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Alternative Position, Navigation, and Timing: The Need for Robust Radionavigation--Mitchell J. Narins, Leo V. Eldredge, Per Enge, Sherman C. Lo, Michael J. Harrison, and Randy Kenagy
Pages 119-136

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From page 119...
... , 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 opera tions are carried out and that it is a robust PNT solution through the development and integration of an Alternative PNT (APNT)
From page 120...
... After exploring a number of sources, the most appropriate definition found, one that applies to processes, organiza tions, 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.
From page 121...
... 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.
From page 122...
... 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." Eldredge_Fig2.eps bitmap FIGURE 3 A few more "personal protection devices." Eldredge_Fig3.eps bitmap
From page 123...
... 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 sup plier you are operating in harm's way.
From page 124...
... 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.
From page 125...
... 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.
From page 126...
... 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.
From page 127...
... 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.
From page 128...
... 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)
From page 129...
... 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)
From page 130...
... 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.
From page 131...
... 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 signifi cant 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)
From page 132...
... 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.
From page 133...
... 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
From page 134...
... Time Synchronization The need to provide time synchronization for both the WAM and DMPL alternatives, as well as the need to provide frequency services for telecommuni cation 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.
From page 135...
... This would allow GBTs or DMPL or both to continue providing multilateration services despite a GNSS service interruption. NEXT STEPS In pursuit of the best APNT solution(s)


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