MITCHELL J. NARINS and LEO V. ELDREDGE
U.S. Federal Aviation Administration
PER ENGE and SHERMAN C. LO
MICHAEL J. HARRISON and RANDY KENAGY
Aviation Management Associates
Positioning, navigation, and timing (PNT) services are the key basis for the provision of both essential (safety and security) and economically beneficial applications worldwide in the 21st century. Whether users are ground-based or sea-based or in the air, their primary/go to source of PNT is a Global Navigation Satellite System (GNSS). While the transition of various users/modes of transport from legacy PNT aids to GNSS is at varying stages, it is of concern that the ability of users to revert from the highly accurate positioning, area navigation (RNAV), and precise time and time difference provided by GNSS back to previous methods, which may provide lower levels of performance, will require higher levels of user skills, knowledge, and abilities—capabilities that may no longer be available when needed without significant investment in equipment sustainment and upgrade and in-depth training and practice.
It is most necessary that the transition from GNSS-provided PNT services to an alternate means of achieving PNT ensures safety and security, precludes significant loss of economic benefits, and requires little change in the way operations are carried out and that it is a robust PNT solution through the development and integration of an Alternative PNT (APNT) capability. The Federal Aviation Administration (FAA) is initiating an APNT program to research various alternative strategies that will ensure that the PNT services necessary to safely, securely, and effectively support the U.S. National Airspace System’s (NAS’s) transition to the Next Generation Air Transportation System (NextGen) are available. This paper discusses the scope of the problem, including the extent of known and
predictable and unknown and unpredictable jamming, and each of the alternative strategies identified so far, and their pros and cons.
To properly address the need for robust radionavigation, it is prudent to first agree upon what is meant by the term robust. After exploring a number of sources, the most appropriate definition found, one that applies to processes, organizations, or systems and best promotes the theme of this discussion, is the ability to withstand or overcome adverse conditions. This then leads us to define robust radionavigation as the provision of PNT services that are strong, sturdy, and able to withstand or overcome adverse conditions.
For radionavigation, the term adverse conditions implies situations where the accuracy, availability, integrity, or continuity of the data or information carried by a radionavigation signal is impacted so as to produce unacceptable, unsafe, or unsecure results. This occurs in the presence of interference.
Interference comes in a number of different varieties. It can be intentional or unintentional. Many, if not most, instances of radionavigation interference have been from sources that were totally unaware that they were causing a problem. Interference can be predictable or unpredictable. Some radio interference is actually planned and mitigations can be put in place to minimize, if not eliminate adverse effects. Interference can be both manmade and environmental. Recently much discussion has occurred in regards to our solar cycle and how increased sun-spot activity has the potential for significant impacts to GNSS-provided services. Interference can be crude or sophisticated (sometimes referred to as jamming or spoofing), the latter being much more problematic. While losing radionavigation services is never pleasant, not knowing that the services have been lost and relying on instrumentation that is faulty can be much worse. Interference can be either widespread, affecting hundreds of square miles and thousands of feet of airspace, or localized, affecting only specific operators and operations. Finally, interference can be continuous or random. While a constant-on jammer causes problems, locating one that randomly “pops up” and stays on for short periods of time can be much more problematic, because it promotes uncertainties in users—the “should I or shouldn’t I” problem. In the case of safety and security operations, the answer is inevitably “I should not,” making the intermittent interferer as effective, but more deceptive than the constant interference source.
Still, when assessing whether a condition is adverse, one must do so in light of the radionavigation system being employed—both on the transmitter and receiver ends. What is adverse for one may not be adverse for another, and that is a basis for determining an appropriate alternative PNT strategy that ensures safety and security and minimizes the impact to the economy. Some PNT systems rely on extremely low-power signals while others employ high-power transmissions. Some rely on line-of-sight signals, while others employ ground waves. Some have
been designed from the start to work in adverse conditions, while others expect every day to be a sunny day.
The message is that the world has already changed and is still changing. Interference occurs more and more often—from both predictable and unpredictable sources. The most prudent course of action by both suppliers and users of radionavigation services is to ensure that they fully appreciate the potential for real-world interference and plan and design accordingly.
GROWING SOURCES OF INTERFERENCE
Certainly the most predictable source of interference to GNSS-provided PNT is exercises conducted by military organizations, whose missions require them to be able to both deny services to opposing forces and operate in GNSS PNT-denied situations. To ensure their readiness, a significant amount of testing is required. Figure 1 denotes the locations, extent, and duration of GNSS interference events originating from the U.S. Department of Defense (DOD) sources. To ensure that neither the FAA nor the DOD mission is impaired, FAA and DOD coordinate these exercises to ensure that the safety, security, and economic benefits of the U.S. NAS are not impaired and that the need for DOD readiness is properly supported.
However, unpredictable interference is much more insidious and is becoming a much bigger problem day by day, driven in part by peoples’ awareness that the GNSS receiver in their car or mobile phone allows others to track their location. In
FIGURE 1 Adverse condition: GPS testing by DOD.
response, a number of manufacturers have produced what they call personal protection devices, small, compact jamming devices that are sold to either interfere only with GNSS signals or to jam both GNSS and cellular telephone transmissions. Figures 2, 3, and 4 provide images of just some of the devices that, while illegal in most parts of the world, are readily available on the Internet.
According to its specifications, also available on the Internet, the jamming device shown in Figure 2 is capable of transmitting 0.5 W of power on the Global
FIGURE 2 So-called “personal protection device.”
FIGURE 3 A few more “personal protection devices.”
FIGURE 4 So-called “super HOT jammer cell phone jammer.”
Positioning System (GPS) L1 frequency (1575.42 MHz). While it claims to be effective for only 2–10 meters, in actuality its range can extend hundreds of meters and cause significant disruption to other GNSS users—even those involved in providing safety and security services. Its price on the Internet is listed as $33.
For a bit more, personal protection devices are available that will jam multiple GNSS and cellular telephone frequencies. Some of these jammers can produce an interference signal that exceeds 5 W.
A recent addition to the jammers available on the Internet is shown in Figure 4. While it does not profess to operate on GNSS frequencies, the ability of this device to do so given the frequency ranges for which it does operate is clear. One can only imagine the effect of these devices if carried aboard planes or trains or ships or buses.
As a provider of safety and security radionavigation services that provide significant economic benefit, the FAA is keenly aware of this ever-emerging problem. That is the first step—to be aware that as a GNSS service user or supplier you are operating in harm’s way. Figure 5 denotes an excellent example of this. Here, the FAA has installed a Local Area Augmentation System (LAAS), the Ground-Based Augmentation System (GBAS), at Newark Liberty International Airport (EWR), an airport that is ringed by major highways. The system’s extremely sensitive GNSS antennae are located close to the New Jersey Turnpike, where literally many thousands of trucks and automobiles pass by each day—a location dictated by siting criteria based on runway configuration. Being aware of the potential problems, the LAAS program has successfully implemented system design aspects to mitigate the effects of interference sources and maintained safe and secure services. It has been a valuable lesson—one that it is hoped will be taken up by PNT users and suppliers worldwide.
FIGURE 5 In harm’s way—FAA GBAS installation at EWR.
Alternative Position Navigation and Time
The FAA, in compliance with U.S. national policy, needs to maintain aviation operations indefinitely in the event of a GPS interference event or outage. This means both maintaining safety and security while minimizing any economic impact. From the FAA’s perspective, a key aspect of any alternative is that NAS services can be continued throughout an interference event. Waiting for the source of the interference to be located and turned off is not an acceptable alternative.
As the FAA migrates today’s NAS to the NextGen, the reliance on GNSS-provided PNT services will only grow. As NextGen evolves from a ground-based system of air traffic control to a satellite-based system of air traffic management it will rely more and more on aviation-specific GNSS-technology applications. These applications will allow more aircraft to safely fly closer together on more direct routes, thus reducing delays and providing unprecedented benefits for the environment and the economy.
To maintain safety and security and minimize impact to the economy, an alternative means of providing position, navigation, and timing services must be sought. The FAA has, therefore, initiated an APNT program to research various alternative strategies that will ensure that the PNT services necessary to safely, securely, and effectively support today’s NAS and its transition to the NextGen will be ensured. An important realization is that today’s air traffic control system cannot simply be scaled up to handle the predicted 2X traffic in the future. Nor
can air traffic controllers handle such an increase using radar vectors. Automation and surveillance systems requiring PNT services will need to separate aircraft performing trajectory-based operations (TBOs) based on area navigation (RNAV) and required navigation performance-based (RNP) routes. Controllers will need to intercede only to provide “control by exception.”
The value of RNAV/RNP is shown in Figures 6 and 7. Figure 6 shows the number of aircraft that can safely “fit” into a 10-nautical mile (nm) airspace depending on the navigation performance available. The navigation performance is a combination of the navigation service provided, the navigation capability of the aircraft avionics, and the ability of the pilot and onboard systems to fly the intended path. As you can see, the number of aircraft capable of safely using the airspace increases dramatically as the capability reaches RNP 0.3. The reason for this increase is further denoted in Figure 7.
A radionavigation/avionics system providing only RNP 1.0 capability would not be sufficient to allow aircraft to safely maintain a three-mile separation standard—the standard desired to support better airspace utilization and advanced procedures under NextGen. With RNP 0.3 capability, not only can three-mile separation be safely achieved, but also it should support procedures for parallel runway operations. It is, therefore, most important that the PNT services that support the safe, secure, and efficient operation of the NAS not be impaired and that an APNT system be developed.
FIGURE 6 The value of RNP to airspace capacity.
FIGURE 7 The benefit of providing RNP 0.3.
The determination of the solution to any problem starts with a description of the problem and a realization that trade-offs will be necessary to reach a realistic and implementable outcome. The problem statement is fairly simple—the NAS operations now and in the future will rely heavily on PNT, most PNT today and more in the future will be derived from GNSS, and GNSS-provided PNT services are vulnerable to adverse conditions. Figure 8 denotes the possible trade-space of solutions.
On the left are the operational contingencies that rely on procedural air traffic control. These alternatives cannot support the “normal” capacity of the NAS, so many aircraft will not be able to fly their intended routes—or in many cases, fly at all. Safety and security will be maintained, but economic impact will be great. On the right are redundant capabilities, which provide all aspects of the systems—in the air and on the ground PNT services equivalent to that provided by GNSS. Safety, security, and economic benefit are maintained for these alternatives, but the costs and resources associated with their implementation may not be realistic—especially in an industry where the potential for avionics and infrastructure changes are measured in decades rather than years. A prudent middle ground are alternatives that provide a backup capability. While not totally
FIGURE 8 APNT trade-space.
eliminating potential economic impact, they minimize the impact to an acceptable level while ensuring safety and security are maintained.
Therefore, the goal of the FAA’s APNT research is to provide a cost-effective alternative PNT service that:
- Ensures continuity of operations in NextGen
- Provides performance-based navigation (PBN)—RNAV/RNP
- Supports dependent surveillance operations (automatic dependent surveillance—broadcast, both out and in)
- Supports trajectory-based operations (TBO) and four-dimensional trajectories (4DT)
- Supports all users (general aviation, business, regional, air carrier, military)
- Minimizes impact on user avionics equipage by leveraging existing or planned equipage as much as possible
- Supports backward compatibility for legacy users
- Minimizes the need for multiple avionics updates for users
- Provides long lead transition time (circa 2020 transition)
It is also important to the FAA to avoid the potential $1.0B costs of having to recapitalize the existing very-high-frequency radio range (VOR) system that currently supplies a non-GNSS backup position and navigation capability, albeit not to the accuracy of GNSS and without area navigation capability. The VOR backup cannot support RNAV/RNP and does not provide a GNSS-independent timing capability. The FAA hopes to disestablish all VORs by 2025.
In order to determine the viability of alternative solutions, the FAA first assessed the minimum PNT requirements an acceptable alternative would need to provide. These requirements are shown in Figure 9. On the left are listed the various airspace domains, i.e., en route, terminal, LNAV (lateral navigation/non-precision approach), LPV (localizer with precision vertical), and GBAS-enabled Cat I and Cat III landings. On the right are the systems that provide the necessary capabilities to support these operations. In the middle are the navigation and surveillance requirements required for each operation—navigation measured in accuracy and containment with integrity and surveillance measured by Navigation Accuracy Category (NAC) and Navigation Integrity Category (NIC). After much analysis and discussion, the requirements for an APNT system were set at the level shown, i.e., an acceptable APNT system will need to support navigation and surveillance down through LNAV/non-precision approach.
Another key metric/parameter not discussed so far is “where.” Where does an APNT system need to provide what performance? It is apparent that neither the U.S. NAS nor any other NAS is homogenous. There are key areas where capacity requirements significantly increase. In the United States, the FAA has identified 135 terminal areas where significant capacity is required and where loss of capacity would cause significant economic impact. Figure 10 denotes these areas as seen from Flight Level (FL) 180 (18,000 feet).
The FAA has categorized the airspace into three zones. Zone 1 is the airspace at FL 180 and above—all the way to FL 600 (60,000 feet). Zone 2 is the airspace that is below FL 180 and above 5,000 feet above mean sea level (MSL). Zone 3 is the
FIGURE 9 Performance-based navigation and surveillance requirements.
airspace that supports terminal operation in high-density areas. It is defined as starting 500 feet above and out to 5 statute miles (sm) from the airport, and then going up at a 2 degree angle to 5,000 feet. Figure 11 shows these three different zones.
Definition of these zones and the PNT requirements within these zones was necessary to be able to appropriately bound solutions that rely on ground-
FIGURE 10 High capacity need areas in the conterminous United States (CONUS).
FIGURE 11 PNT performance zones.
based and line-of-sight assets. Throughout the FAA’s analysis of alternatives and selection of solution(s), safety and security will always be ensured and services provided where economics warrant.
In looking for potential solutions, the FAA has concentrated on the availability of systems onboard aircraft and how to leverage existing and future equipage to facilitate an acceptable solution with a reasonable transition time. Figure 12 shows the various systems on the aircraft and where APNT solution(s) might best fit in.
The FAA has concentrated on three categories of solutions that appear promising, while inviting input from the public and industry at meetings, symposia, and conferences on other potential areas of research. The three categories that are currently being considered are (1) Optimized Distance Measuring Equipment (DME) Network, (2) Wide-Area Multilateration, and (3) DME Pseudolite Network. Each will be described below, along with the pros and cons associated with each potential solution.
Optimized DME Network
Historically DMEs provide pilots with slant range distance from their aircraft to the DME. DMEs that are collocated with VORs traditionally provide pilots with their slant range distance to the end of an airway, while DMEs that are co-located with landing systems at airports provide pilots with their slant range to runway ends. Avionics engineers recognized that because aircraft at altitude could see a number of DMEs, a system using multiple DME ranging sources could provide
FIGURE 12 Potential APNT solutions on aircraft.
pilots with their position. However, because the DME network was not designed or laid out for this function, gaps in service coverage exist—caused by lack of DMEs or lack of necessary geometry between available DMEs. The current population of DMEs in CONUS (continental United States) is shown in Figure 13, many of which are associated with military tactical navigation (TACAN) facilities. DMEs provide high-power transmissions, typically 1,000 W.
While a DME network solution leverages existing technology and systems and will have the least impact on avionics for air carriers, there will be a significant impact on general aviation, where avionics are not available. While the FAA is planning to fill gaps in the DME coverage at FL 180 and above, this assumes that aircraft are equipped with inertial reference units (IRUs) that allow them to coast through gaps in coverage. Aircraft without IRUs are currently not authorized to fly RNAV/RNP routes and even those aircraft with an IRU are not authorized to conduct a published approach procedure requiring less than RNAV/RNP-1.0. There is also a concern that a significant increase in use of the DME network could cause interrogation saturations and impact service delivery. Finally, unless general aviation can be equipped with DME RNAV capability, there may be a need to retain and recapitalize a large number of the VORs at a substantial cost.
Wide Area Multilateration
Wide Area Multilateration (WAM) utilizes signals that are transmitted frequently from an aircraft equipped with ADS-B to determine the aircraft’s position. Figure 14 denotes the sequence of events that occur that would allow an aircraft to learn its position in the event of a loss of GNSS-provided PNT.
FIGURE 13 1,100 DMEs in CONUS.
FIGURE 14 Passive wide area multilateration.
Ground-based transceivers (GBTs) being installed to support ADS-B can utilize this technology to determine aircraft position in the event that the aircraft cannot. The national ADS-B GBT system is shown in Figure 15.
By leveraging the DME installed base and the planned GBT installations, coverage across CONUS would be greatly improved. Figure 16 shows this combined infrastructure.
While the WAM solution will have a minimal impact on existing avionics for surveillance, integrity monitoring and Time-to-Alert necessary to meet navigation requirements may be very challenging. Still, accuracy has been demonstrated to be within target levels and it is compatible with existing WAM systems. There are, however, concerns regarding the availability of bandwidth on the 1090 MHz extended squitter channel so that capacity may be limited in high-density environments. Use of WAM for navigation will also entail changes to existing avionics.
WAM also requires that each of the ground stations maintains a common time reference as WAM is a time-of-arrival system. Current systems utilize a common beacon that can “be seen” by all systems as the synchronizing mechanism. Wider-area systems may encounter issues and certainly additional costs if beacons were the only means to maintain synchronization.
DME Pseudolite Network
DMEs broadcast in the L-band, the same area of the spectrum as GNSS. They work by receiving interrogations from aircraft and replying after a fixed delay, thus allowing the aircraft to determine its slant range to the DME. When a DME is not being interrogated it maintains a “heartbeat” awaiting the next interrogation. The DME Pseudolite (DMPL) solution would include a transmission on the DME
FIGURE 15 800 GBTs to be installed nationwide.
FIGURE 16 Combined DME and GBT network.
heartbeat, which would be maintained continuously, thus identifying the particular DME, its location, and the time-of-day. The aircraft, using the same methodology employed by GNSS and WAM systems, would determine its position. As the aircraft would receive the “raw” data, it would be left to the aircraft to determine the integrity of the derived information, just as it does for GNSS.
The DMPL alternative provides unlimited capacity and an aircraft-based position and integrity solution and could leverage use of existing DMEs and
GBTs. However, it would require modification to the DME transmit signal. It would require a minimum of three sites to compute aircraft position (unless the DMEs interrogation/reply capability was also utilized, and then two would suffice). The DMPL alternative would also require a common GNSS-independent timing reference similar to that needed by the WAM solution. While it would have the greatest impact to aircraft avionics, it could potentially provide the most benefit. There is the potential to include position calculation and integrity monitoring functions in ADS-B in avionics. Because it is the least mature concept, no avionics are yet in development and no standards have been established. It would also require the retention and recapitalization of nearly half the VORs unless general aviation is equipped with Pseudolite avionics.
The need to provide time synchronization for both the WAM and DMPL alternatives, as well as the need to provide frequency services for telecommunication applications, caused the FAA to research alternative time and frequency provision as part of the APNT effort. During the problem analysis phase the FAA determined that if the sources of GNSS interference were so great as to preclude use of any satellite in any direction, the situation would be outside the FAA’s means to mitigate the time service interruption. Therefore, the FAA assumed that the interfering source would arrive from at most a few directions and that by utilizing a steerable null antenna, the jammer could be substantially eliminated and a source of good time and frequency reinforced. Figure 17 shows how this concept would work.
FIGURE 17 Ground-based time synchronization.
Steerable null antennas located at ground facilities (either DME or GBT) should be able to sufficiently null out interfering signals while reinforcing the time and frequency signals from a satellite—whether it be in GEO, medium, or low Earth orbit. This would allow GBTs or DMPL or both to continue providing multilateration services despite a GNSS service interruption.
In pursuit of the best APNT solution(s), the FAA is developing a Project Plan for Full Investigation, the means to validate backup requirements and is performing appropriate system engineering analyses. The FAA plans to develop R&D prototypes along with cost schedule estimates while it completes the analysis of alternatives. The schedule for accomplishing these actions is show in Figure 18.
First and foremost, the APNT remains a research endeavor. The “best” answer is still, as they say, to be determined. What is most important is that the potential problems and impacts have been recognized and steps are being taken to ensure that the safety, security, and efficiency of the U.S. NAS will be maintained in the event of a loss of GNSS-provided PNT.
FIGURE 18 APNT program life cycle.
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