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| Alaska Airlines, which pioneered RNP, has been flying RNP approaches into 27 airports. In 2011, these approaches saved the airline 210,000 gallon of fuel and $19 million. |
It may seem strange, almost unbelievable, that while GPS and its expanding international equivalents have never seemed better, with its worldwide acceptance growing virtually by the minute throughout almost every aspect of human life, there are now concerns, including within its Department of Defense (DoD) owners, about what should replace it. Yet, just 40 years after its basic concepts were defined by a small group of U.S. military officers, that is the case today.
Why is that? How did a military positioning aid, initially conceived as a highly classified American navigation and weapons delivery system, become transformed into a public utility that, if shut off tomorrow, could wreak havoc around the world, due to its reach into the most remote corners of the earth? Tens of millions of words have been written about the system’s applications, from the mundane to the totally extraordinary, but only recently have serious concerns been raised about its limitations.
Ironically, these are a direct result of the system’s original aim of making the GPS satellites’ signals extraordinarily difficult to detect and use, through a combination of very low transmitted signal power and signal concealment techniques, and this remains true today. Low power was desirable in any event to assure long battery, hence long orbital life, but dropping it to -160dBm, a level well below the ambient radio “noise,” called for unique technology to detect and recover its full characteristics, while still preventing its use by adversaries.
Signal concealment relied on the then top secret “frequency hopping” concept, now known as today’s unclassified and widely used spread spectrum technology. But the combination of very low power and spread spectrum continues in use with America’s GPS and compatible foreign systems.
The drawbacks of the system’s low signal power were first made public by the Department of Transportation’s Volpe Development Center in 2001. Volpe scientists were cautioning civil GPS users that the system had shown itself to be vulnerable to interference from more powerful low frequency transmissions. By 2009, however, interference reports increased significantly with the introduction of GPS vehicle tracking systems and the corresponding increase of low-cost portable GPS jamming devices designed to foil them.
While no statistics are available, there are now estimated to be several thousand jammers in daily use on U.S. highways, despite their possession being illegal. Primarily, they are used by long distance truckers, whose vehicles are usually equipped with remote GPS receivers that can be interrogated by company dispatchers to monitor vehicle locations, which the jammer negates. But many other uses have been reported, including concealment of hijacked trucks carrying high value goods, stealing expensive cars and even the avoidance of vehicle monitoring by suspicious spouses.
In themselves, none of these has so far seriously threatened public safety. However, vehicle jammers can affect nearby aviation GPS receivers. For example, a Category 1 GPS landing guidance system undergoing certification at Newark Liberty International Airport in New Jersey suffered frequent shutdowns during acceptance testing due to jammers operating in vehicles on the adjacent turnpike. Modifications to improve jamming resistance have reduced the number of shutdowns at Newark, but Avionics Magazine understands the complete solution may include moving the system’s four tower-mounted GPS monitor receivers much further from the turnpike.
Other than at Newark, there have been few reports of GPS interference that affected aircraft operations. Usually, this is because the majority of current small jammers have quite short ranges, which aircraft quickly pass through. Nevertheless, with the expected increasing use of more powerful jammers, GPS reliability as a key element of FAA’s NextGen will come under pressure and could even risk becoming uncertifiable for essential applications such as RNP in restricted terrain and GPS precision landing guidance, with potential impact on ADS-B position reports at lower altitudes, and during less critical SBAS procedures. And paradoxically, while the steadily increasing numbers of compatible foreign navsats is unquestionably a navigation benefit, their added individual transmissions within the GNSS band has the undesirable side effect of raising the ambient noise “floor,” thereby further weakening the satellite signals.
Interestingly, however, the major concern about aircraft GPS interference has come from the Department of Defense.
“The relatively weak broadcast signal from space can be jammed, precluding UAS operations. Until onboard systems that do not rely on GPS can be fielded, assured position, navigation and timing is a critical UAS concern,” according to the U.S. Air Force’s 2009-2047 Unmanned Aircraft Systems (UAS) policy document.
And while this statement only concerned unmanned systems, adversary jamming at long ranges and high altitudes is an equal DoD concern across its air, land and sea forces, demonstrated by its twice yearly NOTAM’d exercises on the east and west coasts, where very powerful jammers, emulating U.S. and foreign jamming technologies, radiate interference signals out to beyond 350 miles, from ground level to above 40,000 feet.
And now, there’s “spoofing.” While DoD still questions Iran’s December claim that it spoofed a U.S. Air Force RQ-170 Sentinel reconnaissance aircraft by sending it higher powered false GPS coordinates and directing it to crash land in Iran, spoofing’s potential threat has long been recognized as a troubling game changer in military UAV and other operations. Consequently, DoD is studying other, preferably exclusive, navigation alternatives to completely avoid jamming and spoofing.
GPS Alternatives
➤ Enhanced GPS receiver jamming protection: This is part of the current government/industry receiver standards initiative, following the LightSquared affair. But total anti-jam protection is impractical and retrofits to current units could be cost prohibitive. Some corporate aircraft are reported to have installed military controlled reception pattern antennas (CRPA) that evaluate all incoming signals throughout 360 degrees and block reception of segments that appear to include jamming. Unfortunately, jamming is a battle of escalating power and, inevitably, higher powered ground-based jammers will usually win.
➤ Increased satellite transmitter power: The converse of increased receiver protection is to increase the transmit power of the GPS satellites. In fact, the next generation GPS III constellation, with its first launch in 2015, will be able to direct higher power “spot beams” against areas of the earth’s surface where jammers have been detected. But increasing the transmit power of the satellites currently in orbit appears impractical and, even if feasible, would be extremely costly.
➤ Anti-spoofing technology: At present, this does not appear to exist, although undoubtedly research is underway. Reportedly, the military GPS M-Code is virtually immune to spoofing due to its strong encryption and therefore all M Code-equipped aircraft are protected. However, it is understood that most non-front line military aircraft use the unencrypted GPS Standard Positioning Service (SPS), as do virtually all civil aircraft, and are therefore vulnerable to spoofing. (It has been suggested that the RQ-170 UAV reportedly hijacked by Iran would also have been SPS equipped.) Consequently, until anti-spoofing technology is available, GPS-based back up systems depending on other satnav constellations, such as Europe’s Galileo, cannot be relied upon for anti-spoofing protection.
Three anti-jamming and anti-spoofing measures have been proposed.
➤ Inertial reference systems (IRS): IRS does not rely on incoming signals and, in newer airline aircraft, IRS outputs are generally integrated with GPS via the flight management system computer. In a jamming situation causing the loss of GPS, the flight management system would automatically switch to sole IRS guidance until the GPS data returned and was verified, when GPS guidance would resume.
In a spoofing attack, however, there would be no GPS failure alert, and the only warning would be an FMS alert of a disparity between the apparently fully operational GPS and the IRS, showing an increasing divergence from the desired flight path, unless that path was itself defined by GPS coordinates, and could also be spoofed. A route defined by lat/long coordinates would not be easily spoofed.
The two main drawbacks to the IRS backup solution are: 1) The cost of an IRS installation and its FMS integration could put it out of reach for most mid-size regional and corporate aircraft, and all smaller aircraft; and 2) When the IRS is “free running” without continuous position updates from GPS, small heading errors accumulate over time, and these tend to increase more rapidly during maneuvering, such as during radar vectoring in a terminal area, resulting in increasing flight path divergence.
➤ Scanning DME plus VOR: This year, FAA proposed this combination as its solution to GPS jamming. Its appeal lies in the fact that CONUS airspace has a well established DME and VOR network, although some ground installations would have to be re-sited to optimize coverage, and up to 50 additional DME stations would be required to provide sufficient signals to support simultaneous scanning DME positioning from at least two ground stations, but preferably three, to avoid fix ambiguities.. DME and VOR would be supplemented by GPS-like pseudolites and multilateration networks. The drawback here is that scanning DME avionics are common in newer large aircraft, but much less common in mid-size airline and corporate aircraft and rarely found in smaller machines.
As well, both DME and VOR are line of sight navigation aids, with FAA’s analysis proposing they not be used in IFR below 5.000 feet. Additionally, with aviation representing less than 10 percent of the total GPS user community, obtaining funding for additional facilities dedicated exclusively to aviation might be difficult.
➤ eLoran: eLoran is a modernized derivative of the earlier Loran-C system, and still employs high-powered, long range, unjammable low frequency signals extending from the surface to above jet altitudes, transmitted from widely separated ground stations. But while Loran-C stations were arranged in user selected regional groups, eLoran stations operated individually, providing GPS-like “all in view” operation, where the receiver automatically selects those stations with optimum fix geometry and performance.
Earlier FAA analysis of eLoran accuracy indicated its potential to support required navigation performance (RNP) 0.3 throughout the CONUS. However, eLoran’s major drawback is that no avionics equipment is available or in development, although there appears no reason that small, low-cost units could not be produced.
One critically important benefit of eLoran over the other two backup approaches above is that it is one of just three sources — the others are GPS and laboratory atomic clocks — of super accurate time signals. Loran timing units frequently backed up GPS time in a variety of critical national infrastructure applications, including the FAA. The time provision makes eLoran a totally multi-mode, air/land/sea positioning system.
➤ Current military alternatives: Through the years, DoD has investigated a number of positioning systems that could support military GPS-denied situations For example, much work has gone into development of miniaturized IRS units for UAVs and small aircraft, although commercial reliability levels appear elusive.
Area positioning systems used by the surveying industry have particularly been evaluated, due to their high accuracy, GPS jamming and spoofing immunity, rapid set up and breakdown procedures and ease of portability to new areas of concern. Currently, the Australian Locata system is reported to be of interest to DoD.
First, an alternative means to continue operations, even when less efficient, is essential to safe navigation. But second, and perhaps less appreciated, is that the knowledge that an alternative system, having different failure paths, will activate should the primary navaid be disabled, would be a significant deterrent to an attacker from the start. A backup to GPS would therefore not only enhance safety, but would also enhance GPS longevity.
The Future
The one certainty is that no credible near-term GPS replacement, offering all that system’s benefits, is in sight today.
In June, however, the Defense Advanced Research Programs Agency (DARPA) solicited industry bids for the development and tests of systems proposed for its conceptual All Source Positioning and Navigation (ASPN) program. This seeks low cost, robust and seamless navigation solutions for military users on any operational platform and in any environment, with or without GPS, and would rely on selectively integrating a wide range of current or future sensor techniques.
Perhaps coincidentally, BAE Systems recently announced its Navigation Via Signals of Opportunity (NAVSOP), which is claimed to accept signals from GPS satellites plus ATC, TV, radio, Wi-Fi and cellular communications towers and, oddly, GPS jammers. NAVSOP’s accuracy is claimed to be “within a few meters.”
“The technology can also reach areas that GPS is unable to penetrate, such as dense urban areas, deep inside buildings and even underground and underwater,” the company said.