations- typically a pair sharing one of the stations with the first pair.

The navigation receiver then calculates where those distances overlap or triangulate to fix a position. Originally- technicians decoded messages to get information on where to chart the distance from each pairing; today- sophisticated digital computing power does the job – faster- and more accurately.

Over the years the Loran system evolved from the original Loran A to the Loran C and- ultimately- eLoran - and it’s eLoran that Europe and the U.S. both identified as the candidate to back-up GPS.

The U.S. Department of Homeland Security announced in 2008 that the nation would adapt eLoran as its national backup to GPS- but that decision was rendered null and void when the Coast Guard declared Loran C unnecessary for oceanic navigation and a funds-strapped government decided to shut down Loran C. The signals stopped in February – and with the end of Loran C- eLoran development and progress toward a GPS back-up in the U.S. ended.

The decision seems both unfortunate and foolish given the existing history of satellite disruptions- the growing potential for jamming- the inability to totally preclude jamming and the common- sense idea of backing up critical functions with comparably capable technology. The decision also is at odds with steps taken in the wake of the Volpe Report.

eLORAN: CHEAP- AVAILABLE- WORKABLE eLoran builds on the existing hardware and infrastructure erected to support Loran C- involving primarily conversion to digital transmitters and changes in how they work. Receivers built to use GPS or GLONASS or other satellite-based navigation signals can be equipped with a chip that receives and uses the eLoran signal and seamlessly moves in to replace a lost GPS signal.

eLoran is capable of the same accuracy level as WAAS GPS- making it capable of supporting the hosts of new capabilities underpinned by WAAS corrections- and eLoran’s timing signal can also fulfill the same function as the timing functions of GPS. Elsewhere in the world the U.S. announcement from 2008 continues to carry more weight than the 2009 decision to let Loran fall silent. Continuing tests and demonstrations showing the vulnerabilities of GPS continue to impress on other nations the need to leverage a back-up system with none of the same vulnerabilities of satellite-based navigation.

And that’s really the message of their argument. Both Loran and GPS (and all its GNSS parallels) suffer from their own deficiencies and vulnerabilities. For example- just as GPS is vulnerable to very low power interference- Loran is vulnerable to disruption from the electronic noise generated by lightning. GPS is largely immune to lightning interference; similarly- Loran is immune to electronic noise in high frequencies or below its own power level.

The fact that these two technologies share virtually none of their flaws only serves to reinforce the importance of deploying both systems. Admittedly- eLoran isn’t particularly useful out in the vast areas of the world’s oceans out of reach of the 1-200 to 1-500 miles range of the transmitters; but it’s improving and improvable. In the end- most of the risks from losing satellite integrity are primarily landbased – whether navigation or infrastructure support.

For the security of our navigation and the integrity of our systems- the nation’s GPS users must have a conversation with their government and impress officials that a backup is needed.

As the world has seen in the Gulf of Mexico- it’s more than the odds of failure we must address – it’s the price of such a failure that we should be smart enough to avoid paying. A 1/10th of one percent chance of failure with a 100-percent chance of catastrophe is not good odds making.


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